Two-mode multiline laser stabilization using a Fabry-Perot filter under analog and digital computer control

A scanning Fabry-Perot interferometer, servolocked to one of the output frequencies of a multiline laser, is used as a tracking filter so that a second servoloop can accurately stabilize that laser line to features of its intensity-versus-frequency profile. More stable locks occur where two longitudinal modes lase simultaneously than where the single-mode line intensity is maximized. Both analog and digital servoloops have been used for stabilizing a continuous-wave CO laser cavity. The microcomputer-aided digital stabilization yields the more reliable frequency locks and needs negligible frequency dithering. It has produced short-term (less than 1 s) laser linewidths less than 100 kHz and long-term (greater than 100 s) instability estimated as 80 kHz at a laser operating frequency of 55 THz in a noisy laboratory environment.