Principles of optical phase-locking: Application to internal mirror He-Ne lasers phase-locked via fast control of the discharge current

While phase-locking techniques have long been in routine use in the microwave frequency domain, the ten-thousand-fold increase of the carrier frequency in the visible domain brings new problems for effective optical phase control. Particularly robust locks can be based on combining phase-locking under quiescent conditions with an outer frequency-control loop which takes control when phase lock is lost. Electronic strategies for implementing this process are discussed. Diagnostic techniques to allow characterization of phase-lock performance of optical oscillators are presented and discussed using as an example the phase-locking of a low-cost internal mirror He-Ne laser tube. Effectively complete phase lock was possible using small variations of the discharge current to provide fast frequency control.