Laser radar detection statistics: A comparison of coherent and direct detection receivers

Detection statistics for a coherent laser radar are substantially different from those of a direct detection laser radar. Direct detection ladar detection statistics vary depending upon the detection mode (i.e., photon-counting vs. continuous direct detection). Speckle noise also impacts the detection statistics. For a single-pixel single-frequency single-polarization coherent detection transceiver, speckle noise can only be suppressed through temporal averaging. Some degree of speckle averaging can also be achieved in coherent detection systems by using a multiple frequencies or dual polarizations. In addition to these, a direct detection receiver can exploit spatial diversity to suppress the effects of speckle. This paper develops theory useful for describing the performance of these three receiver architectures against diffuse and glint targets and provides example performance comparisons. We show that a photon-counting direct detection receiver can, in principle, provide superior performance, however practical limitations of current detector technology particularly in the near IR spectral region reduces the performance margin and for many applications a coherent detection receiver provides superior performance.