Possible influence of residual amplitude modulation when using modulation transfer with iodine transitions at 543 nm

Abstract

A 543 nm He-Ne laser stabilized to iodine using modulation transfer spectroscopy is described. A beatfrequency comparison between it and a reference iodine-stabilized laser showed that it had a stability better than 2 × 10-12 after 140 seconds. The average frequency difference between the two lasers was only 12 kHz despite the lasers using different stabilization schemes. Two mechanisms for frequency shifts in modulation transfer spectroscopy are discussed, both arising from the presence of amplitude modulation on the phase-modulated saturating beam. The first is produced by the phase modulator itself and the second when the saturating beam propagates through the absorbing medium. For the laser described here, the frequency shifts are smaller than the uncertainty of the frequency measurement (~10 kHz).