The manufacturing of precision aspheres has traditionally been a long-lead-time, labor-intensive process that is made
even more expensive by the need for specific process expertise, dedicated tooling for polishing, and dedicated nulls for
metrology. These challenges have limited the widespread use of optical aspheres. New technology is currently being
developed to enable flexible and lower-cost manufacturing of precision aspheres, without the need for dedicated tools or
null optics. Subaperture Stitching Interferometry (SSI®) combined with Magnetorheological Finishing (MRF®) enable a
flexible and deterministic approach to finishing precision aspheres in a wide variety of materials and geometries. MRF
systems use highly stable, subaperture tools that perfectly conform to the changing curvature of aspheric optics during
the polishing process. This enables a single machine to process plano, spherical, and aspheric surfaces (both convex and
concave) without the delays and costs associated with maintaining and switching between sets of dedicated tooling. SSI
systems mathematically "stitch" together subaperture measurements to generate high-resolution, high-precision, fullaperture
aspheric surface measurements. By locally nulling and using maximum pixel resolution over a subaperture, the
SSI extends general-purpose, null-free interferometry to aspheres with departures from best-fit-sphere on the order of
100ë. When these technologies are combined with either the latest grinding and pre-polishing or diamond-turning
technology, fast, flexible prototyping, or small-batch production of precision aspheres is available at an attractive cost.
Traditionally, the most accurate measurements of aspheric surfaces have relied on interferometric null tests. These
usually require "null correction" optics, which often take significant time and expense to design and fabricate, and are
specific to a particular asphere prescription. Alignment and calibration of the null correction optics can also be quite
difficult. Thus there is a significant benefit to a flexible, accurate, "operator-friendly" alternative to the null test.
Testing aspheres without null correction (using a spherical wavefront) has been very limited. A typical interferometer
can acquire only a few micrometers of fourth-order aspheric departure before the interference fringes become too dense
to resolve. Other "non-null" issues include accounting for the part's aspheric shape and optical aberrations of the
interferometer. QED's SSI-ATM addresses these limitations, allowing a standard Subaperture Stitching Interferometer
(SSI®) to automatically measure mild aspheric surfaces. The basic principles of how subaperture stitching enhances
asphere capability are reviewed. Furthermore, SSI-A measurements from real aspheres are presented, along with null test measurements where available.
Interferometric tests of aspheres have traditionally relied on so-called "null correctors". These usually require significant time and expense to design and fabricate, and are specific to a particular asphere prescription. What's more, they are tedious to align and calibrate. Aspheres can also be tested without null correction (using a spherical wavefront), but such capability is extremely limited. A typical interferometer can acquire only a few micrometers of fourth-order aspheric departure due to high-density interference fringes. Furthermore, standard software packages do not compensate for the impact upon a non-null measurement of (i) the part's aspheric shape or (ii) the interferometer's optical aberrations. While fringe density and asphere compensation severely limit the practical utility of a non-null asphere measurement, subaperture stitching can directly address these issues. In 2004, QED Technologies introduced the Subaperture Stitching Interferometer (SSI<sup>(R)</sup>) to automatically stitch spherical surfaces (including hemispheres). The system also boosts accuracy with in-line calibration of systematic errors. We have recently added aspheric capability, extending non-null aspheric test capability by an order of magnitude or more. As demonstrated in the past on annular zones of nearly nulled data, subaperture stitching can extend the testable aspheric departure. We present a more generally applicable and robust method of stitching non-null aspheric phase measurements. By exploiting novel compensation schemes and in-line system error calibration, our subaperture stitching system can provide significantly better accuracy and increased testable aspheric departure over an unstitched non-null test. Examples of stitched non-null tests are analyzed in this paper, and cross-tested against corresponding null tests.
Interferometers are often used to measure optical surfaces and systems. The accuracy of such measurements is often limited by the ability to calibrate systematic errors such as reference wave and image distortion. Standard techniques for calibrating reference wave include the two-sphere and random-ball test. QED Technologies® (QED) recently introduced a Subaperture Stitching Interferometer (SSI®) that has the integrated ability to perform reference wave calibration. By measuring an optical surface in multiple locations, the stitching algorithm has the ability to compensate for reference wave and imaging distortion. Each of the three reference wave calibration methods has its own limitations that ultimately affect the accuracy of the measurement. The merits of each technique for reference wave calibration are reviewed and analyzed. By using the SSI-computed estimate and the random-ball test in tandem, a composite method for calibrating reference wave error is shown to combine the benefits of both individual techniques. The stitching process also calibrates for distortion, and plots are shown for different transmission optics. Measurements with and without distortion compensation are shown, and the residual difference is compared to theoretical predictions.
Magnetorheological finishing (MRF®) is a deterministic polishing process. Typically, an MRF polishing cycle is used to improve the figure of an optical surface (e.g. reduce the irregularity of a spherical surface to λ/20 PV). The hitmap for this process is based off of a surface (reflection) measurement. However, because MRF polishing is a subaperture process, it is not limited to producing perfectly flat or perfectly spherical surfaces. Indeed, the polishing process can converge to any desired surface shape. This is a particularly useful, enabling feature that can be used to perform transmitted wavefront corrections.
One method to produce a perfect transmitted wavefront is to polish perfect surfaces throughout the system, which assumes perfect material homogeneity. In some instances, this can also be accomplished by measuring the transmitted wavefront of an imperfect system, and correcting it by polishing a compensating surface shape into a single surface. By correcting transmitted wavefront data, rather than a surface measurement, this can be a fairly straightforward process. This process can correct for material inhomogeneities, improve system tolerances, and correct prism angles.
This paper will begin by giving an overview of transmitted wavefront tests. It will explain how this data can be used to perform a correction by an MRF polishing cycle. Finally, we present some results from corrections of optical systems such as laser rods and prisms.
Optical surfaces are routinely measured using phase-shifting interferometry. The fringe imaging and other interferometer optics introduce distortion into the measurements. Distortion causes a change in magnification as a function of field position, and is often not quantified and calibrated during measurements of optical surfaces. When calculating the figure of an optical surface, systematic errors such as distortion will ultimately limit the accuracy of the measurement. We present a method for improving the accuracy in interferometric measurements using subaperture stitching interferometry. QED's Subaperture Stitching Interferometer (SSI®) is a six-axis computer-controlled workstation that incorporates a standard Fizeau interferometer with our own stitching algorithms. The SSI is a commercially available product that automatically performs inline calibration of systematic errors such as reference wave and distortion. By measuring an optical surface in multiple orientations both on and off-axis, our stitching algorithms are shown to have the ability to measure the distortion (and other systematic errors) in an interferometer, and compensate for these errors automatically. Using the compensators obtained from stitched measurements, distortion values are calculated and plots are shown for several different transmission optics. Theoretical simulations displaying the effects of distortion on surface metrology are shown. Measurements are taken with and without distortion compensators, and the residual difference is analyzed.