Characterising semiconductor laser phase noise using delayed self-homodyne measurements with optical coherent reception

We report on the experimental and theoretical analysis of the phase noise of the light emitted by a single-mode laser diode. We analyze the time dependence of the variance of the optical phase obtained in a delayed self-homodyne with optical coherent reception setup. This analysis is of interest since that phase is used for quantum random number generation. We show that the variance linearly grows from the initial time until the time delay corresponding to the extra fiber length of the setup. We show experimentally and theoretically that this initial growth can be used for measuring the phase diffusion coefficient with a delayed self-homodyne setup. This is confirmed by comparing with the results obtained with an heterodyne setup. After that time delay the variance continues to grow more slowly than in the initial growth. We explain that behavior by the progressively important effect of low-frequency noises on the phase noise as the measurement time window increases.