Abstract

The performance of an optical heterodyne receiver is strongly influenced by the degree to which the signal and local oscillator beams can be spatially matched in both amplitude and phase at the detector. Any mismatch reduces the IF signal power and consequently the SNR. The ratio of IF signal power obtained in the presence of a mismatch to that obtained when there is no mismatch is referred to as the spatial mode matching efficiency. Several authors have performed theoretical studies to determine the mode matching efficiency as a function of system parameters, including signal/local oscillator beam angular misalignment, signal/local oscillator beam size differences, and detector-to-spot size ratios. Experiments to quantitatively measure mode matching efficiencies, however, are few. We experimentally measured these efficiencies when two independent GaAlAs semiconductor lasers were heterodyned on a high-speed silicon PIN photo-detector. The experiments have also been repeated using a single frequency-modulated GaAlAs laser, which is self-heterodyned with the aid of an unequal path length Mach-Zehnder interferometer. In the former case, maximum repeatable efficiencies of 75% were obtained. In the latter case, these values increased to ~90 %.

© 1987 Optical Society of America