Suppression of optical feedback effects on saturated absorption signals by phase modulation of the reflected light

In high-resolution spectroscopy, the Doppler broadening is suppressed through the use of two counter-propagating beams so that one of the beams is obviously reflected back into the laser cavity. Conventional optical isolators are efficient enough to reduce the coupling but some intensity fluctuations always remain due to residual feedback. A simple technique is described which does not suppress the residual optic feedback but completely cancels its effect on line shapes and signals.