In this paper, edge filters including short pass (SP) filters and long pass (LP) filters are reviewed in terms of definition and features. The necessity of SP and particularly LP filters and their functions in an optical system are addressed in depth. Principles of defining the OD level in the blocking band are elaborated for different spectra ranges, particularly for long wavelength LP filters. For SP filters, the filter design, performance, and potential applications are discussed. An example is given on a DUV filter via reactive plasma ion assisted deposition of HfO2/SiO2 that suppresses solar background at wavelengths above 300 nm and transmits 260-290 nm radiation. For LP filters, the design principles of the blocking band for the LP filter using the substrate, absorption material in coating, and interference type reflective coating are discussed. Semiconductor materials and doping levels for different bandgap energies and cut-on wavelengths are proposed for blocking band solutions. Examples of design practices cover a broad spectra range including short wavelength infrared (SWIR), mid-wavelength infrared (MWIR), and long wavelength infrared (LWIR). Coating challenges, for example element segregations in the deposition of a compound semiconductor, are discussed. Finally, quality control and related issues are also addressed.
Corning Incorporated has leveraged its industry leading space based hyperspectral technology to create an advanced Low Earth Orbit (LEO) Satellite Payload. We outline the specifications, performance, and capabilities of this new standard in LEO Hyperspectral Imaging (HSI). Corning’s new product platform is capable of ⪅8m GSD imaging across the 400nm-2500nm spectral band with high dispersion. It has onboard computing, storage, and processing capabilities which enhance its exceptional optical and sensor performance. Corning’s design exceeds the launch stress requirements of standard LEO Transporter vehicles, such as the SpaceX Falcon-9, and has been proven on multiple successful LEO missions. The product platform contains a flexible electro-mechanical interface design suitable for a variety of host bus platforms and functions.
Commercial Hyperspectral Imaging (HSI) systems capable of both fine spatial ground resolution and high signal to noise ratio pose challenges to the integration and testing process. The following will describe methods developed for a Low Earth Orbit (LEO) system having an effective focal length of approximately one meter and a ~f/2 optical speed. Internal subsystem testing is outlined as it relates to full-system performance. A sub-aperture testing routine is described to establish baseline performance estimates for spatial and spectral characteristics at a system level. Analysis and testing results are presented as measured during the integration process.
Gradient index (GRIN) lenses have been created for imaging in the infrared regime by diffusion of chalcogenide glasses. The GRIN lenses are shaped using a combination of precision glass molding and single point diamond turning. The precision glass molding step, is known to cause a drop in the index of refraction in both oxide and chalcogenide glasses. This drop is a direct result of the cooling rate during the molding process. Since the GRIN lenses have an index of refraction profile created by diffusion of multiple chalcogenide glasses, we would expect that the index drop would vary as a function of position. In this paper we investigate the expected profile change due to the index drop of the constituent chalcogenide glasses, as well as report performance data on the GRIN lenses.
Metrology of a gradient index (GRIN) material is non-trivial, especially in the realm of infrared and large refractive index. Traditional methods rely on index matching fluids which are not available for indexes as high as those found in the chalcogenide glasses (2.4-3.2). By diffusing chalcogenide glasses of similar composition one can blend the properties in a continuous way. In an effort to measure this we will present data from both x-ray computed tomography scans (CT scans) and Raman mapping scans of the diffusion profiles. Proof of concept measurements on undiffused bonded sheets of chalcogenide glasses were presented previously. The profiles measured will be of axially stacked sheets of chalcogenide glasses diffused to create a linear GRIN profile and nested tubes of chalcogenide glasses diffused to create a radial parabolic GRIN profile. We will show that the x-ray absorption in the CT scan and the intensity of select Raman peaks spatially measured through the material are indicators of the concentration of the diffusion ions and correlate to the spatial change in refractive index. We will also present finite element modeling (FEM) results and compare them to post precision glass molded (PGM) elements that have undergone CT and Raman mapping.
Gradient index (GRIN) optics have been an up-and-coming tool in the world of optics. By combining an index gradient with a surface curvature the number of optical components for a lens system can often be greatly reduced. Their use in the realm of infra-red is only becoming realized as new efforts are being developed to create materials that are suitable and mutually compatible for these optical components. The materials being pursued are the chalcogenide based glasses. Small changes in elemental concentrations in these glasses can have significant effects on physical and optical properties. The commonality between these glasses and their widely different optical properties make them prime candidates for GRIN applications. Traditional methods of metrology are complicated by the combination of the GRIN and the curvature of the element. We will present preliminary data on both destructive and non-destructive means of measuring the GRIN profile. Non-destructive methods may require inference of index through material properties, by careful measurement of the individual materials going into the GRIN optic, followed by, mapping measurements of the GRIN surface. Methods to be pursued are micro Raman mapping and CT scanning. By knowing the properties of the layers and accurately mapping the interfaces between the layers we should be able to back out the index profile of the GRIN optic and then confirm the profile by destructive means.
With the increase in demand for infrared optics for thermal applications and the use of glass molding of chalcogenide materials to support these higher volume optical designs, an investigation of changes to the optical properties of these materials is required. Typical precision glass molding requires specific thermal conditions for proper lens molding of any type of optical glass. With these conditions a change (reduction) of optical index occurs after molding of all oxide glass types and it is presumed that a similar behavior will happen with chalcogenide based materials. We will discuss the effects of a typical molding thermal cycle for use with commercially and newly developed chalcogenide materials and show results of index variation from nominally established material data.
We describe the benefits to camera system SWaP-C associated with the use of aspheric molded glasses and optical
polymers in the design and manufacture of optical components and elements. Both camera objectives and display
eyepieces, typical for night vision man-portable EO/IR systems, are explored. We discuss optical trade-offs, system
performance, and cost reductions associated with this approach in both visible and non-visible wavebands, specifically
NIR and LWIR. Example optical models are presented, studied, and traded using this approach.
QmagiQ LLC, has recently completed building and testing high operability two-color Quantum Well Infrared Photodetector (QWIP) focal plane arrays (FPAs). The 320 x 256 format dual-band FPAs feature 40-micron pixels of spatially registered QWIP detectors based on III-V materials. The vertically stacked detectors in this specific midwave/longwave (MW/LW) design are tuned to absorb in the respective 4-5 and 8-9 micron spectral ranges. The ISC0006 Readout Integrated Circuit (ROIC) developed by FLIR Systems Inc. and used in these FPAs features direct injection (DI) input circuitry for high charge storage with each unit cell containing dual integration capacitors, allowing simultaneous scene sampling and readout for the two distinct wavelength bands. Initial FPAs feature pixel operabilities better than 99%. Focal plane array test results and sample images will be presented.
Infrared Imaging sensors operating in the 3 - 5 um (MW) and 8 - 12 um (LW) spectral bands have long since been traded against one another with respect to mission utility, sensor performance, and system viability (cost factors, technology maturity, etc.). Over the past decade, staring InSb detectors have matured to a refined level (high performance, moderate cost, high yields) and have been used extensively by IR sensor integrators throughout the industry. By the same token, 2-D LW QWIP-based FPA's are fast becoming a viable alternative to traditional LW-scanned technology systems, offering the benefits of mid and large format staring sensor resolution with good sensitivity (even for modest optical F/#'s). With the commercialization of QWIP technology, system viability is rapidly increasing, revealing the need for serious system trade assessments and field measurements to enable the best use of this emerging, complementary detector technology. This paper presents a top-level technical comparison of these two sensor technologies and their use in surveillance/night vision system applications. A variety of technical considerations are discussed to help end users be cognizant of the extent of the trade space that exists between MW and LW staring sensor selection with specific focus on performance comparisons for small, compact militarized IR thermal imaging sensors (including handheld, man-portable and small gimbal products) employing each detector technology in context to various surveillance missions. Application areas include: ground, airborne, and maritime surveillance. Field data is also provided to support the conclusions drawn from these comparisons.
SIRTF requires detector arrays with extremely high sensitivity, limited only by the background irradiance. Especially critical is the near infrared spectral region around 3 micrometers , where the detector current due to the zodiacal background is a minimum. IRAC has two near infrared detector channels centered at 3.6 and 4.5 micrometers . We have developed InSb arrays for these channels that operate with dark currents of < 0.2 e/s and multiply-sampled noise of approximately 7 e at 200 s exposure. With these specifications the zodiacal background limited requirements has been easily met. In addition, the detector quantum efficiency of the InSb devices exceeds 90% over the IRAC wavelength range, they are radiation hard, and they exhibit excellent photometric accuracy and stability. Residual images have been minimized. The Raytheon 256 X 256 InSb arrays incorporate a specially developed (for SIRTF) multiplexer and high-grade InSb material.
Spatial distributions of hole trap sites on a quasipixel level in InSb arrays for SIRTF are examined. The dependence of flux, fluence, and applied bias on image latency is investigated, and experimental results are presented and discussed. Models of linearity and capacitance are compared with experimental results. We find increasing the depletion width in a light exposed pixel by larger reverse biasing decreases the trapped charge (or latency) in that pixel by factors of approximately 3. Assumed pixel geometries lead to an apparent spatial density of active trap sites that falls quickly with distance from the implants.
We have developed a new information-content based look-up table technique for the fast computation of near- monochromatic atmospheric transmittances in the infrared that is well suited for nadir viewing satellite and airplane observations. It allow a user to quickly compute near- monochromatic radiances using a very simple algorithm that is easily ported to many machine architectures. Radiative transfer based on look-up tables of monochromatic absorption coefficients could speed calculations, but they are impractical due to their large size and the need to interpolate long wavenumber vectors in temperature and pressure. We use a singular value decomposition to transform monochromatic look-up tables of absorption coefficients into a compressed representation that is almost 100 times smaller. Moreover, temperature and pressure interpolations can be performed in this compressed representation, resulting in significant savings in computation times and computer I/O. We start with the line-by-line computation of a set of tables of absorption coefficients for each relevant gas. Each 25 wavenumber table has 10,000 wavenumber points and 1,100 temperature/pressure layers. For water vapor we add an extra dimension to these tables that spans 5 water vapor profiles to provide variability in the self-broadening of water vapor spectral lines. On average we need 37 basis vectors for water, 12 for carbon dioxide, and 6 for each of the other required gases in order to reproduce the absorption coefficient tables to an accuracy equivalent to a nadir-viewing monochromatic brightness temperature error of 0.1K.
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