The ultimate performance of any remote sensor is ideally governed by the hardware signal-to-noise capability and allowed signal-averaging time. In real-world scenarios, this may not be realizable and the limiting factors may suggest the need for more advanced capabilities. Moving from passive to active remote sensors offers the advantage of control over the illumination source, the laser. Added capabilities may include polarization discrimination, instantaneous imaging, range resolution, simultaneous multi-spectral measurement, or coherent detection. However, most advanced detection technology has been engineered heavily towards the straightforward passive sensor requirements, measuring an integrated photon flux. The need for focal plane array technology designed specifically for laser sensing has been recognized for some time, but advances have only recently made the engineering possible. This paper will present a few concepts for laser sensing receiver architectures, the driving specifications behind those concepts, and test/modeling results of such designs.
The sensitivity of imaging, hyperspectral, passive remote sensors in the long-wavelength infrared (LWIR) spectral region is currently limited by the ability to achieve an accurate, time-invariant, pixel-to-pixel calibration of the elements composing the Focal Plane Array (FPA). Pursuing conventional techniques to improve the accuracy of the calibration will always be limited by the trade-off between the time required to collect calibration data of improved precision and the drift in the pixel response that occurs on a timescale comparable to the calibration time. This paper will present the results from a study of a method to circumvent these problems. Improvements in detection capability can be realized by applying a quick, repetitive dither of the field of view (FOV) of the imager (by a small angular amount), so that radiance/spectral differences between individual target areas can be measured by a single FPA pixel. By performing this difference measurement repetitively both residual differences in the pixel-to-pixel calibration and 1/f detector drift noise can effectively be eliminated. In addition, variations in the atmosphere and target scene caused by the motion of the sensor platform will cause signal drifts that this technique would not be able to remove. This method allows improvements in sensitivity that could potentially scale as the square root of the observation time.
For infrared laser remote sensing, a direct detection receiver may be optimally designed around a high speed, low noise focal plane array (FPA). Short pulse, high repetition rate operation of the laser transmitter makes it beneficial to operate the detector with short integration times, lowering the limiting integrated background photon flux. With this photon signal (which constitutes a noise contribution) made small enough, improved low-noise readout integrated circuits (ROIC) can be used to realize a significantly improved imaging lidar receiver. A 10 by 10 pixel ROIC has recently been designed and fabricated. Demonstrated capabilities include > 100 kHz frame rate, 50 ns integration time, and less than 100 e- of input-referred readout noise. These ROICs have been mated with long-wavelength HgCdTe infrared detector arrays, with cutoff wavelengths greater than 11 micrometer. Characteristics of the demonstrated ROIC design will be presented, along with testing of the focal plane arrays hybridized to them.
A high sensitivity, CO2 lidar detector, based on recent advances in ultra-low noise, readout integrated circuits (ROIC), is being developed. This detector will combine a high speed, low noise focal plane array with a dispersive grating spectrometer. The spectrometer will filter the large background flux, thereby reducing the limiting background photon shot noise. In order to achieve the desired low noise levels, the HgCdTe FPA will be cooled to approximately 50 K. High speed, short pulse operation of the lidar system should enable the detector to operate with the order of a few noise electrons in the combined detector/ROIC output. Current receiver design concepts will be presented, along with their expected noise performance.
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