3 TESTING OF OPTICAL COMPONENTS

Transcription

1 3 TESTING OF OPTICAL COMPONENTS Measure Optical Figure of Merit, Distortion and Wavefront Quality of Optical Components MODEL OEK-1 00 PROJECT #2

2 r Introduction There are many applications in which one wants to measure the optical quality (flatness, thickness, or mat rial uniformity, curvature properties etc.) of optical components. A very precise technique to perform this measurement is to examine the phase variations across the object as sensed by a light beam. By combining a reference beam of known wavefront with a beam that traversed through or was reflected off the object under study, the resulting interferometer's interference pattern contains all the desired information as to the optical quality of the object under study. Precise measurements to accuracies of a small fraction of a wavelength of light may be achieved. Early applications of optical testing are due to modifications in Mach-Zehnder interferometers due to Twyman-Green and are portrayed in Figure 12. In this form, it is possible to test the homogeneity of glass, flat sided optical components such as mirrors and prisms, and with the appropriate lens or curved mirrors, one can test lens and lens sets. In this experiment, the Twyman-Green configuration will be used to examine various optical components and density fluctuations in transparent media. Accuracies on the order of a fraction of a fringe can be expected from these experiments. ~----'~ MS M2 rt f71.:"'-"' I I MI Figure 1 J ~ - ====~ ~ ~

3 31 L ~ MS BE M2 Figure Equipment list Part Number Description aty Ball Driver Set 1 SK-08A Screw Kit 1 SK-25A Screw Kit 1 RG 'x3' Breadboard 1 BE. Beam Expander Assembly B-2SA Base Plate 1 LC-V Collimator Module 1 M-40X Objective Lens 1 MH-2PM Objective Mount 1 SP-3 3" Post 1 SP-4 4" Post 1 VPH-3 3" Post Holder 1 VPH-4 4" Post Holder 1

4 32 BS. Beamsplitter Assembly 20B20BS.1 2" Beamsplitter 1 U200-A2K Mirror Mount 1 SP-3 3" Post 1 VPH-3 3" Post Holder 1 C1. Lens Assembly AC-1A Lens Mount 2 KPX085ER.1 Lens mm 1 KBX055 Lens mm 1 MSP-3 3" Post 2 MPH-3 3" Post Holder 2 MCF Flat Carrier 2 MRL-6M 6" Micro Rail 1 CT. Collimation Tester Assembly 200S20 2" Collimation Tester 1 AC-2A Lens Mount 1 B-2SA Base Plate 1 SP-3 3" Post 1 VPH-3 3" Post Holder 1 I. Iris Assembly Iris 2 MCF Flat Carrier 2 MH-2P Iris Mount 2 MSP-3 3" Post 2 MPH-3 3" Post Holder 2 MRL-3 Micro Optical Rail 1 MRL-18M Micro Optical Rail 1 L. Laser Assembly 340-RC Clamp 1 40 Rod 1 ULM-TILT Laser Mount 1 R mW HeNe Laser 1 ======~ _ =------_.----

5 33 MS. Steering Mirror Assembly 10D20ER. 1 1" Mirror 1 COR-1 Cntr Of Rotatn Adaptr 1 P100-P Mirror Mount 1 UPA1 1" Mirror Holder 1 SP-3 3" Post 1 VPH-3 3" Post Holder 1 M1 and M2 Mirror Assemblies 20D20ER.1 2" Mirror 2 U200-A2K Mirror Mount 2 SP-3 3" Post 2 VPH-3 3" Post Holder 2 P. Prism Assembly 10BR08 BK7 Prism 1 V100-P2 Platform Mount 1 PT-1 Prism Table 1 SP-3 3" Post 1 VPH-3 3" Post Holder 1 Screen Assembly B-2SA Base Plate 1 BC-5 Base Clamp 1 FC-1 Filter Clamp 1 SP-2 2" Post 1 VPH-2 2" Post Holder Setup Placement of the Breadboard Place the RG-23-4 breadboard on a flat stable surface. Make sure that there is enough surface area near the breadboard to place the power supply units and other items that need not be mounted Laser Setup Mount a 40 Rod on the RG-23-4 breadboard in location L as in Figure 13. Attach a ULM-TILT Laser Mount to a 340-RC Clamp. Slide the 340-RC onto the 40 Rod. Mount the R laser head in the ULM-TILT mount and align

6 34 the laser tube so that the polariz tion plane is perpendicular to the table top ("S" polarization) Laser Beam Alignment Post mount the Iris Assembly I on the MRL-3 Rail. Tum on the laser, point the beam along the long side of the breadboard and adjust the laser height to 6 inches. Place the iris directly in front of the laser head (position I, in Figure 13) with its aperture aligned with the laser beam. Move the iris to the other end of the breadboard (position lz in Figure 13) and adjust tilt and veltical position of the laser on the post to align the beam with the iris aperture. Move the Iris back and forth between positions II and 12 to ensure that the beam is parallel to the surface of the breadboard. Once the tilt of the laser is set the height can be varied by the 340-RC clamp and the beam will still be parallel to the surface of the breadboard Iris Placement Affix ID-0.5 iris I in front of the laser as shown in Figure 14 and adjust the aperture to just allow the laser beam through. The iris will now be used as a reference for retroreflected beams Interferometer Setup Choose one of the setup configurations, Figure 11 or Figure 12 (Figure 12 is an alternative for the setup of Figure 11 which increases the cross section of the optical windows). Place the 20020ER.l 2" diameter mirrors and the 20B20BS.l beam splitter into the U200-A2K mounts and post mount each in place as shown in Figure 12 or Figure 15, to construct the Twyman-Green Interferometer. Use set screws on the SP-3 posts to connect to U200-A2K mirror mounts. Post mount each interferometer mirror 10" from the beam splitter Interferometer Alignment Center the beam on BS optic and on MI by adjusting their post heights. Check the beam height in front of mirror MI. Ifbeam height is not the same before and after the beamsplitter, adjust the tilt of the beam splitter until the beam is horizontal. Place the iris assembly I in front of mirror M2, match the height of the beam by adjusting the beamsplitter and MI respectively Beam Expander Positioning Assemble the beam space expander assembly BE and mount in the path of the laser beam. Attach the SP-3 post to the B-2SA base and mount the LC-V collimating lens directly onto the B-2SA base. Place the VPH-3 post holder on the breadboard so that when the LC-V is put in place there will be some room left to mount the MAOX objective lens. Mount the M-40X objective lens directly behind the LC-V. Tum on the laser and adjust the height of the LC-V until the beam is centered on the lens. Insert the M-40X objective lens in its place and align so that the expanding beam is centered on the collimating lens of the LC-V

7 35 Figure 13 r-ti_~-~ MS L Figure 14 Figure 15

8 Collimation Calibration Place the collimation tester (model No 20QS20) in an A -2A optics mount (use proper support stud tips in the AC-2A). NOTE The collimation tester is a wedged plate with its thicker side marked on the edge. It is desirable to have the thick edge of the plate pointing to the top of the AC-2A. Place the Collimation Tester Assembly CT at a 45 angle in the path of the expanded beam and look for fringes in the reflection. Adjust the position of the collimating lens in the beam expander until horizontal fringes are observed in the reflection. There should be three to five fringes visible in the reflection when fringes are horizontal. At this stage the expanded beam is well collimated. 3.4 Procedure and results Laser C~J-~~MS Two Lens System w:_) - M21<O To Screen Single Lens and Mirror Ml Figure 16 Fringes from the 90 corner of prism Figure 17

9 Prism CAUTION Handle the optics with care. Do not place finger on the smooth transmissive surface of the prism. If the surface gets dusty, use acetone and soft tissue paper to wipe surface, refer to full cleaning instructions in the Newport Catalog. 1. Geometrical integrity of the prism faces- Attach the PT-I Prism Table on a VI 00-P2 Platfonn Mount and post mount with a SP-3 post. 2. Mount the lobr08 prism provided onto the PT-l Prism Table and replace the mirror M2 with the prism assembly. 3. Adjust the tilt of the VIOO-P2 until fringes are observed. 4. Steer the front face reflection away from the beamsplitter so that there will be no interference due to reflection off that surface. The Fresnel reflection off the front face will overlap the fringes due to retroreflection off the back. 5. Adjust the tilting of the I OBR08 prism to display V shaped fringes. When the fringes are vertical, the tilt of the prism can be adjusted to get parallel and equal number of fringes on each side of the center line. Adjust the tilt of mirror Ml to obtain the chevrons (V). The V shaped fringes should touch at the end (Figure 17). 6. Front face flatness- Realign the I OBR08 prism so that the front face is now reflecting back directly onto the beamsplitte r. Cover half of the prism face with a 3 "XS" card or equivalent to eliminate the reflections off the back faces of the prism (only 4% will reflect off the back with this arrangement that cannot be easily eliminated). The fringes from the corner and the surface wili overlap. Arrange the tilt so that the fringes off the front face dominate the other fringes in intensity The fringes we seek are unifonn over the entire surface and are not split like the ones off the wedge. These fringes can be made to dominate by adjusting the tilt of the prism and the first mirror. Density Fluctuations in a Plastic Slab I. Remove 10BR08 prism and replace with 2" mirror to make a Twyman Green Interferometer. 2. Place plastic ruler or a sheet of plastic (i.e. plastic used to wrap food products) halfway in the path of the beam and compare the fringes due the two different paths. 3. The plastic can be bent, twisted, stretched, or squeezed to cause various fringe contortions. The amount of pressure or torsion can be evaluated be observing the pattern of fringes and the spacing.

10 Lenses 1. Single converging lens- Arrange the interferometer as a Twyman-Green. 2. Place lens in test arm of the interferometer and adjust distance from mirror so that the image is focused on the mirror. 3. Make fine alignment of the mirror and lens so that circular fringes are observed on the screen. 4. The optical figure of merit of the lens can be assessed by observing the fringe pattern and the relative thickness of the fringes. 5. Two-lens system- Replace the second mirror with the convex mirror (part No. KPX085ER.l) and align with the beam. 6. Move the bi-convex lens toward the convex mirror and adjust the heights until fringes are obtained. 7. The distance between the two lenses should be of the order of 1 3/8". 3.5 References [3.1] P. Hariharan, Opticalluterferometry, Academic Press, Sydney (1985). [3.2] P. Hariharan, Basics ofinterferometry, Academic Press, San Diego (1992). [3.3] F. A. Jenkins and H. E. White, Fundamentals ofoptics, McGraw Hill, New York (1976). [3.4] E. Hecht, Optics, Addison-Wesley, Reading MA (1987) ~ -~