To demonstrate (benchtop experiment) a DPSK receiver with a free-space interferometer, showing that fiber-optic coupling, associated adaptive optics, and optical preamplification can be avoided, with potential large cost savings; To rigorously quantify the performance (capacity and photon-budget) tradeoff between conventional (adaptive optics + optical preamplification) systems and our proposed free-space photon-counting-array approach; To develop extensions to orthogonal phase-modulated signaling over multiple symbols and associated photon-counting receiver structures, which enable approaching the photon efficiency of high-order pulse position modulation.
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