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Technical Info Technical Information on Optics

This chapter contains plenty of information on optics, from an overview of specialized optic terms to the physical principles of opti- cal resolution, Gaussian beam optics and thin film coatings.

Furthermore, designations and measurement methods for testing optical components and properties of optical materials are explained. Valuable tips on further reading are con- tained in the bibliography and media index.

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Technical Info ______Info Technical Terminology 800 Thin Films 806 Symbols and Sign Convention 807 Explanation of the Legends on Optical Component Drawings 808 Quality Testing of Optical Components and Systems 810 Minimum Spot Size and Resolving Power 810 Focusing and Expanding Laser Beams 812 Literature and Software 814 Optical Glass Data 816

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The following list of definitions is an alphabetically sorted collection of technical terms and their definitions. The terminology provided is to promote a better understanding between customer and manufacturer. For further information you are referred to technical literature. A brief listing of literature is in the section literature. More explanations can be found in the WinLensTM help system, see chapter Optics Software, WinLensTM.

Abbe number Anti-reflection coating Chromatic aberration A term introduced by Ernst Abbe to A single or multi-layer dielectric coating Chromatic aberrations are functions of characterize the dis­per­sion of an optical deposited on the surface of an optical the dispersion charac­te­ristics of optical medium. The Abbe number repre­­sents element to reduce reflection by means of materials. There are two forms of the reciprocal dispersive power and is interference (see also chapter Thin Film chromatic aberration: de­fined as: Coatings). longitudinal chromatic aber­rations which result in different focal points for Aperture stop different wave­lengths and trans­verse Mechanical device which limits the path chromatic aberrations, which cause

where nd, nF nC = indices of refraction of of light rays between the object and different magnifications for different the Fraunhofer d-, F-, and C-lines image planes of an optical imaging wave­lengths. (d=587.6 nm, F=486.1 nm, C=656.3 nm). forming system. Large Abbe numbers correspond to low Coherence disper­sions. Astigmatism The constancy of phase relations Aberration which occurs in an image between two waves. There are two types Aberrations formation due to skew rays. Astigmatism of coherence: temporal and spatial Aberrations occur during image formati- is characterized by two different focal coherence. on with optical systems when the rays positions in two perpendicular planes from the object point do not converge (meridional and sagittal). Coherence length com­pletely at the conjugate image point. The greatest optical path difference Lens aberrations include: spherical Back focal length between two partial waves from a aberrations, coma, astigma­ ­tism, distorti- The distance of the paraxial focus from radiation source where interference can on and chromatic aberrations. the last vertex of an optical system (the still occur. distance from the last surface of a lens or Absorption lens system to its image plane). Unlike Collimator The conversion of light or radiation the effective focal length, the back focal Optical lens system designed to image a energy into another form of energy length can be measured directly. point light source in such a way that all while passing through an optical the emerging rays are parallel to each medium. Birefringence other. Collimation is a general term for In optical anisotropic crystals, the index the imaging of a focal point at infinity Absorption factor of refraction is different for different (in a typical laser collimator a small laser The ratio between the radiant flux in the levels of polarization. A non-polarized­ beam is transposed into an expanded optical medium and the incident radiant light beam is separated into two beams beam of collimated light). flux is called absorption factor. The polarized perpendicular to each other internal absorption factor is the ratio which have two different indices of Coma between the radiant flux penetrating refraction. These are called ordinary and An aberration for skew rays which is not into the medium and the radiant flux extra­ordinary rays. Conse­quently, double rotationally symme­trical. Coma can also absorbed in the medium. images occur in non-polarized light be viewed as an aperture aberration in during trans­mission through an­iso­tropic skew rays whereby the principal ray Airy disc crystals. assumes the function of the optical axis. The central maximum of a diffraction pattern of a circu­lar aperture. The Airy Brewster angle Condenser disc is limited by the first dark ring of the Angle of incidence where the reflected Optical system which is designed to diffraction pattern. and the refracted rays of light striking a collect light sources as completely as transparent optically isotropic medium possible and transfer that light to an Angular dispersion are perpendicular to each other. The object point or plane. The wavelength dependence of the reflected component is linearly polarized diffraction angle of light beams passing and the plane of polarization is perpen- Conjugate points through a dispersive optical element.­ It is dicular to the plane of incidence. Points in both the object and image a function of both the dispersive power plane which are trans­formed into each of the material and the shape of the other by the process of image formation. optical element. with n = refractive index of the surroun- ding medium (i. e. air); n' = refractive index of the refracting medium

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Contrast Dispersion Field of view Contrast is a general term used for Term used to define the process in which The outermost point of the field angle differences in bright­ness. Contrast in the rays of light containing different being transmitted through a lens system context of optical transfer function is wavelengths are deviated angularly by to form an image. This spatial limitation termed modulation (see also modulati- an optical medium. More specifically, can be induced by a field stop. on). dispersion is used to indicate the dependency of the refractive index as a Field of view number

Crown glasses function of wavelength (see also Abbe A characteristic quantity for eyepieces Info Technical Glasses having an Abbe number > 50. number). which gives the diameter of the field of view in millimeters by the equation:­ Depth of field Dispersion curve S = 2 · f · tan w (field of view number) A term used, especially in photography, A graphic representation of the variation f = eyepiece focal length for the plus or minus distance in which of the refractive index of a material as a w = field angle an acceptable focus is attained. The function of wavelength. depth of field (S) for a microscope lens Field stop system can be expressed simply as: Distortion Diaphragm or aperture used to restrict A lens system aberration characterized the useable field by limiting the angle of S = ± n λ / (2 · NA) by the imaging of off-axis straight lines view. An object field stop is lo­­ca­­ted in as curved lines. There are two types of the object area and an image field stop NA = numerical aperture, distortion: pincushion, where off-axis in the image area. n = index of refraction in object space straight lines are imaged curving towards λ = wavelength of light. the center and distortion barrel distorti- Fizeau fringes on, where off-axis straight lines are Fringe pattern which contours the Dielectric films imaged curving away from the center. variation in thickness of thin transparent Dielectric films are typically inorganic objects; i. e., a wedge airgap between materials which are vacuum-deposited Entrance pupil two glass plates viewed at normal onto the surfaces of optical compo­ ­nents The image of the aperture stop in object incidence and illuminated with mono- to increase or decrease reflectivity. space. chromatic light.

Diffraction Entrance window Flatness Deviation of a wavefront from its The image of the field stop in object The measured deviation of a surface with original direction of propagation­ as it space. respect to a reference surface. Deviation passes by an opaque edge or through an in flatness of the test sur­face is typically aperture. Diffraction is not caused by Exit pupil given in units that are a fraction of the refraction, reflec­tion or scatter but by the The image of the aperture stop in image wavelength of the monochromatic light wave nature of light. space. used for measurement.

Diffraction grating Exit window Flint glasses Typically an arrangement of equi-distant The image of the field stop in image Glasses with an Abbe number < 50. parallel lines or elements on a transpa- space. rent or reflecting surface which causes F-number incident light rays to be diffracted. Extinction ratio The ratio of the focal length to the The transmission ratio of a pair of entrance pupil dia­meter of an imaging DIN and ISO standards polarizers in the crossed position to that system. These standards specify dimensions, in the parallel position. tolerances and standard illustrations for Focal length industrial and scientific products.­ Field angle Focal length is defined as the distance Referen­cing the appropriate standard Angle between the optical axis and the between the prin­ci­pal planes and the used in the manu­facturing process principal ray of the object boundary corresponding focal point for paraxial eliminates the need to prepare detailed point. rays. For an individual lens, focal length specifications for individual components. is a function of lens radii, glass type and MIL standards are mostly inactive and Field curvature thickness. Focal length is the most out of date. A lens aberration that causes a flat object important characteristic of any optical surface to be imaged onto a curved imaging system. Direction of polarization surface rather than a plane. Direction of the electric field vector of linearly polarized light. The plane of Field lens polarization and the polarization Lens which is inserted between other direction are parallel to each other. In lenses in an opti­cal system to intercept classical optics, the plane of polarization off-axis rays and bend them to­ward the is always perpendicular to the direction optical axis thus increasing the field of of the beam. view. A field lens has no effect on the magnitude or position of the image.

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Fraunhofer lines Geometric optics Isotropy Fraunhofer observed dark lines in the The field of optics which deals with the A medium is considered to be isotropic solar spectrum. He determined that these propagation of rays in straight lines when its optical proper­ties are indepen- lines were caused by the atomic absorpti- without taking the effect of diffrac­tion dent of direction. Optical glass, for on in specific elements found in the into consideration. Geometric optics only example, is isotropic as its index of chromosphere of the sun. Fraunhofer acknowledges the wave theory of light refraction is the same in all directions. realized that these lines corresponded to with respect to the refractive index as a Many crystals, however, are aniso­ ­tropic, certain wavelengths in the spectrum­ and function of wavelength. as, in their case, the index of refraction is could therefore be used to measure the dependent on direction. disper­sion of optical glasses. He labeled Haidinger fringes the strongest lines A through H. Series of curved interference fringes Koehler illumination which are produced by a constant slope A microscope illumination system Fresnel equations between a test and a reference plate whereby the micros­cope lenses and the They describe the intensity of reflected using monochromatic light. sample are illuminated uniformly. and refracted unpolarized light striking a non-absorbing optical medium having a Half-width Light flux refractive index of n' at an angle of Half-width refers to the full wavelength Power radiated by a luminous source. It incidence α. In the process, the reflected bandwidth of an inter­ference filter at is defined as the product of geometrical ray at the angle of reflection becomes half of the maximum transmission flux, the luminance of the light source partially polarized. The Fresnel equation intensity. (full-width at half maximum, and the transmission efficiency of the gives the intensities of these beams FWHM) optical system. The unit of measurement according to their polarization compo- is the lumen (lm). nents parallel and perpendicular­ to the Illuminance plane of the incident beam. Illuminance is measured as the luminous Light ray (light beam) flux per unit area: lux = 1 lumen/m2 Light ray is the normal to the wavefront Fresnel lens of a wave­train. In general, the direction A Fresnel lens consists of a central thin Index of refraction of the flow of lumious energy. spherical or aspherical lens surface The ratio of the velocity of light in a surrounded by graduated annu­lar rings in vacuum to the velocity in an optical Linearly polarized light the form of prismatic circular zones, all of material at a certain wavelength. Light whose electromagnetic field vector which refract light to the same point. For is restricted to a single plane. all practical purposes, the lens surface has a constant thickness. Fresnel lenses are Line filter commonly made from acrylic plastic and Infrared radiation An optical interference filter exhibiting are used for simple image formation That part of the electromagnetic high trans­mis­sion for atomic or laser where very large apertures are required, spectrum having a wavelength­ between lines. Line filters are usually characterized­ e.g. for overhead projec­tors. Fresnel 0.75 and 1000 micrometers. by a small half-bandwidth (typically on lenses have the advantage of being the order of 1.0 nm) relativ­ely inex­pensive as well as thinner Interference and lighter than an equivalent glass lens . The combining of two or more waves in Luminance such a way that cancellation or amplifica- The luminous intensity per unit area. Fused quartz (fused silica) tion occurs. If amplification occurs it is Luminance is mea­sured in candela per m2 Fused quartz is made by melting and termed constructive interference. If (cd/m2). forming natural or synthetic crystalline cancel­lation occurs it is termed destruc-​ quartz. The melting process destroys­ the tive interference. Luminance indicatrix crystalline structure and there is no The spatial distribution of a luminous longer any birefringence or rotary disper- Interferometer area as a function of the luminous sion. Fused quartz provides better Optical instrument, based on the intensity distribution. transmission especially in the ultraviolet phenomenom of inter­fe­rence of light, and near infrared than normal optical that is typically used to measure length or Luminous intensity glasses. change in length. Interferometers are The luminous flow relative to the solid among the most accurate distance and angle. Gaussian optics length measuring instruments avail­able Gaussian optics is the term used to descri- today. be the optics of paraxial rays and forms the basis of geometric optics. Internal absorption factor See absorption factor. Magnification Geometrical GOTF The ratio of image size u' to object size u See modulation transfer function. Irradiance measured per­pen­­di­cular to the optical The radiant power per unit area in W/ axis: ß' = u'/u cm2. Media plane Plane between two directly adjacent optical mediums.

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Medium Nodal point Optical image formation Medium is a general term to describe any Any ray transversing a lens through its In an optical system, optical image material or space through which light optical center will emerge parallel to the formation is the process of transforming can pass. incident direction. The ex­tensions of the a light beam that emerges from an incoming and outgoing rays will cross object point into a corresponding beam Meridional plane the optical axis at two points called the that creates an image point. Plane through an optical system nodal points. If the lens is surrounded by

containing the optical axis and the object the same medium (i. e. air), then the Optical path difference (OPD) Info Technical point. nodal points coincide with the principal The difference of two optical path points. lengths, e.g. between the optical path Metal films length of a beam travelling through a Thin films of metals designed to increase Numerical aperture medium and that of a beam travelling in reflectivity and/or conductivity. Numerical aperture is defined as the sine vacuum. of the half angle of the widest ray-bund- Minimum deviation le capable of entering a lens σ', multipli- Optical path length (OPL) The smallest angle that light is deviated ed by the index of refraction of the The optical path length of a light ray by an optical component or system. medium n through which the ray-bundle passing through a medium of constant passes. NA = n · sin σ'. Numerical refractive index is the product of the Modulation aperture has special significance for geometrical distance d and the index of In optics modulation is defined as the micros­cope lenses, see chapter "Zoom refraction n. OPL = n·d. ratio of the diffe­ren­ces and the sum of and Microscope Lenses". the maximum and minimum illuminance­ Optical transfer function (OTF) of a series of lines and spaces imaged by Optical activity OTF is the total optical transfer function a lens system. Modulation M is defined A property of certain crystals and liquids. which includes both the MTF and the as: The polarization level of an incident phase transfer function. Usually, only the beam rotates proportional to the path modulation transfer function (MTF) is traversed in the crystal. One distin­g­uishes used to describe the imaging perfor- between substances which rotate mance of a lens system. Modulation is usually considered a clockwise and those which rotate synonym for con­trast. counterclock­wise. Optical tube length A measure used to calculate the distance Modulation transfer function (MTF) Optical axis from the image focal plane of a lens to MTF is a quantitative description of the 1. The symmetrical axis of optical the object focal plane of an eyepiece. It image forming power of an imaging imaging systems. is calculated as t = -β'f, with β' = system. In determining MTF, increasingly 2. The direction where no birefringence magnifi­cation of the lens, and f = focal fine lines of known contrast are imaged occurs in optically birefringent non-cubic length of the lens. by the optical system and the image crystals. modulation is measured in the image Parallel shift plane. The ratio of the image modulati- Optical contact The shifting of the emergent beam on to the object modulation for different The joining of two optical surfaces parallel to the incoming beam which is de­grees of fineness of lines and separa- without the use of an adhesive. When observed when radiation passes through tions (spatial fre­quen­cy) yields the the air between the two surfaces has plane plates obliquely. modulation factor. The MTF is a plot of been completely eliminated, they are this factor versus spatial frequency. An said to be in optical contact. The contact Paraxial space MTF calcula­ted by ray tracing is called a is permanent and can just be separated The space close to the axis where all geometrical optical transfer function through heat. angle functions can be re­­placed by the (GOTF). angle itself (sin α = tan α = α, cos α = 1). Optical density The optics in paraxial space are also Monochromatic radiation Optical density D=log (1 / T), where T is called Gaussian optics. Radiation having a very narrow band- the transmis­sion. If neutral density filters width (for example, laser radiation). are placed in series, the optical density of Phase shift the combination is the sum of the When light travels through a low index Newton rings individual density values. medium and is incident on a medium Circular series of interference fringes of with a higher index, a phase shift is equal thickness (i. e. Fizeau fringes) seen Optical glass observed in the component reflected when two polished surfaces (at least one Optical glasses are transparent, usually from the surface. An additional phase of which is slightly spherical) are brought amorphous, and essentially homogene- shift is observed in the component together with a thin film or air between ous materials whose manufac­tu­ring passing through the denser medium. them. processes are controlled in such a way as to create desi­red variation in characteris- Plane of polarization tics such as refractive index, transmission­ Plane which is perpendicular to the range, dispersion etc.. electric field vector of linearly polarized light.

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Point spread function (PSF) Reflection went on to describe by his theory were The energy distribution in an image The return of radiation upon contact spherical aberration, coma, astigmatism, point P' formed by an illuminated object with a boundary between two different field curvature, and distortion. The Seidel point P. media. There are two types of reflection: zone covers that part of the ray space diffuse (from a rough surface) or direct which can be approximated by this third Polarization (from a smooth surface). The characteris- order theory. The Seidel error sums and A state of oscillation of light waves tics of reflection at the boundary of a coefficients allow for a detailed analysis where the electric or magnetic field weakly or non-absorbing medium are of an imaging system and play an vector vibrations of the wave are summarized by Fresnel's equations. important part in the design of optical restricted to oscillate in a single plane. compo­nents. The specific state of oscillation is Refracting power determined by the position of the Reciprocal value of the focal length of an Sine condition electric field vector. There are three optical imaging system relative to air. Established by Abbe to describe the forms of polarization: linear, elliptical Refracting power is measured in dpt. quality of the image formation of surface and circular. elements lateral to the optical axis. A Refraction satisfactory quality can only be attained Polarization factor The change in direction of an oblique if the magnifi­cation is constant for all The ratio of the intensity of polarized light ray which passes from one medium object zones. In other words, the focal light to that of unpolarized light is called to another having different refractive length must be constant over the entire the polarization factor. indices. aper­ture of the lens. For an infinitely distant object, the sine condition is: Principal planes Resolving power f = const = h / sin σ' (h = incident height, Planes drawn through the principal The measure of the ability of an optical σ' = angle of intersection with the optical points perpendicular to the optical axis component or instrument to image two axis). are the principal planes. The approxi- closely adjacent object details as two mation of a principal plane is applicable separate details. In general, the resolving Spatial frequency only for the paraxial area. power is given as the angular distance at A term used to describe the density of which these details appear or as the regular structures (such as lines of an Principal points number of resolvable lines per mm. optical grating) and is given in lines per Those points of a lens which are imaged mm. onto each other at a magnification of Sagittal plane β'=1. The principal point represents­ the Plane through an optical imaging system Spectrum cardinal point from which the focal which contains the object point and the A spectrum is the entirety of emitted or length, object distance or image distance principle ray of skew rays. It is perpendi­ absorbed radi­ation arranged according is measured. cular to the meridional plane which to wavelength. There are many different contains the object point and the optical types of spectra including continuous Principal ray axis of the system. The sagittal plane band and line spectra. The principal, or chief ray, is a ray from cannot be explained as an independent an object point which passes through the concept beyond the context of a Spherical aberration center of an aperture stop. The ray reference system. Aberrations which occur in widely spread assumes the function of the optical axis beams origi­na­ting from an object point for skew rays. Scattering on the optical axis. They appear as Scattering refers to the deflection of follows: the outer circular lens zones Pupil light by its inter­action with a heteroge- allow image points to develop which do General term used for the paraxial image neous medium. not coincide with the paraxial image of the aperture stop. There are two types point. What results is a rotationally of pupils, entrance and exit pupil. Secondary spectrum sym­metric diversion around the paraxial In a simple achromatic optical imaging image point. Radiant power system, the focal points of two different Radiant power is the energy emitted by a wavelengths will coincide. The remaining Stop radiation source per second and is wavelengths constitute the secondary Diaphragm or aperture used to limit the measured in Watts. spec­trum. ray bundles in optical imaging.

Rayleigh criterion Seidel aberrations Strehl intensity ratio Two Airy interference discs are created Theory of aberrations developed by The ratio of the maximum intensity I of by the image for­ma­tion of two object Seidel went beyond Gaussian optics by an aberrated image in a point P to the

points separated by an angular diffe- no longer equating the sine of an angle intensity I0 of an aberration free image rence. The Rayleigh criterion states that to the angle itself in cases where light in the same point: D = I / I0 the limit of the resolving power of the rays were refracted in an area outside optical system is reached when the the paraxial region. Instead, he represen- maximum of one Airy disc coincides with ted a trigonometric function by power the corresponding first mini­mum of the series and carried the expansion out to a other disc. third order approximation of the function. The aberrations which Seidel

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Surface accuracy errors Thin films Vignetting Deviations of a spherical or plane optical Thin film is a term used to describe either A mechanical limitation of oblique light test surface to a reference surface (test a metal or dielectric film applied to rays passing through an optical system. plate) are known as surface accuracy optical components to increase or This effect cannot be caused by the errors. They are usually given in units of decrease reflection. aperture stop. wavelength. The non-contact methods employed in today's interferometers Total internal reflection Visible light

eliminate the possibility of surface If light is incident on a boundary Radiation which has the capacity to Info Technical damage. between two optical media of different generate visual sensation. The spectral optical densities and is incident from the range lies between 380 nm and 780 nm. Telecentric system denser medium, it will experience total An optical system where the entrance or/ internal reflection. The critical angle for Wave optics and exit pupil is imaged to infinity the two materials is described from Description of optical image formation caused by locating the aperture stop at Snell's law as αc = arc sin (n'/n), where n' taking into account the wave nature of the front or back focal point of the is the index of the denser medium. light. This branch of optics leads to the system. Because of that, the principal investigation of interference. rays are parallel to the optical axis in the Transmission image or/and object space. The passage without frequency change Zonal aberrations of radiation through an optical medium. Zonal aberrations are Seidel aberrations Test plate which occur in zones concentric to the A test plate is a comparison surface of Transmission curves optical axis where the effect due to the extreme precision used to test for surface In general a transmission curve is a change of refractive power has not been accuracy errors. Deviation from the test graphical represen­tation of the transmis- cor­rected to minimize these aberrations. plate profile can be interpreted by sion factor over a given spectral range. careful analy­sis of the fringe pattern created by the close contact, in mono- Transmission factor chromatic light, of the two surfaces. The ratio of transmitted to incident radiation intensity.

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The properties of reflection and trans- The following is a reference of the pri- mission of optical surfaces can be mary thin film coating letter designation affected by thin film deposition. This is (see also chapter Thin Film Coatings): accomplished by evaporating a metal or a dielectric onto a substrate surface in a high vacuum. Antireflection Coatings to minimize reflection for certain The thin film on the boundary of the wavelengths or wavelength­ regions. substrate, and a suitable choice of film Catalog designation AR... thickness, produces interference. The drawing shows the path difference and Metallic Mirror Coatings the effects of the reflected beams of reflective coating, including optional each boundary layer resulting in either over coating. Catalog designation R... constructive or destructive interference. This is how an increase or decrease in Dielectric Mirror Coatings reflection is achieved. achieves maximum reflection, predo- minantly for laser applica­tions, high damage threshold. Catalog designation­ DL...

Reflection as a Function of the Angle of Incidence Beamsplitter Coatings for beamsplitters with a defined reflec- tion and trans­mission ratio. Catalog At the Brewster angle aB, the p-polarized n1 designation T... component goes to a null position, and is transmitted without losses. The reflected beam is completely s-polarized. n2 The transmission of an optical compo- nent is not just a function of coated The reflection properties depend on the surfaces, but also the transmission of following para­meters: n3 the substrate material. We manufacture optical components from many standard Reflection on a Thin Film Surface •• Refractive index of the surrounding materials and also many special materi- medium als. •• Refractive index of the substrate The reflection of the perpendicular (s) •• Refractive index of the vacuum and parallel (p) components of light deposited material entering at an angle of incidence, a ≠ 0, •• Absorption of the vacuum deposited is different. For the air-glass transition material we show the following curve of the •• Film thickness reflection coefficient R for perpendicular and parallel light as a function of the •• Wavelength of the light source angle of incidence: •• Angle of incidence of the light source •• Polarization of the light source

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Symbols and Sign Convention

The symbols used to describe optical In the drawing, we show the system according to the sign convention, f < 0, components, systems and basic opti- parameters and image sizes for an while f' > 0. These symbols are also valid cal quantities in this catalog are listed optical component composed of two for more complex systems composed below: surfaces. The arrows indicate the direc- of many surfaces. The table defines the tion of the paths. In this example and individual symbols: Technical Info Technical •• The sign for paths parallel to the optical axis are determined by the light direction that travels from right Object Plane Image Plane to left. •• Paths measured in the direction of light are positive. Paths which are counter to the direction of light are negative. (lens thickness and system lengths are always positive). •• Symbols in object space are not primed. Symbols in image space are primed. •• The radius of curvature is measured from the surface to the center of curvature. That is why convex surfaces­ Principal Planes in the light direction appear to have a positive radius and concave surfaces F focal point in object space F’ focal point in image space appear to have a negative radius. H principal point in object space H’ principal point in image space •• Paths perpendicular to the optical axis f focal length f’ focal length are positive above the axis and s object to front surface distance s’ image to back surface distance negative below the axis. a object distance a’ image distance z object to focal point distance z’ image to focal point distance

C1 center of curvature, surface 1 C2 center of curvature, surface 2 r1 radius, surface 1 r2 radius, surface 2 O object point O’ image point u object size u’ image size σ object aperture angle σ’ image aperture angle d lens thickness or system length

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Optical components and optical compo- Frequently the testing is done with These written procedures deal with nent drawings are characterised by code test glasses. It is a comparative measu- comparative mea­sure­­ments, always numbers found in the german standard ring procedure and relies on sub­jective referring to a reference flat. Through DIN 3140 and the new international stan- evaluation. A reference glass is placed a combination of many measurements dard ISO 10110, see Literature. Outside over the test surface and the resulting and correspon­ding calculations absolute of the pure geometri­cal tole­ran­ces for interference pattern is observed (Newton testing is possible. thickness and diameter, there are other Rings). The number and the defor­mation properties corres­ponding to similar code of the resulting interference rings are The following illustration shows the numbers. Material properties of glass measure­ments of the deviation between evaluation of a fine form error obtained and form deviations are quantified.­ the reference glass and the glass under with an interferometer. The test com- test. The distance between two interfe­ ponent and the reference surface are In technical drawings the code number rence lines signifies a half wavelength. slightly tilted to generate fringes. From is followed by a slash (/) and then by The accuracy which can be obtained the quantitative representation of the the allowable tolerances. The following with a visual test is in the region of a 100 measured deviations of a surface from example shows the code numbers of a nm form error. For further infor­mation the nominal form we can determine plano­convex lens. please refer to DIN 3140, part 5. and calculate the effects on the image quality.

Bevel

Technical Drawing of a Single Lens F = h / a a

Code number 1 addresses the size and Form error testing with a test glass Fine Form Error Interferogram (top) number of bubbles and inclusions in the medium. The smaller the value, the higher the material requirements. For Computer controlled interferometers further information please refer to DIN provide a vibration free measurement 3140 part 2. with a much higher accuracy. The entire surface can be measured at one time and Code number 2 quantifies cords and the "Peak-to-Valley“ value determined. inhomogenities in glass with the ratio From this value we can understand the of the cords size to that of the entire minimum and maximum deviation from test area. For further information please a reference surface. On a flat surface refer to DIN 3140, part 3. it is the devi­ation of a plane and on a curved surface the relationship is with a Code number 3 minimizes the allowable spherical surface. form error of the effective optical sur- faces. The spherical test produces mostly Fine Form Error Contours (bottom) asymmetrical deviations, which have Form errors describe the deviations from a particular negative effect on optical plane and spherical surfaces. The testing images, and are detected in the Peak-to- Code Number 4 addresses centration occurs inside a predeter­mined test area. Valley value. errors. There are also additional form errors inside these areas which are characte- The centration error is a measurement of rised as fine form errors. the deviation from the optical axis of a lens to its form axis. Frequently,­ the axis Because these form errors deal with very of the boundary cylinder is the refe­ small devia­tion, the testing is accom- rence axis of a lens, because the so called plished with an interferometer. The "Optical Axis" exists as a virtual quantity. wave­length of light is used as the unit of measure­ment; typically 546 nm or 633 nm (HeNe Laser).

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If the centers of curvature of the lens sur- Symbols for Polished Surfaces face are on the reference axis, the lens is centered. Centration errors are normally All optical surfaces are identified with stated in arc minutes. a symbol for surface finish (roughness). The three-diamond symbol signifies a For further information please refer to surface that has minimum stray light. For DIN 3140, part 6. special applications the roughness can be further reduced and this surface finish

Bevel 0.4 is shown with 4 dia­monds signifying a Info Technical centration vertex super fine polish.

reference axis Other Symbols for Plane Optical Residual Reflection <0.3% for 400-800 nm Components

All the previous codes are also valid for Technical Drawing of a Cemented Component beamsplitter cubes. However, there is Lens Centering Error one new factor: the maximum refrac- Symbols for plane optical components tive power. The first value indicates the spherical wavefront distortion and the Code Number 5 addresses the tolerances All the previous code number examples bracketed value astigma­tism. Both values for surface defects. are also valid for plane optical com- are given in diopters. ponents such as mirrors, plane plates, Scratches and digs are considered surface prisms and cemented beamsplitter cubes. Furthermore, the deflection angle and defects and are classified by number and In addition, the angle tolerances are the maximum allow­able deviation is also size. The smaller the value, the cleaner­ specified. stated. For beamsplitter coatings the split the surface. ratio of the transmitted and reflected beam is also specified. For further information please refer to DIN 3140, part 7. after cementing T:R=1:1

black lacquered Code Number 6 classifies the effects of surface strains inside optical glass or optical systems.

This also applies to strains that are a result of cementing lenses. The errors Bevel 0.5 are stated as the optical path difference­ Prism Surface in the glass. The defect is stated as the Slope 4' Bubbles and stress-free cementing allow­able difference in nanometers per max. angular error 8° max. refractive power 0.018 (0.0064) dptr. 10 mm glass path. For further informa- tion please refer to DIN 3140, part 4. Test Areas indicated Code number 15 addresses the purity of by the Arrows Test Areas indicated cement layers and bonded surfaces. by the Arrows

The purity of a cemented optical compo- nent is treated like code numbers 1 for flaws and code number 2 for schlieren. If the purity has not been explicitly spe- cified, then it may not exceed the total value of the acceptable surface defects of both cemented surfaces.

For further information please refer to Technical Drawing for a 90°-Prism Technical Drawing for a Beamsplitter Cube DIN 58170, part 54. In this example, the prism surface slope angle is toleranced to s'. This error is also known as the "Pyra­mid Error". The prism hypotenuse also has a partially transmit- ting coating as is specified in the drawing with a special symbol. This symbol also references the transmission and reflection ratio.

The shaded areas in the lower part of the drawing shows the test area of the prism for which the code numbers and tolerances are valid.

Germany-Phone: +49 (0) 551/ 6935-0 France-Phone: +33 - 47 25 20 420 809 Index Quality Testing of Optical Components and Systems

The quality of optical components path difference which is detected when becomes stronger through the influence and systems are tested using objective a wave passes through an optical system of the diffraction of light on the iris. measurement methods. This includes and is presented as an interferogram This is called a diffraction limited optical diameter and thickness measurements taken over the effective aperture (exit system. with precision apparatuses; surface finish pupil) of the component under test. with microscopes and the measurement This test procedure presents a clear cut In an imaging system there are normally of image performance with interferome- conclusion on the image formation and many optical surfaces. The establishment ters. The optics presented in our catalog the iden­ti­fication of a possible out of of allowable tolerances of each indivi- generally meet or exceed the quality tolerance component in the production dual surface must also be considered for standards specified by DIN ISO. process, especially when the resultant the total system in order to achieve total wavefront is not rotationally symmetric. performance. Imaging systems are tested for their image forming quality by measuring Precision optics can exhibit a wave- Additional information on testing optical the wavefront distortion. As a rule, the front distortion smaller than a quarter components and systems is available in testing is done at a wavelength of wavelength (160 nm). If the beam path the recommended literature section in 633 nm (HeNe Laser) and with an accu- is limited by an aperture, such as an iris this catalog. racy of a fraction of that wavelength. diaphragm, the wavefront distortion One of the quality features is the optical decreases and the imaging performance

Minimum Spot Size and Resolving Power

In geometric optics, the wavelength of light is ignored as the wave theory Aperture stop consideration which lead to diffraction and interference phenomena are not part of geometric optics. That is why it Laser is possible, on the basis of geometrical beam calculation, to generate an infinitely defined image point with a perfect lens. Light Propagation behind a Circular Stop Theoretically, this would result in an infinitely high resolving power. If this were actually possible, electron micro- scopes, for example, would be absolutely Naturally, this re-distribution of the unnecessary. luminous intensity resulting from the diffraction of light occurs in all optical Image The fact is, however, that this is not systems which, of course, represent plane possible. Even the most flawless lens has obstacles (usually of a circular nature) in a finite resolving power deter­mined by the path of the light. If you imagine a wavelength-dependent diffraction. circular aperture filled with glass (as, for example with a lens), then diffraction Brightness Distribution of the Diffraction Pattern Optical diffraction is defined as the will be super­imposed, for the most part, deviation of a wave from its original by refraction. direction of propagation along the normal of the wave surface which is As can be seen in the following pho- The diameter of the first ring is also not caused by refrac­tion, reflection or tograph, the bright­ness distribution of called the diameter of the diffraction scattering but by the wave nature of the the diffraction pattern, when focussed (i. e. Airy disc). This diameter is given by: wavetrain. through a lens, will re-appear in the DAiry = 2.44 λ · k focal or image plane. The principal For example, if you illuminate an obsta- feature of this brightness distribution is with f-number cle such as an iris diaphragm, then light a very distinct central maximum surroun­ will no longer propagate in a parallel ded by additional maxima of decrea- manner behind the obstacle. Instead, its sing brightness. Between each of these λ Wavelength propa­gation will change in accordance maxima, there is a bright­ness minimum f' focal length with the shape of the obstacle. in the form of dark rings. ∅EP Diameter of the free opening

The following drawing shows the result of 84 % of the total light is concentrated in light propaga­tion behind a circular stop the central maximum. Only 16 % is distri- which is illuminated by a laser beam (HeNe buted among the secondary maxima. Laser). The diameter of the stop is about 0.5 mm. As you can see, little remains of the straight parallel propagation of the light from the original laser beam.

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Diffraction discs limit the resolving The resolution limit is reached when the In laser use, the laser beam itself must be power of optical imaging systems. Airy discs exhibit a 50 % overlap in the considered as a "diffracting" aperture. Resolving power can generally be defi- image plane. This is illustrated in the dra- This means that the diameter of the laser ned as the ability of an optical system wing on this page. In this case, the two beam must be included in equations for to separate object points in the image object points can just barely be recogni- minimum spot size and resolving power plane. For example, in a telescope, the zed as two separate points in the image if the optical system in question is not Info Technical resolving power limits the eye's ability plane because the maxi­mum of the one completely illuminated. to recognize two close stars as really disc will coincide with the minimum of two separate stars. One can calculate the other. From this, the size of the smal- the resolving power from the size of the lest structure that can be resolved in the diffraction pattern. image is given by:

In the following illustration, we have an u'min = 1.22 λ · k optical image formation system being acted upon by the light of two separate This determines the minimum angular object points O1 and O2, characterized distance of the two object points which by the two principal rays which pass is generally defined as the resolving through the imaging lens at an angular power of an optical imaging system: distance w.

Each object point now produces its own Airy disc in the image plane. The smaller (assuming small angles, such that the angle σ' or the greater the diameter sin wmin ≈ tan wmin ≈ wmin) of the Airy disc, the more these discs will overlap. The diameter of the disc is Consequently, two object points can be determined by the f-number of the lens. recognized as separate points when their

angular distance is w ≥ wmin.

Furthermore, it follows that it is useless to try to raise the resolving power by increasing the magnification with the aid of focal length f', without simultaniously

redu­cing the f-number, k = f' / ∅EP.

The resolving power of optical instru- ments is rarely ever reached, because the theoretical limit imposed by diffraction is usually further reduced by geometri- cal effects. It is, of an optical imaging system. For lasers, the diffraction limit is a real measure for the efficiency of

Image Plane achromats because of the favorable pro- u'min perties of laser light (monochromaticity, parallelism).

Resolving Power of two separate Object Points

Germany-Phone: +49 (0) 551/ 6935-0 France-Phone: +33 - 47 25 20 420 811 Index Focusing and Expanding Laser Beams

Beam Profile

Laser beams, like other radiation fields, I are composed of one or more beam I modes. In laser technology the radiation 0 field develops a transverse electroma- gnetic mode (TEM). Lasers emit in one single mode (SM) or in multi-mode ope- 2 ration. The TEM Modes are distinguished I0 /e by two indices which indicate how many null positions the electro-magnetic field w0 in the x and y direction exhibits and z is the direction of the beam propagation. Beam Profile 2 Θ

A TEMmn mode has also m null positions in the x direction and n null positions in

the y direction. Whereby I0 is the intensity on the optical axis, also the maximum intensity, and r Beam Divergence

Beams in the TEM00 mode do not exhibit is the distance from the optical axis. The null positions in the transverse direction. beam radius w (beam diameter 2w) is This beam mode is diffraction­ limited; defined as the distance from the optical Where λ is the laser wavelength, z the that means that the product of the beam axis where the intensity has fallen to 1/e2 distance from the beam waist and Θ the divergence and the minimum beam of the maximum intensity. beam divergence defined as: radius takes the smallest possible value compared to any other beam mode. This The optical design program WinLensTM is is why lasers, whenever possible, are especially useful to calculate the expan-

construc­ted to emit in the TEM00 mode. sion and formation of Gaussian beams. A Most gas lasers such as HeNe and Ion concise treatment of the expansion and In the vicinity of the beam waist the lasers, and many low power solid state formation of laser beams is also avail- Gaussian beam maintains an approxima- lasers emit these modes. Diode lasers able by H. Kogelnik, T. Li: Appl. Optics 5 tion of a parallel beam bundle with the frequently in asymmetri­cal (astigma- (1966) 1550-1567. smallest possible cross section. Farther tic) form, compared to high intensi­ ­ty away from the beam waist it approxi- laser beams and special material pro- Beam Expansion mates a spherical wave with the smallest cessing lasers, in general have higher possible angular aperture; the tran­sition beam modes. The TEM00 mode has also Every Gaussian beam has a beam waist in between the two regions results in the considerable meaning in practice and is the direction of propagation where the distance zR (Rayleigh Range): fre­quently used as an approximation for beam radius takes a minimum value w0. higher beam modes. The beam waist can be a virtual beam waist or it can be found beyond the The intensity distribution of the TEM00 beam source. On both sides of the beam mode is described by the Gaussian distri- waist the beam radius increases with For larger distances the following is bution (Gaussian beam): increasing distance z: valid:

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Beam Focusing Collimating Laser Beams Depth of Field

If a TEM00 beam is transmitted through a Laser beams are collimated to produce The depth of fieldD z is the region lens, a new beam waist (real or virtual) is plane waves or to maintain a constant around the beam waist length where produced in the location z’ by: diameter over a large distance. This is the beam waist radius moves within a

achieved by telescopes, where two lenses defined region: Info Technical are separated by the sum of their focal lengths. ,

Special Cases: In principle this is possible using the

beam focusing formula and a single where w/wf is the allowable change of 1. If the beam waist of the incoming lens. In most cases this method results the beam waist radius. If w >> wf this beam lies in the incoming focal point in impractical focal lengths and beam approximation is valid: of the lens (z=0), the new beam waist waist positions as well as extremely high lies in the image side focal point demands on the beam waist length tole- . (z’=0). On the contrary in geometrical rance. That is why in practice and in optics an object in the focal plane is classical optics the predominant tele- imaged to infinity. scope (two lenses separated by their Higher Modes focal lengths) is used when collimating­ light. 2. In the case zR << z, where zR is the For a Gaussian beam (TEM00), the Rayleigh Range in front of the lens, product of the beam waist radius and z’ = f2/z. This is also valid for ima­ The beam parameters of the exiting laser divergence is determined by: gery in geometrical optics. This case beam can be determined by using the occurs, when the beam waist of an two focusing formulas. For most applica- . already strongly focussed beam is far tions it is sufficient to use the telescope away from the lens. expansion formula:

For correspondingly larger TEMmn we 2 2 3. If zR >> z then z’ = z · f /z R. This case , use: is frequently encountered, when a highly collimated­ beam is fo­cused and through a lens. where f2 is the focal length of the exit lens and f1 the focal length of the ent- 4. In addition, if zR >> z and zR >> f, rance lens. In practical terms, in case 3 . then z’ = 0 is valid. Focusing a highly and 4 the beam waist is expanded by this collimated laser beam through a factor and the divergence is decreased short focal length lens the beam by the same factor. The exit beam waist The product of the radius and diver- waist lies again in the image side position can be adjusted by minor gence is larger than the product for the focal point. changes of the lens separation in the TEM00 mode and can be different in the telescope. x and y direction. It then follows that The radius of the image side beam waist, higher modes under the same conditions wf, can be calcu­lated with the following cannot be focussed or collimated as well formula: as Gaussian TEM00 beams.

In practice case 4 is very important and the following is valid:

Θ

F F' 2w0

z f f z'

Beam Focusing

Germany-Phone: +49 (0) 551/ 6935-0 France-Phone: +33 - 47 25 20 420 813 Index Literature and Software

a) Technical Books b) Technical Articles

G. A. Weissler (Hrsg.), "Einführung in die T. Thöniß, "Objektive in der industriel- R. Schuhmann "Standardized Optical Industrielle Bildverarbeitung", Franzis len Bildverarbeitung" in "Einführung in Components for Laser Applications", Verlag GmbH (2007) die industrielle Bildverarbeitung" (see SPIE Vol. 3737 (1999), 644-648 Technical Books) G. Litfin (Hrsg.), "Technische Optik in der M. Schulz-Grosser, R. Schuhmann Praxis", Springer Verlag (1997) N. Henze, "Spektroskopie und Spektro- "Neue Laserspiegel für hohe Ansprüche", meter", Optolines - LINOS Fachmagazin Laser 3, 1999, 32-36 G. Schröder, "Technische Optik", Vogel- für Optomechanik und Optoelektronik, Buchverlag (2002) 10, 2006, 19-22 R. Schuhmann, M. Schulz-Grosser "Laseroptik für den tiefen UV-Bereich", H. Naumann, G. Schröder, "Bauelemente B. Huhnold, M. Ulrich, T. Thöniß "Flexibel LaserOpto 31 (3), 1999, 54-56 der Optik", Fachbuchverlag Leipzig in drei Dimensionen - Miniatur-Labor- (2004) system erlaubt komplexe Optik-Aufbau- T. Thöniß, S. Dreher, R. Schuhmann ten", Laser-Photonik 4, 2005, 32-34 "Optisch auf den Punkt gebracht", H. Haferkorn, "Optik", Wiley - VCH Laser 2, 1999, 10-13 (2002) U. Düwel, M. Ulrich, T. Thöniß "Auswahl- kriterien für präzise Linearpositionierer", R. Schuhmann "Quality of optical Com- M. Born, E. Wolf, "Principles of Optics", Mechatronik F&M 5-6, 2005, 34-37 ponents and Systems for laser applicati- Cambridge University Press (1999) ons", SPIE Vol. 3578 (1998), 672-678 N. Henze, Optische Tischsysteme I-III: Neuauflage "Optik", Oldenbourg Verlag "Die Schwingungsisolation", "Design R. Schuhmann, T. Thöniß "Telezentri- (2005) optischer Tischplatten" und "Tischplat- sche Systeme für die optische Mess- und ten - thermisches Verhalten", Optolines Prüftechnik", tm – Technisches Messen, W. J. Smith, "Modern Optical Enginee- - LINOS Fachmagazin für Optomechanik 65 (1998), 4, 131-135 ring", McGraw-Hill (2000) und Optoelektronik, 7-9, 2005-2006 R. Schuhmann, M. Schulz-Grosser W. J. Smith, "Modern Lens Design", U. Düwel, M. Ulrich, T. Thöniß, "Auswahl- "Multi-glass AR coa­tings in lens designs", McGraw-Hill (2004) kriterien für präzise Linearpositionierer", SPIE Vol. 3133 (1997) 256-262 Mechatronik 5-6, 2005, 34-37 D. Malacara, Z. Malacara, "Handbook of R. Schuhmann "Leistungsstarke Optik- Lens Design", Marcel Dekker (1994) T. Thöniß "Laseraufweitungssysteme - Design-Software für wenig Geld", Grundlagen und Anwendungen", Optoli- F&M 105 (1997) 10, 734-736 M. Young, "Optics and Lasers", Springer- nes - LINOS Fachmagazin für Optomecha- Verlag (1998) nik 1, 2004, 11-14 R. Schuhmannn, M. Goldner "Concepts for Standari­sa-tion of Total Scatter K. Tradowsky, "Laser", Vogel-Verlag T. Thöniß, S. Dreher, R. Schuhmann "Pho- Measurements at 633 nm", Pro­ceeding (1983) tonik-Puzzle - Optische Komponenten of the 4th International Workshop of und Systeme für Laseranwendungen", Laser Beam and Optics Characterization, Laser-Photonik 2, 2003, 14-21 VDI-Verlag, 1997, 298-313

R. Schuhmann "Low Cost Analysis Soft- ware for Optical Design", SPIE Vol. 3780 (1999)

814 US-Phone +1 585 223-2370 UK-Phone +44 2380 744 500 Technical Information on Optics

c) Software

(see chapter "Optics Software" regarding more information) Technical Info Technical WinLensTM LINOS Photonics optical analysis program

WinLens Tolerancer LINOS Photonics optic tolerance calcula- tion program

Glass Manager LINOS Photonics database program for glass types

Material Editor LINOS Photonics utility to create, edit and manage custom materials data

PreDesigner LINOS Photonics software to determine and display key parameters of optical systems

Lens Library LINOS Photonics database including optical systems and modified LINOS components

Germany-Phone: +49 (0) 551/ 6935-0 France-Phone: +33 - 47 25 20 420 815 Index Optical Glass Data Refractive Indices and Internal Transmittances

•• The tables list the internal transmittan- ces, τi , and the refractive indices, n, of A closer look the major types of optical glass and other fused silica used in fabricating All relevant specifications of most components appearing in this catalog of our optics, such as materials, ra- •• Abbe constants may be computed dii and transmittances, are inclu- from the following relation: ded in the database of the LINOS WinLens optical design software. A freeware version of this program is available for •• Refer to Glass Manager on page 592 for download at www.winlens.de. further information on the optical glasses listed above and glass data of all main suppliers.

N-BK7 N-BaK4 N-F2 N-SF10 Fused Silica

λ τi n τi n τi n τi n τi n (nm) d=5mm d=5mm d=5mm d=5mm d=5mm 280.0 - 1.5612 - 1.6289 - 1.7474 - - 0.990 1.4940 290.0 - 1.5567 - 1.6224 - 1.7307 - - 0.993 1.4905 300.0 0.260 1.5529 - 1.6169 - 1.7170 - - 0.997 1.4875 310.0 0.590 1.5495 0.240 1.6121 - 1.7055 - - 0.999 1.4848 320.0 0.810 1.5465 0.530 1.6079 0.200 1.6959 - - 0.999 1.4824 334.1 0.950 1.5427 0.750 1.6028 0.760 1.6845 - - 0.999 1.4795 350.0 0.986 1.5392 0.940 1.5980 0.940 1.6742 - - 0.999 1.4766

365.0 ni 0.994 1.5363 0.981 1.5941 0.981 1.6662 0.060 - 0.999 1.4743 370.0 0.995 1.5354 0.988 1.5929 0.986 1.6639 0.210 1.7978 0.999 1.4736 380.0 0.996 1.5337 0.992 1.5907 0.992 1.6595 0.590 1.7905 0.999 1.4723 390.0 0.998 1.5322 0.995 1.5887 0.995 1.6557 0.830 1.7840 0.999 1.4711 400.0 0.998 1.5308 0.997 1.5869 0.998 1.6521 0.930 1.7783 0.999 1.4700

404.7 nh 0.998 1.5302 0.997 1.5861 0.999 1.6506 0.952 1.7758 0.999 1.4695 420.0 0.998 1.5284 0.998 1.5837 0.999 1.6461 0.981 1.7685 0.999 1.4681

435.8 ng 0.999 1.5267 0.998 1.5815 0.999 1.6420 0.990 1.7620 0.999 1.4667 460.0 0.999 1.5244 0.998 1.5785 0.999 1.6368 0.995 1.7537 0.999 1.4649

480.0 nF’ 0.999 1.5228 0.998 1.5765 0.999 1.6331 0.996 1.7480 0.999 1.4636

486.1 nF 0.999 1.5224 0.999 1.5759 0.999 1.6321 0.997 1.7465 0.999 1.4632 500.0 0.999 1.5214 0.999 1.5747 0.999 1.6299 0.998 1.7432 0.999 1.4625

546.1 ne 0.999 1.5187 0.999 1.5712 0.999 1.6241 0.999 1.7343 0.999 1.4603 580.0 0.999 1.5171 0.999 1.5692 0.999 1.6207 0.999 1.7292 0.999 1.4589

587.6 nd 0.999 1.5168 0.999 1.5688 0.999 1.6200 0.999 1.7282 0.999 1.4587 620.0 0.999 1.5155 0.999 1.5673 0.999 1.6175 0.999 1.7244 0.999 1.4576

632.8 n632.8 0.999 1.5151 0.999 1.5667 0.999 1.6166 0.999 1.7231 0.999 1.4572

643.8 nC’ 0.999 1.5147 0.999 1.5662 0.999 1.6158 0.999 1.7220 0.999 1.4569

656.3 nC 0.999 1.5143 0.999 1.5658 0.999 1.6150 0.999 1.7209 0.999 1.4566 660.0 0.999 1.5142 0.999 1.5656 0.999 1.6148 0.999 1.7205 0.999 1.4565 700.0 0.999 1.5131 0.999 1.5642 0.999 1.6126 0.999 1.7173 0.999 1.4555

1060.0 n1060.0 0.999 1.5067 0.999 1.5569 0.999 1.6019 0.999 1.7023 0.999 1.4498

1529.6 n1529.6 0.997 1.5009 0.998 1.5512 0.998 1.5951 0.999 1.6938 - 1.4442

1970.1 n1970.1 0.968 1.4951 0.983 1.5458 0.975 1.5896 0.990 1.6875 - 1.4384

2325.4 n2325.4 0.890 1.4897 0.940 1.5410 0.930 1.5848 0.959 1.6822 - 1.4330

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Unlike in the visible spectral region (VIS), Comparison Fused Silica / CaF 2 at even shorter UV wavelengths (VUV), the absorption increases proportionally

to the intensity (for intensities < 150 mJ/ Info Technical cm2). This is due to the 2-photon absorp- tion. A degradation test (simple long- term test), starts at medium intensity, and the intensity is then increased and decreased.

In this test, optical glass made of fused

CaF2 silica typically shows early signs of damage (for example, caused by the for- Fused Silica mation of colored points), whereas the

absorption of CaF2 remains unchanged even after a few million pulses.

Measured transmission of CaF2 and Fused Silica in the UV

Comparison of Degradation Fused Silica / CaF2 at 193 nm

Fused Silica

CaF2

Degradation after 8 000 J/cm2 respectively 250 000 pulses

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