The frequencies of two helium-neon lasers stabilized by intracavity saturated absorption of iodine (127I2) built at the National Bureau of Standards (NBS) and the National Physical Laboratory have been compared by beat-frequency measurement. The NBS laser exhibited a power-dependent frequency shift that was reproducible and significant in size (2 × 10-10) over the working range of the power output. The origin of the power shift has not been established, but several possible sources of the shift have been eliminated by our investigations. Specifically, the effect is not electronic in origin. No similar shift was observed in two lasers built at the National Physical Laboratory.
H. P. Layer, W. R. C. Rowley, and B. R. Marx, "National Physical Laboratory-National Bureau of Standards iodine-stabilized helium-neon laser intercomparison," Opt. Lett. 6, 188-190 (1981)