A miniaturized, lightweight turn-key hyperspectral sensor package incorporating a single, monolithic spectrograph, telescope and navigation system is being built for airborne applications on small, Unmanned Aircraft Systems (UAS). The sensor is based on Corning’s existing MicroHSI 410 Vis/NIR Selectable Hyperspectral Airborne Remote sensing Kit (SHARK) currently used for airborne agricultural monitoring. Under DOE sponsorship, we are extending the approach to cover the full spectral range from 0.4-2.5 microns with a single spectrograph. This will enable rapid aerial surveys of vegetative mass, quality, and carbon sequestration. Other applications include mineralogy, agriculture, and intelligence/surveillance/reconnaissance (ISR).
The sensor features an Offner-type spectrograph machined from a single transmissive block. The monolithic construction provides an unprecedented combination of high performance, low cost and low size, weight, and power. It has an f/1.4 aperture, 5 nm resolution, and measures only 46mm x 60mm x 76mm. The spectrograph block is coupled to a sterling-cooled, back-thinned, HgCdTe FPA covering 0.4-2.5 micron spectral range. The flight package, including spectrograph, camera, telescope, and navigation system weighs less than 2.4kg and can fit on group 1 UASs.
In this paper, we present the design and optical performance of the sensor, and a detailed physical model of detection performance in standard, airborne hyperspectral sensing applications. At 100 Hz data rate, the sensor will achieve shotnoise limited performance with SNR > 250 from 0.4-1.7 microns and SNR<100 between 2-2.3 microns. Operating procedures for airborne monitoring of vegetative properties are also discussed. Initial test flights on a UAS are scheduled for next summer.
Prior work has demonstrated that curved focal plan arrays (FPA) enable lenses for traditional cameras with improved image quality and fewer elements as compared to existing flat image planes. There is an increasing interest in the development of curved focal planes for other imaging applications using a variety of surface contours. In this work, the potential performance improvements of a hyperspectral imaging system (HSI) with a curved FPA are explored using Corning’s existing hyperspectral designs. We evaluated the effect of curved FPAs for spectrometers from the free-space and monolithic Offner class as well as the Dyson form. While these designs are already optimized with aspheric mirrors, the addition of the curved FPA provided additional performance improvements. Surface types considered for the FPA include aspheric, toroidal, anamorphic and free-forms. All of these FPA surface types are manufacturable. The surfaces providing optimum performance offer guidelines for future curved FPA development. We found that multiple design parameters, such as size, weight, f-number, field of view and relative cost can be improved compared with current state-of-the-art flat FPAs. A free-space Offner spectrometer, for example, can be reduced 30% in size, or improved by 20% in f-number.
Hyperspectral imaging systems are finding broader applications in both the commercial and aerospace markets. It is becoming clear that to optimize the performance of these systems, their instrument transfer function needs to be tailored for each application. Vis-SWIR systems in the full 400nm to 2500nm waveband present particular design and manufacturing challenges. A single blazed grating is inadequate for a system operating in the full vis-SWIR wavelength range. In addition, optical materials and broad band coatings present a challenge for non-reflective systems. An understanding of the application and wavelengths of interest, combined with a judicious choice of a focal plane array, can then lead to an optimized system for the specific application. The ability to tailor the grating and manufacture a wide variety of grating profiles and substrate shapes becomes a significant performance enabler. This paper will discuss how the use of optical, coating, and grating design/analysis software, combined with grating manufacturing techniques assure meeting high performance requirements for different applications.
By using split-step beam propagation method, self-pumped contra-directional two-beam coupling is simulated in a photorefractive medium for arbitrary shaped beams. The simulation shows a positive influence of the photovoltaic effect on the two-beam coupling efficiency, in agreement with published experimental observations.
Contradirectional two-beam coupling was achieved using a single laser beam incident on fibers grown from congruent iron doped lithium niobate using the laser heated pedestal growth technique. To our knowledge, this is the first time that a grating has been written in a fiber using a single laser beam and its Fresnel reflection in a 180 degree(s) contradirectional two-beam coupling geometry. Beam-coupling efficiencies were compared as a function of beam spot size for fibers and bulk crystals. At small spot sizes, the fibers outperformed the bulk crystals by reducing the diffraction effect.
In this work we present the experimental results of contradirectional two-beam coupling in a bulk crystal and single crystal fibers of iron-doped lithium niobate. Results of a reduction of the grating writing instability, a comparison of the two-beam coupling efficiency of the fibers and bulk crystal as a function of focusing geometry, as well as a comparison of theoretical and experimental results of the two-beam coupling efficiency are presented.
The dead-space carrier multiplication theory properly predicts the reduction in the excess noise factor in a number of APDs. The theory is applied to measurements, obtained from J. C. Campbell and collaborators at the University of Texas, for InP, InAlAs, GaAs, and AlGaAs APDs with multiplication-region widths ranging from 80 nm to 1600 nm. A refined model for the ionization coefficients is reported that is independent of the width of the device multiplication region of each device. In addition, in comparison to predictions from the conventional multiplication theory, the dead-space multiplication theory predicts a reduction in the mean bandwidth as well as a reduction in the power spectral density of the impulse response. In particular, it is shown that the avalanching noise at high-frequencies is reduced as a result of the reduction of the multiplication region width.
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