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March 2017 Newsletter

Upcoming Conferences and Meetings with papers from OPG

OSA Optical Design and Fabrication Congress, Optical Fabrication and Testing

July 9-13, 2017, Denver Marriott City Center

“Computer Generated Holograms as Fixtures for Testing Optical Elements”, Robert E. Parks

DESCRIPTION:

It is common to think of computer generated holograms (CGH) as artifacts for testing aspheres but they can also be used as general calibration artifacts and fixtures for the alignment and test of other more conventional optics.

We show how simple Fresnel zone patterns can be created to simulate centers of curvature or axes in space with dimensional precision associated with microlithography. These centers of curvature and axes can then be located in space to similar sorts of precision with a Point Source Microscope (PSM).

Once the PSM is centered on the center of curvature, or axis, of the Fresnel zone pattern, a ball, or cylinder, respectively, of matching radius can be aligned to the PSM to similar sorts of precision and physically attached to the CGH to serve as kinematic datums against or on which to mount other optical or mechanical components. We give an example of the fixture for the mounting of a rectangular meniscus lens element into a kinematically located mount so the two can be cemented together prior to inserting the bonded pair into an optical assembly.


SPIE Optics and Photonics, Optical System Alignment, Tolerancing, and Verification XI (Conference 10377)

August 6, 2017,

“Optical alignment using a CGH and an autostigmatic microscope”, Robert. E. Parks

DESCRIPTION: 

It is often necessary to locate an optical or mechanical component precisely relative to another as in cementing a doublet or in aligning an off-axis parabola to a mount. We show that one way of making a fixture to do this is to make a computer generated hologram (CGH) that will produce foci or axes in space when illuminated with a point source of quasi-monochromatic light. An ideal instrument to use is an autostigmatic microscope (ASM) because it serves as a means of illuminating the CGH with a point source as well as viewing the reflected focused spot or line.

Once the reflected stigmatic image is located with um precision, the center of curvature of an optical surface or ball, or focus of an off-axis parabola, can be aligned to the image, again using the ASM with the same sort of precision.

We use this scheme to show how one would go about cementing a double or aligning an off-axis parabola to a mount using only the custom CGH and an ASM. It will become obvious from these examples that the same methodology can be used to create fixtures for a variety of alignment cases such as aligning balls to a CGH used for null testing to see that the CGH is precisely and repeatably located relative to a transmission sphere. The method makes full use of the flexible nature of patterning CGHs and of kinematic principles.


SPIE Optifab

SPIE Optifab

October 16-19, 2017,
Rochester Riverside Convention Center, Rochester, New York, USA

“Centering Steep Aspheric Surfaces”, Robert. E. Parks

DESCRIPTION: 

Finding the optical axis of an aspheric surface is an essential part of making an aspheric lens because the center of curvature, or optical axis, of the second side must lie on, or be coincident with, respectively, the optical axis of the first side for maximum optical performance. Looking at the center of an aspheric surface and measuring the tilt and coma as a function of decenter is an obvious means of determining centration, but many aspheric surfaces are relatively spherical over the part of the lens aperture that can be viewed with commercially available optics and there is too little coma to make a useful measurement of decenter.

We describe an alternative method of viewing a small patch of the aspheric surface near the edge of the clear aperture where the asphericity is greatest while the lens is rotated about an axis close to coincident with the optical axis of the surface. By tracking the reflected light image when using a Point Source Microscope (PSM), or using an interferometer to measure the low order Zernike coefficients, as the lens is rotated, both the tilt and decenter of the surface can be determined.

The relationship between the image motion and Zernike coefficients is described for both tilt and decenter of the surface as well as the means of separating the relative amounts of tilt and decenter are given. These methods of determining tilt and decenter seem to work for all the aspheric surfaces we have tried to date.

Preview Paper


American Society for Precision Engineering 32nd Annual Meeting

October 29 - November 3, 2017, The Westin, Charlotte, North Carolina, USA

“Computer Generated Holograms as 3-Dimensional Calibration Artifacts”, Robert. E. Parks, J. Ziegert and J. Groover

DESCRIPTION: 

The positioning accuracy of multi-axis machine tools and coordinate measuring machines are often checked using ball bars or ball plates where the spatial locations of the balls are externally calibrated to provide a traceable artifact. In use, the individual ball surfaces are probed in at least 4 places with a tactile sensor and the points of contact fit to the equation of a sphere to determine the center of the ball. The method is tedious, indirect and semi-static.

A computer generated hologram (CGH) can be fabricated on a fused silica photomask substrate to microlithographic precision that creates a virtual 3-dimensional array of points in space that simulate ball centers when interrogated with a point source of light such as produced by an autostigmatic microscope (ASM). The location of the ball centers in two directions parallel to the plane of the CGH can be read by the ASM to <1 μm and to a few μm in the third dimension.

 A practical calibration CGH has an array of Fresnel zone patterns with regular spacing in the CGH plane, and concentric Fresnel zones within each pattern simulating different ball radii, or heights above the CGH, such as the one shown in Fig. 1 where there is a hexagonal, close packed array of Fresnel zones on 28 mm centers. Within each 28 mm diameter pattern there are Fresnel zones simulating balls centers at heights of 5, 15, 45 and 135 mm above the CGH. The small zones have virtual radii of 6.35 and 12.7 mm to match the radii of common ball sizes.

In use, the CGH is probed at each plane of virtual balls, providing a check of the positioning accuracy of the machine in three dimensions. In principle, scale errors in all three directions can be measured, along with straightness and squareness of the axes. The set up using a Point Source Microscope as the ASM and a MoriSeiki NMV5000 as the test bed are shown in Fig. 2 while the results of these measurements are shown in Fig. 3. Note that the PSM allows the ball center positions to be determined in three dimensions with a single measurement, and with sub-micron accuracy in X and Y, and a few microns in Z. Measurements can also be taken on the fly, speeding the measurement process and providing data on the machine’s dynamic response.

 This paper will provide experimental results for CGH artifacts representing a virtual 3D grid of balls, and also for a virtual cone that can be used to evaluate both components of straightness of the motion of the linear axes. Reversal techniques will be presented to evaluate the “accuracy” of the CGH artifacts. Measurements of the CGH artifacts will also be performed on high-accuracy CMMs, using the PSM as the “probe”.

Case Studies & Testimonials

Why is proper alignment so important?

Here is a case of a very happy customer due to better optics.

A few days ago an astronomer friend of mine commented that he had gotten the optics of his telescope improved and the improvement reduced the time it took to get data by a factor of 3. For an astronomer this is a dramatic improvement since observing time on large telescopes can cost thousands of dollars an hour.

My friend did not say how the optics had been improved, but the important point is that better optics, whether due to figure errors, mounting or alignment mean more productive optics. I generally think of better optics as a better product leaving the manufacturing facility without thinking about how much the better optics mean to the productivity of the customer.