Abstract

We have developed a frequency-multiplex laser spectrometer using the concept of stochastic excitation to provide an effective optical excitation bandwidth of >200 MHz and an inherent spectral resolution of a few kilohertz. A frequency-stabilized Ar⁺ laser is phase-modulated by a pseudorandom binary sequence (PRBS) running at a clock frequency of 200 MHz. The detected system response to the stochastic optical field is cross-correlated with an identical time-delayed PRBS to give the equivalent of a free induction decay signal. A time-to-frequency transformation then gives the transfer function of the optically absorbing/dispersing system. The calibration and linearity of the frequency axis are dependent only on the oscillator used to clock the PRBS generators, thus avoiding the problems of drift and nonlinearities in ordinary sequential-scan spectroscopy. The multiplexing scheme allows a spectrum to be obtained in much less time than sequential scanning or, equivalently, gives a higher SNR for the same amount of data taking time. We have demonstrated the spectrometer by observing Doppler-free transitions in molecular iodine. We also describe the use of digitally filtered binary sequences and active control of residual amplitude modulation to improve the SNR of the spectrometer and possibilities for increasing the excitation bandwidth of the system.

© 1987 Optical Society of America