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Dual-Band Lens SWAP Reduction and Increased Optical Throughput with Calcium Lanthanum Sulphide (CLS)

Description:

TECHNOLOGY AREA(S): Electronics, 

OBJECTIVE: Design and fabricate a dual-band relay imager lens assembly which leverages newly discovered infrared material, Calcium Lanthanum Sulphide (CLS), and/or a curved focal plane array (FPA). The relay imager lens assembly should demonstrate the impact of CLS or a curved FPA on overall size, weight, number of elements, and performance. 

DESCRIPTION: The Army is acquiring dual-band FLIR thermal imaging systems for use on airborne platforms and ground-based platforms wherein the optics image both the MWIR and LWIR infrared spectrums. This includes the 3rd Gen FLIR Program of Record that supports the Next Generation Combat Vehicle (NGCV) Cross Functional Team (CFT). There are a limited number of materials which transmit both the MWIR and LWIR. Dual-band optical elements must correct aberrations to ensure all desired rays entering the optical assembly image correctly onto the focal plane. Of those aberrations, chromatic aberration contributes the most to the optical blur observed on the FPA. Chromatic aberration is a wavelength-dependent aberration where each wavelength focuses to a different location. Lens materials with complementary chromatic dispersions, i.e., change in index of refraction versus wavelength, are needed to correct for chromatic aberration in dual-band systems. While existing optics demonstrate the capability of achieving high performance in dual-band sensors, the number of optical surfaces needed to establish such performance is also high because the available materials do not possess the ideal dispersion relationships. The number of elements in an optical system impacts the transmission of the assembly. Each optical element will attenuate the energy impinging on the focal plane, thus limiting the system range performance. In addition, these elements contribute to the assemblies' overall weight, size, and cost. Recently NVESD has discovered that the optical properties of Calcium Lanthanum Sulphide (CLS) may be well-suited for use in dual-band sensors. Introducing CLS or a curved focal plane into the imaging system may reduce the number of elements required to meet diffraction-limited optical performance. Because of the limited number of materials which transmit in the MWIR and LWIR, the new CLS material properties may directly reduce the chromatic aberration in the optical system. Furthermore, a curved focal plane could aid in aberration correction by defining a field dependent focus location. Research of optical designs that take advantage of the new material and curved focal plane architectures is required to identify optimal forms which meet sensor needs. The following table of first-order parameters represents a typical system level requirement: Focal length 94.4mm Entrance pupil location (ref L1) 35mm Entrance pupil diameter 39.1mm Waveband 3.5-5um & 7.6-10um Total length 94.5mm Cold stop diameter 10.4mm Cold stop height 25.15mm Image plane diagonal format 17.62mm Distortion (f tan(theta)) <3% Table 1.1 First Order Optical Parameters 

PHASE I: Perform trade studies and develop optical designs using CLS material and curved focal planes for re-imaging optics per Table 1.1. Trade analysis shall address issues of reducing lens count, ease of fabrication, athermalization, total optical transmission, and minimization of “Narcissus” back-reflections assuming a cryogenic dewar around the cold stop. Size, weight, and cost shall also be criteria for evaluating best possible design options. The design forms shall assume simple aluminum housings, and passive athermalization techniques using materials with differing coefficients of thermal expansion may be considered. Optical elements near the intermediate image plane shall avoid beam footprint diameters less than 1 mm. Designs shall have diffraction-limited performance across most of the image plane footprint. The results of these efforts shall be documented in a deliverable electronic format final report. 

PHASE II: Using the results of Phase I, choose the best design approach for proceeding to fabrication and test of a “proof of design” demonstrator hardware optical system. Execute prototype fabrication and deliver the assembly to the government. A test plan shall be developed and executed in this phase to confirm performance and assess compliance with designed performance parameters. In addition, the offeror shall determine a path for hardware fabrication to include identifying material vendors, coating shops, and component integrators. The Government does not intend to provide a focal plane dewar assembly; therefore, commercially available prodcts may be considered for test & demonstration purposes. Deliver a final report containing final as-built design and test information. 

PHASE III: Transition applicable techniques, processes, and material sources of supply to a production environment with the support of an industry partner if needed. Finalize a sensor design with appropriate SWAP-C and form factor based on human factors and operational testing. Determine the best integration path as a capability upgrade to existing or future systems, including firmware and interfaces required to meet sensor interoperability protocols for integration into candidate systems as identified by the Army. This topic supports NGCV (e.g., OMFV, RCV) through the 3rd Gen FLIR POR and FVL (e.g., FARA, and any combined pilotage + ASE missions such as DDUS). 

REFERENCES: 

1: Gentilman, R. L. (1988). Calcium Lanthanum Sulfide as a Long Wavelength IR Material. SPIE, 929, 57th ser., 57-64

KEYWORDS: Sensors, Optics, Imaging, Lens, Thermal 

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