Optical pumping of an atomic vapor with a frequency-modulated laser

Abstract

Rubidium maser and passive frequency standards performances rely on the efficiency of the optical pumping scheme. When using a semiconductor laser as the pumping source in such experiments, one found that the sharpness of the laser spectrum allows only the excitation of a small class of atomic velocities. The use of a frequency-modulated laser has shown great promise by largely increasing the overall pumping efficiency.1 Broadening of the narrow laser spectrum ensures the contribution of all classes of atomic velocities present in the vapor.2 Different modulation waveforms, amplitudes and frequencies have very important effects in overall optical pumping efficiency. We theoretically evaluate the efficiency of a frequency-modulated laser pumping source by studying deterministic as well as random modulation. Important considerations are obtained regarding the optimal conditions concerning the magnitude and the bandwidth of the modulation signal. Moreover a deterministic modulation must be used to lock the laser frequency by interrogating an atomic cell and synchronously detecting the transmitted intensity. Experiments conducted on a rubidium 87 maser optically pumped with a semiconductor laser confirm the predictions. In fact the oscillation of the maser was only possible with the help of the laser frequency modulation.