Frequency-stabilized 3.39-μm He-Ne laser with no frequency modulation

The frequency of the 3.39-μm He-Ne laser whose output beam does not contain any artificial frequency modulation was newly stabilized in reference to the saturated absorption line of CH₄ gas contained in an external cell. Instead of modulating frequency by vibrating the resonator length, the frequency modulation was given by an external vibrating mirror on which the laser beam was reflected. A 3.39-μm low-noise optical amplifier was also used for observing saturated absorption in the external cell. A signal proportional to a frequency deviation from the reference absorption line was fed back to the resonator to control its length such that the frequency deviation was minimized. The short-term stability so achieved under a proportional feedback control was ±1.25×10^-12/min, the frequency drift was 2.3×10^-10/h, and the long-term stability under an integral feedback control was ±3.5×10^-11 over 5 h.