A control system has been devised for stabilizing the output wavelength of a laser by reference to an external passive optical element. This element, consisting of two spherical mirrors, forms an off-axis resonator that, when broadly illuminated by a laser beam, functions as a wavelength-sensitive discriminator. The stabilization control loop is closed by using a signal from this discriminator to tune the laser by moving one of its mirrors with a piezoelectric transducer. The error signal is proportional to changes in wavelength of the incident laser beam and is derived from the discriminator without deliberate frequency or amplitude modulation of the laser The optical arrangement does not return any light in the direction of the source and thus avoids wavelength pulling due to spurious reflections. Two independent helium—neon lasers operating at 6328 Å were stabilized against a common reference interferometer using this system. Their relative stability was studied by heterodyning the two outputs and analyzing the beat spectrum. Each feedback loop had a gain of 60 dB at dc falling off to 46 dB at 200 c/s and to 20 dB at 2 kc/sec. The resulting wavelength stability was about Δλ/λ = 2 × 10-10. The residual instability was mainly owing to room noise occurring at frequencies above the response of the feedback loops.
Morley S. Lipsett and Paul H. Lee, "Laser Wavelength Stabilization with a Passive Interferometer," Appl. Opt. 5, 823-826 (1966)