OPTICAL INTERFERENCE FRINGE REDUCTION IN FREQUENCY-MODULATION SPECTROSCOPY EXPERIMENTS

We show both theoretically and experimentally that interferer;ce fring e signals can always be suppressed to improve the signal-to-noise rati o, provided that the modulation frequency is of the order of the absor ption linewidth or higher. Suppression of optical interference fringes by more than 1 order of magnitude and signal-to-noise ratio enhanceme nt of more than 13 dB is demonstrated by use of a proper choice of las er modulation frequency. A further fringe reduction of 10 dB is possib le by adjustment of the local oscillator phase. (C) 1995 Optical Socie ty of America