Conversion of Laser Phase Noise to Amplitude Noise in a Resonant Atomic Vapor: The Role of Laser Linewidth

When laser light propagates through a resonant medium, the transmitted beam can exhibit excess intensity noise AMPLITUDE MODULATION (AM). In a semiclassical description of the phenomenon, laser phase noise (PM) induces fluctuations in the medium's electric susceptibility, which in turn cause fluctuations in the transmitted intensity. The process provides an efficient means for PM-to-AM conversion, and intuition suggests that large linewidth lasers should exhibit much greater PM-to-AM conversion than narrow linewidth lasers. Here we measure the relative intensity noise (RIN) for two diode lasers whose linewidth delta-v sub L differ by more than 10(exp 2), after the lasers have propagated through a resonant rubidium vapor. Though the RIN of the narrow linewidth laser is only reduced by a factor of about 6 compared to the broad linewidth laser, our results are nonetheless consistent with numerical simulations of the PM-to-AM conversion process. In particular, both computation and analytical theory indicate that RIN is a nonlinear function of delta-v sub L. For single-mode laser linewidth less than the atomic dephasing rate, RIN increases like square roots of delta-v sub L, while for linewidth greater than the atomic dephasing rate RIN is a decreasing function of delta-v sub L.