ministry of higher education & scientific research University of Technology & Optoelectronics Engineering Department

Laser Beam Analysis Using Image Processing

A Project Submitted to the Laser and optoelectronics Engineering Department / University of Technology in Partial Fulfillment of The Requirements for the Degree of B.Sc in Laser and Optoelectronics Engineering/Laser Engineering

By

Mayss Khudhur Abbass

Supervised by

Dr.Husham Muhamed Ahmed

ﺟﻤﺎدي اﻟﺜﺎﻧﯿﺔ ١٤٣١ may 2010 IV

Abstract

The laser beam profiles captured by CCD camera processed and analyzed by using 2Dgraphic and displayed on the monitor. The analysis of the He-Ne profiles and profiles are presented. The results showed that the He-Ne laser emits a pure , whereas the diode laser emits an elliptical beam shape. The laser beam profile analysis developed to study the intensity distribution, the laser power, number of modes and the laser beam, these parameters are quite important in many applications such as the medical, industrial, military, , Nonlinear , Alignment, Laser monitoring, Laser and laser amplifier development, Far-field measurement, Education. The analysis of the image profile can be used in determination the distance of the objects depending on the distribution of the spot contours of the laser beam with its width. As well as it can be used to capturing images to the objects and determining their distance and the shape of the objects. V

contents

Chapter one

1.1 Introduction

Chapter two ( Laser beam profile-principles and definition )

2.1 Transverse electromagnetic modes …………...….…………….……….3

2.2 Beam diameter and spot propagation ……..……..…………..…….…...4

2.3 Gaussian beam propagation …………………………...……..…..…..….5

2.3.1 Beam waist and divergence.

2.3.2 Near – field Vs. Far – field divergence.

2.4 ………………………………………….….…....…....10

2.5 The beam profile and how dose if work ……….………….….…….….12

2.5.1 Gaussian beam profiling.

2.5.2 Measurement of beam profiles.

2.5.3 Camera – based profilers.

2.5.4 Knife – Edge and slit profilers.

2.6 M² factor ...... 17

2.7 Laser beam profile analysis system ……………………..…….………20

2.7.1 The laser source.

2.7.2 The laser splitters.

2.7.3 Variable attenuators for high power .

2.7.4 Beam optics controller.

2.7.5 Laser image Analysis.

Chapter One: Introduction 1 Chapter One INTRODUCTION

1.1 Introduction The laser radiation is used in the broadest sense to include the entire electromagnetic spectrum. Laser today spans the range from approximately 200 nm to 400 μm, since the generation of the first laser beam in 1960, the detection techniques are developed in order to recognize and analyze the discovered beam. In general the analysis of laser beam is based on energy measurement, the intensity distribution of the laser beam, beam divergence, waist parameter, number of modes , 1/e² width , Beam quality parameter, M², Beam astigmatism, Beam wander or jitter…etc [1],[2]. Usually the above parameters were measured individually using separate setting. Later on when the computer processor has been highly developed most of the above parameters could be measured by using the computer system. The progress of the mosaic detector leads to the production of the CCD (Charge Coupled Device) camera, which make the above measurement feasible. There are several methods for spatial profiling of laser beam. In many applications it is important to know the energy distribution in the laser beam. There are number of standard beam profiling methods involving detector arrays [3].Scanning aperture [4], scanning knife-edges [5], burn patterns and photographic technique. All of these methods have disadvantages in certain situations. The detector array is difficult to be used when the beam size exceeds the size of the array. The scanning apertures require a Chapter One: Introduction 2 very large number of shots to cover a beam in two dimensions with reasonable resolution .Where as the scanning knife-edge reduces the number of shots required but assumes a circularly symmetric profile. The burn patterns are unsuitable for precise work, being only a rough guide and require a certain threshold energy density. There are several disadvantages associated with photographic, e.g. chemical processing, non-linearity of the film response, wavelength sensitivity and the need to use a micro density-meter to reach a quantitative result. When the beam to be profiled has a large diameter, a relatively low energy density and a wavelength outside the range of photograph, non-of the above techniques are desirable. In many laser applications precise measurements of laser beam profiles are essential for estimating laser performance in that application. One typically needs to quantify the circular symmetry and intensity smoothness of the beam energy profile. The beam profile tells all about the beam’s spatial characteristics, which in turn describe the propagation, beam quality and utility of the beam. In addition, it can tell how effectively optics are succeeding in modifying and shaping the laser’s output. Profiling is particularly helpful in building optical systems for laser printers and fiber optic collimators [6].

Chapter two: Laser Beam Profile-Principles and Definitions 3 Chapter Two Laser Beam Profile-Principles and Definitions

Most laser applications involve the laser output beam striking an object or interact with the material at the target surface. The target may be an object to which a distance is measured, a detector that receives information form the laser beam, or some material that is transformed by heating, melting, or vaporization. The effectiveness of the interaction depends on the intensity of the laser radiation and how this intensity varies across the beam. If a laser beam radiates against a plane perpendicular to the direction of beam propagation, the cross section of the beam would normally form a round spot. The Intensity of the spot would be highest in the center and taper off near the edge . The ideal beam propagates in the Transverse Electromagnetic Mode (TEM), which is generally preferred for most applications. However, many lasers do not operate in this mode and may have irregular intensity variation across the beam.

2.1 Transverse Electromagnetic Modes Since is composed of electromagnetic waves, the laws governing electric and magnetic fields in reflecting cavities control the spatial distribution of laser light resonating within a laser cavity. Each allowable distribution of an electromagnetic field across the long axis of the resonator is called a transverse electromagnetic mode. A laser may operate in one or many TEMs at the same time [5].

Chapter two: Laser Beam Profile-Principles and Definitions 4

Many lasers are designed to operate in the TEM00 mode. In this mode, the beam has a circular cross section with a maximum intensity in the center and decreasing intensity at point’s radial removed from the center. The TEM00 mode produces the smallest-diameter and lowest divergence laser beam. Multimode operation increases the output power but decreases the coherence and increases divergence [7].

2.2 Beam Diameter and Spot Size: Figure 1 shows the irradiance of the TEM00 mode as a function of distance across the laser beam. The shape of this curve is Gaussian; therefore, TEM00 laser beams are called Gaussian beams. The irradiance of such beams drops off away from the center and the exact edges cannot be located. For this reason, beam diameter is defined as shown in Fig. 1. The shaded portion shows the effect of passing a Gaussian beam through an aperture smaller than the diameter of the beam [8,9]. The diameter of a Gaussian beam is defined as the distance across the center of the beam for which the irradiance equals 1/e² of the maximum irradiance (1/e²=0.135). The area inside a circle of this size centered on the beam center will contain 86.5% of the total energy (or power) of the Gaussian beam. The remaining 13.5% of the energy lies in the edge of the beam beyond this arbitrarily chosen distance.

Chapter two: Laser Beam Profile-Principles and Definitions 5

2.3 Gaussian Beam Propagation 2.3.1 Beam Waist and Divergence In order to an appreciation of the principles and limitations of Gaussian beam optics, it is necessary to understand the nature of the laser output beam. In TEM00 mode, the beam emitted from a laser begins as a perfect plane wave with a Gaussian transverse irradiance profile as shown in the figure below. The Gaussian shape is truncated at some diameter either by the internal dimensions of the laser or by some limiting aperture in the optical train. To specify and discuss the propagation characteristics of a laser beam, we must define its diameter in some way. The commonly adopted definition is the diameter at which the beam irradiance (intensity) has fallen to 1/e2 (13.5%) of its peak, or axial, value.

Chapter two: Laser Beam Profile-Principles and Definitions 6

Gaussian beam profile (theoretical TEM00 mode) causes light waves to spread transversely as they propagate, and it is therefore impossible to have a perfectly . The spreading of a laser beam is in precise accord with the predictions of pure diffraction theory; aberration is totally insignificant in the present context. Under quite ordinary circumstances, the beam spreading can be so small it can go unnoticed. The following formulas accurately describe beam spreading, making it easy to see the capabilities and limitation of laser beams. Even if a Gaussian TEM00 laser-beam wavefront were made perfectly flat at some plane, it would quickly acquire curvature and begin spreading in accordance with

And

Chapter two: Laser Beam Profile-Principles and Definitions 7 where z is the distance propagated from the plane where the wavefront is flat, λ is the wavelength of light, w0 is the radius of the 1/e2 irradiance contour at the plane where the wavefront is flat, w(z) is the radius of the 1/e2 contour after the wave has propagated a distance z, and R(z) is the wavefront radius of curvature after propagating a distance z. R(z) is infinite at z = 0, passes through a minimum at some finite z, and rises again toward infinity as z is further increased, asymptotically approaching the value of z itself. The plane z = 0 marks the location of a Gaussian waist, or a place where the wavefront is flat, and w0 is called the beam waist radius.

The irradiance distribution of the Gaussian TEM00 beam, namely,

where w = w(z) and P is the total power in the beam, is the same at all cross sections of the beam. The invariance of the form of the distribution is a special consequence of the presumed Gaussian distribution at z = 0. If a uniform irradiance distribution had been presumed at z = 0, the pattern at z = ∞ would have been the familiar Airy disc pattern given by a Bessel function, while the pattern at intermediate z values would have been enormously complicated.

Chapter two: Laser Beam Profile-Principles and Definitions 8

Simultaneously, as R(z) asymptotically approaches z for large z, w(z) asymptotically approaches the value

2 where z is presumed to be much larger than πw0 /λ so that the 1/e irradiance contours asymptotically approach a cone of angular radius

This value is the far-field angular radius (half-angle

divergence) of the Gaussian TEM00 beam. The vertex of the cone lies at the center of the waist, as shown in the figure below.

Growth in beam diameter as a function of distance from the beam waist. It is important to note that, for a given value of λ variations of beam diameter and divergence with distance z are functions of a single parameter, w0, the beam waist radius.

Chapter two: Laser Beam Profile-Principles and Definitions 9

2.3.2 Near-Field vs. Far-Field Divergence Unlike conventional light beams, Gaussian beams do not diverge linearly. Near the laser, the divergence angle is extremely small; far from the laser, the divergence angle approaches the asymptotic limit described above. The Raleigh range (zR), defined as the distance over which the beam radius spreads by a factor of the square-root of 2, is given by

At the beam waist (z = 0), the wavefront is planer (R(0) = ∞). Likewise, at z = ∞, the wavefront is planer (R(∞) = ∞). As the beam propagates from the waist, the wavefront curvature, therefore, must increase to a maximum and then begin to decrease, as shown in the figure below. The Raleigh range, considered to be the dividing line between near-field divergence and mid-range divergence, is the distance from the waist at which the wavefront curvature is a maximum. Far-field divergence (the number quoted in laser specifications) must be measured at a distance much greater than zR (usually >10 × zR will suffice). This is a very important distinction because calculations for spot size and other parameters in an optical train will be inaccurate if near- or mid-field divergence values are used. For a tightly focused beam, the distance from the waist (the focal point) to the far field can be a few millimeters or less. For beams coming directly from the laser, the far-field distance can be measured in meters [10].

Chapter two: Laser Beam Profile-Principles and Definitions 10

2.4 Beam Divergence All light beams spread out, or diverge, as they travel away from their sources. Lasers are the most directional light sources available, but even the beams diverge with distance. Figure 2 shows a beam diverging as it leaves a laser [11].

The lines representing the edges of the beam connect the 1/e² point along the expanding beam. The full angle beam divergence is the angle, Ө, between the 1/e² lines of the beam edge and is approximated by the following equation; Ө = (d2-d1)/(L2- L1) Where Ө is the beam divergence in radians, d1 is the beam diameter at point 1, d2 is the beam diameter at 2, L1 is the distance from the laser to point 1 and L2 is the distance from the laser to

Chapter two: Laser Beam Profile-Principles and Definitions 11 point2 The beam divergence of a laser beam is a measure for how fast the beam expands far from the beam waist. It is usually defined as the derivative of the beam radius with respect to the axial position in the far field, i.e., in a distance from the beam waist which is much larger than the Rayleigh length. This definition yields a divergence half-angle. (Sometimes, full angles are used in the literature; these are twice as large.) For a diffraction-limited Gaussian beam, the beam divergence is λ/(πw0), where λ is the wavelength (in the medium) and w0 the beam radius (radius with 1/e² intensity) at the beam waist. A large beam divergence for a given beam radius corresponds to poor beam quality. A low beam divergence can be important for applications such as pointing or free-space optical communications. Beams with very small divergence, i.e., with approximately constant beam radius over significant propagation distances, are called collimated beams. For the measurement of beam divergence, one usually measures the beam radius at different positions, using e.g. a beam profiler. It is also possible to derive the beam divergence from the complex amplitude profile of the beam in a single plane: spatial Fourier transforms deliver the distribution of transverse spatial frequencies, which are directly related to propagation angles. [2].

Chapter two: Laser Beam Profile-Principles and Definitions 12

2.5 The Beam Profiler and How Does it Work 2.5.1 Gaussian Beam Profiling The standard beam profiler is a device comprising a photo detector with a scanning pinhole, knife-edge, or slit, or a 2 dimensional array, such as a CCD (charge coupled device) camera. The laser spot is focused on the profiler and measurements taken. For the purposes of discussion we will describe the scanning slit profiler for measurement of Gaussian paraxial beams. The slit has some distinct advantages over the pinhole, knife- edge and camera methods. The scanning slit is a natural attenuator,which makes it possible to measure a fairly high-powered beam directly without the need for external attenuation. By comparison a CCD camera array can only accept a few picowatts of direct power. By using two mounted slits it is quite simple to align them to measure the x and y axes of the beam. If the profiler is rotated to have the slits pass through the long and short axes, the ellipticity of a beam can be accurately measured. It is this XY orthogonal scan that is the basis of the Photon BeamScan and NanoScan instruments. The advantage that the slit has over the knife-edge profiler is twofold. First, because the knife edge only blocks one side of the beam—it does not attenuate it—additional attenuation will be necessary to avoid detector saturation. Second, to observe a profile from a knife-edge the trace must be differentiated. The differentiated noise contained on the laser is amplified and must be filtered to arrive at a legibly smooth profile. This smoothing may remove important information about the

Chapter two: Laser Beam Profile-Principles and Definitions 13 structure of the beam, thus making the beam look better than it actually is [12]. Pinholes also attenuate the beam, but they are much more difficult to align in order to obtain a good profile. A pinhole will give better spatial detail than a slit, but only marginally so, and it must be aligned to pass through the maximum chord of the beam. This is difficult to do, particularly with a focused beam [13]. The beam width is determined by measuring the profile between the clip level points on either side of the maximum energy of the beam. Although clip levels are arbitrary, they do define the beam. By setting the desired clip level (usually 1/e² or 13.5%), the scanning slit instrument will then report a direct beam width reading. With the XY scan head, the slit method reports a beam diameter for both the x- and y-axes. Computer graphics programs can then extrapolate these data into a pseudo 3 dimensional representation of the beam profile.

Chapter two: Laser Beam Profile-Principles and Definitions 14

2.5.2 Measurement of Beam Profiles As a laser beam propagates, its width and spatial intensity distribution will change in space and time due to changes in the laser cavity and divergence, as well as interaction with optical elements. Spatial intensity distribution is one of the fundamental parameters that indicates how a laser beam will behave in an application. , material processing, fiber-optic coupling, optical data storage, , and photochemistry are some of the applications whose efficiency depends on a laser’s spatial intensity profile and beamwidth. Theory can sometimes predict the behavior of a beam, but manufacturing tolerances in , and , as well as ambient conditions affecting the laser cavity, necessitate verification. Consequently, it is crucial for researchers, system designers, and

Chapter two: Laser Beam Profile-Principles and Definitions 15 laser manufacturers to be able to accurately measure these parameters.

Defining Beam Width The boundaries of optical beams are not clearly defined and, in theory at least, extend to infinity. Consequently, the dimensions of a beam cannot be defined as easily as the dimensions of hard physical objects. The commonly used definition of beam width is the width at which the beam intensity has fallen to 1/e² (13.5%) of its peak value when measured in a plane that is orthogonal to the optical axis. This is derived from the propagation of a Gaussian beam and is appropriate for lasers operating in the fundamental TEM00 mode. Many lasers, however, exhibit a significant amount of beam structure, and applying this simple definition leads to problems. Therefore, the ISO standard specifies the beam width as the 1/e² point of the second moment of intensity, a value that is calculated from the raw intensity data and which reduces to the common definition for a Gaussian beam.

Chapter two: Laser Beam Profile-Principles and Definitions 16

Measuring Beam Width There are three main types of beam-profiling instrumentation: camera-based systems, knife-edge scanners, and slit scanners. Each has specific advantages and disadvantages. Different measurement techniques may result in slightly different results; therefore, it is critical that comparative measurements be made with the same technique.

2.5.3 Camera-Based Profilers Camera-based profilers, which use a two-dimensional array of square or rectangular pixels as the imaging element, instantly record and display the spatial and intensity information that impinges on the detector surface, providing a true three- dimensional profile of the beam. Camera-based profilers may be

Chapter two: Laser Beam Profile-Principles and Definitions 17 used with pulsed or cw lasers. Their resolution, in most cases, is limited by pixel size, restricting measurements to beams in excess of 60 μm. The μ Beam™ profiler measures beams as small as 5 μm in diameter but is limited to a maximum beam size of 50 μm or less.

2.5.4 Knife-Edge and Slit Profilers These instruments generate a profile by mechanically moving a slit aperture or knife-edge across the beam. Knife-edge profilers measure the portion of the beam that is not blocked by the knife and differentiate the result; slit profilers measure the amount of light passing through a narrow slit and integrate the result. A calculated beam profile is then displayed, based on the raw power data. In both systems, assumptions are made about the shape and uniformity of the beam being measured. These systems can measure beams as small as 0.5 μm, but they do not give information as complete as that give by camera-based systems. Three-dimensional tomographic reconstructions based on seven- blade scans are much more accurate than the three-dimensional reconstructions made by three-blade scans or seven-blade nontomographic scans.

2.6 M² Factore Definition: a parameter for quantifying the beam quality of laser beams. The M² factor, also called beam quality factor or beam propagation factor, is a common measure of the beam quality of a laser beam. According to ISO Standard 11146 [14], it is defined as the beam parameter product divided by λ / π, the latter being the

Chapter two: Laser Beam Profile-Principles and Definitions 18 beam parameter product for a diffraction-limited Gaussian beam with the same wavelength. In other words, the half-angle beam divergence is

where w0 is the beam radius at the beam waist and λ the wavelength. A laser beam is often said to be “M² times diffraction- limited”. A diffraction-limited beam has an M² factor of 1, and is a Gaussian beam. Smaller values of M² are physically not possible. A Hermite–Gaussian beam, related to a TEMnm resonator mode, has an M² factor of (2n + 1) in the x direction, and (2m + 1) in the y direction.

The M² factor of a laser beam limits the degree to which the beam can be focused for a given beam divergence angle, which is often limited by the numerical aperture of the focusing . Together with the optical power, the beam quality factor determines the brightness (more precisely, the radiance) of a laser beam. For not circularly symmetric beams, the M² factor can be different for two directions orthogonal to the beam axis and to each other. This is particularly the case for the output of diode bars, where the M² factor is fairly low for the fast axis and much higher for the slow axis. According to ISO Standard 11146 [14], the M² factor can be calculated from the measured evolution of the beam radius along the propagation direction. A number of rules have to be observed, e.g. concerning the exact definition of the beam radius and details of the fitting procedure. Alternative methods are based on

Chapter two: Laser Beam Profile-Principles and Definitions 19 wavefront sensors, e.g. Shack–Hartmann sensors, which require the characterization of the beam only in a single plane. Note that the M² factor, being a single number, cannot be considered as a complete characterization of beam quality. The actual quality of a beam for a certain application can depend on details which are not captured with such a single number. The concept of the M² factor not only allows one to quantify the beam quality with a single number, but also to predict the evolution of the beam radius with a technically very simple extension of the Gaussian beam analysis: one simply has to replace the wavelength with M² times the wavelength in all equations. This is very convenient for, e.g., designing the pump optics of diode- pumped lasers. Note, however, that this method works only when a certain definition of beam radius is used which is suitable also for non-Gaussian beam shapes. [15]. M² attachments are designed to measure the beam propagation factor. k =1/M²=λ/ ( πWoӨo ) Where λ is the wavelength of the beam, Wo is the beam waist, and Өо is the far-field divergence of the beam. If k =M =1, the beam is Gaussian. If k<1 (M²>1), the beam is not Gaussian, but all of the standard Gaussian propagating formulas may be used with appropriate modifications. To calculate the propagation factor, M² meters first focus the beam and then measure the beam diameter both at the focus and at multiple points on either side of the focal point. A hyperbolic curve is fitted to the data points and various parameters are calculated including M², far-field divergence, beam waist, and beam waist location. The higher number of data points, the greater the accuracy [16].

Chapter two: Laser Beam Profile-Principles and Definitions 20

2.7 Laser Beam Profile Analysis System The block diagram of the laser Beam profile analysis system is shown in Fig. 3. Figure 4 shows the photographic image of the system, which consist of: 2.7.1 The Laser Source There are two laser sources are used in this paper, a He-Ne laser of wavelength 632.8 nm and 1 mW power. A semiconductor laser of the type GaAs of wavelength 635.9 nm and power of 5 mW was used for the same analysis. The beam splitter, which is a glass coating plate, reflects half of laser beam power to the radiometer (power meter) and transmits the second half to the beam optics controller.

2.7.2 Beam Splitters Definition: devices for splitting a laser beam into two or more beams. A beam splitter (or beam splitter, power splitter) is an optical device which can split an incident light beam (e.g. a laser beam) into two or more beams, which may or may not have the same optical power. Different types of beam splitters exist, as described in the following, and are used for very different purposes. For example, beam splitters are required for interferometers, autocorrelators, cameras, projectors and laser systems.

Chapter two: Laser Beam Profile-Principles and Definitions 21

Types of Beam Splitters Dielectric Mirrors

Figure 1: A partially reflecting , used as a beam splitter.

Any partially reflecting mirror can be used for splitting light beams. In laser technology, dielectric mirrors are often used for such purposes. The angle of incidence, also determining the angular separation of the output beams, may be 45° (as in Figure 1), which is often convenient, but it can also have other values, and influences the characteristics of the beam splitter. A wide range of power splitting ratios can be achieved via different designs of the dielectric coating. In general, the reflectivity of a dichroic mirror depends on the state of the beam. Such a device can be optimized to function as a thin film , where in some wavelength range a beam with a certain polarization can be nearly totally reflected, while a beam with different polarization is largely transmitted. On the other hand, it is also possible to optimize for a minimized polarization dependence to obtain a non-polarizing beam splitter. This is most easily achieved for near normal incidence. Dielectric beam splitters can also have a strongly wavelength-dependent reflectivity. This can be used for dichroic beam splitters (dichroic mirrors), which can separate spectral components of a beam. For example, such a device may be used after a frequency doubler for separating the

Chapter two: Laser Beam Profile-Principles and Definitions 22 harmonic beam from residual pump light. The separation may occur based on the difference in wavelength or polarization.

Beam Splitter Cubes

Figure 2: A beam splitter cube, which may be polarizing or non- polarizing.

Many beam splitters have the form of a cube, where the beam separation occurs at an interface within the cube (Figure 2). Such a cube is often made of two triangular glass prisms which are glued together with some transparent resin or cement. The thickness of that layer can be used to adjust the power splitting ratio for a given wavelength. Instead of glass, crystalline media can be used, which can be birefringent. This allows the construction of various types of polarizing beam splitter cubes such as Wollaston prisms and Nomarski prisms, where the two output beams emerge from the same face, and the angle between these beams is typically between 15° and 45°, i.e., much smaller than shown in Figure 2. Other types are the Glan–Thompson prism, and the Nicol prism, the latter having a rhombohedral form (i.e., not that of a cube). It is also possible to use a multilayer coating within a cube. This further expands the possible device characteristics, e.g. in terms of operation bandwidth or polarizing properties.Beam splitter cubes can be used not only for simple light beams, but also for beams carrying images, e.g. in various types of cameras and projectors.

Chapter two: Laser Beam Profile-Principles and Definitions 23

Fiber-optic Beam Splitters

Figure 3: A fiber-optic beam splitter with a single input port and two output ports. Various types of fiber couplers can be used as fiber-optic beam splitters. Such a device can be made by fusion-combining fibers, and may have two or more output ports. As for bulk devices, the splitting ratio may or may not strongly depend on the wavelength and polarization of the input. Fiber-optic splitters are required for fiber-optic interferometers, as used e.g. for optical coherence . Splitters with many outputs are required for the distribution of data from a single source to many subscribers in a fiber-optic network, e.g. for cable-TV [17]. The power meter was used to measure the power of the laser beam incident in the CCD camera. The type of this power meter is (Metro-logic Radiometer), ranging from 0.3 μW to 30 mW. The image of laser spot was appeared on a transmission screen. A ground glass was used a transmission screen ground glass had transmission of 50% at visible wavelength. In order to attenuate the incident because of the CCD camera in the short distance application a special type of disk attenuation was used. It consists of a coating glass of a different attenuation factor. The attenuation factor was calibrated and registered on the attenuate disk.

Chapter two: Laser Beam Profile-Principles and Definitions 24

2.7.3 Variable Attenuators for High Power Lasers Diffractive attenuators control the power of laser radiation using diffraction gratings. Since phase diffraction gratings do not absorb light they can be used for high-power laser radiation (cw, pulsed). Variation of grating parameters along the substrate results in variable transmission at a given wavelength. Diffractive attenuators can be designed for wavelength from deep UV till IR for laser powers up to 250W/cm2 in CW and 1000mJ/cm2 in 10ns pulses.

Principle of operation Light beam passing through a diffractive grating is partially deflected into several diffractive orders, which get subsequently blocked by diaphragm. Thus the zero order beam (does not change the direction of propagation) is attenuated. The intensity of output radiation Ioutput depends on the grating design and wavelength of input beam. By changing the grating parameters one can control the power of output radiation1. The transmission coefficient of such an attenuator is h = Ioutput/Iinput ..[18].

Chapter two: Laser Beam Profile-Principles and Definitions 25

Design of diffractive attenuator on the base of circular diffractive grating.

2.7.4 Beam Optics Controller It consist of a computerized motor controller giving rise to control the horizontal movement in X direction of the collecting optics lowered the laser source. The other controllers are responsible for scanning the laser in Y direction via a reflecting mirror.

Chapter two: Laser Beam Profile-Principles and Definitions 26

2.7.5 Laser Image Analysis CCD camera technique The CCD camera technique is simple: attenuate and shine a laser onto a CCD and measure the beam profile directly. It is for this reason that the camera technique is the most popular method for laser beam profiling. The most popular cameras used are silicon CCDs that have sensor diameters that range up to 25 mm (1 inch) and pixel sizes down to a few micrometres. These cameras are also sensitive to a broad range of , from deep UV, 200 nm, to near , 1100 nm; this range of wavelengths encompass a broad range of laser gain media.

The use of CCD Camera For low repetition rate pulsed beams, it is usually necessary to use a 2-dimensional array to profile the beam. For single pulsed lasers, the short duration of the beam precludes the use of the slit to profile it. A CCD camera can be synchronized to “catch” the pulse. It will give a true 2D image of the beam. All the same clip level parameters can be used with CCDs to give good value for the peak diameter. The advantages of the CCD camera technique are:  It captures the 2D beam profile in real-time  Sensitive CCD detectors can capture the beam profiles of weak lasers  Resolution down to about 5 μm  CCD cameras with trigger inputs can be used to capture beam profiles of low-duty-cycle pulsed lasers

Chapter two: Laser Beam Profile-Principles and Definitions 27

 CCD’s have broad wavelength sensitivities from 200 to 1100nm The disadvantages of the CCD camera technique are that attenuation is required for high power lasers, and CCD sensor size limited to about 1 inch [2]. The principle disadvantage of the CCD camera approach is that the direct power that the CCD can accept without saturation is very small. This means that one will necessarily have to use external attenuators to control the amount of light hitting the array. In addition, the pixels on the array determine the precision of the measurement. The best CCDs available for profiling have pixels that are 5-10μm in diameter. For very small beam diameters, fine detail may be invisible. CCD based profilers are fine for beams that are greater than 100μm in diameter. Although silicon CCDs are quite inexpensive, their spectral response is confined to the visible range. For the “telecom wavelengths” between 1300-1700nm, it is necessary to use In GaAs arrays, and these are very costly and still have fairly large pixel sizes (>30μm), which limit their spatial resolution. Longer wavelengths require even more costly approaches to measure them with arrays [6]. The CCD camera output images of the laser spot are recorded at different attenuation factor using He-Ne and semiconductor laser are shown in Fig. 5 and 6, respectively. The two-figures verify the common fact that as the attenuation increases the laser spot becomes poorer.

Chapter two: Laser Beam Profile-Principles and Definitions 28

Chapter two: Laser Beam Profile-Principles and Definitions 29

Chapter two: Laser Beam Profile-Principles and Definitions 30

The spot images of He-Ne are shown in Fig. 7 for different distances. When the distance increases, the cross-section of laser spot increases and the laser power decreases.This relation is shown in Fig. 8 and 9 for He-Ne and semiconductor laser, respectively.

Chapter two: Laser Beam Profile-Principles and Definitions 31

Chapter two: Laser Beam Profile-Principles and Definitions 32

Figure 10 shows the theoretical result of the intensity profile for the Gaussian He-Ne Laser Beam and Fig. 11 shows the experimental result of the intensity profile of a Gaussian He-Ne Laser Beam.

Chapter two: Laser Beam Profile-Principles and Definitions 33

With the help of the above results, we could study the intensity distribution of the laser beam, the laser energy, number of mode and the effect of laser alignment. The traditional methods of measuring the laser beam intensity profile such as burn spots, mode burns and viewing of the reflected beam do not provide sufficient information to enable a scientist to achieve optimum laser performance. On the other hand the availability of computerized the laser beam profile analysis system has enabled scientists and engineers to tune lasers to the higher standards. The important property of the computerized laser beam profile analysis system is the ability to provide illuminating beam profile displays. These display both 2D and 3D beam views often provide sufficient intuition to enable the laser operator to make significant improvement to the laser beam in a very short time. The benefit of pseudo contour is to enable an intuitive visualization of laser beam intensity distribution. In these contours the background signal level corresponds to white colors and colors from blue (lowest) to red (highest) display the intensities of laser beam. It can be clearly noted the cross section of He-Ne laser is a circular, whereas it’s elliptical in diode laser. This is because the divergence of He-Ne laser beam is symmetric, but in the case of diode laser, the divergence in the X direction is different from that in the Y direction. Thus because the cavity of diode laser is rectangular and its circular in the He- Ne laser. This result shows the capability of our system in analyzing the laser sources.

Chapter two: Laser Beam Profile-Principles and Definitions 34

CONCLUSION The cost of building the proposed system in this paper is very cheap. From the work we can conclude, that the image processing to the laser beam profile and its energy distribution can be computerized in a very short time, the developed technique is compact and accurate in measuring the laser beam characteristics instead of using the traditional system. The proposed method of analysis provides a better view of the laser beam pattern showing even minor deviation in the uniformity. And provide accurate quantitative analysis that helps to reduce operator error and misalignment. The accurate checking or the beam quality (controlling the uniformity of the top hat and determination of the laser beam energy) is very important in medical applications especially in surgical operations. The image processing of the laser beam is related to the power source, can be used in determination the distance of the objects.

References 35

REFERENCES [1] Heard, H.G., 1968. Laser Parameter Measurements Handbook. John Wiley and sons Inc. [2] E:\ - Wikipedia, the free encyclopedia.mht. [3] Fleischer, J.M. and D. Doggett, 1989. Spectral Profiling with a single photodiode. Laser and Optronics , pp: 47-52. [4] Talmi, Y., 1979. TV-Type Multichannel Detector. Anal. Chem., 47: 7. [5] Hull, D.M . And A. Stewart, 1985. Laser beam profiles principles and definitions. Laser and Application, pp: 75. [6] By Allen Cary- photon Profiling Inc. [7] Chen, M.L., 1999. Spatial and Temporal Properties of Optical Radiation Produced by Stepped Ladders. Geophysical Research Atmospheres, 104: 27573. [8] Thattey, S.S., 1998. Digital Image Processing. 9: 4. [9] Dickey, F.M.1996. Gaussian Laser Beam Profile Shaping. Optical Eng., 35: 3285-3295. [10] E:\laser 1\CVI Melles Griot Optics Guide - Beam Waist and Divergence.mht. [11] Suzaki, Y., 1977. Measurement of the Gaussian laser beam divergence. Appl. Optics, 16: 1481- 1482. [12] Fleischer, J.M., “Beam Profiling Monitors Laser Health” Industrial Laser Solutions, September 1999. [13] Ibid. [14] ISO Standard 11146, “Lasers and laser-related equipment – Test methods for laser beam widths, divergence angles and References 36 beam propagation ratios” (2005). [15] E:\ccd\Encyclopedia of Laser Physics and Technology - M² factor, laser beam, quality factor, beam divergence, caustic, ISO Standard 11146.mht. [16] www.mellesgriot.com / Measurement of Beam Profiles. [17] E:\ccd\Encyclopedia of Laser Physics and Technology - beam splitters, power splitter, beam splitter, thin film polarizer, non-polarizing.mht. [18] E:\laser 1\Laser Beam Attenuator.mht.