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Coherent lidar sensors provide a useful resource in a sensor suite, allowing high dynamic range, near quantum limited sensitivity, and derivation of phase, frequency and intensity information from the received light. However, a conventional system can only provide single pixel detection, requiring the pixel to be scanned over the scene to produce an image, the rate of which is limited by mechanical bandwidths. Alternatively, a direct detection focal plane array (FPA) can provide many pixels to image a full scene but lacks the required bandwidth for militarily useful data product. The HiPAR (High Pixel Acquisition Rate) program is developing a proof of principle system to combine the advantages of coherent lidar with the simultaneous multi-pixel acquisition advantages of an FPA. In a novel optical arrangement, a line of laser light is projected onto the scene. Backscatter from the line is then mixed in the receive path with a local oscillator which matches the received image and the two, interfering fields are imaged onto an FPA producing a single coherent line image. Phase differences between the scene and local oscillator fields – e.g. due to Doppler shift or intentional modulation – cause the short-term irradiance in the line to fluctuate both spatially (along the line) and in time. Spatial variation is adequately sampled by FPA pixels but, in general, temporal fluctuation will significantly exceed FPA frame rate. To overcome the bandwidth limitations, the system is configured to rapidly sweep the line across the FPA so that the temporal fluctuations are captured as spatial variations in the second dimension. The FPA behaves as an “optical storage oscilloscope”, similar in concept to a streak camera. High sweep rates allow interference modulation rates far in excess of frame rate to be captured. Thus, high bandwidth coherent information can be derived from a line of pixels across the scene. By using a scanning mirror in the transmit/receive chain, the line imager can be scanned over an entire scene to build up a full image for Doppler or range-finding purposes, with line acquisition proceeding at FPA frame rate. This paper presents the physical principles of the system, the progress to date and the initial results.
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