Fundamental Optics Material Properties <strong>Optical</strong> Specifications Gaussian Beam Optics 0.8-mm-diameter input beam. In order to achieve the spot size wanted, the beam must first be expanded by a factor of 10 before it is focused. The 10# expander described in the previous example could perform the task, as could any of the standard 10# expanders offered by Melles Griot. For focusing, we now have an 8-mm-diameter beam going into the 100-mm-focal-length lens, so we are operating at f/12.5. At this f-number we can probably use a plano-convex lens, but it is a good idea to check the spherical aberration to make sure. 0.067 × 100 spot size (spherical aberration) = = 3 mm. 3 12.5 The plano-convex lens, oriented with its convex surface toward the beam expander, will provide diffraction-limited performance in this case. Although the effects of manufacturing tolerances should always be taken into account when choosing a standard catalog lens, they are not significant for the input lens of this beam expander because the aperture is so small. With a diameter of 1 mm or less, virtually any of the lenses in this catalog introduce only a fraction of a wave of wavefront distortion as a result of manufacturing errors. However, with a larger beam, lens quality is a consideration. One of the precision-grade lenses, in this case the 01 LLP 017, should be used for this precision application. Example 3: Collimate a diode laser Collect and collimate the output of a diode laser to a 25-mmdiameter diffraction-limited beam. The output wavelength is 780 nm and has a full-angle divergence of 60°!20° (see figure 2.15). The first step is to determine the numerical aperture needed to collect all the light from a source with a 60-degree divergence angle. Since numerical aperture is defined to be the sine of the half angle of divergence, NA = sin 30º = 0.5. Stated in terms of f-number, 1/(2 NA), this is f/1. At this low f-number we can immediately rule out virtually any simple lens or achromat; even if a simple lens were available at this low f-number, it would not provide the performance level required. The best choice would be a highly corrected, multielement diode laser collimating lens, such as the 06 GLC 002, which has a numerical aperture of 0.5. The 06 GLC 002 yields a collimated elliptical beam with dimensions of 8 mm ! 2.7 mm. The smaller dimension of this beam must be expanded to match the larger dimension; otherwise, it will have a larger beam divergence because of diffraction. Since there is approximately a 3:1 ratio in the two dimensions, we will use a 3# anamorphic prism pair, 06 GPA 004, to accomplish the expansion. This will now yield a collimated beam 8 mm in diameter. The next step is to expand the beam by a factor of 3.125#in order to get to the desired 25-mm beam diameter. Since no constraint has been given on the length of our optical system, we’ll play it safe and operate our beam expander at a minimum of f/10. This virtually ensures diffraction-limited performance, even with singlets. At f/10 and an 8-mm-diameter input beam, we would need a focal length of 80 mm for the input lens of our collimator. Since we are looking for diffraction-limited performance, our best choice would be one of the precision diode laser singlets (06 LXP series). Once again, we choose a high-precision lens because our beam has a fairly large diameter and the effects of manufacturing tolerances must be considered. The closest focal length we have in this series of lenses is the 06 LXP 009 with a focal length of 110 mm. Operating at f/13.75, we will have diffraction-limited performance, which can be verified by using the formula for spherical aberration. We now need a collimating lens with a focal length of 3.125 ! 110 mm = 344 mm. The best choice is probably the 01 LAO 277 because there is no precision singlet lens with the necessary focal length. The achromat is also manufactured to tighter tolerances. The final system would then consist of the 06 GLC 002 mated directly to the 06 GPA 004, followed by the 06 LXP 009 with its curved surface facing toward the diode laser. The spacing between the 06 LXP 009 and 06 GPA 004 is not critical. Finally, the 01 LAO 277 would follow, spaced approximately 455 mm from the singlet, with its flint surface facing toward the diode laser. Since the standard coating supplied with the 01 LAO series achromats does not perform very well at 780 nm, this lens should be specified with a /076 coating, which is optimized for performance at 780 nm. 06 GPA 004 06 LXP 009 01 LAO 277 06 GLC 002 <strong>Optical</strong> <strong>Coatings</strong> Figure 2.15 1.1 mm 455 mm Melles Griot diode laser components, showing how they may be used in relation to each other 2.12 1 Visit Us OnLine! www.mellesgriot.com
<strong>Optical</strong> Specifications 3 Wavefront Distortion 3.2 Centration 3.3 Modulation Transfer Function 3.4 Cosmetic Surface Quality – U.S. Military Specifications 3.6 Surface Accuracy 3.8 3.1 1 Fundamental Optics Gaussian Beam Optics <strong>Optical</strong> Specifications Material Properties <strong>Optical</strong> <strong>Coatings</strong>
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