Recent advances in polishing and metrology have addressed many of the challenges in the fabrication and metrology of freeform surfaces, and the manufacture of these surfaces is possible today. However, achieving the form and mid-spatial frequency (MSF) specifications that are typical of visible imaging systems remains a challenge. Interferometric metrology for freeform surfaces is thus highly desirable for such applications, but the capability is currently quite limited for freeforms. In this paper, we provide preliminary results that demonstrate accurate, high-resolution measurements of freeform surfaces using prototype software on QED’s ASI™ (Aspheric Stitching Interferometer).
This paper describes the manufacturing steps necessary to manufacture hemispherical concave aspheric mirrors for high- NA systems. The process chain is considered from generation to final figuring and includes metrology testing during the various manufacturing steps. Corning Incorporated has developed this process by taking advantage of recent advances in commercially available Satisloh and QED Technologies equipment. Results are presented on a 100 mm concave radius nearly hemispherical (NA = 0.94) fused silica sphere with a better than 5 nm RMS figure. Part interferometric metrology was obtained on a QED stitching interferometer. Final figure was made possible by the implementation of a high-NA rotational MRF mode recently developed by QED Technologies which is used at Corning Incorporated for production. We also present results from a 75 mm concave radius (NA = 0.88) Corning ULE sphere that was produced using sub-aperture tools from generation to final figuring. This part demonstrates the production chain from blank to finished optics for high-NA concave asphere.
Many optical system designs rely on high numerical aperture (NA) optics, including lithography and defense systems. Lithography systems require high-NA optics to image the fine patterns from a photomask, and many defense systems require the use of domes. The methods for manufacturing such optics with large half angles have often been treated as proprietary by most manufacturers due to the challenges involved. In the past, many high-NA concave surfaces could not be polished by magnetorheological finishing (MRF) due to collisions with the hardware underneath the polishing head. By leveraging concepts that were developed to enable freeform raster MRF capabilities, QED Technologies has implemented a novel toolpath to facilitate a new high-NA rotational MRF mode. This concept involves the use of the B-axis (rotational axis) in combination with a “virtual-axis” that utilizes the geometry of the polishing head. Hardware collisions that previously restricted the concave half angle limit can now be avoided and the new functionality has been seamlessly integrated into the software. This new MRF mode overcomes past limitations for polishing concave surfaces to now accommodate full concave hemispheres as well as extend the capabilities for full convex hemispheres. We discuss some of the previous limitations, and demonstrate the extended capabilities using this novel toolpath. Polishing results are used to qualify the new toolpath to ensure similar results to the “standard” rotational MRF mode.
As applications for freeform optics continue to grow, the need for high-precision metrology is becoming more of a necessity. Currently, coordinate measuring machines (CMM) that implement touch probes or optical probes can measure the widest ranges of shapes of freeform optics, but these measurement solutions often lack sufficient lateral resolution and accuracy. Subaperture stitching interferometry (SSI™) extends traditional Fizeau interferometry to provide accurate, high-resolution measurements of flats, spheres, and aspheres, and development is currently on-going to enable measurements of freeform surfaces. We will present recent freeform metrology results, including repeatability and cross-test data. We will also present MRF® polishing results where the stitched data was used as the input “hitmap” to the deterministic polishing process.
The subaperture and conformal nature of the MRF® polishing tool has proven its unmatched production capability and efficiency for more than a decade at leading optics manufacturers worldwide. The introduction of the third generation of manufacturing systems combined with newly developed MRF fluids pushes the limits of the technology to extend its benefits to very low roughness surfaces and high-precision freeform surfaces. In this article, after reviewing the benefits of the new platforms, two specific examples of very advanced capability will be discussed:
- A new super-fine MRF polishing fluid that is able to meet both form and roughness specifications for very demanding optics required for EUV applications and high power laser systems.
EUV optics, made of calcium fluoride or similar materials, ideally require sub-Angstrom surface roughness while achieving nm level form error. To achieve the above specifications, optics must undergo iterative global and local polishing processes. The new MRF polishing fluid minimizes the number of steps required if MRF® is used as the final step, or a reduction in the post-processing if a final smoothing step is performed.
- The total manufacturing process, including generation, pre-polishing, MRF and metrology, of a very steep freeform surface, highlighting the capabilities available in today’s optical fabrication companies.
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 the fabrication results of a high-precision freeform surface.
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®, and metrology, highlighting the capabilities available in today’s optical fabrication companies.
Aspheric surfaces can provide significant benefits to optical systems, but manufacturing high-precision aspheric surfaces
is often limited by the availability of surface metrology. The lack of 3D surface data required to drive aspheric
manufacturing equipment can create risk and unwanted variation in the manufacturing process. One typical approach to
gathering this 3D data is using dedicated null correction optics in addition to the interferometer itself. However, the
cost, lead time, inflexibility, and calibration difficulty of such null optics makes interferometric aspheric testing a far less
attractive solution than the relatively simple spherical test. Subaperture stitching interferometry was originally developed
to allow for the full-aperture 3D measurement of large-aperture spheres and flats using commercially available
interferometers and transmission elements1, 2, 3 The method was then extended to the measurement of mild aspheric
surfaces, by exploiting the local best-fitting and magnification of the high density fringe patterns associated with nonnull
interferometry.4 Subaperture stitching interferometry was then extended by an order of magnitude through the use
of a Variable Optical Null (VON) that allowed the measurement of high-departure aspheres. The automated VON has an
optical system with a range of motion control that generates an optical wavefront that closely matches the surface of the
asphere for each subaperture. The residual wavefront error is measured with a standard interferometer, and the fullaperture
surface profile of the asphere is reconstructed using advanced stitching algorithms. This method allows for the
accurate measurement of aspheres with more than 1000 waves of departure from best-fit sphere, without the use of
dedicated null lenses.
Fabrication of large optics has been a topic of discussion for decades. As early as the late 1980s, computer-controlled
equipment has been used to semi-deterministically correct the figure error of large optics over a number of process
iterations. Magnetorheological Finishing, MRF®, was developed and commercialized in the late 1990's to predictably
and reliably allow the user to achieve deterministic results on a variety of optical glasses, ceramics and other common
optical materials. Large and small optics such as primary mirrors, conformal optics and off-axis components are
efficiently fabricated using this approach. More recently, specific processes, MR Fluids and equipment have been
developed and implemented to enhance results when finishing large aperture sapphire windows.
MRF, by virtue of its unique removal process, overcomes many of the drawbacks of a conventional polishing process.
For example, lightweighted optics often exhibit a quilted pattern coincident with their pocket cell structure following
conventional pad-based polishing. MRF does not induce mid-frequency errors and is capable of removing existing quilt
patterns. Further, odd aperture shapes and part geometries which can represent significant challenges to conventional
polish processing are simply and easily corrected with MRF tools. Similarly, aspheric optics which can often present
multiple obstacles-particularly when lightweighted and off-axis−typically have a departure from best-fit sphere that is
not well matched with to static pad-based polishing tools resulting in pad misfit and associated variations in removal.
The conformal subaperture polishing tool inherent to the QED process works as well on typical circular apertures as it
does on irregular shapes such as rectangles, petals and trapezoids for example and matches the surface perfectly at all
points. Flats, spheres, aspheres and off-axis sections are easily corrected. The schedule uncertainties driven by edge
roll and edge control are virtually eliminated with the MRF process.
This paper presents some recent results of the deterministic finishing typified by the QED product line and more
specifically of its large-aperture machines, presently capable of finishing optics up to one meter in size. Examples of
large sapphire windows and meter-class aspheric glass optics will be reviewed. Associated metrology concerns will also
Optics manufactured for infrared (IR) applications are commonly produced using single point diamond turning (SPDT).
SPDT can efficiently produce spherical and aspheric surfaces with microroughness and figure error that is often
acceptable for use in this region of the spectrum. The tool marks left by the diamond turning process cause high surface
microroughness that can degrade performance when used in the visible region of the spectrum. For multispectral and
high precision IR applications, surface figure may also need to be improved beyond the capabilities of the SPDT
process. Magnetorheological finishing (MRF®) is a deterministic, subaperture polishing technology that has proven to be
very successful at simultaneously improving both surface microroughness and surface figure on spherical, aspheric, and
most recently, freeform surfaces. MRF has been used on many diamond turned IR materials to significantly reduce
surface microroughness from tens of nanometers to below 1 nm. MRF has also been used to successfully correct figure
error on several IR materials that are not diamond turnable.
This paper will show that the combination of SPDT and MRF technologies enable the manufacture of high precision
surfaces on a variety of materials including calcium fluoride, silicon, and nickel-plated aluminum. Results will be
presented for microroughness reduction and surface figure improvement, as well as for smoothing of diamond turning
marks on an off-axis part. Figure correction results using MRF will also be presented for several other IR materials
including sapphire, germanium, AMTIR, zinc sulfide, and polycrystalline alumina (PCA).