Sensitivity Improvement of 2.5 Gb/s Receivers Using AlGaAsSb Avalanche Photodiodes

At a 1550 nm wavelength, the optical sensitivity of conventional indium gallium arsenide (InGaAs)-based avalanche photodiodes (APDs) is restricted by their high excess noise, hindering their performance in long-range free-space optical communication links. Al0.85Ga0.15As0.56Sb0.44 (AlGaAsSb), lattice-matched to indium phosphide (InP) substrates, has a much lower excess noise factor than InP, the conventional avalanche material. In this work, we evaluated the performance of optical APD-TIA receivers utilizing InGaAs/AlGaAsSb APDs through simulations and experiments. Simulations confirmed their optimum gain is much higher than conventional APDs. InGaAs/AlGaAsSb APD dies and transimpedance amplifier (TIA) chips were integrated, yielding four optical receivers for experimental evaluation. At 2.5 Gb/s and BER = 10−9, these receivers operated at a high optimal gain of 56 (as predicted in simulations) and produced a mean sensitivity of −38.5 dBm, with the best sensitivity at −39.2 dBm. These sensitivity values are at least 2.7 (or, in the best case, 3.4) dB better than those of typical commercial receivers with InGaAs APDs. This work quantifies the significant performance improvement that InGaAs/AlGaAsSb APDs provide to long-range free-space optical communication links.