Synthetic heterodyne interferometry for semiconductor laser spectral analysis

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Abstract
The laser is frequency shift keyed by superimposing a small amplitude square-wave signal on a bias injection current. The modulation period 2 T0 is chosen to be much longer than the dynamic response time of the laser and than the correlation time of its phase-noise process. Then the laser is time-shared between two steady states with different frequencies and uncorrelated phases. The optical field is analyzed by an unbalanced Mach-Zehnder single mode fiber interferometer of which the optical path difference is adjusted to be CT0. A detector at the output of the interferometer acts as an optical product detector. Therefore a spectrum analyser gives a transposition at a radioelectric frequency of the correlation product of the two states lineshapes. The one can easily measure the linewidth and dependence of the optical frequency on the injection current.