Cancellation of local oscillator intensity noise caused by the relaxation oscillation of GaAlAs lasers with a dual-detector heterodyne receiver

Abstract

Coherent optical communication systems using semiconductor lasers can provide improved performance over direct detection systems due to higher receiver sensitivities. However, excessive local oscillator intensity fluctuations of present state-of-the-art semiconductor lasers usually prevent conventional heterodyne receivers from operating at the quantum limit. The amount of degradation can be substantial (2-40 dB).1.2 For wide-bandwidth systems in particular intensity noise due to the re- laxation oscillation of the local oscillator laser can degrade receiver sensitivities at frequencies well below the relaxation resonance frequency.3 A dual-detector heterodyne receiver has been proposed for the purpose of local oscillator intensity noise cancellation.4 The concept has been demonstrated for small receiver bandwidths (<150 MHz).5 In this paper we describe techniques and results of noise cancellation over a 3-GHz bandwidth. Intensity noise at the relaxation oscillation resonance has been successfully suppressed by 20 dB. Analytical performance bounds based on measurable device and circuit parameters are found to give excellent predictions of experimental results