Free-space optical communication (FSOC) offers the advantages of high bandwidth, directional signal propagation, and deployment flexibility without the infrastructure costs of fiber optics or the spectrum limits of radio frequency links. At the heart of many high-speed FSOC systems operating at 1550 nm, indium gallium arsenide (InGaAs) avalanche photodiodes (APDs) are commonly employed as detectors, but their performance is hindered by excess noise, limiting sensitivity and, consequently, feasible link distances. This paper investigates the integration of AlGaAsSb alloy APDs—lattice-matched to indium phosphide substrates—with commercial transimpedance amplifiers (TIAs), leveraging the alloy’s substantially lower excess noise to enhance sensitivity in 2.5 Gb/s digital optical receivers. Through simulations and experiments, the authors demonstrate that receivers built with AlGaAsSb APDs deliver mean sensitivities surpassing typical InGaAs APD-based commercial receivers by at least 2.7 dB, with the best observed improvement exceeding 3.4 dB at a bit-error rate of 10-9. The resulting high-gain, low-noise architecture offers significant advantages for enabling long-distance FSOC links, including applications in satellite, terrestrial, and space communications, especially under challenging atmospheric conditions. The findings underscore the technological potential of AlGaAsSb APDs to advance FSOC receivers and broaden the practicality of optical links in diverse environments.
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