Previous work has demonstrated the feasibility of using ultrafast laser generated stress to deform fused silica substrates to a desired flatness in a process called ultrafast laser stress figuring (ULSF). Materials other than fused silica may offer superior optomechanical properties that are more suited to certain applications or environments. In this work we explore the stress generated by focused ultrafast laser pulses in several common optical materials: Corning Ultra Low Expansion (ULE) glass, Corning Eagle XG glass, fused silica, and sapphire. Using a laser polarization state perpendicular to the writing direction, we find that the laser induced stress depends on the energy of the ultrafast laser pulses, the distance between two adjacent focused pulses, and the repetition rate at which the pulses are delivered into the material. Each material explored showcases unique dependence on these parameters. The results from this investigation will be used to characterize the potential equivalent material removal rates that would be theoretically achievable by ultrafast laser stress figuring for commercially available sapphire and Eagle XG substrates.
We compare the optical performance, alignment sensitivity, and thermal stability of a Non-Uniform Rational B-Spline (NURBS) freeform telescope design to two more conventional design forms with the goal of facilitating acceptance of this new optical surface for aerospace applications. We present the designs of three three-mirror anastigmat (TMA) wide field (4°) telescopes with identical first order optical design parameters. These TMAs consist of a conventional design using off-axis aspheric mirrors, a freeform design using off-axis Zernike polynomial surfaces, and a freeform design using NURBS surfaces. Of the three, the NURBS design gives the best image quality and lowest geometrical design residual. The three designs have similar misalignment sensitivities and sensitivity to thermal soaks, countering a common misconception that freeform designs are more sensitive to misalignment than conventional designs.
The alignment of precision optical assemblies can be time-consuming and labor-intensive, particularly for ap- plications that need to maintain performance through harsh environments. To achieve a rugged design, op- tomechanical elements are frequently aligned and locked in place with shims that are ground and lapped to extremely tight tolerances. The grinding and lapping process can take days, weeks, or even months in select instances that require extremely tight tolerances for alignment. In this work, we present an alternative actua- tion and lock approach that can shorten alignment times without sacrificing ruggedness or alignment resolution. The faster optical alignment is achieved with an Adjustable, Re-lockable, Ruggedized, and Kinematic (ARRK) mount principle. Select experiments demonstrate the working principle of an ARRK mount, evaluate ease-of-use, and demonstrate stability through a random vibration environment. Our results suggest ARRK mounting as a promising approach for fast, robust optical mounting in applications that need to withstand vibration.
An afocal freeform telescope has been prototyped at MIT Lincoln Laboratory. The design leverages an upgraded version of the Laboratory’s FANO design code to produce designs in afocal-space rather than imaging-space. The FANO code is unique, as it optimizes the surfaces using a Non-Uniform Rational B-Spline (NURBS) description rather than a polynomial-based equation. This approach allows the mirrors to take the most optimal shape without the limit of number of polynomial terms often hard-coded into commercial design software. Design comparisons with polynomial-based approaches are discussed showing improved performance when designing with NURBS shapes. Test results from the 150-mm diameter, 6x afocal magnification, 4° field-of-view prototype unit operating in the near-infrared band are shown.
The Panoramic Annular Lens (PAL) form provides a hyper-hemispheric field of view in a monolithic element. The lens uses two refractive and two or more internally reflective surfaces to create an afocal or near-afocal imaging system. The resulting field of view loses some coverage off of the central axis due to obscurations from the multiple surfaces, resulting in a doughnut-like image. The PAL form provides significant size and complexity advantages over fish-eye and other wide-angle lenses, as multiple individual elements are combined into the single block monolith. The design form also provides ideal locations for mounting and blocking based on the intersection of the various curves into the surface. Up to this point, PAL designs have been limited to small spectral regions based on the substrate material. We developed several visible through long-wave variants of this form in a variety of multiple multi-spectral materials, each providing an annular coverage from 30-110° off of the central axis. Results are shown from a fabricated prototype in water-clear Zinc Sulfide, including imaging tests in the visible and LWIR.
For an optical surface to be properly prepared, the amount of material removed during polishing must be greater
than the volume of grinding damage. An intermediate stage between loose abrasive grinding and polishing can
reduce the total volume of subsurface damage. This results in less time and expense needed during the polishing
phase. We have characterized the Prestos's coefficient and subsurface damage depth for 3M Trizact™ diamond
tile pads and believe it can fit this intermediary role. Trizact shows a sizeable reduction in the overall subsurface
damage compared to similar sized loose abrasives. This understanding of the abrasive behavior allows us to
create a better grinding schedule that more efficiently removes material and finishing with less overall damage
than traditional loose abrasives.
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