The Book on the Technologies of Polymicro

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The Book on the technologies of Polymicro

• Flexible Fused Silica Capillary Tubing

• Fiber Optic and Capillary Assemblies

• Fused Silica and Quartz Micro-Components www.polymicro.com Forward

Welcome to Polymicro Technologies’ catalog. We pride ourselves on the quality of our products and our service. This catalog represents an effort to provide a tool to aid our customers in understanding our products, the materials with which we work, and the capabilities we can provide. We are unique in our ability to work with the customer from design, to prototype, to large-scale production. We provide a breadth of expertise unparalleled in the industry with custom preform capabilities, tower capabilities to draw fiber and tubing, laser capabilities to create micro-components, and assembly capabilities to put it all together. It is our goal to continually improve our products, our capabilities, and ourselves to meet the challenges of our customers’ technological requirements. Production, Engineering, and Sales at Polymicro are dedicated to your success. We believe firmly in the basic idea that our customers’ success translates to our success. For the latest in new procedures, application notes, and general information visit our website.

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THE PEOPLE OF POLYMICRO TECHNOLOGIES

Polymicro Technologies, a Subsidiary of Molex 18019 North 25th Avenue Phoenix, Arizona 85023-1200 Phone: (602) 375-4100 Fax: (602) 375-4110 e-mail: [email protected] URL:http://www.polymicro.com

Technical Data Disclaimer The information given herein, including drawings, illustrations and schematics (that are intended for illustration purposes only), is believed to be reliable. However, Polymicro Technologies, makes no warranties as to its accuracy or completeness and disclaims any liability in connection with its use. Polymicro Technologies, only obligation shall be as set forth in Polymicro Technologies, standard terms and conditions of sale for this product and in no way will Polymicro Technologies, be liable for any incidental, indirect or consequential damages arising out of the sale, resale, use or misuse of the product. Users of Polymicro Technologies, products should make their own evaluation to determine the suitability of each such product for the specific application.

Page# 1 Introduction History - from 3000 BC ...... 1 - 1 Glass ...... 1 - 2 Quartz and Synthetic Fused Silica ...... 1 - 2 Drawn Glass And Glass Tubing ...... 1 - 3 Physical Properties ...... 1 - 3 High Technology Synthetic Fused Silica and Common Quartz ...... 1 - 3 Difference in Types ...... 1 - 4 Solarized Glass ...... 1 - 4 2 Fiber Optics and Optical Fiber Optical Concepts ...... 2 - 1 Light Sources and Distributions ...... 2 - 1 Getting Light Where You Want It ...... 2 - 2 The Science of Optical Fiber ...... 2 - 2 Total Internal Reflection ...... 2 - 2 Numerical Aperture ...... 2 - 3 Input-Output Phenomena ...... 2 - 4 Tapered Fibers ...... 2 - 5 Matching Numerical Apertures ...... 2 - 5 Fiber Types & Modes of Transmission ...... 2 - 7 Dispersion ...... 2 - 9 Bandwidth ...... 2 - 9 Focal Ratio Degradation (FRD) ...... 2-10 Attenuation & Transmission ...... 2-10 Low -OH and High -OH Optical Fiber ...... 2 - 11 Internal & External Losses ...... 2 - 11 Bend Radius - Optical Effect ...... 2-12 Evanescent Wave Losses in Small Diameter Fibers ...... 2-12 Power Transmission ...... 2-13 Fluorescence in Fiber Optical Materials ...... 2-14 UV Fiber Performance Solarization ...... 2-14 Infrared Waveguides ...... 2-16 Coatings and Buffers ...... 2-16 Broadband Fiber (FBP) ...... 2-17 Mechanical and Environmental ...... 2-18 Mechanical Stress and Fiber Strength ...... 2-18 Radiation Resistance ...... 2-20 3 Flexible Fused Silica Capillary Tubing What is a Capillary? ...... 3 - 1 Applications Employing Capillary ...... 3 - 1 Characteristics Offered by Capillary ...... 3 - 2 Gas Chromatography ...... 3 - 2 Capillary Electrophoresis ...... 3 - 3 Capillary Liquid Chromatography ...... 3 - 3 Mass Flow Control ...... 3 - 4 Other Uses for Synthetic Fused Silica Capillary Tubing ...... 3 - 4 Capillary Products ...... 3 - 5 Cutting and Cleaving Capabilities ...... 3 - 5 Coupling and Connecting Capillary ...... 3 - 6 Pressure Handling Capabilities ...... 3 - 6 Estimating the Flow Rate in Capillaries ...... 3 - 6 Bending Stress in Capillary ...... 3 - 7 Optical Properties of a Capillary ...... 3 - 7

Page# Light Guiding Capillary ...... 3 - 8 Internal and External Coatings and Chemistries: Interior Surface Chemistry of Capillary ...... 3 - 9 Internal and External Coatings and Chemistries: Coatings, Interior ...... 3 - 9 Internal and External Coatings and Chemistries: Coatings, Exterior ...... 3-10 Alternative Cross-Sectional Geometries ...... 3-10

4 Fiber Optic & Capillary Assemblies What are Assemblies? ...... 4 - 1 Applications ...... 4 - 1 Design Considerations ...... 4 - 2 What Does Polymicro Need From You? ...... 4 - 2 The Assembly Designer Will Determine ...... 4 - 2 Low Fiber Count Assemblies ...... 4 - 2 High Power Laser Connectors ...... 4 - 3 Fiber Optic Cable ...... 4 - 4 Bundles and High Fiber Count Assemblies ...... 4 - 5 Capillary Assemblies ...... 4 - 8 The Polymicro Advantage ...... 4 - 8

5 Fused Silica Micro-Components Sculpted Fiber Tips - Tapers, Cones, Diffusers and Ball Lenses ...... 5 - 1 Sculpted Tips Integral with Optical Fibers ...... 5 - 2 Polishing, Shaping and Finishing ...... 5 - 2 Tapered Fibers ...... 5 - 3 High Power Lasers ...... 5 - 3 Beam Expansion ...... 5 - 4 Ferrules & Splices - For Optical Fibers ...... 5 - 4 Connectors, Ferrules & Splices - For Capillaries ...... 5 - 5 Special Capillaries: Multi-lumen ...... 5 - 5 Special Capillaries: Square/Rectangular ...... 5 - 5 Special Capillaries: Windowed Capillaries ...... 5 - 5 MEMS/NEMS Technologies ...... 5 - 5

G Technical Glossary ...... G-1 to G-6

A Appendix Polyimide Removal from Silica Fibers or Capillary Tubing ...... A - 1 Cleaving Procedure ...... A - 2 General Handling...... A - 3 Units of Measure - Useful Conversion Factors ...... A - 4 Units of Measure - Conversion Units for Light ...... A - 5 Units of Measure - Wavelength Units, Wavenumbers and Photon Energy...... A - 5 Polyimide Characteristics - Polyimide Physical Properties ...... A - 6 Polyimide Characteristics - Physical Properties - Chemical Resistance...... A-6 Quartz/Silica Characteristics - Typical Trace Elements...... A - 7 Quartz/Silica Characteristics - Typical Thermal Properties ...... A - 7 Quartz/Silica Characteristics - Typical Electrical Properties ...... A - 8 Quartz/Silica Characteristics - Typical Mechanical Properties ...... A - 8

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Optical Information - Refractive Index versus Wavelength Reference Table measured at 20o C...... A - 9 Optical Information - Product Application Spectrum ...... A - 9 Optical Information - Optical Properties Optical Window Transmittance, Corning #7980, Fused Silica, 10mm thick ...... A-10 D Product Reference Descriptor Optical Fiber SILICA/SILICA Optical Fiber - High -OH Core ...... FV ...... D - 1 SILICA/SILICA Optical Fiber - Ultra Low -OH Core ...... FI ...... D - 2 SILICA/HARD CLAD Optical Fiber - Low -OH Core ...... JTFLH ...... D - 3 SILICA/HARD CLAD Optical Fiber - Ultra Low -OH Core . . .JTFIH ...... D - 4 SILICA/SILICA Optical Fiber - Broadband Optical Fiber . . . .FBP ...... D - 5 SILICA/SILICA Optical Fiber - Broadband Optical Fiber . . . .FBPI ...... D - 6 for Industrial Applications SILICA/SILICA Optical Fiber - High-OH Deep UV Enhanced .FDP ...... D - 7 SILICA/SILICA Optical Fiber - Solarization Resistant ...... DUV ...... D - 8 SILICA/TEFLON® Clad Optical Fiber ...... FSU, FLU ...... D - 9 High -OH or Low -OH Core SILICA/SILICA Optical Fiber - Hollow Silica Waveguide . . . .HSW ...... D - 11 Capillary Tubing Flexible Fused Silica Capillary Tubing ...... TSP/TSG ...... D-12 Thick Wall Flexible Fused Silica Capillary Tubing ...... TSP ...... D-13 Flexible Fused Silica Cap.Tubing – Deep UV Transparent Coating .TSU ...... D-14 Flexible Fused Silica Cap.Tubing – UV Transparent Coating . . . . .TSH ...... D-15 Precision Windowed Silica Capillary Tubing ...... WIN ...... D-16 Flexible Fused Silica Capillary Tubing Pieces – Cleaving/Cutting ...... D-17 Square Flexible Fused Silica Capillary Tubing ...... WWP ...... D-18 Light Guiding Flexible Fused Silica Capillary Tubing ...... LTSP ...... D-19 Polyimide Coated Light Guiding Capillary Fiber Optic and Capillary Assemblies ...... FOA, CTA, FOC ...... D-20 Fused Silica Micro-Components Sculpted Silica Fiber Tips ...... Custom ...... D-22 Fused Quartz/Silica Ferrules and Sleeves ...... Various ...... D-23 Inner-LokTM ...... MLC ...... D-24 Equations Optical Fiber Eq. 2-1 & 2-2 ...... Snell’s Law - Law of Refraction ...... 2 - 3 Eq. 2-3 ...... Numerical Aperture ...... 2 - 4 Eq. 2-4 & 2-5 ...... Fiber End/Angle vs. Output ...... 2 - 4 Eq. 2-6 ...... Tapered Fiber Input - Output Equation ...... 2 - 5 Eq. 2-7 ...... Numerical Aperture Mismatch ...... 2 - 6 Eq. 2-8 ...... F-number ...... 2 - 6 Eq. 2-9 ...... Relation Between F# and NA ...... 2 - 6 Eq. 2-10 & 2-11 ...... NA Match Between Lens System/Optical Fiber . .2 - 6 Eq. 2-12 ...... Attenuation ...... 2-10 Eq. 2-13 ...... Evanescent Wave Field ...... 2-12 Eq. 2-14 ...... Depth of Wave Penetration ...... 2-12 Eq. 2-15 & 2-16 ...... Power Density ...... 2-13 Eq. 2-17 ...... Raleigh Range Perimeter ...... 2-13 Eq. 2-18 ...... Weibull Plot Function ...... 2-18

Inner-lokTM is a registered trademark of Polymicro Technologies Teflon® AF is a trademark of E.I. du Pont de Nemours and Company

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Eq. 2-19 ...... Static Fatigue or Stress Corrosion ...... 2-19 Eq. 2-20 ...... Stress - Strain Equation ...... 2-19 Eq. 2-21 ...... Fiber Bend Stress ...... 2-19 Eq. 2-22 ...... Fiber Bend Radius ...... 2-19

Capillary Tubing Eq. 3-1 ...... Flow Rate ...... 3 - 6 Eq. 3-2 ...... Poiseuille’s Law ...... 3 - 7 Eq. 3-3 ...... Flow Rate ...... 3 - 7

Fused Silica Micro-Components Eq. 5-1 ...... Fiber Taper Beam Expansion ...... 5 - 4 Eq. 5-2 ...... Spot Size to NA Relationship ...... 5 - 4 Eq. 5-3 ...... Recommended Maximum Input NA ...... 5 - 4 Eq. 5-4 ...... Maximum Acceptance Angle ...... 5 - 4

Figures Optical Fiber Figure 1-1 ...... "Line Drawing, Glass Blower At Work" ...... 1 - 1 Figure 1-2 ...... Photophone, A. G. Bell ...... 1 - 1 Figure 1-3 ...... Modern Fiber Drawing Tower ...... 1 - 3 Figure 2-1 ...... Light and Nature ...... 2 - 1 Figure 2-2 ...... Reflections of Some Metal Coatings ...... 2 - 3 Figure 2-3 ...... Refraction, Reflection and Numerical Aperture ...... 2 - 3 Figure 2-4 ...... Numerical Aperture - Acceptance Cone ...... 2 - 4 Figure 2-5 ...... Conical Output ...... 2 - 4 Figure 2-6 ...... Bevel Cut Fiber ...... 2 - 4 Figure 2-7 ...... Bent Fiber ...... 2 - 4 Figure 2-8 ...... Tapered Fiber Transmission ...... 2 - 5 Figure 2-9 ...... Large-end to Small-end Transmission ...... 2 - 5 Figure 2-10 ...... Mae West - Fiber Collimator ...... 2 - 5 Figure 2-11 ...... Stray Light Injection ...... 2 - 5 Figure 2-12 ...... Quartz Refractive Index ...... 2 - 6 Figure 2-13 ...... Fused Quartz Lens, 100mm, f/2 ...... 2 - 7 Figure 2-14 ...... Effects of Focal Length Change with Wavelength and Optical Fiber Coupling ...... 2 - 7 Figure 2-15 ...... Optical Fiber Modes ...... 2 - 8 Figure 2-16 ...... Dispersion ...... 2 - 9 Figure 2-17 ...... Attenuation versus Transmission ...... 2-10 Figure 2-18 ...... Attenuation, Type FI - Low -OH ...... 2 - 11 Figure 2-19 ...... Attenuation, Type FV - High -OH ...... 2 - 11 Figure 2-20 ...... Effects of Cladding Thickness ...... 2-13 Figure 2-21 ...... Typical Attenuation of Polymicro High -OH UV Fiber Products ...... 2-14 Figure 2-22 ...... UV Effects from Deuterium Lamp on Transmission of Various Optical Fibers ...... 2-15 Figure 2-23 ...... UV Damage Following 4 Hour UV Exposure ...... 2-15 Figure 2-24 ...... FVP-UVM Fiber 214nm Degradation/Recovery ...... 2 - 15 Figure 2-25 ...... FDP Fiber 214nm Degradation/Recovery ...... 2 - 15 Figure 2-26 ...... Comparison of Polymicro UV Fibers ...... 2 - 16 Figure 2-27 ...... Spectral Attenuation of Fiber Types ...... 2 - 17 Figure 2-28 ...... Characteristics of Fiber Types ...... 2 - 17 Figure 2-29 ...... Weibull Plot for Fused Silica Optical Fiber ...... 2 - 18 Figure 2-30 ...... Bend Radius vs Stress Level For Different Core Sizes ...... 2 - 19 Figure 2-31 ...... Radiation Sensitivity of Optical Fibers ...... 2 - 20

Capillary Tubing Figure 3-1 ...... Capillary Tubing Bend Stress ...... 3 - 7 Figure 3-2 ...... Light Path Through Capillary Windows ...... 3-10 Figure 3-3 ...... Examples of Capillary Geometries ...... 3 - 11

Fiber Optic & Capillary Assemblies Figure 4-1 ...... Polymicro Technologies Low Fibert Count Assemblies ...... 4-3 Figure 4-2 ...... Polymicro Technologies Laser High Power Connector ...... 4-4 Figure 4-3 ...... Termination Configurations ...... 4 - 4 Figure 4-4 ...... Sample Cable Constructions ...... 4 - 5 Figure 4-5 ...... Example Hex-pack Characteristics ...... 4 - 6 Figure 4-6 ...... Fiber Hex-pack ...... 4 - 6 Figure 4-7 ...... Bundle Diameter vs Number of Fibers ...... 4 - 7 Figure 4-8 ...... Tight vs Linear Fiber Packing ...... 4 - 7

Fused Silica Micro-Components Figure 5-1 ...... Fiber and Focusing Sphere ...... 5 - 1 Figure 5-2 ...... Two Sphere Lenses as Fiber Coupler ...... 5 - 1 Figure 5-3 ...... Sculpted Fiber Tip Examples ...... 5 - 2 Figure 5-4 ...... Laser Coupling into Taper ...... 5 - 3 Figure 5-5 ...... Laser Induced Damage Threshold Variables ...... 5 - 3 Figure 5-6 ...... Taper Optical Characteristics ...... 5 - 4 Figure 5-7 ...... Mechanical Splice ...... 5 - 4 Figure 5-8 ...... Dual Fiber Ferrule ...... 5 - 5 Figure 5-9 ...... Capillary Inner-Lok™ ...... 5 - 5

History – from 3000 BC Occasionally history is reshaped by a single invention that changes some aspect of our world or environment. Optical fiber is one of those landmark inventions that is having a positive influence on many technological developments world wide.

Humankind has communicated with light long before scientists invented the first low-loss optical fiber. Many thousands of years ago signal fires were lit on prehistoric hills. Ancient Egyptians reflected the sun’s light to send solar signals. Revolving lenses magnified small flames in light- houses long before electricity was harnessed or Alexander Graham Bell said his historic words to Watson.

Naturally occurring glasses have been used by man as far back as there is archeological evidence. Glasses are known to be manufactured as early as 1200 B.C.1 The oldest pieces of glaze and glass were dis- covered in Egypt, but it is unclear whether they originated in the Middle East or in Asia. Initially, glasses were used only as decorative and orna- mental objects, such as jewelry. As the techniques for manufacturing glass developed, so did practical applications. Vessels were manufactured through molding and pressing of the glass. The first known glass vessels date back to the reign of Thutmose III (1504-1450 B.C.).2

Figure 1-1 “Line Drawing, Glass Blower At Work,” The invention of glassblowing in the first century B.C. greatly increased Curtesy of Corning Glass Museum the use of glass for practical applications. Applications extended from ornaments to vessels to windows. Glassblowing soon spread as the standard method of shaping glass until the 19th century. Skilled craftsmen further developed the techniques and tools used in glassblowing. A common technique utilized a hollow iron pipe approximately 4ft long with a fitted mouthpiece on one end. The craftsman, known as a “gaffer,” would collect a small amount of molten glass or “gather” on the far end of the pipe. The gaffer would mold the exterior of the gather on a paddle or metal plate. This shaping is known as “marvering.” When sufficiently cooled, the gaffer would blow into the pipe, thereby expanding the gather into a bubble or “parison.” Through reheating and marvering, the gaffer could control the form of the glass piece.3 Glass manufacturing developed into a major industry, and by the year 1903, a fully automated glass blowing machine had been perfected.

As glass was being developed for use primarily in vessels and window panes in the 1700 and 1800’s, other technologies were being developed for communication. In the 1790’s, Claude Chappe invented “optical telegraphy.” Using towers mounted on hilltops, messages could be relayed from tower to tower through light signals. This was eventually replaced in the mid-19th century by the electric telegraph. The idea of using light for a communication signal came up again in 1880 when Alexander Graham Bell patented the photophone, an optical telephone system.

Separately in 1870, John Tyndall demonstrated the principle of total internal reflection to the British Royal Society. This principle demonstrates how light could be directed around curves, illustrated by Tyndall through light traveling down a stream of pouring water. The concept of using light to send communication signals, the principle of total internal reflection, and the development of high purity glass were combined in the mid to late 1900’s into the field of fiber optics.

By the late 1950’s, glass optical fibers were developed similar to those now used in science, medicine, and industry. During the 1950’s, image-transmitting fibers were developed by Brian O’Brien at the American Optical Company and by Narinder S. Kapany and colleagues at the Imperial College of Science and Technology in London. Many credit Kapany with the invention of Figure 1-2 Photophone, A. G. Bell the glass-coated glass rod and coining the term fiber optics in 1956.

1 G.W. Morey, The Properties of Glass, 2nd ed., Reinhold, New York, (1954) 2 "Glass," Microsoft® Encarta® 97 Encyclopedia, Microsoft Corporation (1993-1996) 3 Reference the Internet at www.pennynet.org/glmuseum/glgloss.htm

Glass-clad fibers had attenuation of about one decibel per meter by 1960, fine for medical imaging, but much too high for communications.4 In 1966, Charles Kao and Charles Hockham, of Standard Telecommunication Laboratory in England published a paper proposing that optical fibers could be used as a transmission medium if their losses could be reduced to 20dB/km. They speculated that the current high losses of over 1000dB/km were the result of impurities in the glass, not of the glass itself. Reducing these impurities would produce low-loss fibers suited for communications.

In 1970, Robert Maurer and colleagues at Corning Glass Works produced the first fiber with losses under 20dB/km. By 1972 losses were reduced to 4dB/km in laboratory samples, well below the level Kao and Hockham suggested was required for a practical communication system. Today, losses in the best fibers are less than 0.2dB/km.

Application of glass and optics continues to grow at a rapid pace. As a leading manufacturer of fused silica glass products, Polymicro has taken high purity glass materials and optics to even higher levels in fields such as gas chromatography, capillary electrophoresis, and specialty fiber optics. Our flexible manufacturing process provides the product you need in diameters from 30µm to 6.5mm, in lengths from 1mm to several kilometers. Stringent in-line dimensional controls yield tolerances as tight as ±1µm. Polymicro began business in 1984 and is now the leading supplier in many specialized market applications of synthetic fused silica optical fiber, capillary tubing and precision synthetic fused silica components.

Glass The word “glass“ refers to the solid phase of a material with no long-chain molecular order. It is used almost interchangeably with “amorphous,” “non-crystalline,” and “vitreous.” Glass is a disordered structure, as compared to a crystalline material that exhibits a symmetrical, ordered structure. The most common glasses are oxide based, such as Silicates (SiO2), Borates (B2O3), Germanates (GeO2) or mixtures of these. Examples of glass in nature are volcanic material obsidian and tektites. Fused silica used in optical fiber and capillaries is a very high quality, synthetic glass of almost pure SiO2 (Silicon dioxide). Glass is usually transparent, but can also be translucent or opaque. The color may vary with the constituents of the particular glass.

Due to its structure, glass materials do not have specific melting points, but transition from solid to molten over a temperature range. This allows glasses to be easily shaped and formed. Glasses can also be modified with additives to adjust certain physical properties such as strength, thermal expansion, transparency, index of refraction, viscosity, dielectric strength, or other characteristics. The ease of processing and modification has led to many useful applications of glass from windowpanes to insulators to optical fibers. Quartz and Synthetic Fused Silica

Modern quartz products can be many things to many people. The purest of quartz is Silicon dioxide (SiO2). Natural quartz has many impurities; some are good and some not so good for high technology use. For use in industry, science and communications, it is rarely pure SiO2, but rather a mixture of SiO2 with controlled trace impurities. Most of the trace impurities are introduced on purpose to give the quartz specific properties that it needs to do the task at hand.

The name used to describe the end-product may vary as much as the trace impurity amounts. Names like quartz, silica, fused silica, fused quartz, synthetic quartz, synthetic fused silica or synthetic fused quartz have been used in various publications, sometimes to describe the same product. Much of the terminology used in silicate glasses is inconsistent, and tends to be confusing when discussed on an introductory level. To help in this handbook, we will use the following definitions.

• Quartz is a natural grade of crystalline Silicon dioxide (SiO2). This is the most common phase of SiO2. This is also referred to as “rock crystal.”

4 See "History of Fiber Optics,” Jeff Hetcht at www.sff.net/people/Jeff.Hecht/index.html

• Fused Quartz is a natural grade of amorphous SiO2. Typically produced from the melting (fusing) of crystalline quartz and refined such that an amorphous (glass) is formed.

• Silica is silicon dioxide (SiO2).

• Fused Silica is silicon dioxide (SiO2) in its amorphous (glassy) state. • Synthetic Fused Silica is amorphous silicon dioxide that has been produced through chemical deposition rather than refinement of natural ore. This synthetic material is of much higher purity and quality as compared to fused quartz made from natural minerals. • Doped (Synthetic) Fused Silica is amorphous silicon dioxide that has been produced through chemical deposition. It has been intentionally doped with trace elements such as Germanium, Fluorine, Boron, Phosphorous, Titanium, etc. to adjust the optical properties of the glass. Drawn Glass and Glass Tubing Molten glass can be drawn directly from the furnace to make tubing, sheets, fibers, and rods of glass that must have a uniform cross section. Large tubing can be formed by extrusion or casting. Smaller tubing is most often produced by drawing or pulling a larger cylinder of semi- fluid glass down to the desired dimension. Fused quartz/synthetic fused silica tubing is a little more difficult to manufacturer than borosilicate glass because of the higher required temperature. Temperature and draw speed or rate must be controlled precisely to maintain accurate final dimensions. An example of a modern-day draw tower is shown in Figure 1-3.

Physical Properties Depending on the composition, Figure 1-3 Modern Draw Tower some “glasses” will soften at temperatures as low as 500oC (900oF); others only soften above 1,650oC (3180oF). Tensile strength, normally between 280 and 560kg/cm2 (4.0 and 8.0kpsi), can exceed 7,000kg/cm2 (100kpsi) for a material such as high quality bulk fused silica. High performance optical fibers made from high grade synthetic fused silica typically exceed 49,000kg/cm2 (700kpsi) break strength. Specific gravity ranges from 2 to 8, or from less than that of aluminum to more than that of steel. Similarly wide variations occur in optical and electrical properties. High Technology Synthetic Fused Silica and Common Quartz Many manufacturers define silica glass as a generic term to describe both fused quartz (made by melting natural quartz crystals) and synthetic fused silica (created through a chemical deposition process). Both forms provide an exceptional range of properties including:

• Very low expansion coefficient • Broad transparency range from the deep ultraviolet to the near infrared • High corrosion resistance • High temperature capability

Difference in Types A comparison of the basic trace elements in fused quartz from rock crystal and synthetic fused silica is a good starting point in describing a base line for typical trace impurities in quartz materials. Different manufacturers as well as different measurement techniques will give slightly different values for the various constituents. Buyers should be aware that there are differences and care should be taken when selecting one for use.

In addition to the trace elements shown in the Appendix there are other characteristics that are important in many applications. One such characteristic is the -OH content in the material used for optical fiber (especially the core) and sometimes for capillary tubing use.

For optical fiber use, certain hydroxyl (-OH) absorption bands may limit the performance. Material with a high concentration of -OH (180 to 1150ppm) offers excellent transmission in the UV wavelength range from 190nm to 1064nm. It is well suited for broadband and laser applications in the UV if the energy densities are not too high. The transmission in the 800nm region is excellent even with high doses of Gamma radiation. Certain regions in the visible and near IR are limited due to absorption around 724nm, 880nm, 944nm and 1242nm. There are useful windows (minimums) around 670nm, 800nm and 1030nm.

For longer visible and near-IR wavelengths a Low -OH material is typically used. Typical -OH contents are frequently below 1ppm and give only one minor absorption peak around 1385nm in the near-IR range. This type of material is used in optical fiber from 500nm to 2100nm. Solarized Glass Certain types of colorless, transparent glasses, when exposed to sunlight for extended periods, develop a pink or pale purplish color. Bottles, insulators, and other objects having color are often called “desert glass,” but the scientist prefers the term “solarized glass.”

The major constituent of most glasses is silica which is usually introduced as a raw material in the form of sand. Although silica itself is colorless in glass form, most sands contain iron as an impurity, and this imparts a greenish tint to glass. By adding other ingredients to molten glass, it is possible to offset the greenish color and produce colorless glasses. Such ingredients are known as decolorizers, and one of the most common is Manganese dioxide (MnO2). In chemical terms, the Manganese acts as an oxidizing agent and converts the iron from its reduced state (which is a strong greenish blue colorant) to an oxidized state (which has a yellowish, but much less intense, color). In the course of the chemical reaction the Manganese goes into a chemically reduced state, which is virtually colorless.

If pieces of decolorized glass containing reduced Manganese are exposed to ultraviolet radiation for long periods of time the Manganese may become photo-oxidized. This converts it back into an oxidized form, which, even in rather low concentrations, imparts a pink or purplish color to glass. The ultraviolet rays of the sun can promote this process over a matter of a few years or decades, thus accounting for the color of desert glass. The effect has been repro- duced in the laboratory.

Other chemical elements that are subject to photo-oxidation can also undergo color changes in glasses when exposed to ultraviolet light. Some of these elements, such as Selenium and Cerium have occasionally been used as a decolorizer and can produce solarization colors similar to Manganese. The colors developed by these two elements are said to range from yellow to amber.5

Solarization becomes more than an aesthetic problem when it occurs on glass components of an optical system. The solarization causes deterioration of the UV performance and eventual failure of the glass and the system.

Polymicro has developed solarization resistant optical fiber. It resists darkening as a result of high UV radiation levels. More about solarization resistant optical fiber characteristics in the Fiber Optics & Optical Fiber chapter.

5 Education Department, The Corning Museum of Glass, One Museum Way, Corning, NY 14830-2253 U.S.A.

Optical Concepts A few general optical concepts and definitions should be covered before going into more specific details on fiber optics and optical fibers. Optics is a branch of physical science dealing with the propagation and behavior of light and its interaction with materials. In a general sense, light is that part of the electromagnetic spectrum that extends from X-rays (~0.1nm) to millimeter waves (~1mm). It includes the radiant energy of the visible spectrum (~400 to 750nm) that produces the sensation of vision.

In our discussion, we will focus on an expansion of the visible spectrum from the ultraviolet (200nm) to the infrared (20µm). This is the optical spectral range where most of the major applications have been for optical fibers, quartz capillaries and hollow silica waveguides (HSW). We will cover a few basics in optics before reviewing some more specific areas related to optical fibers.

If you find yourself in need of more information after perusing this handbook and do not want to become an optical engineer, you might try The Photonics Design & Applications Handbook.1 Light Sources and Distributions White light to lasers – collimated, coherent, Lambertian; light comes in all colors, shapes and distributions. One source defines light as “the electromagnetic radiation that may be perceived by the human eye.” Light can also be defined as the form of radiant energy acting on the retina of the eye to make sight possible. The eye is remarkable in its ability to see color as well as handle more than six orders of magnitude in brightness automatically.

Humans only see the light from a portion of the optical spectrum. The rest of the optical spectrum is in the ultraviolet (UV) or in the infrared (IR). Several animals use their IR capability to hunt. Insects use UV reflection to locate pollen sources.

The main characterization of light is by its wavelength, most often specified in nanometers (nm). We can also define a frequency to each wavelength by dividing the speed of light by the wavelength. The human eye is most sensitive to light at a wavelength of 555nm, which is equal to 5.4 x 1014 Hz. This high frequency (and its inherent bandwidth) is what makes light such a good information carrier in optical fibers.

The range of light wavelengths we see is called Figure 2-1 Light and Nature the Visible Spectrum. It covers a range from about 400nm to 555nm to 750nm (violet to green to red). The near-UV wavelength range just shorter than the visible is the range from which we most often want to protect our eyes and skin. Varying amounts are present in sources like high power quartz halogen lamps, fluorescence lamps, the sun and lasers used in medicine. The range just longer than the visible, the near-IR, is most commonly used in IR remote controls and is the dominant wavelength in the night sky spectrum. Even though we can not see these near-visible wavelengths our technology advancement over the past few centuries has led to the development of electro-optical sensors to detect and measure them, new light sources to produce them, and optical fiber to transmit them.

Light comes in all colors. If it is only one color or wavelength, it is called monochromatic, like a laser. If it is a blend of many colors, it is called polychromatic, such as the sun. For a good reason there is a very good match between the sun and the eye. Nocturnal animals usually have peak eye sensitivity shifted toward the near infrared where the night sky illumination peaks.

1 The Photonics Design & Applications Handbook, 43rd Edition, 1997, Laurin Publishing Co., Inc., Pittsfield, MA

Two additional properties of light that are important in many areas of optics are the spatial distribution of the light and the coherence of the lightwaves. The spatial distribution describes the direction(s) that the light is traveling. If it appears uniform in all directions, that is it has uniform brightness from any viewing angle, it is called Lambertian. The other extreme is collimated light, which essentially travels in only one direction such as a laser. The coherence of light refers to the way the light waves are ordered or phased with each other. Common incandescent lamps are inco- herent (random phase) while lasers are coherent (in-phase).

A Lambertian source is a plane surface that emits (reflects) a flux proportional to the cosine of the angle to the normal to the surface, but appears to have uniform brightness at all angles. Matte “white” paint and phosphors are approximate examples of such source planes and the light diffused by opal glass is a close approximation for most measurements. Getting Light Where You Want It We are all familiar with a mirror reflecting light. Unfortunately there are significant losses associated with this reflection. The loss at the glass-air interface is about 4% and there is approximately another 4% loss at the glass-mirror surface interface. This means for each reflection of light there is about a 10% loss. As a waveguide, this would not be a very effective means to direct light for any distance since after 50 reflections only about 0.5% of the light would remain.

We can also get reflection under certain conditions from a glass-air or glass-glass interface. These losses can be lower than 0.1% per reflection. For the case where each reflection gives 99% of the light back, there will be about 37% left after 100 reflections. That’s a little improvement. None of these losses, however, included the effects of any light lost due to the light beam spreading and not hitting the next surface. We could use a hollow glass tube or capillary to prevent this but we still need better reflection. Under the right conditions a glass rod with a different optical material surrounding it can be produced in a long fiber with a much higher reflection per bounce.

With today’s optical fiber and thousands of reflections it is now quite common to have an average power loss of less than 5dB per kilometer. This equates to about 32% of the input power remaining at the output end, after 1km. Astonishingly better transmission than a thousand very good mirrors or 10 sheets of window glass. The Science of Fiber Optics The science of fiber optics deals with the transmission or guidance of light (rays or waveguide modes in the optical region of the spectrum) along transparent fibers of glass, plastic, or a similar medium. The phenomenon responsible for the optical fiber or light-pipe performance is the law of Total Internal Reflection (TIR). Total Internal Reflection A ray of light incident upon the interface between two transparent optical materials of different indices of refraction will be totally internally reflected under certain conditions. When the ray is incident from the direction of the more dense material and the angle made by the ray with the normal to the interface is greater than the critical angle, the light will be reflected, not refracted. The critical angle is dependent only on the indices of refraction of the media.

Rays may be classified as meridional and skew. Meridional rays are those that pass through the axis of a fiber while being internally reflected. Skew rays are those that never intersect the fiber axis although their behavior patterns resemble those of meridional rays in all other respects. For convenience, we will deal only with the geometric optics of meridional ray tracing.

An off-axis ray of light traversing a fiber 50µm in diameter may be reflected 3,000 times per foot of fiber length. This number increases in direct proportion to diameter decrease. In principal, total internal reflection between two transparent optical media results in a loss of zero loss per reflection; thus a useful quantity of illumination can be transported. This spectral reflectance differs significantly from that of metallic coatings shown graphically. An aluminum mirror cladding on a glass fiber core would sustain a loss of approximately 10 percent per reflection, a level that could not be tolerated in practical fiber optics as shown in Figure 2-2. The reflection at the core-clad interface must be much higher in optical fiber to obtain useful light transmission over long distances.

As indicated in Figure 2-3, the angle of reflection is equal to the angle of incidence. (By definition, the angle is that measured between the ray and the normal to the interface at the point of reflection.)

Light is transmitted over the length of a fiber at a constant angle with respect to the fiber axis. Scattering from the true geometric path can occur due to:

• Irregularities in the core/clad interface of the fiber. • Surface scattering at the interfaces. • Scattering in the bulk material.

Light will be scattered in proportion to fiber length and depends on the angle of incidence. To be Figure 2-2 Reflection of Some Metal Coatings functional long fibers must have an optical quality superior to that of short fibers. End face scattering occurs readily if optical polishing has not produced a smooth surface. Pits, scratches, and scuffs scatter light significantly. Additional losses such as bending are covered in the Internal & External Losses section.

The speed of light in matter is less than the speed of light in air. The change in velocity that occurs when light passes from one medium to another results in refraction. A portion of the light incident on a boundary surface from a higher index media to air is not transmitted but is instead reflected back into the air. The majority that is transmitted is totally reflected from the interface, assuming that the angle is less than the critical angle as in Figure 2-3.

The relationship between the angle of incidence, I, and the angle Figure 2-3 Refraction, Reflection and Numerical Aperture of refraction, R, is expressed by Snell’s law as:

ni • sin I = nr • sin R (Eq. 2-1)

where, ni is the index of refraction of first media (air in many cases) and nr the index of refraction in which the light continues to travel (in this case the core). When ni = 1(air) for all practical purposes, the refractive index of the core would be calculated from: sin I n = r sin R (Eq. 2-2) Numerical Aperture Numerical aperture (NA) is a basic optical characteristic of a specific fiber configuration. It can be thought of as rep- resenting the size or “degree of openness” of the input acceptance cone in Figure 2-4. Mathematically, numerical aperture is defined as the sine of the half-angle of the acceptance cone (sin θ).

The light-gathering power or flux-carrying capacity of a fiber is proportional to the square of the numerical aperture. This is the ratio between the area of a unit sphere within the acceptance cone and the area of a hemisphere (2π solid angle). A fiber with a numerical aperture of 0.66 has 43 percent of the flux-carrying capacity of a fiber with a numerical aperture of 1.0.

Snell’s law can be used to calculate the maximum angle within which light will be accepted into and conducted through a fiber:

(Eq. 2-3)

θ Where, MAX is the half-angle, n0 the refractive index outside the Figure 2-4 Numerical Aperture - Acceptance Cone fiber end (air =1.0), nCO the refractive index of the core, and nCL the refractive index of the clad.

As light emerges from the more dense glass medium into a less dense medium such as air, it is again refracted. The angle of refraction is greater than the angle of incidence, R > I, when emerging into a lower index media. Because R is by necessity less than 90 degrees, there must be a limiting value of I, the incident angle, beyond which no incident ray is refracted. This becomes the critical angle, and rays that strike at a greater angle are reflected. This is the principle behind Total Internal Reflection (TIR) in optical fiber.

It should be noted that this formula, for the calculation of numerical aperture, does not take into account striae, surface irregularities, and diffraction, all of which tend to decollimate the beam bundle. Decreasing the clad index and/or increasing the core index will increase the NA, which increases the Full Acceptance Angle and the Field of View. Input-Output Phenomena If a ray is incident at angle θ, it will ideally emerge from a fiber at angle θ. In practice, however, the azimuthal angle on emergence varies so rapidly with θ, the length and diameter of the fiber, etc. that the emerging ray spreads to fill an annulus of a cone twice angle θ, as shown in Figure 2-5.

The exit end of a fiber will act as a prism if it is not cut perpendicu- Figure 2-5 Conical Output lar to the fiber axis. A bias cut will tip the exit cone as shown in Figure 2-6. Thus,

(Eq. 2-4)

(for small angles) (Eq. 2-5)

Figure 2-6 Bevel Cut Fiber

where α is the axis of the deflected ray and β is the cut angle to the normal of the fiber.

The preservation of angle θ on exit is only an approximation. Diffraction at the ends, bending, striae, and surface roughness will cause decollimation or opening of the annulus. The striae and roughness cause progressive decollimation; diffraction and bending may be regarded as end finish factors (Figure 2-7). The effect is most apparent in systems in which collimated transmission is emphasized. Figure 2-7 Bent Fiber

Tapered Fibers Tapered fibers are governed by one important relationship,

(Eq. 2-6) where diameters and angles are as shown in Figure 2-8. The angle of reflection of a light ray is equal to the angle of incidence.

Therefore, light entering the small end of a fiber becomes more colli- Figure 2-8 Tapered Fiber Transmission mated as the diameter increases because the reflecting surface is not parallel to the fiber axis. Collimated light entering tapered fibers at the large end, on the other hand, becomes decollimated, and if the angle of incidence exceeds the acceptance angle, it will pass through the side of the fiber. The error perhaps most frequently made by the novice is to attempt to condense an area of light that is Lambertian using a taper. As illustrated in Figure 2-9, this merely “throws” light out the sides. If the incoming light is in a small angle, the outgoing flux per unit area can be increased.

Figure 2-9 Large-end to Small-end Transmission

When working with a system of the “Mae West” variety (Figure 2-10), it is important to remember that the smallest diameter determines the acceptance angle of the system. This is in conformity with the relationship previously cited that governs tapered fibers.

Figure 2-10 Mae West - Fiber Collimator

Light entering the side of a fiber or taper can be trapped if the angle of incidence is greater than the critical angle. The stray-light cone thus produced forms the basis for one type of injection lighting (Figure 2-11).

Figure 2-11 Stray Light Injection

Matching Numerical Apertures In optical fiber use, it is extremely important to attempt to match mating fiber NA’s. With the same fiber core diameter it is usually best to center the two fiber ends using a tapered sleeve or set of mating connectors. If there is a large mismatch in the fiber diameters, a tapered optical fiber may be a good solution. Two different diameter spheres in a coupler may also achieve the needed result.

The target is to match the NA’s as best possible since the light loss in a lens coupling will vary as the square of the NA. You should also take care that the NA’s of two coupled fibers are matched. Optical loss due to fiber NA mismatch is represented by:

(Eq. 2-7)

Where NA1 is transmitting fiber and NA2 is the receiving fiber. If NA1 is less than NA2 then the mismatch loss is zero. When there is a difference in NA between the two fibers, the higher NA should be the second in line so that all or most of the flux from the first fiber is accepted by the second.

The total flux lost will be a function of how well the input acceptance cone of the second fiber is filled (without over- filling) by the output source fiber. These fiber-to-fiber matches are also a function of other conditions besides NA, such as core alignment, angle, distance between the fibers, surface cuts, angles and finishes, as well as any optical coupling media characteristics.

When using optical fiber in an optical system, the acceptance cones should match at each interface. The f-number, f#, of a lens is given by:

f (Eq. 2-8) where efl is the lens effective focal length and D is the diameter of its entrance pupil. The relation between the f# and the numerical aperture (NA) is:

f (Eq. 2-9)

So in order to match NA’s between the lens system and the optical fiber, we need:

(Eq. 2-10) or, (Eq. 2-11)

This means, if we have a lens with an efl of 50mm and a 18mm aperture (f# = 2.8), we would need an optical fiber with an NA of at least 0.18 to get an ideal match. This does not include any losses due to Fresnel reflections and lens focal spot size versus core size mismatches.

Because of the rapid change of the refractive index of quartz in the UV (Figure 2-12), great care should be taken when designing spectrometers or other systems that must work in both the visible and the UV. Either focus adjustments need to be included or a compromise focus position must be chosen. Since the focal length decreases with wavelength (about 13% from 550nm to 220nm), the NA increases. Matching the Figure 2-12 Quartz Refractive Index NA’s and assuring the focus spot is just inside the fiber core at the shortest wavelength would be the best situation. The balance between these two characteristics may still leave some losses.

Figure 2-13 Fused Quartz Lens, 100mm, f/2

The efl and NA characteristics for a fused quartz plano-convex, singlet lens versus wavelength can be seen in Figure 2-13.

This axial chromatic aberration problem is solved in visible and IR optical systems, to some extent, by adding color correction using optical elements with complementary characteristics. In the UV there are very few choices of materials to obtain color correction. The refractive index versus wavelength curve in the graph illustrates the reason for some of the problem.

A generalized illustration of lens coupling into an optical fiber for two different wavelengths may help to clarify the phenomena of UV chromatic aberrations in quartz. As can be seen in the illustration, not only is the focal length shorter at 220nm (~13% less), but the exit cone angle is larger. Both of these changes from the 550nm case can cause losses when trying to use optical fiber over an extended wavelength range.

Figure 2-14 Effects of Focal Length Change with Wavelength and Optical Fiber Coupling

Fiber Types & Modes of Transmission In fiber optics, the term “mode” refers to a stable propagation state of light down the fiber. Fibers can have any number of stable propagation states (modes), giving rise to two basic types of optical fibers, multimode and singlemode. Multimode obviously refers to a fiber that has many modes of propagation, while a singlemode, by design, only has one. Whether a particular fiber is multimode or singlemode depends on the fiber geometry, core/clad refractive indices, and the wavelength of operation. Multimode fibers can be further broken down into two subcategories, step-index and graded-index. Each type has distinctive advantages and disadvantages, which will be discussed in more detail.

Step-Index Multimode Fiber This was the first fiber type to find practical application, and continues to be in wide use today. A step-index multimode fiber allows the light to travel at many different angles within the fiber, thereby allowing many modes of propagation. The term “step” refers to the step function the refractive index takes at the core/clad interface.

The advantages of a step-index multimode fiber are related to the relatively large core area and high numerical apertures. Both of these properties allow light to be easily coupled into the fiber. In turn, this allows the use of inexpen- sive termination techniques, low cost diodes, and high power handling capability. These fibers are therefore widely used in high power laser delivery applications (medical procedures, material processing), industrial process control links (factory automation), short distance data communications, and fiber sensors.

A disadvantage to step-index multimode is bandwidth. Referring to Figure 2-15, the path the light takes down a step-index multimode fiber will be longer or shorter depending on the angle of propagation. This difference in path length causes the pulse of light to spread out during its journey down the fiber. This is known as modal dispersion (see Dispersion section). As one pulse spreads, it eventually interferes with neighboring pulses, distorting the transmission signal. The longer the fiber length the more severe this pulse spreading will become. However, this is only a problem in applications that require a coherent signal, as in communications links. Power delivery or sensor systems do not require coherent transmission and many data communication or industrial process control links are relatively short distances (less than 2km) allowing the widespread use of step-index multimode fiber.

There is a wide selection of step-index multimode fiber available. Sizes vary from ~50 to >2000 µm core diameters. Their construction can be silica or plastic cladding using silica, plastic, or liquid as a core. There are also applications with no core called hollow waveguides. The silica constructions allow lower attenuation, Figure 2-15 Optical Fiber Modes greater spectral range, higher power handling capability, and greater environmental range. Plastic fibers offer lower cost and greater flexibility but are limited in transmission and environmental properties. Hollow waveguides are used princi- pally in the IR, with some recent developmental work to construct hollow waveguides for use in the UV range.

Polymicro offers a wide selection of step-index, multimode fibers, particularly for laser power delivery and stringent or harsh environmental conditions. Please refer to our data sheets located in the back of this handbook.

Graded-Index Multimode Fiber As the name implies, the refractive index of this fiber gradually decreases from the core out through the cladding, as opposed to the abrupt step change of step-index. Instead of taking a zigzag path down the fiber, the gradual change in refractive index directs the light in a sinusoidal path as previously illustrated. Since the light travels faster in a material of lower refractive index, the light traveling on the outer reaches of the graded region moves more quickly, thereby reducing the amount of pulse spreading. The result is a dramatic >25- fold increase in bandwidth over step-index multimode fibers.

Graded-index is actually a compromise between step-index multimode and singlemode fibers, trading off bandwidth for ease of termination and light launch. The graded profile and smaller core increases bandwidth over step-index multimode, but the core sizes are still large enough for convenient termination and use of lower cost diodes. In more recent years, the components and techniques for terminating singlemode has improved dramatically, so graded-index has seen a decline in market share. However, graded-index remains a popular standard for use in medium distance (2-15km) data communication links.

The most common core sizes for graded-index multimode fibers are 50, 62.5, and 100µm. These sizes have become industry standards. The construction is always silica core/silica clad based, with dopants (typically Ge, B, P, and F) used to adjust the refractive index in the graded profile. This fiber is used almost exclusively for medium distance data communication (local area networks), although it is sometimes used for fiber sensor systems. The smaller core

© Polymicro Technologies, a Subsidiary of molex. 2-8 2 Fiber Optics & Optical Fiber area makes this fiber less useful for power delivery applications, however new special, larger core designs specifically for high power applications are available.

Singlemode Fiber In singlemode fiber the core size is reduced to the point (5-10µm diameter) where only one mode, the primary mode, can be guided. This mode essentially travels straight through the fiber and thus is not subject to the pulse spreading seen in multimode fiber due to different path lengths. The net effect is a substantial increase in bandwidth since all the light is traveling at the same speed for the fiber length. In addition, using the primary mode and higher operational wavelengths (1310 and 1550nm) results in very low attenuation. For these reasons, singlemode is the fiber of choice for long distance data and voice communication.

Singlemode does experience some distortion of the signal, but this is due primarily to chromatic dispersion, which is variation in light speed due to the pulse not being purely monochromatic. This type of dispersion is very small when compared to the modal dispersion experienced in multimode fibers.

Singlemode fiber typically consists of a silica core/silica clad construction with a step-index refractive index profile. The core and/or clad is doped to obtain the index difference between the core and clad. The core size, being very small, is more difficult and costly to terminate versus the multimode fibers, but for long distance systems this cost is acceptable. In contrast, the small core size does not allow a great deal of power input, and therefore this fiber is generally not suitable for power delivery and many sensor applications. Dispersion As they travel down an optical fiber, optical pulses will broaden. This is called dispersion. Since eventually the pulses will overlap and the data will be lost, dispersion determines the data carrying capacity.

Step-index optical fibers generally have many modes of propagation. Modes are represented by different angles Figure 2-16 Dispersion with which the light rays hit and reflect from the core-clad interface. These various angles can be caused by the various angles that light enters the fiber or by other effects in the fiber such as scattering or changes in angle due to bending. The time it takes the light in a particular mode to travel from one end to the other is directly proportional to the distance it has traveled in the optical fiber. Low order modes get to the output end quicker than higher order modes. There are three main types of dispersion: • Chromatic dispersion … is a result of different wavelengths of light traveling at different velocities in the core material. Since typical light sources provide power distributed over a range of wavelengths, rather than a single, discrete "line," the pulses spread out as they travel through the fiber. This is predominant in single-mode fibers. • Modal dispersion … is due to different fiber modes traveling along different paths through multimode fiber. The result is broadening of the pulse as the higher-order modes reach the output behind the fundamental mode. This is predominant in step-index, multimode fibers. • Waveguide dispersion … is due to the geometry of the fiber and results in different velocities of the various modes for different wavelengths. It is the least important of the causes of dispersion.

Bandwidth The data-carrying capacity of an optical fiber (just as with a wire or coax) is called bandwidth. It is normally expressed as a distance-frequency product such as MHz-km (megahertz-kilometers) or more often as GHz-km (GigaHertz-kilometers). What this really means is, if you need to transmit a 27MHz signal (common frequency for CB radio) up to 1 kilometer (0.6 miles) and still detect it as a useful signal at the other end, you need an optical fiber with a bandwidth of at least 27MHz-km. Step-index, multi-mode fiber would just barely do it. If you wanted to transmit at the frequency of the newer wireless telephones, 900MHz, you would probably have to go to step-index, singlemode fiber to get a 1km range.

The main cause of bandwidth limiting is pulse broadening caused by modal and chromatic dispersion in the fiber. This is why step-index, singlemode fiber does so well over long distances. But, due to the very small core diameter, it does have major limitations when it comes to getting light into the fiber efficiently.

For step-index, multimode fiber pulse-broadening is a very difficult parameter to calculate, since macro-bending, fiber length and the number of modes initially injected, all have a bearing on the pulse spreading.

Typical bandwidths for the different types of fiber are:

FIBER TYPE BANDWIDTH Step-Index,Single mode 100 GHz-km Graded-Index,Multimode 500 MHz-km @ 1300nm 160 MHz-km @ 850nm Step-Index,Multimode 20 MHz-km

Focal Ratio Degradation (FRD) Focal Ratio Degradation (FRD) is the decrease in focal ratio (decrease in effective F-number) in an optical fiber versus its length. This can be a significant parameter of an optical fiber when matching NAs in telescope-spectrometer systems such as used in astronomy. The ability of a fiber to preserve the angular distribution of the input beam from the telescope to the spectrometer is very important.

The major causes of FRD are mechanical variations in the fiber dimensions with length (under the manufacturer’s control) and the mechanical set-up of the instrumentation (under the control of the user). Small variations in the fiber core diameter or core-clad interface can cause mode stripping, resulting in FRD. Both macro-bending and micro- bending will cause FRD.

Without proper NA matching between the telescope, spectrometer and the fiber, energy may also be lost from over filling the acceptance cone of the optical fiber or not filling the NA of the spectrometer. Ramsey found that f-ratios of f/3.0 to f/7.0 were the best matches for glass fibers depending on the fiber diameter.2 Over-filling or under-filling the optical fiber acceptance cone appeared to produce higher losses. Macro-bending did not appear to have any major effect on FRD while micro-bending did. Attenuation & Transmission The attenuation in an optical fiber is usually expressed in decibels per kilometer (dB/km) in the visible and IR spec- trums and in dB/m in the UV. The attenuation is calculated as:

(Eq. 2-12)

Where Pin is the input power injected into the fiber core and Pout is the attenuation optical output collected from the distal end. The NA of the input and output optical test system must be matched and effects of lens focal lengths at the test wavelength must be considered so that the acceptance cone of the fiber under test is not erro- neously over-filled. This can cause excessive attenuation values since input energy present in the test standardization or cali- bration may not enter the input fiber under test. Since both the core and the cladding have transmission characteristics that vary with wavelength, attenuation is normally Figure 2-17 Attentuation versus Transmission plotted with respect to optical input wavelength.

2 Lawrence W. Ramsey, “Focal Ratio Degradation in Optical Fibers of Astronomical Interest,” Pennsylvania State University

Low -OH and High -OH Optical Fiber The optical attenuation characteristics are quite different for High -OH and Low -OH optical fiber core material. The -OH content of the core fused silica must be formulated into the raw boule material before the optical fiber is made. The choice is dependent on the user’s application. The Low -OH type optical fiber has very low attenuation throughout the near-IR wavelength range from 700nm to beyond 1800nm, except for a small peak at 1385nm.

The normal absorption peaks at 726nm, 880nm, 950nm, 1136nm and longer, that exist in the High -OH material used in UV applications are not present in Low -OH material. Conversely, the Low attenuation of the High -OH in the UV is significantly better than the Low -OH material. Typical curves for Polymicro’s type FI fiber (ultra Low -OH) used for visible and NIR and FV fiber (High -OH core) used primarily for UV applications follow. Polymicro has recently introduced a new broad-spectrum FB fiber that operates between 275-2100nm. Please refer to FBP specification sheet under Optical Fiber in the Product Reference section for more information.

Figure 2-18 Attenuation, Type FI - Low -OH

Figure 2-19 Attenuation FV - High -OH

Internal & External Losses There are several sources of attenuation in optical fibers. In addition to losses due to mechanical variations in the fiber dimensions, there will be losses due to: • Rayleigh scattering ... microscopic irregularities in the index of refraction of the glass are the most important mechanism for attenuation in modern fiber. Rayleigh scattering is wavelength dependent (proportional to 1/λ4) and is thus less significant at longer wavelengths. • Absorption ... in silica can be significant at short-wavelengths due to UV (electronic) absorption, and at long-wavelengths (infrared) due to multiphoton (atomic vibrational) absorption. In addition, unwanted impurities and intentional dopants in the fiber absorb optical energy around specific wavelengths. State-of-the-art manufacturing methods have reduced impurity levels to the point that their effect is almost insignificant.

• Bending ... Micro-bending, the result of microscopic imperfections in the fiber geometry, is caused by the manufacturing process or mechanical stress. Macro-bending occurs when the fiber is bent into a larger, visible curvature. In both cases, light rays hit the core-cladding interface with angles greater than the critical angle and optical energy is lost into the cladding. Bend sensitivity of fiber includes the attenuation losses from both macro and micro-bending. It can be given in terms of dB of loss for a particular bend radius and wavelength. Proper selection of coating materials and optical properties can dramatically improve microbending effects on transmission. Bend Radius – Optical Effect Any bend in a multimode optical fiber will cause mode stripping or loss of higher order modes in the fiber. The smaller the radius the higher the induced loss.

Long term exposure of optical fiber to bending stress will shorten the mechanical lifetime of the fiber but generally will not change the attenuation until the fiber fails. See bending loss in the glossary.

The bend radius is generally defined as the radius of the drum or mandrel on which the fiber is to be wound or bent.

For a given radius of curvature, the loss due to bending becomes significant when the effective index of a mode is below a “cut-off” value. The actual loss in a step-index fiber is very complex to calculate. There are usually so many assumptions by the time the calculation is finished that is better to depend on rule of thumb or actual measurements rather than depend upon calculations. See Mechanical Bend Radius Limits & Stress section. Evanescent Wave Losses in Small Diameter Fibers In the basic discussions on total internal reflection, it was indicated that under certain conditions between the core and cladding indexes, light is reflected at the interface between the two surfaces. When the light wave meets the boundary between the two indexes the light standing wave actually penetrates about a quarter wavelength into the second media while being reflected. This penetrating wave is called an evanescent wave. It actually decays exponentially with distance from the interface, with a characteristic penetration depth of 50 to 100nm.

Since the field cannot go to zero immediately, it decays exponentially into the lower cladding according to the relation,

(Eq. 2-13) where δ is the distance from the interface. This exponentially decaying wave is the evanescent wave. The decay of this wave at an angle α, between the incidence ray and the normal to the surface, is characterized by a penetration 3 depth, dp, the distance from the interface at which the wave amplitude falls to 1/e of its initial value at the interface.

(Eq. 2-14)

The referenced article gives an excellent application for using the evanescent wave. Care should be taken, however, when specifying small fibers to make sure that the cladding is thick enough to contain the evanescent wave and thereby not create unwanted losses. As a rule of thumb, for multimode fibers, the cladding thickness should be at least 10 times the operational wavelength.

An optical fiber that has a core to clad ratio of 1:1.1, i.e., a core of 50µm and an OD of 55µm has a cladding thickness of only 2.5µm. This situation is barely 10 times the wavelength if used in the UV and only about 4 times if used at 600nm. In this situation, it is advisable to use a larger fiber (thicker cladding and/or larger core) if possible. Since the loss in fused silica is higher in the UV, some additional cladding losses may be contributed if the cladding is too thin, but they will be even more noticeable in the red part of the spectrum and NIR. The FVP100110125 fiber has a 5µm cladding thickness. FV-data is typical of High -OH silica/silica fibers with cladding of 10µm or greater.

3 See N. Nath & S. Anand, “Evanescent wave fiber optic fluorosensor: effect of tapering configuration on signal acquisition,” Optical Engineering, 37(1) p220-228, January 1998, for excellent discussion on evanescent waves, tapers and V-number.

The effect of having too thin a cladding can be seen in the chart below.

Figure 2-20 Effects of Cladding Thickness

Power Transmission The amount of laser power that a fiber is transmitting without damage is specified as the power output of the laser divided by the area of the laser spot,

(Eq. 2-15) where ω is the beam radius of the laser beam at distance L from the fiber surface. The radius ω is given by:

(Eq. 2-16)

ω When the position of 0, the focal waist, is inside an optical fiber of index n, then the Rayleigh range parameter z0 is given by: (Eq. 2-17)

The output of a pulsed laser, typically specified in millijoules (mJ) of energy per pulse, must first be converted to the power per pulse. For example, a pulsed laser that produces 50mJ in a 10ns pulse produces an output power of 5MW.

For a Nd:YAG laser (1064nm) emitting 10ns pulses, synthetic fused silica fibers have survived pulse energies up to 0.4 - 0.6 Giga-Watts/cm2.

To achieve maximum power transmission the end face of the fiber must be optically smooth and perpendicular to the beam. The beam must be focused properly to prevent light from leaking into the cladding or else the polymer cladding or buffer coating may be damaged. The epoxy used for bonding the fiber to the connector can be affected as well. For this reason, a silica/silica fiber is often used in a specially designed mounting or connector to better tolerate the high power densities and rapid heating levels. It is also recommended that the beam fill no more than 70% of the core diameter, and with a uniform power distribution across the face of the fiber.

Configurations that help spread or reduce the energy density at the fiber surfaces have helped reduce laser damage at high energy levels. Options such as tapered end-tips, spheres, and other shaped end tips manufactured onto the end of the fiber are additional options offered by Polymicro. Some of the many possible configurations are shown and discussed in the Fused Silica Micro-Components chapter.

Fluorescence in Fiber Optical Materials Fused silica glass, especially the lower -OH types, can exhibit fluorescence when illuminated with 254nm light with a Schott UG-5 glass filter. From Heraeus Amersil data, this apparently does not occur in the High -OH materials in the Suprasil® family. Slight blue to blue-violet appears in materials such as Homosil®, Herasil®, Infrasil® and HOQ 310. Quartz glass is free from visible fluorescence at excitation wavelength greater than 290nm.

Some varieties of sapphire have been reported to exhibit fluorescence under UV excitation. Cladding and buffer materials used in the manufacturing of optical fiber may exhibit similar emission characteristics under some types of excitation, UV, soft X-ray or energetic particles. Polymicro offers a variety of fiber types that use low fluorescence silica.

UV Fiber Performance (Solarization) Polymicro manufactures several categories of silica fibers that are capable of operation in the UV wavelength range. These include: 1) FVP (Standard High -OH) 2) FVP-UVM (Modified Core High -OH) 3) FVP-UVMI (Hydrogen Loaded UVM) 4) FDP (Deep UV Fiber)

In selecting the best product for a UV application, there are 3 main performance criteria that should be considered. These are: 1) Initial Attenuation. 2) Additional attenuation caused by UV radiation. 3) Stability following initial UV degradation.

Initial Attenuation. This is the attenuation as a function of wavelength for new fiber prior to any UV exposure. This is typically measured in dB/km, and it can be changed to dB/m by dividing by 1000. Figure 2-21 below shows a typical initial attenuation for all of Polymicro’s UV products.

UV Induced Additional Attenuation. This is the damage induced by exposure to UV radiation. It is commonly known as solarization. Most of this damage occurs at wavelengths less than 250nm, with peak damage occurring at approximately 214nm. The degree of damage varies greatly with the type of fiber. Figure 2-22 shows typical solarization levels as a function of time for various Polymicro UV products as shown in linear units for 1m length. Figure 2-23 shows comparison of 4hr exposure levels as a function of wavelength. Typically, this additional attenuation is measured in units of dB/m and is scalable to any length.

Figure 2-21 Typical Attenuation of Polymicro High -OH UV Fiber Products FVP-UVM fiber has improved performance compared to the standard High -OH FVP fiber. The hydrogen loaded fiber, FVP-UVMI, has almost no degradation, but this fiber has a limited lifetime and eventually reverts to typical FVP-UVM performance. This lifetime can be extended by keeping the fiber refrigerated and by using larger diameter fibers. The deep-UV enhanced FDP fiber has low solarization degradation, but with no limit on lifetime and no need for refrigeration.

Herasil®, Homosil®, Infrasil® and Suprasil® are registered trademarks of Heraeus Quarzglas GmbH & Co.

Figure 2-22 UV Effects frm Deuterium Lamp on Transmission of Various Optical Fibers Figure 2-23 UV Damage Following 4 Hour Exposure

Stability (Recovery). When UV radiation is removed from a fiber, some of the solarization damage can recover over the following several hours. When maximum transmission is a priority, this improvement in transmission is useful. However, for applications where consistent, stable output is critical, this recovery can be detrimental. Figure 2-24 shows the initial degradation as well as the subsequent degradation following 20 hours of recovery for a typical FVP-UVM fiber. Figure 2-25 contains similar data for a typical FDP fiber.

Figure 2-24 FVP-UVM Fiber 214nm Degradation/Recovery Figure 2-25 FDP Fiber 214nm Degradation/Recovery

Summary. The table in Figure 2-26 on the next page discusses each of the Polymicro UV fibers in detail with the characteristics of each type.

In selecting a fiber type for an application, the performance characteristics must be balanced against one another as well as against the price. For example, applications that only require transmission from 250nm up through the visible range, standard FVP would work well and is very economical. For applications that have stringent stability requirements deep into the UV, but have a short period of usage, FVP-UVMI would work very well. For applications that require stability and a long lifetime in the deep UV, FDP would be the best option.

Fiber Type Wavelength Range Characteristics Cost

FVP 240-850nm • Economical Very Low • High Solarization • Damage below 240nm • Minimal Solarization Recovery • All Sizes Available • Alternate Coatings Available FVP-UVM 200-850nm • Moderate Solarization Damage Low • Minimal Solarization Recovery • All Sizes Available • Alternate Coatings Available FVP-UVMI <200-850nm • Very Small Solarization Damage Moderate • Diameter and Temperature Dependent Degradation with Time • Only Larger Diameters Recommended (>400µm) • Refrigeration Recommended When Not In Use • Reverts to FVP-UVM Over Time • Available with Polyimide Coating Only FDP <200-850nm • Small Solarization Damage Moderate • Minimal Solarization Recovery • No Shelf Life Issues • Diameters 100µm to 600µm Available • Available with Polyimide Coating Only

Figure 2-26: Comparison of Polymicro UV Fibers

Infrared Waveguides At near-IR to IR wavelengths (2.1 to 20µm), fused silica does not transmit light particularly well due to multiphoton (atomic vibrational) absorbance. However, there are several materials that do transmit well at these wavelengths. Chalcogenide glasses, fluoride glasses, polycrystalline metal halides, and some germanate oxides perform well beyond 2.1µm and have all been fabricated into optical fibers. Unfortunately, these materials tend to be very difficult to process, and have inferior mechanical and durability properties when compared to fused silica. This has limited their widespread use, but they are still utilized in specialized applications.

There is also a class of optical fiber known as Hollow Waveguides (HSW). This type of waveguide can effectively transmit wavelengths in the IR (out to 20µm). A HSW can be one of two types of light guides:

1) “leaky” guide, or 2) attenuated total reflectance guide (ATR)

The HSW guide consists of a hollow tube coated on its inner surface with a reflective metal (e.g. Ag). This coating may or may not be followed by dielectric coating which serves to improve reflectivity and provide protection to the metallic surface. This type of waveguide has been produced using plastic, metal, and glass capillary tubing. Tubing produced from synthetic fused silica is most effective due to its tight dimensional control, smooth surface properties, high strength, and lower cost. HSW fabricated with synthetic fused silica tubing approach the theoretical limits of transmission for this type of waveguide. They are finding increased utilization in sensing and in Er:YAG (2.9µm) and µ CO2 (10.6 m) laser systems as well as in the medical and industrial fields. Please refer to our Product Reference section for more information on HSW. Coatings and Buffers Buffer coatings are applied to optical fiber while the fiber is being drawn. The purpose of the buffer is to protect the fiber from environmental conditions, especially moisture and abrasion, which might accelerate the generation of stress-cracks in the silica surface. Several types of coatings are used depending on the application. The most durable is polyimide but acrylate, silicone, and fluoropolymers are also used.

Broadband Fiber (FBP)

Traditionally, fibers with high -OH content perform better at UV wavelengths. However, the -OH content creates very large absorption regions in the Near Infrared (NIR) wavelengths. Conversely, fibers with Low -OH content can perform very well in the NIR region of the spectrum, but tend to have very poor UV performance. Both types of traditional fibers transmit well in most of the visible spectrum.

Polymicro has developed a fiber that combines the benefits of both types of fiber. The FBP series of fibers has good transmission from below 275nm to beyond 2100nm. A typical attenuation spectrum of the FBP fiber is shown in Figure 2-27 compared with a typical Low -OH (FIP) fiber and a typical High -OH (FVP) fiber.

1000 Spectral Attenuation

900

800

FIP 700 FVP FBP 600

500

400

Attenuation (dB/km) 300

200

100

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Wavelength (nm) Figure 2-27 Spectral Attenuation of Fiber Types

The FBP fiber is solarization resistant down to its specified low wavelength of 275nm. It can be produced in core diameters from 50µm to 600µm and requires no refrigeration or other special storage or handling. Figure 2-28 below is a table comparing FBP properties with those of High and Low -OH fibers. FBP fiber has found use in astronomical applications and spectroscopic applications, among others. FBP fiber is recommended in applications where transmission in both UV and NIR wavelengths is needed.

Fiber Type Wavelength Range Characteristics Cost High -OH 200-850nm • High -OH – Good for UV and Visible Transmission Varies by type • Solarization Varies by Type • All Diameters Available • Various Coatings Available Low -OH 500-2400nm • Very Low -OH - Good for Visible and NIR Transmission Very Low • All Diameters Available • Various Coatings Available FBP 275-2100nm • Low -OH - Good UV, Visible and NIR Transmission Moderate • Minimal Solarization in Specified Wavelength Range • All Diameters Available • Polyimide Coating Needed • Unspecified Attenuation below 275nm

Figure 2-28 Characteristics of Fiber Types

Polyimide is generally applied in a thickness from 5µm to 25µm depending on the fiber size. Acrylate and silicone thickness can be varied from 10µm to 140µm and depend on the application and fiber size. Aluminum generally varies from 20µm to 45µm, increasing with increasing fiber size. Mechanical and Environmental In extreme mechanical, thermal and other environmental conditions the survival of synthetic fused silica optical fiber depends on the packaging, handling and additional protective coatings used, not so much on the glass itself. In addition to buffer coatings optical fiber is often cabled in assemblies or given a second buffer coating or over-jacket. These materials and the cable package design are key to the survival of the fibers in their final application. It is best to team with someone experienced in designing optical fiber assemblies or cables to assure a reliable end product. Polymicro has years of experience in this type of custom and production assembly design work and is ready to assist you with your design.

Acrylate buffered synthetic fused silica optical fiber can easily survive –55oC to 95oC required by many specifications. For more extreme temperature performance, there are other buffer materials available. Depending on the buffer coating, temperatures up to 400oC are survivable as well as below –55oC. Polymicro’s polyimide buffered fibers have been used in liquid nitrogen environments, as well as at 300oC continuous operation with intermittent use up to 400oC.

Mechanical Stress and Fiber Strength Optical fibers produced with synthetic fused silica have remarkable strength. Based on the Si-O bond strength, the fiber has a theoretical strength of ~2,000kpsi, which is stronger than steel! In practice the observed strength is con- siderably lower (typically 700kpsi) due to the presence of small flaws in the bulk and on the surface of the silica. In order to produce a reliable fiber these flaws must be minimized or eliminated. The size of the flaw determines the stress level needed to fracture the fiber, a larger flaw causing a lower strength fiber.

The dynamic strength of an optical fiber refers to the force required for an instantaneous break (as opposed to a delayed failure). Since failure in brittle materials, like glass, is a statistical process, many samples must be measured in order to adequately represent the distribution of flaws (and strength) throughout the fiber. This distribution is commonly presented as a Weibull plot. The Weibull function is represented by:

(Eq. 2-18)

Where F is the fractional failure probability, m is the Weibull slope, S is the failure stress, So is the Weibull characteristic stress (stress at F~0.632), L is the gage length, and Lo is the unit gage length. In a Weibull plot, a near vertical slope (m) with high stress values represents a tight strength distribution and a consistently strong product. This is demonstrated in the presented Weibull plot of Polymicro’s FVP050055065 High -OH fiber type (Figure 2-23). It is a 50µm core, 55µm clad OD with a 65µm OD polyimide buffer coating.

The mechanical strength of glass optical fibers will also degrade over time. This effect is known as static fatigue or stress corrosion. The mechanism in silica based optical fiber is the propagation of surface flaws due primarily to a combination of stress, moisture, temperature, and time. With a fiber under stress a surface flaw acts as a stress concentrator with the maximum stress being at the tip of

Figure 2-29 Weibull Plot for Fused Silica Optical Fiber

© Polymicro Technologies, a Subsidiary of molex. 2-18 2 Fiber Optics & Optical Fiber the flaw. Water attacks and breaks the silica bonds preferentially at the high stress area at this type. This causes the flaw, and consequently the stress, to increases in magnitude until the fiber catastrophically fails. Temperature, of course, speeds up the reaction.

This mechanism is well described by Charles4 and can be represented by:

log(time-to-failure) = n x [log(1 sec failure-stress) – log(failure-stress)] (Eq. 2-19) Where time to failure is in seconds and failure stress in kpsi. The static fatigue parameter, n, is an important constant that is very dependent on the specific manufacturer’s fiber and the materials used to produce it. In most analyses, the safest failure stress to use is the proof test of the fiber since the fiber has been confirmed to be at least that strong.

Static fatigue results are typically plotted on a log time to failure vs. log stress curve. In this case the fatigue curve will be straight with the slope being equal to n. Values for n range on the low end of ~10 for borosilicates up to >100 for hermetic fibers. Typical n values for polymer coated synthetic fused silica fibers range from 20 to 28.

An effective method to assure fiber strength is to perform a proof test on the fiber. The proof test is used to filter out flaws of a given size or larger. This assures the fiber will meet a minimum strength requirement, typically 100kpsi for most optical fibers. This proof test value can be adjusted higher or lower to meet the strength and lifetime requirements of the application. Proof testing can be performed by applying either a bend or a tensile force on the fiber. Polymicro proof tests 100% of their optical fibers and capillary to insure a strong, high quality product.

Stress on an optical fiber can be generated by tension, bending, or torsion. The calculation of the stress and the proof test method is typically based on either tensile force or bending stress. In tension the stress is simply the force divided by the cross sectional area of the glass. Note that the fiber coatings have Young’s moduli that are typically several orders of magnitude lower than the glass, and therefore do not bear a significant portion of the tensile load. Although the coatings do not add strength, they have the important function of protecting the glass surface from abrasion and chemical damage, which in turn would degrade fiber strength.

Bending stress can be determined through the following equations. These equations are used to determine the bend stress imposed on a fiber during use as well as the wheel radius needed to perform a specific level proof test. If we σ ε define the applied stress, a, as the strain, , times Young’s Modulus, E, we can derive the relationship:

(Eq. 2-20) or (Eq. 2-21)

Where r is the fiber radius, R is bending radius, and Cth is the coating thickness. Thus, R is given by the following:

(Eq. 2-22)

Figure 2-30 Bend Radius vs Stress Level For Different Core Sizes

4 R. J. Charles, J. Appl. Phys., 29, 1547, 1657 (1958)

Radiation Resistance Pure Silica core optical fibers generally exhibit superior radiation resistance as compared to doped Silica fibers. Unlike optical fiber used for telecommunication, Polymicro’s step-index multimode optical fibers are constructed of a pure silica core material which is generally considered radiation resistant. These fibers are available in both high and Low -OH versions. For ionizing radiation, such as gamma and X-ray, the higher -OH content fiber is far superior. From recent technical articles radiation resistance, UV, and near-IR attenuation are all inter-related with the optical fiber production parameters and buffer coating. Some of these results are presented in summary form in the table below. For additional information, see the article “Influence of preform and draw conditions on UV transmission and .5 transient radiation sensitivity of optical fiber”

In the referenced article, all of the fiber tested was drawn from the same lot of raw material by one university (Rutgers University) and one manufacturer (Polymicro Technologies). At Polymicro (PT), the draw conditions were varied to analyze the effects. Draw rates ranged from 5-30 meters per minute at a tension of 55-130 grams and 2- 56m/min at 25-185 grams tension. Fiber was drawn at 1950oC and 2100oC at both facilities. Rutgers University (RU) used draw rates of 15-45m/min at tensions from 12 to 148 grams. Polymicro applied both polyimide and acrylate buffers while Rutgers only supplied acrylate. Portions of the results are given in the following table.

Figure 2-31 Radiation Sensitivity of Optical Fibers

5 “Influence of preform and draw conditions on UV transmission and transient radiation sensitivity of optical fiber,” P. Lyons and L.Looney, LANL, H. Henschel, O. Kohn, H.U. Schmidt, Fraunhofer INT, K. Klein and H. Fabian, Heraeus Quarzschmeize, M. Mills, Rutgers University, G. Nelson, Polymicro Technologies

What is a Capillary?

According to The American Heritage Dictionary, Capillary is defined as "Any tube with a small internal diameter".1 In the realm of analytical chemistry and spectroscopy, capillary is understood to be small internal diameter glass tubing. The most highly specialized glass capillary is referred to as Flexible Fused Silica Capillary Tubing. It is drawn from high purity silica and externally coated with a protective polymer to yield a strong, durable, and yet flexible tube that is used across a wide range of applications.

Flexible synthetic fused silica capillary tubing from Polymicro is manufactured across a range of internal diameters from less than 1.0µm to over 2000µm. PT standard products range from 2µmto 700µm. Standard capillary is fabricated from synthetic fused silica preforms using sophisticated draw towers and following closely controlled manufacturing processes. The outside silica diameter typically ranges from 90µm to >3500µm. PT standard products range from 90µm to 850µm. In most applications, an external polymer coating is applied to protect the silica outer surface from abrasion. The coating thickness will depend on the material and application. Similar tubing can be manufactured using natural quartz, borosilicate glass, or doped synthetic fused silica materials depending on the application requirements. Polymicro manufactures only the highest quality fused silica, doped fused silica, and specialty natural quartz capillary. Applications Employing Capillary When summarizing uses for capillary, it is difficult to make an all-inclusive list. As technology expands, so does the number and variety of applications. As a starting point for new ideas, here are a few well established techniques that rely on capillary:

• Analytical Chemistry... Capillary is a fundamental component in Gas Chromatography, Capillary Electrophoresis, Capillary Liquid Chromatography, Flow Cytometry, and a number of other techniques. • Combined-Method Analytical Chemistry... Hyphenated techniques rely on capillary for easy, compact interface designs. Examples include: GC-MS, LC-MS, CE-MS, and LC-NMR. • Gas & Fluid Delivery... Capillary can withstand high temperature and corrosive or hazardous environments, making it a good choice for transfer lines and connecting to micro-accessible areas. • Drug Delivery Systems.... The high purity, non-contaminating interior surface, and wide range of internal diameters make capillary a key component in these systems. • Flow Cells... Small ID capillary make excellent, low band-broadening transfer lines to flow cells. Unique spectral properties make them ideal for on-column detection, even into the deep UV. • Flow Restrictors... Capillary provide accurate mass flow control, with strong adherence to Poiseuille’s Equation. • Micro-pipettes... Whether used as produced, or “pulled” to a Pasteur-type tip, capillary is widely used as pipettes, predominately in neurological sciences. • Liquid Light Guides... Capillary offers flexible fluid containment for energy transmission via high index fluids. • Hollow Wave Guides... Dielectric interior coating of capillary generates low cost, long wavelength IR Waveguides. • Micro Insulators... Capillary can be used over conductors to provide 25 kV/mm or more electrical insulation. Polymer coatings withstand up to 400oC (short term). Polymicro capillary with no external coating can withstand temperatures in excess of 1000oC. • Coupling Ferrules and Connectors... Laser micro-machined capillary is ideal for coupling optical fibers as well as capillary. The popular Inner-LokTM used for coupling GC columns is an example of this technology. • Micro-optical Elements... Capillary is found in a variety of instrument applications, from optical detection cells to fiber optic alignment ferrules.

1 The American Heritage Dictionary, Houghton Mifflin Company, Boston, 1981 Inner-lokTM is a registered trademark of Polymicro Technologies

Characteristics Offered by Capillary Synthetic fused silica capillary has several fundamental characteristics that make it such a unique material: • Very strong and flexible • Abrasion resistant when externally coated • Mirror smooth interior surface • Inert surfaces and high purity • Accurately drawn to a wide range of internal and external diameters • Internal surface modification possible • Dimensionally stable over long lengths • Transparent from the deep UV to near IR (coating removed) • Superior temperature resistance • High dielectric strength, very high breakdown voltage • Easily cleaved or cut • Variety of external coatings available

Capillary is strong and abrasion resistant, but can be easily cleaved. It has inert, high purity surfaces, yet stable chemical modifications are routinely produced. It can be made to a wide range of sizes, yet each size can be held to exacting tolerances over long production lengths. Because of this impressive list of characteristics, synthetic fused silica capillary has claimed a unique position in the material sciences as an ideal product for an expansive range of applications. A discussion on key applications follows below. Gas Chromatography Although capillary tubing is now used over a broad range of applications, its history is inseparable from that of Gas Chromatography. According to the International Union of Pure and Applied Chemistry (IUPAC), “Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (stationary phase) while the other (the mobile phase) moves in a definite direction”.2 In Gas Chromatography (GC), the mobile phase is a gas and the stationary phase is either a solid or a liquid, depending on the type of separation column selected. The actual separation of components in a mixture is achieved due to the differential distribution of solutes between the mobile and stationary phases. Capillary is fundamental to the technique, as it is the support for the stationary phase and the conduit through which the mobile phase travels.

One of the first applications for capillary tubing was in the Gas Chromatography industry. Dandeneau and Zerenner’s pioneering efforts are recognized by many as the cornerstone for the introduction of fused silica based capillary as a support for GC columns.3 Continued improvements in GC analysis have been aided by the unique properties of synthetic fused capillary tubing, with the majority of new methods rooted in proprietary new stationary phase coatings. Since its first use in the late 70’s, the range of capillary tubing internal diameters (ID) has grown, and specification tolerances have continually tightened to meet the exacting demands of the column manufacturing industry. Numerous GC column manufacturers offer a broad selection of columns used in the analysis of food products, pharmaceuticals, clinical samples, petroleum, and a variety of other applications.

Polymicro produces a wide range of capillary designed specifically for GC applications. Capillary with ID’s ranging from 50µm up to 750µm are routinely used, with the majority of columns employing capillary between 180µm and 530µm. Two product lines are commonly used, these being the standard TSP and high temperature TSG. Although Polymicro communicates the specifications in micrometers, most column manufacturers list column diameters in millimeters. It is common to name a column by a combination of the ID and stationary phase.

2 McNair, H. M., Miller, J. M., Basic Gas Chromatography, Chapter 1, John Wiley & Sons, Inc. New York, (1997) 3 Dandeneau, R. D., Zerenner, E. H., HRC&CC 2(6), (1979) 351-356

Capillary Electrophoresis “Capillary Electrophoresis (CE) was born of the marriage of the powerful separation mechanisms of electrophoresis with the instrumentation and automation concepts of chromatography.”4 CE is a separation method based on the differential migration rates of sample components within a capillary when an electrical field is applied axially to that capillary. The detection of sample components is usually “on-column” using UV spectrometric or fluorescence analysis through a “window” in the capillary. CE has become a powerful technique, finding a wide range of applications areas, including the analysis of proteins, peptides, chiral compounds, pharmaceuticals, inorganic ions, and DNA to mention a few. CE played a pivotal role in the Human Genome Project’s ultimate goal of unraveling the sequence of human DNA. It is routinely used in sequencing facilities and DNA forensic laboratories worldwide.

Performing electrophoresis in small-diameter polyimide coated capillary allows the use of very high electric fields, and the efficient dissipation of the Joule heating that results. High electric field strengths produce very efficient separations while minimizing separation times. Key features that make fused silica capillary tubing the column material of choice in CE include:

• The surface ionization characteristics of fused silica when in an applied electrical field generates elec- troendomosis, also known as Electro-Osmotic Flow, or EOF. This basic pumping action drives most CE applications and is responsible for the techniques high chromatographic efficiencies. • Capillary tubing has a high dielectric strength, giving ample isolation for the high voltages (up to 30kV) used in CE. • The polyimide coating can be easily removed in a process called “windowing”. Windowed capillary has high transmission in the UV-Vis spectral region, allowing for direct on-column absorbance measurements. • Windowed capillary exhibits low background fluorescence, making it ideal for fluorescent detection schemes, including LIF (Laser Induced Fluorescence). • Capillary can be easily rinsed and filled with solutions and buffers by either positive pressure or vacuum. • Modification of capillary surfaces through dynamic coatings, covalently bonded phases, or stabilized gels, allow for control of EOF and can impart unique separation mechanisms. • The durability and abrasion resistance of polyimide coated capillary allow for great flexibility in instrumental design and hyphenation interfacing. • Capillary can be easily cleaved to length, providing users great latitude in adjusting column length and economies of scale in tubing procurement.

CE has carved out a unique niche in the analytical laboratory, both as a fundamental separation tool and a complementary technique to established methodologies. Polymicro continues to be the primary supplier of tubing to this market, providing capillary ranging in ID from 2µm to 150µm. Polymicro set the standard in outside diameters (OD) by introduction of the 375µm and 150µm TSP product lines, and offers capillary for CE in a variety of formats including: bulk capillary on spools, capillary cut to prescribed lengths, windowed capillary, capillary assemblies, and custom capillary arrays. A number of CE manufacturers turn to Polymicro for custom sizes to meet their specific instrumental design and rely on Polymicro’s expertise for design and development assistance. Capillary Liquid Chromatography Chromatographic techniques that employ a liquid mobile phase loosely fall into a category referred to as Liquid Chromatography (LC). In LC, the stationary phase is either a solid surface or a surface that has been modified, or coated, to allow for partitioning of solutes into this thin layer of coating. As in GC, separation is achieved due to the differential distribution of solutes between mobile and stationary phases. In LC, the stationary phase is typically in the form of non-porous, semi-porous or porous particles that are “packed” into a metal or glass tube, referred to as the chromatographic column. New developments in this field have introduced the use of monolithic columns, a construct wherein a porous bed is formed in situ, as opposed to the packing of particles into the tubing.

4 Heiger, D. N., High Performance Capillary Electrophoresis - An Introduction, (Forward by J. W. Jorgenson), Hewlett Packard GmbH, Waldbronn, Germany (1992)

Capillary fills two fundamental roles in LC applications, as connection lines and as columns. First, it is often used for making fluidic connections between the chromatography columns and the other components of the system, i.e. flow cells, injectors, detectors, etc. Capillary is durable, yet easily cut to the exact length needed for such interfacing. The wide range of available ID’s allows chromatographers great latitude in selecting tubing that offers the optimum performance under their experimental and instrumental design. To minimize system dead volume in Nano LC applications precision cleaving is required. Polymicro offers precision Cleaving & Cutting Capabilities.

Second, trends toward miniaturization of systems has lead to the introduction of capillary-based columns, and thus the advent of Capillary Liquid Chromatography (CapLC). In this technique, a capillary is either packed with particles or used as the substrate for construction of a monolithic bed. The particles, or monolithic bed, contain the stationary phase and the capillary has become the chromatographic column. Capillary diameters most commonly range from 50µm up to 500µm, with a variety of different designs available. Standard TSP products are commonly used with TSU and TSH products gaining interest for some monolithic columns. Polymicro now offers a “Thick Wall TSP” product line when internal diameters of 150µm or larger are desired. Mass Flow Control A wide range of applications take advantage of capillary for precision control of liquid and gas flow rates. This general application area is referred to as Mass Flow Control, and relies on several unique properties of capillary.

• The smooth internal surface of capillary results in stable laminar flow profiles and good adherence to Poiseuille’s Law (Eq. 3-1), allowing accurate prediction of flow rates. • Capillary flow rates can be adjusted by changing either the ID or overall length. Polymicro offers a broad range of standard products with different ID’s so that users can select one that best fits their flow rate requirements. • Capillary can be easily cleaved, providing users great latitude in manipulating flow rates through length adjustments. • The tight tolerances of Polymicro’s capillary provide the needed dimensional consistency, both within each lot and from lot to lot. • Capillary, if handled with care, has excellent strength and durability. This, coupled with high temperature capabilities and chemical resistance properties, allows for usage in harsh and challenging environments. • Combinations of capillary are often utilized in flow splitting devices, with LC/MS interfaces being a classic example. Capillary is a robust solution in any application that demands stable, reproducible control of fluid or gas flow rates. Insertion of a segment of appropriate length and ID generates a fixed restriction, given that the other key parameters of viscosity and pressure drop are controlled. Tips on estimating the expected flow rate are discussed in this chapter (see Estimating the Flow Rate in Capillary). Other Uses for Synthetic Fused Silica Capillary Tubing At the beginning of this chapter, a number of key uses of capillary were summarized. This is certainly not a complete list. There are a significant number of other uses, many involve variations in the basic design and composition of capillary itself. Some examples include:

• An outer coating can be applied that is deep-UV transparent and can be used up to 160oC. These are referred to as TSU products. The alternate TSH products are also UV transparent. • Square, rectangular, and other shaped cross sections offer new opportunities. The square capillary line is popular in Flow Cytometry instruments, and is referred to as the WWP line of products. These, and other unique geometries, are further discussed in detail later in this chapter (see Alternative Cross-Sectional Geometries). • Other coatings, both internal and external, expand the potential uses of capillary. These are discussed later in this chapter (see Internal and External Coatings and Chemistries). • Custom sizes of both unique ID’s and OD’s are commonly produced. Standard sizes drawn to tighter tolerances are also considered custom products. • Capillary can be micro-machined into fittings and ferrules, used for coupling capillary as well as optical fiber (discussed in the Fused Silica Micro-Components chapter).

Capillary Products Capillary can be purchased in a variety of formats, including:

• Bulk: The majority of capillary is supplied to the market in long, continuous lengths on Styrofoam® or plastic spools. Spool lengths depend on production specifications and customer order requirements; each spool is individually bagged. Capillary ends are fused closed to maintain a clean, inert internal surface. All material is supplied with an appropriate label that includes the part number, lot number, length, and other pertinent information. • Precut: Capillary is often precut to customer specified dimensions. Length can vary from a few millimeters up to tens or hundreds of meters. Attention is given to critical parameters, such as length tolerance, end finish quality, cleanliness, packaging, and lot traceability. Parts are packaged in groups, but can be individually packaged if required. • Windowed Capillaries: Polymicro also supplies precision windowed capillary tubing. Parts are supplied with the polyimide coating removed over a defined length at a specified distance from one end of the capillary section. This provides an optical quality window that has excellent mechanical properties as well. In addition to the window location and quality, attention is given to other critical parameters such as overall length, end finish quality, cleanliness, packaging, and lot traceability. • Coils: GC columns can be pre-coiled to length, or more commonly multiples of their final length. Polymicro produces these products, referred to as coils, which allows for efficient production of finished GC columns. • Arrays and Assemblies: Capillary is routinely used to build arrays and other assemblies, just as with optical fiber. It is even possible to combine capillary tubing with optical fiber into the same assembly, reducing the amount of assembly required in the final instrument. Handling techniques become very important when dealing with capillary arrays and assemblies (discussed in the Appendix). Bonding materials and other potential contaminates must be kept from the interior of the capillary while building a sturdy assembly that will survive the application requirements. Polymicro’s experience with handling and building capillary assemblies can save product development time and optimize production efficiency, reducing overall costs. Cutting & Cleaving Capabilities Fused silica capillary tubing is commonly produced in long, continuous lengths; in many cases these lengths can exceed 2km. Nearly every application requires sectioning of the capillary into lengths that appropriately match the application requirements. Lengths can vary from a few millimeters to over 100 meters. When selecting an appropriate option for producing the desired length there are a number of issues to consider. Although the ultimate performance of the part in the target application is most critical, issues such as cost, packaging, durability, end-face finish, length tolerance and general ease of use should be considered. It is also important to understand the impact that capillary attributes (i.e. wall thickness, i.d., and o.d.) have on potential utilization of different cutting and cleaving options. The above issues and impacts have been discussed previously.5 Common methods employed for segment- ing capillary are (a) standard cleaving, (b) precision cleaving, (c) saw cutting and (d) laser cutting.

•Standard Cleaving - Standard Cleaving is done manually with a cleaving stone or diamond blade and often results in a slightly uneven end-face. For detailed instructions on how you can cleave your tubing by this method, refer to Cleaving Procedure in the Appendix.

•Precision Cleaving - Precision cleaving is done using proprietary robotic cleaving systems. These high-precision systems provide outstanding repeatability. This method generates minimal debris; end-face quality and perpendicularity are excellent.

•Saw Cutting - Multiple capillary segments are bouled together and then sawed to length as a group. Sawing leaves a matte end finish; chips and cracks in the end-face are common. Subsequent lapping and then polishing can improve surface quality in most cases.

•Laser Cutting - A proprietary CNC style, multi-axis high power laser workstation is utilized to perform the cutting operation. End-faces are typically defect-free, with no sharp edges, chips, or cracks. Due to the use of high energy laser light, some polyimide is ablated from near the end face of the capillary. 5Macomber, J., Lui, P., Acuna, R., LCGC Applications Notebook, (September 2009) 57 Styrofoam® is a registered trademark of the Dow Chemical Company © Polymicro Technologies, a Subsidiary of molex. 3-5 3 Flexible Fused Silica Capillary

Coupling and Connecting Capillary

There are a number of methods and devices used for connection of capillary. Selecting an appropriate connector depends on the application(s) and size of capillary being used. Suggestions are discussed below.

There are two basic connection types used in GC applications. 1) A Swagelok® or equivalent compression fitting with appropriate Vespel® insert is common for end of column connections. The capillary is held by a compression fit to the polyimide buffer, making this a robust connection. A number of options are available from most GC Column manufacturers. 2) Capillary to capillary connections are often accomplished using tapered silica or quartz micro components called Inner-LoksTM. These connectors accept a range of capillary sizes and offer a simple, clean method for coupling. Using silica or quartz, as opposed to borosilicate, as the coupling material provides a good thermal expansion match. The capillary can be bonded after insertion into the Inner-LokTM making a more permanent connection.

Connections in LC and related fluidic applications normally employ compression fittings made of polymeric materials, such as PEEKTM. A number of manufacturers offer connector products compatible with Polymicro capillary. Many of these fittings are pressure rated, and are designed for simple, fast assembly of capillary into instrumentation.

Custom connections and terminations of capillary are available from Polymicro. Key design considerations include end finish, ID cleanliness, strain relief, pressure requirements, operating temperature, solvent compatibility, and construction of the mating fitting.

Pressure Handling Capabilities CAUTION: Great care should be taken when using capillary tubing with internally applied pressure. Although capillary is very robust, a small particle or a minute chemical etching can create an internal stress crack or flaw. The combination of pressure and heat should be given special consideration. If handling, environmental, and/or chemical exposure after manufacturing has changed the internal surface or generated flaws, the added stress of high internal pressure can result in failure. When there is a possible chance for abnormally high internal pressure, we suggest testing a sample of the actual material that will be used to obtain some confidence in its durability under increased pressures. Always use safe laboratory practices when working with any pressurized devices.

NOTE: The material is proof tested in production to verify strength and quality (see Bending Stress in Capillary); it is important not to interpret proof testing as a measure of internal pressure capability.

Estimating the Flow Rate in Capillary When a viscous fluid flows through a tube of fixed length and ID, a resistance to fluid flow exists. So long as the Reynolds Number is less than 2000 and the fluid is considered to be Newtonian, some relatively simple equations can be used to describe the system. Under these conditions, the maximum speed is at the center and near the walls the fluid tends to remain nearly stationary. In terms of the viscosity, the resistance to fluid flow, R, for steady flow through a circular tube of radius, r, can be shown to be:

(Eq. 3-1) where η is the coefficient of viscosity, and L is the length. The SI unit for the coefficient of viscosity is poise, after the French physician Jean Poiseuille (1797-1869), who is often credited with deriving the above expression.

As can be seen, the resistance is inversely proportional to the square of the radius of the tube. For example, a change from 200µm inside diameter capillary to 100µm inside diameter means an increase in resistance to flow by a factor of 4.

Inner-LokTM is a trademark of Polymicro Technologies PEEKTM is a Trademark of Victrex plc Swagelok® is a registered trademark of Swagelok Co. Vespel® is a registered trademark of DuPont Dow Elastomers

By rearrangement of Equation 3-1 on previous page, Poiseuille’s Equation is derived. It follows that the flow rate through a capillary can be expressed in terms of pressure drop. More importantly, the flow rate is expressed in terms of the capillary ID, and is given by: (Eq. 3-2)

The flow rate, F, varies as the 4th power of the radius, i.e., a 2-fold change in radius creates a 16-fold change in flow rate. Equation 3-3 offers an easy to use version that gives the flow rate, F, in mL/min: F = [(r4 · ∆P) / (η · L)] · 1.625x10-8 (Eq. 3-3) where, the radius (r) is in µm, the pressure drop (∆P) is in psi, coefficient of the viscosity (η) is in cp, and the length (L) is in cm. Bending Stress in Capillary The bending stress on capillary is nearly identical to that of optical fiber, and as such, the same bending stress prin- ciples and test methods can be applied. A lengthy discussion can be found in the Fiber Optics & Optical Fiber chapter under Mechanical Stress and Fiber Strength. Polymicro proof tests most products to a nominal bending stress level of 100kpsi. However, since the inner surface can be exposed to the elements, it is easy for users to affect the strength of capillary during routine handling. If chemical reactions or particulate debris cause stress cracks on the interior surface, they may reduce the resistance to bending stress damage. Proper storage of the capillary and simple laboratory practices such as filtering solvents and gases, using quality connectors, and insuring quality cleaves on capillary ends, can have direct impact on maintaining capillary strength. Storage, handling, and cleaving are discussed in the Appendix. Figure 3-1 shows the relationship of OD (including polyimide), bend radius, and applied bending stress for a number of standard polyimide coated capillary.

Applied Bending Stress (kpsi)

Bend Radius (mm) Total OD (µm) 4 6 8 10 15 20 25 30 40 50 60 80 100 130 160 200 90 875843352317141297643322 105 106 71534328211714119754332 150 165 110 83664433262217131187543 164 184 123 92744937292518151297654 238 270 180 135 108 72 54 43 36 27 22 18 14 11 8 7 5 340 399 266 200 160 106 80 64 53 40 32 27 20 16 12 10 8 360 425 284 213 170 113 85 68 57 43 34 28 21 17 13 11 9 363 424 283 212 170 113 85 68 57 42 34 28 21 17 13 11 8 435 524 349 262 209 140 105 84 70 52 42 35 26 21 16 13 10 665 * 540 405 324 216 162 130 108 81 65 54 40 32 25 20 16 700 * 571 428 342 228 171 137 114 86 68 57 43 34 26 21 17 850 **526 421 281 211 168 140 105 84 70 53 42 32 26 21 * exceeds capillary break strength

Figure 3-1 Capillary Tubing Bend Stress

Optical Properties of a Capillary A review of the optical properties of capillary involves close examination of not only the glass substrate itself, but also the protective coatings often employed. These will be discussed further below. Optical properties such as transmission, fluorescence, refractive index, and in some cases numerical aperture are all important to consider. It should be noted that capillary is used as a substrate for liquid light guides and hollow silica waveguides. Further, if appropriate coatings or claddings are added, capillary can embody both fluidic and fiber optic properties simultaneously such as in Polymicro’s Light Guiding Capillary.

When examining transmission of light through Polymicro's standard capillary products, it is important to consider both the direction of light propagation and the silica type. Since non-axial, or orthogonal, transmission (i.e. on-column detection

© Polymicro Technologies, a Subsidiary of molex. 3-7 3 Flexible Fused Silica Capillary schemes) is normally of key interest, the extremely thin walls of capillary make the actual transmission losses in the silica negligible. However, this is dependent upon the type of silica used to make the capillary. Polymicro has selected an appropriate -OH content glass for applications in the deep UV and Visible spectral regions.

Fused silica glass, especially the lower -OH types, can exhibit fluorescence when illuminated with 254nm light passed through a Schott UG-5 color glass filter. From Heraeus data, this apparently does not occur in the high -OH materials. High -OH fused silica glass is essentially free from visible fluorescence at excitation wavelengths greater than 290nm. Slight blue-violet fluorescence appears in materials such as natural fused quartz products, and significant fluorescence is observed in most borosilicate glasses. It can be concluded that fused silica is indeed the material of choice for most capillary products.

The refractive index of the fused silica used to make capillary is often of interest to researchers. The actual value is wavelength dependent. A table of fused silica refractive index versus wavelength can be found in the Appendix. On occasion, the numerical aperture of capillary is sought. It is of course dependent upon the gas, fluid, or coating that has been applied to the capillary surface. A discussion of numerical aperture and related topics is found in Fiber Optics & Optical Fiber.

As most capillary is coated with a thin layer of polyimide, this material will receive consideration here. Polyimide transmission varies significantly from that of the silica. Standard polyimide is relatively translucent down to about 550nm, with select custom polyimides exhibiting as much as 90% transmission at 425nm. All polyimides evaluated have shown less than 2% transmission below 350nm. This fact, when coupled with polyimide’s inherent fluorescence, has lead to the wide spread use of windowed capillary for on-column spectral analysis applications. Polyimide exhibits significant fluorescence across a broad range of excitation wavelengths. This has been studied and is summarized in a Polymicro application note.6 The polyimide coating on capillary has fluorescence essentially equivalent to that of a 1mM solution of Rhodamine B in a 50µm ID capillary.

As mentioned earlier, Polymicro does employ other protective coatings, and their optical properties vary significantly from polyimide. Acrylate coated capillary (TSA products) are more translucent in general and offer lower fluorescence. When UV transmission through the coating is needed, Polymicro offers two product solutions. A novel Fluoro- polymer coated capillary (TSH products) offers >10% transmission at 310nm. This coating also offers improved background fluorescence when compared to the TSA products. Depending upon the signal to noise ratio requirements, TSH capillary has proven useful for on-column fluorescence detection. Teflon® AF coated capillary (TSU products) are produced specifically for their unique optical properties. This fluoro-polymer coating is UV transparent, with transmission at 214nm typically greater than 90%. The refractive index yields an NA of 0.66 and this, coupled with the low absorbance properties of the coating, make it an excellent optical cladding material. A number of applications take advantage of this by using TSU products as a light-guiding capillary. Others capitalize on the transmission properties by using TSU for on-column detection applications, as removal of the coating is not required. Unfortunately, these unique optical properties are offset by the lack of abrasion resistance of the fluoro-polymer. TSU products must be handled with care to avoid breakage during use. In addition to fluid handling capabilities for analytical instruments, capillary tubing can be used as a waveguide. In hollow waveguides, the internal surface must be coated with an appropriate dielectric coating for the wavelength of interest. In this hollow core configuration, both metal and dielectric coatings have been applied to create a waveguide for wavelengths further into the infrared than would be possible with conventional optical fiber.

It is also possible to use silica capillary tubing as the cladding for a fluidic waveguide, or liquid light-guide. As long as the fluid has a refractive index higher than the capillary, a very effective light guide can be produced. Light Guiding Capillary

The addition of an external coating or cladding that has a refractive index lower than that of the capillary itself will produce a light guiding capillary. As mentioned above, Teflon® AF coated capillary TSU has inherent light guiding properties. TSH will also act as a light guiding capillary. More durable light guiding capillary that employ an inorganic cladding can be manufactured. This line of products, referred to as LTSP, is produced with durable polyimide coating and an NA of 0.22. Custom draws are available upon request.

6Macomber, J. et.al., LCGC North America, June Suppl. (2004) 72 Teflon® AF is a trademark of E.I. du Pont de Nemours and Company

Internal and External Coatings and Chemistries:

Interior Surface Chemistry of Capillary

The interior surface of silica capillary is pristine when it comes from the initial draw process. It is considered to be inert as compared to other glasses, such as borosilicate and quartz, due to the low level of impurities. A list of impurities and their typical abundance can be found in the Appendix. During the manufacturing process, the silica has been heated to a temperature at which its viscosity allows it to be drawn down to the specified dimensions, typically in excess of 1,800o C . It is well understood that after exposure to these elevated temperatures, the surface of the capillary is devoid of both chemically and physically absorbed water. The surface is rich in siloxane bridges (Si-O-Si) and the residual silanol groups (Si-OH) present are isolated. In many applications, the initial conditioning of the surface before use is extremely important and should be given due consideration. Development of a protocol that achieves the desired surface chemistry is well advised.

Common terms used when discussing conditioning of capillary are surface activity and deactivation. These words have different meanings depending on the application and the background of the user. Some would say that the drawn capillary has high surface activity because it readily incorporates water back into the surface structure. It follows that a fully hydroxylated surface would be deactivated.

In GC column production, surfaces are sometimes said to be “too active”, and steps are taken to address this during internal coating operations. Some refer to these initial steps as deactivation. In other instances, such as in the case of capillary column connectors for GC, deactivation refers to the addition of covalently bound ligands to the surface to reduce analyte adsorption.

In the case of CE, surfaces are treated to generate a uniform silanol population, a process some refer to as activa- tion, or more commonly as conditioning. Deactivation in CE is accomplished by the addition of coatings and dynamic additives that eliminate EOF or unwanted analyte adsorption. Hewlett Packard (HP), in High Performance Capillary Electrophoresis, suggests that “The most reproducible conditions are encountered when no conditioning other than with buffer is employed. However, adsorption of sample to the surface and changes in EOF often do not allow this.” The author further suggests “Base conditioning to remove adsorbates and refresh the surface by deprotonation of the silanol groups is most commonly employed. A typical wash method includes flushing a new capillary with 1N NaOH, followed by 0.1N NaOH and then buffer”.7

Silica related topics of common interest are the dissolution rate of silica, silica solubility, and their dependence on pH. Molecular diffusion in silica is also of interest to many researchers. A body of work on these topics is available.8, 9, 10, 11 Coatings, Interior Interior surface coating of capillary is routinely performed. These modifications fall into three basic categories.

• Wall Coated Open Tubular (WCOT) columns are used routinely in GC. The coating process involves deposition and subsequent cross-linking of the stationary phase. Some chemical bonding to silanol groups is desired, and gives improved temperature stability and reduced bleed. Companies that market GC columns are the primary source for these materials. • Covalently bonded chemistries find broad use in CE, both as finished columns and polymeric anchors for gel matrices. Bonding reaction schemes are numerous, with silanization using methoxy-, ethoxy-, or chloro- silanes being most common. Grignard reaction schemes are also popular. Often the goal of such coatings is to stabilize, eliminate, or adjust EOF. CE instrument manufacturers and a few after-market suppliers offer these types of coated columns. Polymicro can produce bulk capillary with covalently bonded chemistries. This type of internally coated capillary can be produced on a custom basis upon request. • Dynamic coatings involve the adsorption of specially designed solutions onto the walls of capillary and routine, periodic re-application is required. These typically target the same goals as covalently bonded chemistries.

7 Heiger, D. N., High Performance Capillary Electrophoresis - An Introduction, Chapter 4 (88) Hewlett Packard Gmbh, Waldbronn, Germany (1992) 8 Sjoberg, S., J of Non-Crystalline Solids 196, (1996) 51-57 9 Dove, P. M., Crerar, D. A. Geochimica et Cosmochimica Acta 54, (1990) 955-969 10 Majors, R. E., LCGC North America 18 (12), (2000) 1215-1227 11 Doremus, R. H., Glass Science, John Wiley & Sons, inc., New York (1973) 134-142

Coatings, Exterior Just as in optical fibers, many types of materials can be applied as the external capillary coating. These are applied either during the drawing operation, or as a secondary process. Further, the addition of specific agents to the coating material can bring unique functionality to the resulting capillary. A general discussion of Coatings and Buffers can be found in the Fiber Optics & Optical Fiber chapter and in the literature. Alternative Cross-Sectional Geometries The most common geometry for capillary is circular or round. Non-circular geometries such as square cross-section capillary have been produced in response to customer requests, with square shaped capillary first introduced for use in Capillary Electrophoresis and later gaining popularity for Flow Cytometry devices. Rectangular designs are popular as ferrules in telecom devices, such as DWDM. Other geometries, such as triangular, oval, and “race track” designs are being explored for cutting edge applications. Custom configured cross-sections are often explored through cooperative feasibility studies.

• Square Capillary: Of the many geometric variations possible, the square cross-section tubing has seen the most interest and a line of standard products, referred to as WWP, is available. The "Square" inside dimension of the tubing has sides of 50µm, 75µm or 100µm. The outside glass dimension is nominally 320µm per side and the outer surface is protected by Polymicro’s standard polyimide. This polyimide coating is nearly round, but some irregularity can be expected and can be felt by rolling the tubing in one’s fingers. In most instances, standard polymeric fittings will form sufficient seals to the polyimide even though it is not perfectly round.

Reported Advantages of Square Capillary:

• Eliminates the need to correct for the optical effect of curved capillary surfaces in Capillary Electrophoresis, Flow Cytometry and other capillary based detection devices. • Offers larger effective internal surface area. • Flat sides create 27% more volume and 2X the transverse optical interaction path length (using a collimated beam) than that of a round capillary configuration. • Behaves like a cuvette in standard fluorescent sensing devices such as those used in Flow Cytometry. • With polyimide coating WWP capillary will seal in most fittings designed for Polymicro capillary tubing products. • The glass substrate is the same high purity synthetic fused silica used in Polymicro’s wide range of standard capillary products (i.e. TSP series); as a result, one can expect equivalent surface chemistries.

Figure 3-2 Light Path Through Capillary Windows

• Other Potential Geometries and Associated Design Issues: A variety of different geometric variations are possible; it should be noted that the inside and outside geometries need not be the same. Several different combinations are shown in the figures below. Polymicro welcomes inquiries into other unique internal/external combinations. Beyond the basic geometry, there are a number of important design considerations; some key ones are discussed below.

Basic specifications to define include inner and outer dimensions, corner radii, flatness of sides, parallelism between opposing sides, angles between adjacent sides, and the concentricity of the inner and outer geometries. Another key consideration is the final product wall thickness. In general, the minimum allowable wall thickness at any radial point is ~100µm. Although most shaped capillary is made from synthetic fused silica, in some cases quartz has been specified. Coatings are commonly employed to help protect the outer surface, but there are limitations. For products up to ~600µm in outer dimension, polyimide is normally the coating of choice. As the outer dimension increases, or if the aspect ratio exceeds ~2.5:1, acrylate is often suggested. For outer dimensions in excess of 1.5mm it is common to produce the drawn product without a coating. Although these shaped geometry capillaries are uniquely different from standard round tubing products, nearly every common post-processing technique can be performed equally on either type of capillary tubing. This includes value added operations such as windowing, precision cleaving, laser cutting, ID coating, and custom capillary assembly.

Figure 3-3 Examples of Capillary Geometries

What are Assemblies? Assemblies are easy to install units which can include optical, electrical, mechanical, and fluid functions, which are economical to produce. They can combine optical fibers, mechanical holding and mounting hardware, as well as fused silica capillaries, to transfer or manipulate light, gases, or fluids from one location to another. Most assemblies are based on specific user requirements. These can vary in complexity from a single terminated optical fiber to a complex branching assembly. These can combine the advantages of fused silica optical fiber and capillary tubing into an integral unit.

Optical fiber or fused silica capillaries can be clustered into bundles at one end and fanned out into various configurations at the opposite end. Special end shapes can be micro-machined directly onto the optical fiber or capillary, saving bonding and coupling losses. This handbook section will cover the basics in assembly configurations.

There are a number of opto-mechanical considerations when designing a fiber and/or capillary assembly that may not be apparent. Products are very often proprietary to a customer. Because each assembly is intended to provide a unique interface or solution we will describe technical considerations and guidelines rather than specific details of “standard” products. It is important to choose a manufacturer with materials and product experience. This will help to insure a high quality, successful result. Applications Assemblies consist of opto-mechanical and electro-optical components that incorporate standard products in combination with custom configured fused silica and precision optical products. Added optical or fluid features such as micro-machined end tips, metal terminations, or precision coup- lings can be included. Customer requirements and/or specifications help to determine whether standard products or custom designed optical and mechanical parts are required.

Optical fiber cables are specifically designed for applications demanding high optical transmission for the ultraviolet (UV), near infrared (NIR), and infrared (IR). Specially designed fiber optic cables are ideally suited for applications ranging from instrumentation and process control, to short distance data communication and laser power transmission. The wide variety of specialty optical fibers and cabling materials give the optical designer unparalleled flexibility in system design. The fiber’s high mechanical strength is enhanced by the use of “state-of-the-art” buffer and cabling materials.

Polymicro offers the system designer unparalleled flexibility to incorporate a wide range of design options into their assemblies. Customers have incorporated our assemblies into the manufacturing of devices and systems in the following areas:

* Polymicro Technologies, manufactures optical fiber cable, components, and assemblies only. Polymicro Technologies does not design or market medical devices.

Design Considerations Polymicro offers the system designer unparalleled flexibility to incorporate a wide range of design options into their assemblies. Everything from the termination style and technique, jacketing, and fiber type can be specified to ensure optimum system performance. Our experience in designing and manufacturing assemblies for the industrial and scientific industries is a valuable resource for applications ranging from sensing, to laser power transmission, to process control instrumentation, to short distance data communication. In order to do our best job for you, we will need answers to as many of the following items as possible.

* The end manufacturer is responsible for bio-compatibility and sterilization testing and validation studies. What Does Polymicro Need From You? In response to the details of the assembly parameters listed above, our engineers will be able to take your requirements, specifications and designs to suggest solutions. Before any design is finalized, the following items will generally have to be specified. This helps to generate performance, cost, and test requirements. It will become the defini- tive guide when production of the assemblies begins.

The Assembly Designer Will Determine:

Low Fiber Count Assemblies Because fused silica fibers are available in a broad range of sizes, simple assemblies are quite often made using only a few large fibers rather than a large number of smaller fibers. Combiners or fan-outs may have all channels using the same fiber diameter or may use different fiber sizes. These types of assemblies include simple patch cables for single fiber connections using most of the standard optical fiber connectors. The example in the photograph on page 4-3 combines three fiber channels and connectors into one single connector. Low fiber count assemblies such as this are very simple and compact. They are most often used when the light source can be focused small enough, the active areas are all circular, and the sensor(s) are small. With large light

sources, larger detectors, odd geometry sources, and sensors, fiber bundles are commonly used. Being able to do special cable connections in an efficient, reliable package will result in maximum performance in the end use system.

Fiber pigtails (optical fiber cables with a connector at one end only) are supplied for customers to direct couple to their own source or sensor. Simple patch cables, using industry standard connector types, can be supplied. A few of the standard connector types are shown in the illustration below.

All of the commercially available connectors used for step-index optical fiber can be supplied in patch cords or assemblies. Some types are SMA, ST, FC and custom designs for specified matches. Mating parts, including bulk- head connectors and types with special alignment features, can be supplied and will determine the total overall efficiency.

Polymicro has several end termination and connector designs to help solve unique customer optical fiber application requirements including proven high power laser connectors designs.

Common Multimode Connectors

Less Common Multimode Connectors

Figure 4-1 Polymicro Technologies’ Low Fiber Count Assemblies High Power Laser Connectors The design of connectors for high power laser use is a specialized area. If the design is not done correctly by someone with experience in laser/fiber interface, the chances are very high that the heat load from the laser power will destroy the optical fiber and the connector assembly. Polymicro has provided several high-power connector designs over the years. The simplest design, for low to medium laser power, involves the proper choice of bonding material, a well-designed stainless steel connector assembly, and an optional quartz isolation sleeve. The choice of bonding the fiber into the sleeve at the end tip will depend to some extent on customer requirements, as well as on how tightly focused and positioned the laser beam is relative to the fiber tip. In some applications, the fiber is laser

© Polymicro Technologies, a Subsidiary of molex. 4-3 4 Fiber Optic & Capillary Assemblies welded to the quartz sleeve to eliminate epoxy at the tip altogether. In other high power applications, a special connector may be used with a counter-bored tip to allow the end of the fiber to be cantilevered away from the metal and epoxy. Temperature ranges depend on the materials chosen; however, designs for continuous operation at 200oC with short term excursions to 300-400oC have been supplied. A general schematic of a single, quartz sleeve, connector design is shown in the following illustration.

Figure 4-2 Polymicro Technologies’ Laser High Power Connector

Polymicro has a “Standard Termination” and three types of “High Power Terminations.” The term “High Power” is relative to the customer’s application since 10 Watts may be high to one customer but low to another. These are the most common types of terminations, but others can be designed based on specific customer requirements.

Standard Termination: High Power: Quartz Sleeve: • Standard low stress epoxy • Special epoxy • Epoxy to tip of connector • Polyimide left on fiber • Polyimide left on fiber • Quartz sleeve • Temperature Limit 150°C • Temperature Limit 200°C Continuous

High Power: Quartz Sleeve/Welded: High Power: Cantilevered Connector: • Special epoxy • Special epoxy • No epoxy at tip of connector • No epoxy at tip of connector • Polyimide removed from fiber • Polyimide removed from fiber • Fiber Laser welded into quartz • Cantilevered/counter-bored connector tip sleeve • Temperature Limit >200°C • Temperature Limit >200°C continuous (>400°C intermittent)

Figure 4-3 Termination Configurations Fiber Optic Cable

Optical fiber or fibers can be jacketed into cable form for added protection from environmental hazards including mechanical damage, chemical attack and water erosion. The cabling materials and design together with optical fiber will determine the environmental performance of the final cable. A fiber optic cable can be designed to work in a temperature range from -50°C up to >200°C. Besides cabling design, optical fiber can be a limiting factor to the maximum temperature fiber cable can withstand. For example, hard polymer clad fiber has an operating upper temperature limit of ~125°C, thus limiting the maximum temperature the cable can withstand.

The cabling materials are typically polymer but sometimes metal is also used. Common cabling materials include:

1) Polymer jacket materials: PE (polyethylene), Hytrel® (polyester), Nylon, Tefzel® (ETFE), Teflon® PFA, TFE, PU (polyurethane), and PVC

Kevlar® (aramid yarn), epoxy/fiberglass central members, and stainless steel tube.

Hytrel®, Kevlar®, Teflon®, and Tefzel® are trademarks of E.I. du Pont de Nemours and Company

There are a variety of cable configurations: simplex, duplex, multi-fiber breakout, ruggedized, high temperature aerospace, armored, tactical, loose tube, zipcord, optical coil cord, and other custom designs. Hybrid cables containing both electric conductor and optical fiber are increasingly common for medical applications.

• Primary Jacket Only: Simplest fiber cable design comes only with a jacket over a buffered fiber.

• Simplex Construction: Buffered fiber with an outer-jacket strengthened with linear aramid yarn between the buffered fiber and the outer jacket. In addition to improved mechanical pull strength, the yarn permits the fiber to be relatively loose inside the outer jacket, reducing or eliminating possible micro-bending loss due to the over jacketing. For increased durability, the aramid yarn can be braided.

• High-temperature Aerospace Style Dual Jacketed Cable: Polyimide coated fiber with an extruded high temperature fluoropolymer jacket in a simplex construction with braided Figure 4-4 Sample Cable Constructions aramid yarn. The final outer jacket over the yarn is also a high temperature fluoropolymer material. This design is capable of high temperature up to >200°C, while being compact and lightweight with low micro-bending loss.

• Multi-cable Polymeric Conduit Construction: Multiple simplex fiber cables placed inside a polymer conduit to maintain low transmission loss for each individual fiber cable.

• Stainless Steel Stripwound Construction: Fiber is loosely placed inside stainless stripwound tubing that gives the fiber mechanical protection. A polymer tubing with or without braided aramid yarn may be used to enclose the fiber and be placed inside the stripwound. This fiber optic cable construction is often used in high end fiber optic assemblies that offer good mechanical and other environmental protection and quality finish. The stainless stripwound can additionally be covered with polymer material. Bundles and High Fiber Count Assemblies A fiber optical bundle is a cluster of optical fibers with some geometric arrangement at each end to accomplish the transport of light from one place to another (or several others), usually along a non-linear path. Optical fiber is very useful in this instance because of its ability to carry light very efficiently over paths with many turns.

The end configurations can be almost any geometry … round, square, rectangular, or some other shape. In some applications, the only requirement is to get the light from one place to another. In this case, a fiber bundle is constructed with the optical fibers packed into circular ferrules or end tips. The most efficient packing geometry is hexagonal. If some randomizing of the light intensity is wanted between the input and output, the fibers are put in a quasi-random position on one end relative to the other. In the photograph is a rectangular output bundle with stainless steel flexible armor sheath. It is used to transmit high intensity UV from the source to the rectangular output end.

An example of a circular to rectangular assembly would be one where the output of a high intensity lamp is needed to illuminate a linear area. At one end, the fibers would form a circular bundle; and at the other, the fibers would be lined-up in a linear stack, square or staggered. The linear stack could then be focused on the area to be illuminated with a simple cylindrical lens or used in direct contact without a lens. In most applications, this method is much more efficient than using conventional optics to get the light where it is needed.

Applications may require the source light to be split into several different positions. This can be done with a multiple branch optical fiber assembly. The light is divided from the input to the output, in proportion to the number of fibers in each branch. The fiber could be arranged in a circle at one end and a ring at the other to make a ring light, which is now quite commonly used on microscopes.

An important feature of synthetic fused silica fibers is that wavelengths can be transmitted over the entire spectrum from the ultraviolet to the near infrared. Borosilicate and plastic fibers do not have this ability. In addition, Polymicro’s multimode, step-index, all-silica fiber has extremely low fluorescence characteristics, low scattering losses, and a stable index of refraction. Hollow waveguides can even be used in the Mid-Infrared region. See the HW specification sheet in the Product Reference section under Optical Fiber.

Light at one or more wavelengths can be sent down fibers and reflections or interactions (e.g., fluorescence) returned through the same or additional fibers in the same fiber bundle assembly. This type of assembly is found in reflective readers as well as remote sensing applications. The operating temperature range will depend on the materials used in the construction of the bundle, including the fiber buffer material. Jacketing materials, to protect the fiber bundle from damage, may also put some limitations on the type of environment in which the assembly can be used.

A bundle assembly need not be limited to just optical fibers; fused silica capillary tubing, wiring or other filaments can be integrated into the assembly as well. This feature might be used in applications requiring venting or flushing by gasses, fluid transfer or wire insulation via flexible silica tubing. For example, thermocouple or heater wires could be incorporated into the assembly.

In applications where several optical fibers are clustered into a bundle to form an end-tip, the true active area is the core area. The percentage of active area is calculated by dividing the total core area by the total geometrical area of the bundle. This active area is always less than 100% due to area taken up by the buffer coating, cladding, packing geometry and bonding material.

The most cost-effective method in most fiber bundle applications is to use an optical fiber with a thin buffer or polymer cladding. This is the most reliable choice as well since once the buffer is removed from the fiber end the fiber becomes very prone to damage or breakage. The little extra gain in active area is often lost due to broken Figure 4-5 Example Hex-pack Characteristics fibers and usually results in reduced yield (increased cost) and reduced reliability and performance.

The fiber-bundling approach is the best for large area applications or where flexibility is required. Routing light via optical fiber is much more efficient than lenses and mirrors. Using lenses to relay a light spot with an f/1.0 lens (at a 1:1 ratio), you will still lose 50% or more of the light. Whether it is 50% or more depends on the lens assembly transmission losses.

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,QWHUVWLWLDO VSDFHV

Figure 4-6 Fiber Hex-pack

Figure 4-7 Bundle Diameter vs Number of Fibers

For a bundle with 91 fibers or greater, the tolerance build-up makes it very difficult to get a perfect hex pack. To give the designer a gauge as to how many fibers of a specific diameter yield a certain bundle diameter, a chart has been generated. The nominal circumscribed diameter for some polyimide buffered silica fibers with perfect hex pack is shown.

Figure 4-8 Tight vs Linear Fiber Packing

In general, the smaller the number of fibers in the bundle the less expensive the final assembly. For instance 37 fibers, 500µm in diameter gives approximately a 4µm diameter as does 91 fibers, 300µm in diameter or 217 fibers, 200mm diameter. The 37 fiber bundle will also take less time to assemble.

If a linear array is needed at one end of the assembly, the fibers can be arranged in rows and columns or every other row nested in between the previous row to obtain tighter packing. In either case, the tolerance build-up in outside diameter of the fibers must be accounted for in designing the termination hardware configuration. The fiber size may have to be selected based on the resolution required in the linear array. Still, a fewer number of larger fibers will turn out to be the least expensive, all other requirements being equal.

From these two simple illustrations, it can be seen that we can dramatically change the optical distribution by going from say a circle on one end to a rectangle or line on the other. The choice of stack- ing method on the rectangular end will depend on the application requirements. A general light gathering or illumination application will most likely use the hex pack for maximum packing density; the in-line stack is more likely to be used for imaging such as spectrometer imaging relay or other coherent applications.

Other losses that should be taken into account are:

1. Media Losses (attenuation) – will depend on the length and type of fiber used. 2. Fresnel Losses – typically 4% per polished surface. 3. Potential fiber breakage – typically less than 2% of bundle area. The total loss from input to output will be the summation of all four factors: Packing, media, Fresnel and fiber breakage.

Capillary Assemblies In many ways, fused silica capillaries can be handled the same as fibers are in assemblies. They can be bundled and divided into several paths. The major difference is in the end finishing. If the interior of the capillaries needs to be kept clean, special handling and materials are needed. Cleaving, end finishing and bonding must be based on the application requirements and prior experience. It is important to choose a manufacturer and designer familiar with special handling required for capillary tubing assemblies and finishing. Polymicro, being experienced in design, proto- typing, and production of these assemblies, can save you time and material.

The Polymicro Advantage Designing and fabricating optical fiber assemblies can be tricky and expensive for the first time user. Polymicro offers a custom engineering service to help with new requirements or to help improve the cost or performance of customers’ current configurations.

To best serve customers’ needs, Polymicro has developed an excellent customer service and engineering support organization. Working with your design and manufacturing engineers at our facility and yours, we can quickly and cost effectively solve your problems. Using concurrent engineering methods and our capabilities in standard products as well as customized products yields a result that is readily manufactured at a reasonable cost. Polymicro can cus- tomize preforms, materials, and/or processes to meet the customer design and manufacturing requirements. Resource limited companies will especially appreciate this engineering outsourcing opportunity.

Polymicro’s fiber optic assemblies are specifically designed for applications demanding high optical transmission from the ultraviolet (UV) to the infrared (IR). Polymicro offers the system designer unparalleled flexibility to incorporate a wide range of design options into their assemblies. Everything from the termination style and technique, jacketing, and fiber type can be specified to ensure optimum system performance. Scientific applications in designing and manufacturing assemblies for industrial and scientific industries is a valuable resource, Polymicro can assist you in designing and building fiber optic assemblies for applications ranging from sensing, to laser power transmission, to process control instrumentation, and to short distance data communication.

Sculpted Fiber Tips – Tapers, Cones, Diffusers and Ball Lenses

With the latest technological advances in laser micro-machining, it is now possible to incorporate special optical features onto the end of an optical fiber. Fused silica capillary tubing can also be reshaped to modify the end shape or to generate custom couplers for capillaries or optical fibers. In this section, we will refer to these devices as micro-components. They generally have diameters less than 2mm.

There are many applications for optical fiber where a simple, flat, fiber end is not ideal. The ability to obtain an optical fiber incorporating optical coupling features can alleviate design problems. The problems associated with getting light in or out can be greatly improved by having interface features micro-machined into the fiber material.

Similar coupling problems exist with fused silica capillaries. The design can be made simpler and more efficient using micro-machined ends or couplers. Fused silica or quartz couplers offer a good solution for connecting fused silica capillary tubing. They also offer a good thermal match as well as a clean, non-contaminating, chemically durable connection.

Before going into a few of the various shapes that can be sculpted on optical fiber ends, a quick review of spherical lenses might be useful. When we discuss spherical lenses relative to optical fibers, we are usually talking about a complete sphere of quartz, glass or other optical material. The choice of material will be based on the wavelength range for which it is to be used. It will also depend on how strong a focusing effect is needed. Higher refractive index materials will give shorter effective focal lengths, but may have other problems relative to cost, durability or mounting.

A “Ball Lens” or full sphere is often used to couple optical fibers. The back focal length, BFL, increases as the ratio of D/d increases. The collimated beam diameter is d and the Sphere diameter is D. The market has these ball lenses available in various materials to allow choices of wavelength range, diameter and focal length. Materials such as BK-7, SF-8 or LaSFN-9, quartz, sapphire, ruby, doped Al2O3, and fused silica are used. High index materials normally must be anti-reflection coated due to higher Fresnel reflections. For many applications, an optical fiber with an integral spherical end surface Figure 5-1 Fiber and Focusing Sphere may be less expensive and give better performance results with less cost.

The most common fiber coupling, after the use of direct coupling, is the double sphere. The first sphere roughly collimates the light exiting one fiber and the second refo- cuses the collimated light into the second fiber. This method reduces the lateral alignment requirement between the two fibers. They can also be used to reduce Figure 5-2 Two Sphere Lenses as Fiber Coupler the divergence of a light source, such as an LED, to help couple more light into the fiber.

Sculpted Tips Integral with Optical Fibers Tapers or cones can be formed at the end of an optical fiber. As mentioned before in Fiber Optics and Optical Fiber, tapers can be used to reduce or increase the NA (the output divergence). The evanescence wave effect in a taper with the cladding partially or entirely removed can be used as a fluorosensor component.1 Sculpted tips can also include a spherical surface such as mentioned previously. These end configurations are precision micro-formed on an optical fiber end using a laser. Some typical shapes are shown below.

From the drawings, the optical fiber tip-end shape and characteristic are listed in the table. Keep in mind that the typical optical fiber diameter in these devices is typically in the range of 200µm to 2mm. The shapes are made from the fiber itself, thus there are no coupling losses or interface contamination that might exist if the shape was bonded or fusion spliced onto the fiber end. Many other shapes that combine prism surfaces, spherical lensing surfaces, and flat facets have been produced.

The components can also be cut to specific lengths and used as micro- optical components. They can be coupled to existing optical fibers or used as individual optical elements. Several of the configurations shown are used for laser beam manipulation to re-shape the beam pattern on silica glass. Polishing, Shaping and Finishing Industrial standard mechanical polishing techniques are often used. There are special applications where even these high quality polishing procedures are not ade- quate. In these extremely demand- Figure 5-3 Sculpted Fiber Tip Examples ing cases, automated laser machining methods developed by Polymicro has been used for custom and production optical fiber finishing. Some of the shapes are shown in the previous section on sculpted tips. When silica core with doped silica clad optical fiber is used, the cladding is left intact on several of the configurations. Most often, the configuration is rotationally symmetric about the fiber axis. This is not a rigid requirement. Many surface shapes and finishes can be applied to meet specific optical characteristic requirements. Features such as spiral grooves through the cladding, hemispherical or other shapes on the end, and various combinations of mechanical shapes can be engineered.

1 N. Nath, et al., "Evanescent Wave Fiber Optic Fluorosensor: Effect of Tapering Configuration on Signal Acquisition"

Tapered Fibers

Tapered optical fibers are fibers that have a diameter that varies along their length. Optical tapers have properties that make them useful as input/output devices. They can be used as a passive optical component to alter the fiber’s input and/or output divergence (NA). They can also be used as a high power coupler for laser energy (decreases the power density by allowing a larger input spot size), or simply as a device to loosen alignment tolerances in an optical system.

In general, there are several facts about tapers that should be kept in mind when considering their use. When light travels in an “Up” taper towards a smaller radius, the angle the light makes with the taper axis will increase with each reflection. It is important to remember that if this angle exceeds the critical total internal reflection angle of Figure 5-4 Laser Coupling into Taper the fiber at the bottom of the taper, then the light will not be con- tained in the core. This means that the NA at the entrance of the taper must be reduced to avoid significant loss at the end of the taper. A first order approximation is that to avoid losses, the input NA must be reduced by the taper down ratio. For example, with a 2:1 “Up” taper on a 0.22NA fiber, one should keep the input NA at or below 0.11. In the case of a “Down” taper, as light travels towards a larger radius, the angle will decrease with each reflection.

Polymicro can fabricate tapers using two separate methods. Laser machined tapers are formed out of the existing glass material at the end of a fiber and tend to be relatively short, typically 5 to 15mm in length. The other method involves forming the fiber taper on the draw tower by automatically controlling the draw parameters. These tapers are longer, typically in the range of one to two meters. High Power Lasers

Another consideration is the concentration effect of tapers as the diameter decreases. Q-switched laser systems produce high peak powers that are difficult to couple into fiber. Typically, the bulk fiber material can withstand this power, but the fiber surface may become damaged due to dielectric breakdown of the surrounding medium or surface contamination may initiate the breakdown. Both effects degrade the fiber surface rapidly.

With an “Up” taper integrated into the proximal end of the fiber, the power density at the larger input surface can be reduced to levels well below the damage threshold in many cases. In cases where the minimum beam waist is larger than the fiber diameter, the use of a taper can greatly improve the coupling efficiency.

Depending on conditions, some of which are listed in the table following, the laser induced-damage threshold of Polymicro’s fiber at 1064nm is up to 1 Giga-Watt/cm2 for pulsed lasers and up to 2 Mega-Watts/cm2 for CW. This threshold decreases at shorter wavelengths.

Some of the significant factors that affect optical fiber damage in laser applications are given in the following table. Depending on the specific application, there may be other effects as well.

Figure 5-5 Some Laser Induced Damage Threshold Variables

Beam Expansion An optical taper can be used on the output-end of an optical fiber using its angle changing property to alter the angular distribution of the output intensity. For example, if a lower output divergence than the fiber normally exhibits is desired, an “Up” taper can be used at the distal end. Alternately, if a larger divergence is required, a “Down” taper can be used.

The well-known concept of conservation of brightness states that if light losses are negligible, the spatial and angular content of the light anywhere within or at either end of a taper are described by:

(Eq. 5-1) Figure 5-6 Taper Optical Characteristics

Where subscript i refers to input, o to output parameters.

A = cross sectional area of the light distribution normal to the taper axis θ = maximum angular extent of the light distribution n = refractive index of the medium where θ is measured di = input diameter do = output diameter

θ 2 2 Since n sin = NA and Ai/Ao = di /do , it follows that:

(Eq. 5-2)

However, if the product of the input NA and the ratio of the diameters exceeds the greatest NA that the taper can support, light will escape into the cladding and be lost. This relation will no longer be valid. Therefore, the recommended maximum input NA is:

(Eq. 5-3)

As an example, choose a 2:1 input taper and the output NA is 0.22:

NAin = 0.22 (1/3), thus we have NAin = 0.11

Light is confined to the core if it strikes the interface between the core and the cladding at an angle to the surface, θ c, of equal to or less than:

(Eq. 5-4)

To obtain the best possible coupling efficiency, the launch NA must be 0.073 or less, and the focal point of the beam should be in the neck down region of the taper.

Ferrules & Splices - For Optical Fibers One type of optical fiber coupling ferrule is a precision quartz tube in which the center region is sized to allow a precision fit for the diameter of the optical fiber being used. The ends are flared to Figure 5-7 Mechanical Splice

allow easy fiber insertion with minimal risk of damaging the cleaved or polished fiber ends. The precision bore allows excellent alignment of two fiber ends when they are inserted from opposite ends of the coupler.

Figure 5-8 Dual Fiber Ferrule Another type of coupler ferrule is used in WDM and other applications where two fibers need to be aligned side by side very accurately. Thermal, mechanical and optical parameters must be taken into account. A quartz capillary with a center that has a rectangular shape or precision separate dual ID’s can be fabricated. The entrance has a taper to simplify alignment when the two fibers are inserted. For other multiple fiber applications, the quartz tube can have a variety of shapes. With appropriately engineered preforms, Polymicro can produce various geometries including square, circular, elliptical, rectangular and triangular shapes.

Connectors, Ferrules & Splices - For Capillaries For coupling capillary tubing Polymicro offers Inner-Lok™ Capillary Connectors. These connectors are designed for quick, contamination-free coupling of polyimide coated silica capillary tubing in applications such as Gas Chromatography. Two standard products are available, a universal union and a Y-shaped splitter. The capillaries are inserted into the tapered ends and press fit to seal. If needed, a drop of adhesive or polyimide can be applied into the tapered ends to help hold the capillary (appropriate curing may be required). Proper cleaving is essential to form a good seal and is discussed in the Cleaving Procedure section of this Handbook’s Appendix. Polymicro’s standard Inner- Lok™ products have a 2mm outside diameter and are nominally 38mm long. They are designed to accept tubing with OD’s ranging from 360µm to 670µm. Custom sizes of

Inner-Lok™ style connectors can be manufactured. Figure 5-9 Capillary Inner-LokTM Although less common, Ferrules and Splices like those used on Optical Fibers can be employed with capillary tubing. In many cases an existing larger size capillary tubing can be used as the ferrule or splice component. Custom tubing can be drawn to provide an exact fit if needed. Polymicro is well experienced with cutting, polishing, and flaring ferrules prior to use. These preparatory operations can reduce manufacturing time and improve final product quality. In addition, Polymicro can do the final assembly operations if needed; see Fiber Optic & Capillary Assemblies.

Special Capillaries - Multi-lumen

Multi-lumen or multi-element capillaries are a class of flexible quartz or glass capillary tubing having multiple passageways. The number, size, and shape of the individual passageways depend on the application requirements. One can be used to guide an optical fiber for illumination at the distal end, while another can guide a gas or another instrument. The openings do not have to all be the same size. Special Capillaries - Square/Rectangular Polymicro introduced a flexible silica square capillary in response to requests from the capillary electrophoresis (CE) scientific community. This square capillary offers an improved optical beam-sampling path over traditional circular cross-section silica capillaries for in line spectrometry. This square configuration allows for a simplified optical system for the spectrometer due to the lack of cylindrical lens effects found in normal round tubing. See the Flexible Fused Silica Capillary Tubing section of this handbook under Alternative Cross-Sectional Geometries for more details. Special Capillaries - Windowed Capillaries Polymicro also supplies windowed silica capillary. The window is made using a laser-based technique to completely remove the polyimide while leaving the silica capillary extremely strong with excellent optical surface qualities. See the Flexible Fused Silica Capillary Tubing section of this handbook under Capillary Productss for more details. MEMS/NEMS Technologies Polymicro is constantly evaluating and supporting emerging technologies, including the areas of Micro- and Nano- Electro Mechanical Systems. Polymicro is integrally involved in a number of applications in this arena.

Inner-LokTM is a registered trademark of Polymicro Technologies

Polymicro capillary is finding wide usage in fluidic interfacing between MEMS/NEMS devices and the macro world. In fact, a number of companies have developed connectors for MEMS/NEMS devices that dimensionally mate with Polymicro capillary. Some researchers bond capillary directly into their Microfabricated devices instead of using connectors. Polymicro draws capillary to the specifications needed in order to meet the application requirements. Further, Polymicro can provide pre-cut capillary, simplifying the production requirements of the customer.

Polymicro optical fiber is also being used in the MEMS/NEMS research market. These fibers efficiently guide light to and from the devices, with significant use in microfluidic structures. Polymicro has drawn a number of custom optical fibers to satisfy the specific demands that each device requires. In many instances, Polymicro builds the finished assemblies to the customer's specifications. When needed, Polymicro can assist in basic design issues related to fiber optics.

Absorption Cable, fiber optic In optics, the loss or attenuation that is due to material proper- A package or assembly for an optical fiber or bundle of fibers ties of an optical fiber. Absorption is quite often wavelength that may include buffering, strength members and/or an outer dependent. jacket.

Acceptance angle Capillary Electrophoresis (CE) The maximum cone half-angle for which incident light is cap- Capillary Electrophoresis is a separation method based on the tured by and will travel through the optical fiber. If the accept- differential electrophoretic migration rate of sample compo- ance angle is θ then the acceptance cone is defined by a solid nents in a capillary when a voltage is applied. The detection cone of 2θ. See NA for more details. method is usually “on-column” using UV spectrometric or fluorescence analysis through a window in the capillary. Acrylate Performing electrophoresis in small-diameter capillaries allows A polymer material used in optical fibers as a buffer coating the use of very high electric fields because the small capillar- or cladding or in capillary as a coating. ies efficiently dissipate the heat that is produced. Increasing the electric fields produces very efficient separations and Adsorption reduces separation times. CE detection includes absorbance, In chemistry, the taking up by the surface of a solid or liquid fluorescence, electrochemical, and mass spectrometry. (adsorbent) of the atoms, ions, or molecules of a gas or other liquid (adsorbate). Porous or finely divided solids can Capillary tubing hold more adsorbate because of the relatively large surface Quartz or glass tubing which has internal diameters from area exposed. less than 2µm to more than 2000µm. The outside diameter can range from 90µm to greater than 3500µm, depending Attenuation on the application requirements. An outside buffer coating The amount of light loss experienced in an optical fiber or of polyimide, silicone, acrylate or fluoropolymers can be optical media as a function of length. For optical fiber it is added. usually expressed in dB (decibels) per kilometer (km). See Transmission. Chromatic dispersion The separation of a beam into its various wavelength compo- Bandwidth nents. In an optical fiber, dispersion occurs because of the dif- The range of frequencies (or wavelengths) which is useful for fering wavelengths propagating at different speeds. This a device or system. In optical fiber, it is a measure of informa- causes pulse spreading or broadening. See Dispersion. tion carrying capacity. The frequency bandwidth is usually described as the frequency where the signal power is one-half Chromatography the power at zero frequency. The wavelength bandwidth is In chemistry, analytical technique used for the chemical sepa- usually expressed in terms of spectral wavelength-dependent ration of mixtures and substances. The technique depends on attenuation or transmission, and is not necessarily related to the differential distribution of solutes between the mobile and the signal bandwidth. stationary phases.

Bending loss Cladding Loss in an optical fiber caused by bending of the fiber. This A low refractive index optical material that surrounds the core loss is usually due to internal light paths exceeding the critical of an optical fiber. It is used to cause reflection of the core light angle for TIR. Both micro-bending and macro-bending are while preventing scattering from surface contact. In all-glass loss mechanisms in optical fibers. fibers, the cladding is glass. In plastic-clad Silica fibers, the plastic cladding also may serve as the buffer coating.1 In some Bend Radius applications, multiple cladding layers can be used. The radius of a drum or mandrel around which an optical fiber or cable is wrapped or wound. The radius at the center of the Color fiber or cable is the bend radius plus one-half the fiber or The attribute of visual perception that can be described as cable diameter. having characteristics of hue, saturation, and brightness. It does not include aspects of extent (e.g., size, shape, texture, Broad Spectrum fiber etc.) and duration (e.g., movement, flicker, etc.). A color that is An optical fiber that has a relatively wide transmission seen can be a single wavelength or a combination of wave- spectrum window ranging from ~300nm to ~2µm. lengths. Most colors are a very complex combinations of many wavelengths of various amplitudes. Buffer The buffer is an outer coating on an optical fiber. Typically a Core plastic material, it protects the fiber from external stresses and The light-guiding portion of an optical fiber having a higher abrasion. refractive index than the cladding. It is usually made of a pure synthetic Silica material, but can be a doped material to provide special fiber characteristics.

1The Photonics DictionaryTM, 43rd Edition, 1997, Laurin Publishing Co., Inc., Pittsfield, MA

Cut-off wavelength cence that persists for less than about 10ns after excitation. 1. In detector technology, the long wavelength at which detec- Radiation which persists for longer time is known as phospho- tor response falls to a set percentage (usually 20 or 50 rescence. percent). 2. In fiber optics, the wavelength below which a waveguide can transmit multiple modes rather than purely Focal ratio degradation (FRD) single mode. The reduction of relative f-number (or f-ratio) in an optical fiber due to the characteristics of the fiber. Basically, the emittance Decibel light cone is always greater than or equal to the incident light The standard unit used to express gain or loss and relative cone in an optical fiber. The effect of FRD results in the output power levels. The decibel (dB) = -10 log (Po/Pi), where Po is cone of light being larger than the input cone of light. Because the output power and Pi is the input power. of this effect, insertion of optical fiber in an f-number matched optical train (e.g., a astronomical fiber optic spectrometer) will Diffuse cause a signal loss greater than that expected from the normal The type of reflection/transmission from many powders (e.g., loss per unit length values. Failure to account for FRD in transmission measurements, e.g., measuring light output with phosphors, MgO2 or BaSO4), matte surfaces and transmitting materials such as ground quartz, flashed opal glass or Teflon® an integrating sphere, will give optimistically high transmission (PTFE). Flat white paint is an example of a nearly Lambertian, values for higher f-number systems. diffuse coating. Diffusers are often used to remove imaging characteristics from an optical beam. See also Specular Fused quartz reflection, Spread and Lambertian. See Fused silica.

Dispersion Fused Silica The separation of a beam into its various wavelength compo- Fused Silica is Silicon dioxide (SiO2) in its amorphous nents. In an optical fiber, dispersion occurs because the differ- (glassy) state. Silica is Silicon dioxide (SiO2). Synthetic ing wavelengths propagate at differing speeds. In fibers, it is fused Silica is amorphous Silicon dioxide that has been the cumulative effect of three types: chromatic, modal and produced through chemical deposition rather than refinement waveguide dispersion. of natural ore. This synthetic material is of much higher purity and quality as compared to fused quartz made from natural Doped (synthetic) fused silica minerals. Doped (synthetic) fused Silica has been intention- See Fused silica. ally doped with trace elements such as Germanium, Boron, Phosphorous, Titanium, Fluorine or other elements to adjust Dual Clad the optical properties of the glass. Quartz is a natural grade An optical fiber constructed with Silica core and doped Silica of crystalline Silicon dioxide (SiO2), the most common phase cladding coated with optical quality polymer that has a lower of SiO2. Fused quartz is a natural grade of amorphous SiO2. refractive index than the doped Silica cladding. Dual clad is Typically produced from the melting (fusing) of crystalline designed to transmit higher optical power as compared to a quartz and refined such that an amorphous (glass) is formed. single clad. Gas Chromatography (GC) Effective numerical aperture Gas Chromatography is a method for separating substances See Numerical aperture (NA). in a mixture and measuring the relative quantities of substances. It is a useful technique for substances that do not Elasto-optic effect decompose at high temperatures and when a very small quan- A change in the refractive index of an optical fiber caused by tity of sample (micrograms) is available. variation in the length of the fiber core in response to mechanical stress. In this type of chromatography, a sample is rapidly heated and vaporized, and then a stream of gas carries it along a column Fiber optics that contains a stationary phase. The sample becomes dis- A branch of optics that deals with the transmission of light tributed between the mobile gas phase and the stationary through fibers, tubes or thin rods of a transparent material. If phase. The higher a substance’s affinity for the stationary light is injected into one end of an optical fiber or rod, it can phase, the more slowly it travels through the column. travel through it with very little loss, even if the fiber is curved. The amount of loss depends on the color of light (wave- Glass length), the optical fiber design, the materials used, and the The word “Glass” refers to the solid phase of a material with manufacturing process. no long-range molecular order. It is used almost interchangeably with “amorphous,” “non-crystalline,” and “vitreous.” Glass Fluorescence is a disordered structure, as opposed to a crystalline material The emission of light or other electromagnetic radiation of that exhibits a symmetrical, ordered structure. The most longer wavelengths by a substance as a result of the absorp- common glasses are oxide based, such as Silicates (SiO2), tion of some other radiation of shorter wavelengths, provided Borates (B2O3), Germinates (GeO2) or mixtures of these. the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the lumines- Teflon® AF is a trademark of E.I. du Pont de Nemours and Company

Graded index Lambertian Descriptive of an optical fiber having a core refractive index A relation between the illumination on to a surface and the that decreases almost parabolically and radially outward light flux from a surface versus angle. Lambert’s Cosine Law toward the cladding. This type of fiber combines high band- states that the light flux, E , at some angle, , is equal to the width with moderately high coupling efficiency. Sometimes illumination perpendicular to the surface, E, times the cosine called gradient-index. of the angle.

Hard clad Light An optical quality fluorinated polymer that has a lower refrac- 1. Electromagnetic radiation in the visual spectrum. 2. a. A tive index than pure Silica glass. It is applied in a thin layer µ source of light, especially a lamp or electric fixture. b. The (5-15 m thick) and has a hardness in the range of 60-70 illumination derived from such a source. Shore A. Often, it also refers to the fiber constructed with a pure Silica core surrounded with hard clad. Liquid Chromatography (LC) Liquid Chromatography (LC) is an analytical chromatographic High-Performance Liquid Chromatography (HPLC) technique that is useful for separating ions or molecules that High-Performance Liquid Chromatography (HPLC) is a form of are dissolved in a solvent. If the sample solution is in contact Liquid Chromatography used to separate compounds that are with a second solid or liquid phase, the different solutes will dissolved in solution. HPLC instruments consist of a reservoir interact with the other phase to differing degrees due to differ- of mobile phase, a pump, an injector, a separation column, ences in adsorption, ion-exchange, partitioning, or size. These and a detector. Compounds are separated by injecting a plug differences allow the mixture components to be separated of the sample mixture onto the column. The different compo- from each other by using these differences to determine the nents in the mixture pass through the column at different rates transit time of the solutes through a column. due to differences in their partitioning behavior between the mobile liquid phase and the stationary phase. Solvents must Macro-bending be degassed to eliminate formation of bubbles. The pumps Bending of an optical fiber or fiber bundle at a radius compara- provide a steady high pressure with no pulsating, and can be tively larger than the fiber diameter. Attenuation is increased programmed to vary the composition of the solvent during the due to light escaping beyond the critical angle at the cladding course of the separation. Typical detectors rely on a change in interface. refractive index, UV-VIS absorption, or fluorescence after excitation with a suitable wavelength. MEMS/NEMS Acronyms for "Micro- and Nano- Electro Mechanical Systems". Hollow waveguide (HSW) Micro-bending A flexible hollow capillary with an internal surface coating Microscopic curvatures in optical fiber that create local axial which is highly reflective at the wavelength unlike optical displacements of a few microns. One frequent cause is longi- fibers, this does not utilize total internal reflection. It is usually tudinal shrinking of the fiber buffer or jacket. But it can also used where there are no transmissive materials that can be result from poor fiber or cable manufacturing methods or formed into a flexible optical fiber at the wavelength of interest. installation issues. Micro-bending causes transmission loss Most commonly used in the IR. through a power-coupling effect from the guided modes to the radiation modes. HPCS Acronym for "Hard Polymer Clad Silica". Another term for Micron (µm) hard clad fiber. Alternative name for micrometer (µm or 1x10-6 meters).

Isotropic Modal dispersion Invariant with respect to direction. The property of an optical Synonym for multimode distortion or modal distortion. material that allows the velocity of propagation of electro-mag- netic radiation to be the same for all directions. Modal distortion Synonym for multimode distortion. Jacketing Mode Usually the outer material used on an optical fiber or fiber 1. The characteristic of the propagation of light through a bundle. Inner jackets are also used in multifiber assemblies. waveguide that can be designated by a radiation pattern in a plane transverse to the direction of travel. 2. The state of a kpsi laser that corresponds to a particular field pattern and one of Kilo-pounds per square inch is a common unit used for tensile the possible resonant frequencies of the system. strength testing of optical fiber. Multimode distortion Lambert In an optical waveguide, the distortion resulting from differen- A unit of luminance (brightness) equal to 1/π candela per tial mode delay. Axial rays, with the shortest path length, will square centimeter. have the shortest transmission time, while rays entering the fiber at its maximum acceptance angle will travel further and require the maximum time. As a result, narrow light pulses, will broaden as they propagate along the fiber.

Multimode fiber Packing fraction (Pf ) An optical waveguide that will allow more than one mode to The fraction of the area of an optical fiber bundle surface that propagate. is actual core area. The actual number of fibers, N, which can be packed in a circle of diameter, D = (2m+1)d, is given by the Nanometer (nm) geometric summation: A unit of measure most often used for light in the visible -9 (1x10 meters). The peak of the human eye sensitivity is at a where m is the number of rings around the central fiber in the wavelength of 550 nm (green light). In the UV range where pack. Angstroms are still sometimes used, 200 nm is 2000 Å. In the near infrared where microns (µm) are used, 1000 nm equals µ 1 micron ( m). &RUH Numerical Aperture (NA) The larger the NA, the greater the amount of light that is &ODG accepted into the fiber for propagation to the distal end. %XIIHU =

The NA for a glass-core to glass-clad interface is often derived from calculations using the equation:

,QWHUVWLWLDO where nco is the core index and ncl is the cladding index. VSDFHV

Values calculated in this manner do NOT take into account losses from Fresnel reflection or degradation of NA with fiber For circular fibers packed in a tight hexagonal array, the length. See Fresnel loss and FRD. number of fibers, N, is given approximately by:

Optics A branch of physical science dealing with the propagation and Where d is the fiber core diameter and D is the fiber bundle behavior of light. diameter. The packing fraction is thus given by:

Optical fiber Drawn filament made of glass or plastic with a high refractive index core surrounded by a lower index cladding Where A = πD2/4, is the area of the bundle. See the Fiber through which light can be transmitted using the principle of Optics & Capillary Assemblies chapter for additional infor- Total Internal Reflection (TIR). When light traveling inside a mation. fiber strikes the surface at an angle of incidence greater than the critical angle, the light is reflected back toward the PCS center of the fiber with negligible loss. Thus, light can be Acronym for a "Polymer Clad Silica" optical fiber. PCS fiber is transmitted over long distances by being reflected many usually constructed from Silica core and Silicone clad. times, as long as the loss per reflection is extremely low. Sometimes HPCF is referred as PCS.

Polyimide Polyimide is an aromatic, linear polymer typically produced by condensation reaction, such as polymerizing aromatic dian- hydride and aromatic diamine. The most notable properties The most common optical fiber configuration is based on the are its solvent resistance, barrier properties, and performance use of two materials concentrically arranged as a center at both high and low temperatures. core and outer tubing (cladding). This arrangement avoids losses that would result from the scattering of light by impu- Preform rities on the surface of the fiber. The optical fiber core is The starting form of glass or silica that is used to generate much higher in refractive index (nCO), than the cladding fiber or capillary by heating and drawing to produce the final index (nCL); the reflections occur at the interface of the glass smaller product size. fiber core and the cladding. Sometimes a second outer layer (secondary-cladding) is added to the fiber to increase trans- Proof testing mission, prevent lost light from coupling into adjacent fibers, A non-destructive means of applying tensile stress to an or to increase the strength or environmental durability of the optical fiber or capillary during the manufacturing process to fiber. See Fiber optics. identify mechanical flaws that might otherwise exhibit or breakage during later use thereby assuring a minimal strength level.

4 "Optics", Microsoft® Encarta® 97 Encyclopedia, Microsoft Corporation (1993-1996)

Quartz Solarization

A natural grade of crystalline SiO2. A change in material characteristics due to illumination of a material with ultraviolet light. High intensities of UV illumina- Raman spectroscopy tion can cause photo-thermal damage in Silica optical fibers, The field of spectroscopy that uses optical frequency shifts dramatically increasing the scattering and attenuation. and intensity changes in the Raman chromophore(s) produced by monochromatic illumination, to determine the characteris- Spectrum tics of the sample. A range of wavelengths. In optics, the electromagnetic spectrum includes the wavelength region extending from the Rayleigh scattering vacuum ultraviolet at 40nm to the far-infrared at 20µm. Scattering of light from particles smaller than the wavelength of the radiation incident. A feature of Rayleigh scattering is that Spectrum, Visible the scattered flux is inversely proportional to the fourth power The region of the electromagnetic spectrum to which the of the wavelength. Thus in the visible region, blue light is scat- human retina is sensitive. It covers the range from about 400 tered more strongly in air than longer wavelengths, accounting to 750nm in wavelength. for the blue color of the sky.6 Specular reflection Refractive index When light obeys the law of reflection, it is termed to be The ratio of the speed of light in a vacuum to the speed of specular reflection. See also diffuse, and Lambertian. light in the material. Also called the Index of Refraction, it is dependent on wavelength for optical materials. Static fatigue Degradation of the strength over time of an optical fiber that is Secondary cladding under stress. The stress can be due to a bend, tension, An optical material surrounding the primary cladding which torsion or a combination thereof. Also, see tensile strength. has a lower index than the cladding. Sometimes the buffer material can act as a secondary cladding, giving rise to unex- Step-index fiber pected higher order modes. Some bonding materials as well An optical fiber that has a uniform core index and a lower as liquids can also act as secondary claddings. uniform cladding index, creating a step change in refractive index profile. Also, see graded index. SI Systeme Internationale d’Unites, the international metric Synthetic Fused Silica system of units. See Fused Silica.

Silica Taper Silicon dioxide (SiO ) A section of optical fiber or a micro-component that has a con- 2 tinuously changing outer dimension, along its length, from one end to the other. It can be a separate component or an inte- Silicone(s) gral part of the optical fiber tip. Although most have a circular A class of polymer materials. In optical fiber, some are used as cross-section, they can be made in other shapes. buffer materials and others for optical cladding. Not to be con- fused with silica (SiO ) or silicon (the element, Si). 2 Tensile strength The strength of an optical fiber when placed in tension. Singlemode fiber Usually given in units of kilo-pounds per square inch (kpsi). An optical fiber in which only one mode, the fundamental Also, see proof testing and kpsi. mode, is transmitted. This mode travels straight through the fiber without reflection at the core-clad interface. Core dia- µ Total Internal Reflection (TIR) meters are typically 5-10 m, making the alignment very criti- The condition resulting in total reflection at an interface. In cal. Coupling losses tend to be higher than with multimode optical fiber, it is the angle defined as sin-1 (n /n ), where n is fibers. 2 1 2 the lower index media and n1 is the higher index media. For optical fiber, the cladding is n and the core is n . As the angle Snell’s Law 2 1 Snell’s Law of incidence is increased from the normal to the surface, the The relationship between an incident ray at angle I in refrac- critical angle is that angle where total internal reflection tive index media n , and the refracted at angle R in refractive 1 begins to take place. index media n2 is: Transmission In optics it is often given as the percentage of light or energy propagating through an optical system (light output divided by the light input). Transmission data may or may not include surface losses, such as Fresnel reflections.

6 The Photonics DictionaryTM, 43rd Edition, Laurin Publishing Co., Inc., Pittsfield, MA (1997)

Ultraviolet (UV) The wavelength range below the lower end of the visible spectrum. The UV most often refers to the range from 400nm down to 40nm. Below 200nm a vacuum system is used to get useful transmission. In addition to fused Silica, only a few crystal materials transmit UV below about 350nm such as, Magnesium fluoride, sapphire, Calcium fluoride and Lithium fluoride.

Waveguide dispersion For each mode in an optical waveguide, the term used to describe the process by which an electromagnetic signal is distorted by virtue of the dependence of the phase and group velocities on wavelength as a consequence of the geometric properties of the waveguide. Also, see dispersion.

Wavelength The length of one wave cycle of a light wave. It is most commonly expressed in nanometers (nm) for the visible and UV, and “microns” (1000nm/µm) in the IR spectral regions. The frequency is inversely proportional to the wavelength.

Wave Number The frequency of a wave divided by its velocity of propagation; the reciprocal of the wavelength.

Xenon arc The arc formed when the Xenon gas is excited electrically and emits a brilliant white light. Xenon is used to fill electronic and stroboscopic flashlamps, and also large discharge tubes for lighting large areas.

X-ray A region of the electromagnetic spectrum at wavelengths shorter than ultraviolet (shorter wavelengths). X-rays are cus- tomarily expressed in energy units rather than wavelength. Wavelengths range from <1nm to >0.01nm.

Polyimide Removal from Silica Fibers or Capillary Tubing The majority of the optical fiber and capillary sold by Polymicro is externally coated with polyimide to provide abrasion resistance and maintain product strength. Occasionally the need arises for the controlled removal of polyimide. A variety of methods can be employed to remove polyimide and care should be taken in selecting the appropriate technique. Some methods leave the glass surface relatively unaffected, while others embrittle the glass making the product extremely fragile and prone to breakage during handling. Listed below are a variety of removal methods and associated comments. In many cases Polymicro may have direct experience using the technique and a Polymicro sales technician can provide assistance.

Thermal Techniques • Open flame: Matches and lighters are quick, easy, and effective at removing polyimide, but they tend to leave the glass surface brittle and are not recommended if strength of the final product is important. • Gas torch: Oxygen/hydrogen flames do a good job of removing polyimide, leaving the final product strong. Care should be taken regarding the inherent dangers in using this type of torch and the potential for distortion of the filament from overheating does exist. Propane, etc. are often acceptable, but are not as good as the oxygen/hydrogen flame. If the flame temperature is not high enough residual polyimide can be present. • Oven: At temperatures >600°C the polyimide will carbonize and flake off. This generally takes 30 to 60 minutes, and can be expedited with higher temperature or the addition of oxygen. This method works well to remove large sections of polyimide. The finished product retains excellent strength after processing. • Electric coil heater: Coiled NiChrome® wire, or a NiChrome® wire wrapped around a quartz insulating tube, makes a resistive coil heater capable of rapidly burning off the polyimide. The coil heater approach works, but one must be careful not to touch the glass to the wire or insulating tube. This will damage the glass surface, making the glass brittle. Residue is common and post-process cleaning is generally required. • Electric arc: Plasma is effective at burning off the polyimide and leaves the glass strong. However, plasma removal can be challenging to control and overheating is difficult to avoid. In fact, this overheating can be useful; Polymicro uses an electric arc plasma technique to melt and seal all capillary ends prior to product release.

• CO2 Laser: Removes the polyimide thermally, just as the above techniques. This method is excellent due to the clean heat source and the fine control over the hot zone. Distortion from overheating should be monitored. Chemical Techniques • Sulfuric acid*: When heated to approximately 130°C, sulfuric acid (concentrated) removes the polyimide very rapidly. Multiple applications are recommended and the finished product should be rinsed with DI water after the polyimide is removed. • Strong bases*: Caustic solutions, such as Sodium hydroxide, will also attack the polyimide. Although these will remove polyimide, they generally etch the filament surface and are generally not recommended as a removal method. *Caution: Proper laboratory safety practices should be followed when working with these types of chemical reagents.

Laser Techniques • Excimer laser: Ablates the polyimide without heat, providing a clean, undamaged silica surface. Polymicro uses this technique routinely, especially on products where a thermal char line is undesirable. This is the method of choice for volume production, but is not very practical for general lab use due to laser expenses.

• CO2 Laser: See above discussion under Thermal Techniques.

NiChrome® is a registered trademark of Driver Harris Corp.

Mechanical Stripping • Machining: Removing the polyimide with a mechanical technique, such as an X-ACTO® knife, razor blade, or cutting tool, can work, but damage to the glass surface by the cutting tool will cause brittleness. • Wire strippers do not work. Generally the polyimide is bonded to the glass surface. Wire strippers will damage the glass during stripping of the polyimide and breakage is almost certain.

Cleaving Procedure Cutting capillary tubing and optical fiber can be accomplished by a number of methods. Matching the cutting method quality to the application requirements is essential and should be given due consideration. Cleaving is a quick, simple method that can yield a high quality end finish and works well for many applications.

The goal of any cleaving tool is to penetrate through the polyimide and impart a sub-micron defect into the outer glass surface. Ceramic cleaving stones and diamond tip devices are common and effective tools for imparting the required defect. Once a defect is generated, applying a linear tension to the defect separates the capillary or optical fiber. This is the preferred method and leads to the highest quality end faces. The most common error in cleaving is to bend the capillary or fiber, which normally yields a low quality cleave with an uneven and sometimes jagged end finish.

A general misconception when dealing with capillary tubing is that cleaving and breakage are unrelated. A poor cleave generates excessive glass debris inside of the capillary which can lead to internal flaws and subsequent breakage. It is not uncommon for this debris to be swept down the capillary by gases or liquids that are introduced, leading to flaws and breakage some distance down the capillary from the cleave itself. This effect is most common in large ID capillary, but can happen in any capillary product.

When dealing with optical fiber, a subsequent lap and polish is commonly employed to provide a final end finish with optimal transmission properties. Alternately, laser cutting of optical fiber and capillary tubing has proven to be a reliable method for many applications that require a flaw free end face.

The following general procedure should be followed when cleaving capillary tubing and optical fiber with a Polymicro ceramic cleaving stone.

Procedure:

1. Place the capillary tubing or optical fiber on a clean, flat surface.

2. Holding the cleaving stone at approximately 30° angle to the tubing or fiber, draw the non-serrated edge of the cleaving stone across the tubing or fiber. Apply just enough pressure to penetrate through the polyimide coating.

3. Pull the tubing or fiber axially until it breaks. If it won't break, the polyimide coating has not been fully penetrated. Repeat the above steps, pressing down with slightly more force while drawing the cleaving stone across the tubing or fiber.

4. Once cleaved, inspect the end finish to ensure the cleave quality meets the application requirements.

Note: If end finish is not of concern, the tubing or fiber can be bent as opposed to pulling axially. The tubing will break more easily, but the end finish will be of lesser quality and excessive debris may be generated.

Useful Tip: It is not uncommon for users to practice the above cleaving procedure in order to become familiar with proper technique.

X-ACTO® is a registered trademark of Hunt Corporation.

General Handling Careful consideration should be given to the general handling of capillary tubing and optical fiber. A few key guidelines are discussed below.

Storage of capillary tubing and optical fiber can be critical, depending on the application. • Most Polymicro products are packaged with a protective foam wrapped around the outside layer of product. The product is then shipped in a sealed plastic baggie. Efforts to reduce exposure to moisture will prolong lifetime, therefore keep the shipping baggie sealed until the product is ready to be used. Purchasing in spool lengths that appropriately match consumption is recommended. To minimize collec- tion of debris and dust onto stored material, replace the protective foam after removing product from the spool.

• If exposure of capillary tubing internal surfaces to the atmosphere is of concern, make sure to reseal the ends after removing product. This can be done by thermal fusing or by placing a septa or similar material over the end of the capillary tubing.

• When purchasing large diameter tubing or fiber, be sure to store the product so it is setting on the flange edges. This will avoid cascading and subsequent entanglement during product removal.

Cleanliness of any surface that comes into contact with the capillary tubing or optical fiber is critical. • Debris on work surfaces, such as glass particles from previous cleaving operations, can lead to breakage and is often perceived as apparent brittleness. Especially troublesome, are small particles that become embedded in the polyimide, and lead to breakage during further processing or use. Consider placing butcher paper on your workbench and change it regularly to provide a clean work area. If this is not possible, clean the work surface frequently.

• If tubing or fiber is placed onto, or routed through, a manufacturing device, consider all surfaces or features that could contact the product and make sure these are routinely cleaned of any debris, especially after any breakage. Surfaces should be smooth and free of manufacturing defects such as burrs or sharp edges. Keep this in mind during fixture design and manufacture.

Bending stress is a key handling issue that should be given careful consideration. • Capillary tubing and optical fiber are often exposed to bending during manufacturing processes and subsequent use. Bending these products produces localized tension, often referred to as bending stress. The smaller the bending radius, the greater the imparted bending stress. The acceptable bending radius for a given application should always be taken into account. For further discussion on bending stress refer to Fiber Optics & Optical Fiber and Flexible Fused Capillary Tubing chapters of this handbook.

• Note that product lifetime is directly related to the bend radius. The smaller the bending radius either during handling, or in the final product design, the shorter the lifetime of the product.

• A common handling oversight is the incorporation of rollers or guides that expose the tubing or fiber to excessively high stresses. It is recommended that the applied stress be calculated for each component of the system. Related equations and tables are found in Fiber Optics & Optical Fiber and Flexible Fused Capillary Tubing chapters of this handbook.

Units of Measure

Useful Conversion Factors

Parameters SI Unit SI Symbol Common Conversions or Definitions

1 m = 1.0936 yd = 39.37 in = 3.281 ft, Length meter m 1 in = 2.54 cm, 1 km = 0.621 mi, 1 mi = 5280 ft 1 mm = 1000 µm, 1000 nm = 1 µm

Mass kilogram kg 1 kg = 2.2046 lb, 1 lb = 453.59 g, 1 lb = 16 oz

Time second s 1 h = 3600 s, 1 day = 8.64 x 104 s

0 K = -273.15 °C = -479.67 °F, K = °C + 273.15 °C = Temperature Kelvin K 5/9 (°F – 32), °F = 1.8 °C + 32

cubic 1 ft3 = 2.832 x 10-2m3, 1 L = 1000 cm3, 1 mL = 1 cm3, Volume m3 meter 1 in3 = 16.4 cm3

1 J = 0.239 cal = 0.738 ft•lb = 107 erg = Energy Joule J 6.24 x 1018 eV = 9.487 x 10-4 Btu

1 N = 0.2248 lb-force, 1 N = 0.1019716 kilopond, Force Newton N 1 N = 100,000 dynes

1 Pa = 1.45 x 10-4 psi, 1 atm = 101,325 Pa, Pressure Pascal Pa 1 atm = 760 Torr, 1 bar = 105 Pa

1 W = J•s = 0.738 ft•lb-force/s = 3.412 Btu/h = Power Watt W 2.65522 x 103 ft•lb-force/h

Units of Measure, cont.

Conversion Units for Light

Power & Energy Radiance

1 lm/m2/sr = 6.83x 106 lm/m2/sr at 555nm 1 W (watt) = 683.0 Im at 555nm = 683 cd/cm2 at 555nm = 1700.0 scotopic Im at 507nm Luminance 1 J (joule) = 1 W-s (watt-second) = 107 erg 1 lm/m2/sr = 0.2388 gram-calories = 1 cd/m2 (candela/m2) = 1 nt (nit) Luminous Flux = 10-4 lm/cm2/sr = 10-4 sb (stilb) 1 lm (lumen) = 1.464x10-3 W at 555nm = 9.290x10-2 cd/ft2 = 1/4πcd (candela) if isotopic = 9.290x 10-2 lm/ft/sr 1 lm-s = 1 T (talbot) = π asb (apostilbs) = 1.464 x10-3 J at 555nm = π x10-4 L (Lamberts) = 2.919 x 10-1fL (foot-Lamberts) Irradiance = 2.919 x10-1 lm/π/ft2/sr

1 W/cm2 = 104 W/m2 = 6.83x 106 lux at 555nm Radiant Intensity 2 = 14.33 g-cal/cm /minute 1 W/sr = 12.556 W (isotropic) = 4π W Illuminance = 683 cd at 555nm 1 lm/m2 = 1 lux Illuminance = 10-4 lm/cm2 = 10-4 ph (phots) 1 lm/sr = 1 cd = 9.290 x10-2 lm/ft2 = 4π lm (isotropic) = 9.290 x10-2 fc (foot-candles) = 1.464x 10-3 W/sr at 555nm

Wavelength Units, Wavenumbers and Photon Energy

Angstroms Nanometers Micrometers Wavenumbers Energy Units (Å) (nm) (µm) (cm-1) (keV)

Near IR 10000 1000 1 10,000 0.0012398

Visible 5000 500 0.5 20,000 0.0024796

UV 2500 250 0.25 40,000 0.0049592

X-ray 10 1 0.001 10,000,000 1.2398

X-ray 0.1 0.01 0.00001 1,000,000,000 120.4

Polyimide Characteristics

Polyimide Physical Properties

Mechanical Data Electrical data Density 1.42 g/cm3 Dielectric Strength 4000 volts/mil

Flexibility 180° bend, no cracks Volume Resistivity 1016ohm-cm

Elongation >10% Surface Resistivity 1015ohm

Tensile Strength 15,000 psi Dielectric Constant 3.5

Optical Data Gas Permeability

2) Refractive Index 1.78 Carbon Dioxide [cc/100in 45 (24 hours) (atm / mil)] Thermal Data Hydrogen 250 Melting Point None Final Nitrogen 6 Decomposition 560°C Oxygen 25 Temperature Coefficient of Helium 415 Thermal 2x10-5 /°C Expansion Coefficient of Thermal 37x10-5 cal/(cm) (sec)(°C) Conductivity Flammability Self-extinguishing

Specific Heat 0.26cal/gm/°C

Polyimide Physical Properties - Chemical Resistance

Resistance To: # of Days % of Tensile % of Elongation

Benzene 365 @ 23°C 100 82 Toluene 365 @ 23°C 99 91 Methanol 365 @ 23°C 100 73 Acetone 365 @ 23°C 67 62

10% Sodium 5 @ 23°C Degrades Degrades Hydroxide Transformer Oil 180 @ 150°C 100 100 WaterpH = 1 14 @ 150°C 65 30 pH = 7 166 @ 100°C 65 20 pH = 10 5 @ 23°C 60 10

Quartz/Silica Characteristics

Typical Trace Elements: Fused Quartz and Synthetic Fused Silica Parts per Million (ppm) by Weight

Synthetic Fused Trace Elements Fused Quartz Silica Aluminum Al 15 < 0.04 Calcium Ca 0.5 < 0.02 Chromium Cr < 0.05 < 0.001 Copper Cu < 0.05 < 0.001 Iron Fe 0.1 < 0.03 Lithium Li 0.6 < 0.002 Magnesium Mg 0.05 < 0.01 Manganese Mn < 0.05 < 0.0005 Potassium K 0.4 < 0.01 Sodium Na 0.3 < 0.01 Titanium Ti 1.1 < 0.03 Zirconium Zr 0.7 < 0.04

Source: Heraeus Quarzglas

Typical Thermal Properties

Thermal Data Units Fused Quartz Fused Silica Softening temperature °C 1710 1600 Annealing temperature °C 1220 1100 Strain temperature °C 1125 1000 Max. working Temp. Continuous °C 1160 950 Short-term °C 1300 1200 0 ... 100 °C 772 772 Mean specific heat 0 ... 500 °C 964 964 J/kg-K 0 ... 900 °C 1052 1052 20 °C 1.38 1.38 Heat conductivity 100 °C 1.47 1.46 W/m-K 200 °C 1.55 1.55 300 °C 1.67 1.67 400 °C 1.84 1.84 950 °C 2.68 2.68

7 7 Mean expansion 0 ... 100 °C 5.1 x 10- 5.1 x 10- 7 7 coefficient 0 ... 200 °C 5.8 x 10- 5.8 x 10- Source: Heraeus Quarzglas

Quartz/Silica Characteristics, cont.

Typical Electrical Properties Electrical Data Fused Quartz Fused Silica

Source: Heraeus Quarzglas

Typical Mechanical Properties Mechanical Units Fused Quartz Fused Silica Density g/cm3 2.203 2.201 Mohs hardness 5.5 ... 6.5 5.5 ... 6.5 Micro hardness N/mm2 8600 ... 9800 8600 ... 9800 Knoop hardness N/mm2 5800 ... 6000 5800 ... 6200

4 4 Modulus of elasticity at 20°C N/mm2 7.25 x 10 7.0 x 10

4 4 Modulus of torsion N/mm2 3.1 x 10 3 x 10 Poisson’s ratio 0.17 0.17

Compression strength N/mm2 1150 1150

Tensile strength N/mm2 50 50

Bending strength (approx.) N/mm2 67 67

Torsion strength (approx.) N/mm2 30 30

Sound velocity m/s 5720 5720

Source: Heraeus Quarzglas

Optical Information

Refractive Index versus Wavelength for Synthetic Fused Silica at 20°C

Wavelength Index of Wavelength Index of Wavelength Index of R K R K R K in µm Refraction in µm Refraction in µm Refraction .185 1.57486 .0498 .903 .45 1.46557 .0356 .930 2.25 1.43420 .0318 .937 .193 1.56007 .0480 .906 .50 1.46233 .0352 .931 2.35 1.43250 .0316 .938 .195 1.55788 .0475 .907 .55 1.45991 .0349 .931 2.40 1.43163 .0315 .938 .200 1.55051 .0466 .909 .60 1.45804 .0347 .932 2.60 1.42789 .0311 .939 .205 1.54411 .0457 .911 .63 1.45702 .0346 .932 2.70 1.42588 .0308 .939 .210 1.53836 .0449 .912 .65 1.45654 .0345 .932 2.75 1.42484 .0307 .939 .215 1.53316 .0443 .913 .70 1.45529 .0344 .932 2.80 1.42377 .0306 .940 .220 1.52845 .0437 .914 .75 1.45424 .0342 .933 2.90 1.42156 .0303 .940 .225 1.52416 .0431 .916 .80 1.45332 .0341 .933 3.00 1.41925 .0300 .941 .230 1.52024 .0426 .916 .85 1.45250 .0340 .933 3.10 1.41682 .0297 .941 .235 1.51664 .0421 .917 .90 1.45175 .0339 .933 3.20 1.41427 .0294 .942 .240 1.51333 .0417 .918 .95 1.45107 .0339 .933 3.30 1.41161 .0291 .943 .242 1.51208 .0416 .919 1.06 1.44968 .0338 .934 3.40 1.40881 .0288 .943 .248 1.50855 .0411 .919 1.30 1.44692 .0334 .934 3.50 1.40589 .0285 .945 .250 1.50745 .0409 .920 1.40 1.44578 .0332 .935 3.60 1.40282 .0281 .945 .300 1.48779 .0384 .925 1.45 1.44520 .0332 .935 3.70 1.39961 .0277 .945 .320 1.48274 .0376 .926 1.5 1.44462 .0331 .935 .35 1.47689 .0371 .927 1.7 1.44217 .0328 .936 .365 1.47454 .0365 .928 2.00 1.43809 .0323 .936 .40 1.47012 .0362 .929 2.15 1.43581 .0320 .937 K= Maximum possible transmittance assuming absorption = 0 R = Single surface reflectance

Source: Malittson I.H. Journal of the Optical Society of America, 1965

Product Application Spectrum

Optical Information, cont.

Optical Window Transmittance, Corning #7980, Fused Silica, 10mm thick

Theoretical Reflection Loss

Typical for 1 cm thickness, surface reflection losses included. Typical production variation for 1 cm thickness, surface reflection losses included.

(t2) T T1 To calculate transmittance (T2) at a thickness other than 1 cm: 2 = K (t2-1) T1 = Transmittance from plot K = Max. possible transmittance from refractive index table t2 = New thickness in centimeters

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