Arithmetic averaging of interferometric phase measurements is a well-established method for reducing the effects of time varying disturbances, such as air turbulence and vibration. Calculating a map of the standard deviation for each pixel in the average map can provide a useful estimate of its variability. However, phase maps of complex and/or high density fringe fields frequently contain defects that severely impair the effectiveness of simple phase averaging and bias the variability estimate. These defects include large or small-area phase unwrapping artifacts, large alignment components, and voids that change in number, location, or size. Inclusion of a single phase map with a large area defect into the average is usually sufficient to spoil the entire result. Small-area phase unwrapping and void defects may not render the average map metrologically useless, but they pessimistically bias the variance estimate for the overwhelming majority of the data. We present an algorithm that obtains phase average and variance estimates that are robust against both large and small-area phase defects. It identifies and rejects phase maps containing large area voids or unwrapping artifacts. It also identifies and prunes the unreliable areas of otherwise useful phase maps, and removes the effect of alignment drift from the variance estimate. The algorithm has several run-time adjustable parameters to adjust the rejection criteria for bad data. However, a single nominal setting has been effective over a wide range of conditions. This enhanced averaging algorithm can be efficiently integrated with the phase map acquisition process to minimize the number of phase samples required to approach the practical noise floor of the metrology environment.
Freeform applications are growing and include helmet-mounted displays, conformal optics (e.g. windows integrated into airplane wings), and those requiring the extreme precision of EUV. These non-rotationally symmetric surfaces pose challenges to optical fabrication, mostly in the areas of polishing and metrology. The varying curvature of freeform surfaces drives the need for smaller, more “conformal”, tools for polishing and reference beams for interferometry. In this paper, we present fabrication results of a high-precision freeform surface. We will discuss the total manufacturing process, including generation, pre-polishing, MRF<sup>®</sup>, and metrology, highlighting the capabilities available in today’s optical fabrication companies.
Long measurement times can be a bottleneck in an optics production environment. Ideally the measurement time will be
quicker than polishing times. Large aperture and high precision parts, however, tend toward slower measurement times.
Additionally, such parts usually need dedicated and expensive test setups. In 2004, QED Technologies introduced the
Subaperture Stitching Interferometer (SSI®) to automatically stitch spherical surfaces (including hemispheres) up to 280
mm. The system also reduces measurement uncertainty with in-line calibration of systematic errors.
With stitching, measurement time is a variable that can impact measurement uncertainty. The user can control such
parameters as lattice design, systematic error calibration, and acquisition speed to optimally balance measurement speed
and quality. We empirically demonstrate the trade-offs between measurement uncertainty and cycle time on the SSI.
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.
New optical designs containing freeform optics have recently begun appearing in systems. Applications have
incorporated parts ranging in size from small (e.g.: ~5 – 10 mm rectangles) to large (e.g.: astronomical applications).
To meet these needs, QED Technologies recently introduced two solutions using its Q22-Y and Q22-950F platforms.
Magnetorheological Finishing® (MRF®) is a production proven technology for deterministically finishing symmetric
parts (flats, spheres, and on-axis aspheres) using a rotational toolpath, and rectangular flats and cylinders using a raster
toolpath. The new freeform toolpath expands the raster capabilities of the Q22-Y and Q22-950F machines to include
spheres, aspheres, off-axis sections, and true freeform geometries.
The freeform raster toolpath was first introduced on a meter-class optic platform, the Q22-950F. As optics grow in size,
the mass typically scales as well. This in turn increases the demands on the machine dynamics to meet rotational
polishing requirements. The raster freeform toolpath solution greatly reduces the machine dynamics and is employed to
polish a wide variety of part shapes, sizes, and geometries. A similar version of the toolpath was subsequently
implemented on the smaller Q22-Y platform. This paper will compare the implementations on each platform, describe
the benefits of the toolpath for existing and new applications, and present results from demonstrations on the two platforms.