SPECTRAL FILTERING WITHIN THE SCHAWLOW-TOWNES LINEWIDTH AS A DIAGNOSTIC-TOOL FOR STUDYING LASER PHASE NOISE

We report on spectral filtering within the fundamental or Schawlow-Townes linewidth of a laser. Intensity correlation functions of the filtered laser light have been calculated assuming the phase-diffusion model (PDM), the phase-jump model (PJM) and the phase-increment model (PIM) as the model underlying the statistics of the laser phase noise. Experiments have been performed on semiconductor lasers; either a one-line or a two-line filter has been positioned underneath the Schawlow-Townes linewidth of the semiconductor laser. We observe that a one-line filter converts (amplitude-stable) laser light into chaotically fluctuating light in the limit that the filter linewidth is much smaller than the Schawlow-Townes linewidth. Two such narrow filters, when sufficiently separated spectrally, lead to two independent chaotic sources. Distribution functions and correlation functions of the filtered light have also been measured as a function of the filter bandwidth. In this case the various models for laser-phase noise lead to different theoretical predictions. We find that the experimental results agree with the phase-diffusion model and not with the other models.