PROBABILITY-DENSITY FUNCTION OF NOISE AT THE OUTPUT OF A 2-BEAM INTERFEROMETER

The probability-density function (PDF) of the fluctuating output intensity of a two-beam interferometer, illuminated by a semiconductor laser, is described theoretically and experimentally. The PDF, while dominated by the laser phase noise, may be substantially modified both by the laser intensity noise and by the thermal noise and bandwidth limitations imposed by the detection system. Two regimes of operation are explored. In the coherent regime, for which the interferometer's differential delay-tau is much smaller than the laser coherence time tau-c, the form of the PDF is highly sensitive to the mean phase difference between the interfering beams: The PDF is strongly asymmetrical for mean phase differences of 0 and pi (experimentally reported here for the first time to our knowledge) but is symmetrical when the interferometer is in quadrature. In the incoherent regime, for which tau >> tau-c, the PDF is insensitive to such phase differences. The experimental results are in excellent agreement with a numerical simulation for which it is assumed that the laser phase noise is a Wiener process and for which the finite bandwidth of the detection system, as well as the effects of laser intensity noise and system thermal noise, is taken into account.