A phase-sensitive technique to measure small changes in laser frequency: application to measure the shift and broadening of a saturated absorption line of OsO4

Abstract

A phase-sensitive technique was used to measure the sensitivity to operating conditions of the output frequency of a highly stable carbon dioxide laser, which operated on the 10-μmm R(0) transition (28.8 THz) and was locked to the saturated absorption of the Q(15) transition of OsO4. Periodic changes in the operating conditions were imposed on the laser and the resulting changes in the output frequency were measured through phase-sensitive demodulation of the heterodyne beat with a second CO2 laser of much lower stability. Shifts as small as 50Hz could be detected by this technique. The results are in good agreement with previous measurements which employed an infrared frequency chain.