Heterodyne detection: phase front alignment, beam spot size, and detector uniformity.

We consider the effects of signal and local oscillator phase front misalignment, beam spot sizes, and electric field distributions on heterodyne detection. The signal and local oscillator fields that we consider are various combinations of Airy, Gaussian, and uniform distributions. We show that the values of the beam radii that maximize the heterodyne SNR are sensitive to phase front misalignment and that the degradation with misalignment angle is somewhat less severe for Airy received signals than for uniform. We also prove that for small optical spot sizes and perfect alignment, the optimal ratio of local oscillator Gaussian l/e field radius to signal Airy F number is approximately 0.7lambda. We next consider the effects of nonuniform detector quantum efficiency. Simple examples show that quantum efficiencies averaged over the detector surface give only crude estimates of the sensitivity of a heterodyne system. For accurate estimates full account must be made of the electric field parameters and the detector re ponse at each point on its photosurface.