There is an increasing number of applications for rapid deployment mid-wave infrared (MWIR) and long-wave infrared (LWIR) optical systems, especially for proof-of-concept. These systems are typically placed in test environments where they need an environmental enclosure window for protection. Traditional IR windows, like Germanium and Zinc Selenide, often have high costs and long fabrication lead times, especially for custom designs. Thin, readily available polymers have potential of solving this problem where they may have high enough transmission in the IR to be of use as an inexpensive environmental enclosure window. This paper outlines 33 polymer materials that have been tested as candidates for IR windows by measuring transmission and reflection and then calculating values of refractive index and extinction ratio from the measurements. We have identified 7 polymer materials as having high enough transmission to be used as an IR window. Further testing was done to characterize wavefront error and image quality of MWIR and LWIR cameras with these polymer windows. The combined results show 3 promising materials in the MWIR and LWIR.
The intense development in imaging spectrometers and related technology has yielded systems that are highly performing. Current grating-based designs utilize focal plane arrays with aberrations controlled to a fraction of a detector element and low F-numbers for high étendue to maximize the signal to noise performance. Tailored grating facets using two or more blaze angles optimize the optical efficiency across the full 400-2500 nm solar reflective spectral range. Two commonly used forms, the Offner-Chrisp and Dyson designs, are adaptations of microlithographic projectors with a concave or convex mirror replaced by a shaped grating; maintain a high degree of spatial-spectral uniformity. These gratings are relatively difficult to manufacture using either e-beam lithography or diamond machining. The challenge for optical designers is to create optical forms with reduced size, weight, and power (SWaP) requirements while maintaining high performance. We have focused our work in this area and are developing a breadboard prototype imaging spectrometer that covers the full VNIR/SWIR spectral range at 10 nm spectral sampling, has a large swath of 1500 spatial samples, and is compact. The current prototype is for an F/3.3 system that is 7 cm long with an 8 cm diameter with aberration control better than 0.1 pixel assuming an 18 μm pixel pitch. The form utilizes a catadioptric lens and a flat dual-blaze immersion grating. The flat grating simplifies manufacturing and we are currently exploring the manufacture of the grating through grayscale optical lithography where the entire pattern can be exposed at once without stitching errors.
The Transiting Exoplanet Survey Satellite (TESS) will carry four visible waveband, seven-element, refractive F/1.4 lenses, each with a 34 degree diagonal field of view. This paper describes the methods used for the assembly, alignment and test of the four flight optical assemblies. Prior to commencing the build of the four flight optical assemblies, a Risk Reduction Unit (RRU) was successfully assembled and tested [1]. The lessons learned from the RRU were applied to the build of the flight assemblies. The main modifications to the flight assemblies include the inking of the third lens element stray light mitigation, tighter alignment tolerances, and diamond turning for critical mechanical surfaces. Each of the optical assemblies was tested interferometrically and measured with a low coherence distance measuring interferometer (DMI) to predict the optimal shim thickness between the lens assembly and detector before -75°C environmental testing. In addition to individual test data, environmental test results from prior assemblies allow for the exploration of marginal performance differences between each of the optical assemblies.
The Transiting Exoplanet Survey Satellite (TESS) will carry four visible waveband seven-element refractive f/1.4 lenses, each with a 34 degree diagonal field of view. This paper describes the tolerancing, assembly and alignment methods developed during the build of the TESS Risk Reduction Unit optical system. Lens assembly tolerances were derived from a sensitivity analysis using an image quality metric customized for mission performance. The optomechanical design consists of a two-stage lens housing that provides access for active alignment of each lens using a Trioptics OptiCentric measurement system. Thermal stresses and alignment shifts are mitigated by mounting the optics with cast RTV silicone spacers into individually aligned bezels, and custom fixtures were developed to aid in RTV bonding with reduced alignment error. The lens assembly was tested interferometrically over the field of view at room temperature and results were used to successfully predict lens performance and compensator adjustments and detector shim thickness for the -75C operational temperature and pressure.
B. Kruschwitz, J. Kelly, C. Dorrer, A. Okishev, L. Waxer, G. Balonek, I. Begishev, W. Bittle, A. Consentino, R. Cuffney, E. Hill, J. Marozas, M. Moore, R. Roides, J. Zuegel
A one-dimensional smoothing by spectral dispersion (SSD) demonstration system for smoothing focal-spot nonuniformities using multiple modulation frequencies (multi-FM SSD) was commissioned on one long-pulse beamline of OMEGA EP—the first use of such a system in a high-energy laser. System models of frequency modulation-to-amplitude modulation (FM-to-AM) conversion in the OMEGA EP beamline and final optics were used to develop an
AM budget. The AM budget in turn provided a UV power limit of 0.85 TW, based on accumulation of B-integral in the final optics. The front end of the demonstration system utilized a National Ignition Facility preamplifier module (PAM) with a custom SSD grating inserted into the PAM’s multipass amplifier section. The dispersion of the SSD grating was selected to cleanly propagate the dispersed SSD bandwidth through various pinholes in the system while maintaining sufficient focal-spot smoothing performance. A commissioning plan was executed that systematically introduced the new features of the demonstration system into OMEGA EP. Ultimately, the OMEGA EP beamline was ramped to the UV power limit with various pulse shapes. The front-end system was designed to provide flexibility in pulse shaping. Various combinations of pickets and nanosecond-scale drive pulses were demonstrated, with multi-FM SSD selectively applied to portions of the pulse. Analysis of the dispersion measured by the far-field diagnostics at the outputs of the infrared beamline and the frequency-conversion crystals indicated that the SSD modulation spectrum was maintained through both the beamline and the frequency-conversion process. At the completion of the plan, a series of equivalent-target-plane measurements with distributed phase plates installed were conducted that confirmed the expected timeintegrated smoothing of the focal spot.
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