Measurement of the optical phase statistics of single-mode semiconductor lasers biased around the threshold current

We report on the experimental analysis of the statistics of the optical phase of the light emitted by a distributed-feedback (DFB) laser. We measure the dependence of the phase diffusion coefficient on the applied bias current in the region around the threshold current. We measure the phase noise statistics using three different methods based on i) averages of the noise phase trajectories over different temporal windows, ii) the variance of the phase noise difference, and iii) the frequency noise spectral density. The comparison between the results obtained with these methods is succesful. We obtain that the phase diffusion coefficient monotonously decreases with the bias current in the region around threshold. We finally compare our experimental results with simple analytical expressions for the below and above threshold cases based on Schawlow-Townes laws.