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White Paper: Modifying Laser Beams – No Way Around It, So Here's White Paper: Modifying Laser Beams – No Way Around It, So Here’s How By John McCauley, Product Specialist, Ophir Photonics There are many applications for lasers in the world today with even more on the horizon. In order to apply a laser to an industrial process, medical therapy or communications technology, it is usually necessary to modify the laser beam to achieve the desired results. Laser beam measurement tools make this possible. In this article we will discuss how to match an appropriate laser beam profiling technique to a laser application. The topics will include building an aligned optical system, collimation and focusing, high power laser quality measurements, and proper attenuation techniques for making accurate laser measurements. Building an Aligned Optical System Many laser applications are based on a system that includes a laser with associated optics to deliver a laser beam of known size and position to do something. Examples of these are as diverse as laser printers and marking machines, laser range finder, LIDAR or military targeting systems, and laser projection systems for movie theatres. What all of these have in common is the need to put a beam of a certain size at a certain place in space. Using a beam profiler allows this to be done quickly and efficiently — simply place the profiler where the beam is to be aimed and measure the size and position. These parameters can be adjusted with direct feedback from the profiler. Beam profilers to do this can be either based on CCD or CMOS array or scanning slit technology. Each has its advantages. Arrays will provide a true two dimensional image of the beam and show any irregularities in power distribution; scanning slits will provide an XY profile, but can measure much smaller beams, as small as 10µm, and will require little or no attenuation or adjustment to the profiler itself. In addition, the scan plane of the slit profiler is very well known, allowing the z-location of a beam waist to be located very accurately. Both camera and slit profilers can determine the x-y position of the beam in space to less than a µm. Ophir Photonics Group 3050 North 300 West North Logan, UT 84341 Tel: 435-753-3729 www.ophiropt.com/photonics Fig 1. Scanning slit and array profilers. For applications where pointing accuracy is very important it is necessary to isolate linear and angular pointing error and also to eliminate external influences on the measurement. Pointing accuracy is dependent on the profiler, the fixturing of the laser system and external influences. Localized heating, air currents, vibration all contribute to uncertainty in pointing measurements. One way to isolate external influences is to shrink the measurement to as short a distance as possible. Rather than measuring pointing over a distance of 10s of meters, using a very accurate profiler on the bench top can deliver better results. By using scan averaging with a NanoScan slit scanning profiler it is possible to measure pointing accuracy to submicron precision, allowing a range finder to be aligned in a very small test fixture. Isolating linear from angular pointing error can be done with a two profiler method. By splitting the beam and looking at the near-field and far-field through an f-theta lens these motions can be separated. Angular motion will show up as a shift in the near-field measurement, but will not be seen in the far-field due to the f-theta correction. Linear motion, on the other hand will be seen in the far-field as a shift in position on the far-field profiler. Ophir Photonics Group 3050 North 300 West North Logan, UT 84341 Tel: 435-753-3729 www.ophiropt.com/photonics Fig 2. Angular vs linear pointing measurement set up. This may be more than most applications need, and simply placing the profiler where the beam is to be delivered is usually enough. Collimation and Focusing Other applications often require that a laser either be focused to a particular beam waist or collimated to a minimally diverging or converging beam. Focusing can be a fairly simple process for low power beams, such as those used in fiber optic telecommunication or point-of-sale scanners. It is much more complicated for higher power applications like ophthalmic surgery, medical therapeutics or precision welding. For a low power application, one can simply place the profiler at the focus point and adjust the optics until the desired beam size if obtained. This is routinely done with passive optical components for free space coupling of fiber. Fig 3. Focusing fiber optic components. Measuring a higher power focused beam can be much more difficult because the power density of the laser increases geometrically as beam size shrinks. Although the slit scanner can often handle direct measurement of fairly high Ophir Photonics Group 3050 North 300 West North Logan, UT 84341 Tel: 435-753-3729 www.ophiropt.com/photonics power beams, often the power density can exceed the damage threshold for the slit material. Camera measurement of focused beams is always a bit more complicated, because arrays always need attenuation to measure lasers without saturating the signal. With a focused beam it is usually impossible to get the focused beam to the array through attenuators without distortion. Fig 4. Focused beam has different path lengths through attenuator. The attenuation in Fig. 4 is much simpler than is normally used and it would still cause distortion. The solution to this problem is to image the beam waist with a lens, with the attenuation added to the image. Fig 5. Focused spot profiler. Ophir Photonics Group 3050 North 300 West North Logan, UT 84341 Tel: 435-753-3729 www.ophiropt.com/photonics This method can also be used with a scanning slit profiler to reduce the power density of the beam, again geometrically. For example if one has a 15W laser beam that is focused to 10µm, the power density would exceed the damage threshold of the slit material. However, by expanding the beam 10X, the power density will be reduced by 100-fold, bringing it back to the safe level. Fig 6. Beam expander with slit profiler. Collimation Collimation is the opposite of focusing, that is, it is the process of making the laser have the same diameter for as long a distance as possible. Laser applications that need this type of adjustment include laser point-of-sale scanners, LIDAR and rangefinders, guide stars and free-space optical communications; basically anything that needs a beam that maintains a uniform beam size over a long distance. Another application is the collimation of a large beam that will then be focused to a small diameter. There are actually two types of collimation; one can be described as the minimum divergence, and the other as the maximum distance to the waist. When collimating for minimum divergence, the following formula defines the divergence angle, θ = Dƒ / ƒ, using a lens of known focal length. By placing a beam profiler at the geometric focus of the lens, adjusting the collimation to produce the minimum spot size at this point achieves the best collimation. This type of collimation is important for situations where the mathematically accurate value for θ is important, such as when creating a collimated beam to be refocused to a specific size and Rayleigh range. In many cases, simply adjusting for the maximum distance to the waist, or focusing to infinity, is sufficient to provide the performance required. Ophir Photonics Group 3050 North 300 West North Logan, UT 84341 Tel: 435-753-3729 www.ophiropt.com/photonics Fig. 7. Lens collimation method. It is also possible to use the above divergence measurement technique to achieve the divergence that will provide the proper distance-to-the-waist desired. There is a more thorough discussion of collimation of Gaussian beams in Donald O’Shea’s Elements of Modern Optical Design.1 High Power Beam Quality Measurements High power lasers are used for many industrial applications, particularly high precision welding, brazing and cutting operations. The laser is well suited to these operations and can be a rapid and efficient method, but only if the laser is operation properly and delivering the correct beam shape and power to the work piece. In order to guarantee this, there are several things that can and should be measured periodically. • Output power or energy • Spatial or Beam Profile • Waist position • Temporal profile (for pulsed beams) All of these beam quality parameters can affect the performance of the laser in accomplishing its assigned task. Measurements should be made in order to establish the criteria of a laser process to ensure stable, reliable and consistent performance and to monitor this performance over time. Lasers and optical delivery systems degrade over time, and monitoring the laser criteria can help to prevent waste and rework if the laser performance no longer meets the requirements of the process. Measurements should be made whenever one is moving or expanding a process to another system, for batch validation and for laser maintenance and troubleshooting. Many 1 O’Shea, Donald C., Elements of Modern Optical Design, John Wiley and Sons, 1985, pp. 241-5 Ophir Photonics Group 3050 North 300 West North Logan, UT 84341 Tel: 435-753-3729 www.ophiropt.com/photonics organizations operating under ISO 9001, GMP or GLP protocols will mandate routine periodic measurement to ensure consistent and traceable results. The technologies available for laser measurement include power and energy meters, which indicate the total power or energy being delivered by the laser system; beam profilers, which show the shape and size of the beam; and fast photodiodes to measure the pulse shape of pulsed laser beams.
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