Broad maxima and cusp-like minima can be observed in the laser intensity noise spectrum when light from a fixed-frequency, solitary diode laser tuned near the rubidium D2 hyperfine components is transmitted through a rubidium cell. The spectrum of amplitude noise extends from DC to at least 9 GHz. The lowest-noise feature is smaller than the intrinsic diode laser noise and approaches the shot-noise limit. Broad noise maxima are due to laser FM noise converted to AM noise by dispersive phase shifts; noise minima result either when the phase shift returns the AM noise to FM noise or when both laser noise sidebands are strongly absorbed. Complex experimental lineshapes compared with a simple model based on dispersive phase-shift conversion between laser FM noise and AM noise show surprisingly good agreement. The figure shows the computed laser noise spectrum as a function of Fourier frequency and laser tuning through ⁸⁷Rb D2 hyperfine components. The calculation assumes a cell of 10 cm length at a temperature of 130 C.

Hugh Robinson, NIST, hgr@boulder.nist.gov