Military imaging is the largest application sector for shortwave infrared (SWIR) detectors, followed by spectroscopy (for the sorting of products and materials), and thermal sensing. Each application places different demands on the detectors, and fulfilling these requirements has driven the production of higher-quality, lower-cost imagers. The Visible SWIR Camera images digital pictures under day and starlight-only conditions, enabling the transmission of those images between soldiers on the battlefield. Additional functions are a windowing capability for comm link reception, and range-gating ability (viewing a specific depth of field at a specified range.) The combination of gated and video imaging is achieved through a high bandwidth pixel with a capacitive transimpedance amplifier (CTIA) design. Two different sensitivities in the CTIA pixel are achieved by switching between two feedback capacitor sizes, allowing for different illumination conditions. Anti-blooming is provided in the all solidstate gated camera, to prevent charge spreading from oversaturated pixels. All pixels are gated simultaneously for "snapshot" exposure. The low dark current and high bandwidth of the InGaAs photodetectors enables both high sensitivity imaging at long exposure times and high bandwidth at short exposure times. The spectral response of InGaAs extends from 0.9 μm to 1.7 μm, The Visible SWIR Camera is very reliable, in addition to being small and lightweight.
We report on a 640 x 512 pixel, 25 μm pitch, InGaAs focal plane array based camera with the ability to perform range-gated imaging, while also allowing integration times longer than 32 ms for imaging in a staring mode at video rates. The combination of gated and video imaging is achieved through a high bandwidth pixel with a capacitive transimpedance amplifier (CTIA) design. The CTIA pixel may be switched between two feedback capacitor sizes to allow two different sensitivities and capacities, depending on the illumination conditions. Anti-blooming is included in the pixel to prevent charge spreading from oversaturated pixels. All pixels are gated simultaneously for "snapshot" exposure. The all solid-state gated camera is very reliable, in addition to being small and lightweight. The low dark current and high bandwidth of the InGaAs photodetectors enables both high sensitivity imaging at long exposure times and high bandwidth at short exposure times. The spectral response of InGaAs extends from 0.9 μm to 1.7 μm, allowing the use of eye-safe commercially available pulsed lasers with 1.5 μm wavelength, several millijoule pulse energies, and nanosecond scale pulse durations.
We report on our 640x512 pixel InGaAs/InP focal plane array camera for visible and short-wavelength infrared imaging. For this camera, we have fabricated a 640x512 element substrate-removed backside-illuminated InGaAs/InP photodiode array (PDA) with a 25 mm pixel pitch. The PDA is indium bump bonded to a silicon read out integrated circuit. Removing the InP substrate from the focal plane array allows visible wavelengths, which would otherwise be absorbed by the InP substrate due to its 920 nm wavelength cut-off, to reach the pixels' active region. The quantum efficiency is approximately 15% at 500 nm, 70% at 850 nm, 85% at 1310 nm, and 80% at 1550 nm.
Features incorporated into this video-rate, 14-bit output camera include external triggering, windowing, individual pixel correction, 8 operational settings of gain and exposure time, and gamma correction. The readout circuit uses a gate-modulated pixel for high sensitivity imaging over a wide illumination range. This camera is useable for visible imaging as well as imaging eye-safe lasers and is of particular interest seeing laser designators and night vision as well as hyperspectral imaging.
We report on the recent production release of our 320x240 pixel InGaAs/InP focal plane array and camera for visible and short-wavelength infrared light imaging. For this camera, we have fabricated a substrate-removed backside-illuminated InGaAs/InP photodiode array hybridized to a silicon read out integrated circuit (ROIC). Removing the InP substrate from the focal plane array allows visible wavelengths, which would otherwise be absorbed by the InP substrate due to its 920 nm wavelength cut-off, to reach the pixels’ active region. Quantum efficiency is approximately 15% at 500 nm, 70% at 850 nm, 85% at 1310 nm and 80% at 1550 nm. This focal plane array is useable for visible imaging as well as imaging eye-safe lasers and is of particular interest for day and low light level imaging as well as hyperspectral imaging.
We report on recent results in using InGaAs/InP focal plane arrays for visible light imaging. We have fabricated substrate-removed backside illuminated InGaAs/InP focal plane arrays down to a 10 μm pitch with high quantum efficiency from 0.4 μm through 1.7 μm. This focal plane array can be used for visible imaging as well as imaging eye-safe lasers. Using the InGaAs/InP materials system for visible imaging applications has several advantages over silicon based CMOS or CCD imagers including inherent radiation hardness, the ability to simultaneously achieve low crosstalk (less than 1%), and bandwidths exceeding 1 GHz, as well as the ability to image out to 1.7 μm.
KEYWORDS: Avalanche photodetectors, Signal to noise ratio, Avalanche photodiodes, Receivers, Sensors, Indium gallium arsenide, Staring arrays, LIDAR, Capacitance, Chemical elements
We report on recent progress in developing 2-dimensional arrays of InGaAs/InP avalanche photodiodes. Advances in compound semiconductor epitaxy and device processing technologies enable large (128x128) element focal plane arrays with breakdown voltage standard deviations < 0.3%. The uniformity in breakdown voltage simplifies readout integrated circuit designs, in that a single bias voltage may be used for all elements in the array. Each element in the array achieves responsivities greater than 10 A/W at a wavelength of 1550 nm, while maintaining dark currents less than 20 nA. The APD arrays stand to enable new cameras for such applications as three-dimensional imaging, and various other laser radar and communications systems. In particular, the improved responsivity of avalanche photodiodes over their pin photodiode counterparts can improve sensitivities by as much as 6 - 10 dB depending upon the readout integrated circuit bandwidth. So-called "flash" laser radar systems wherein a single high energy laser pulse is used to image a target require the extra sensitivity afforded by avalanche photodiodes due to the low return photon count from distant targets.
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