Frequency Stabilization of Internal-Mirror He-Ne (λ=633 nm) Lasers Using the Polarization Properties

The laser frequency of an internal-mirror He-Ne (λ=633 nm) laser has been stabilized with a new scheme. The difference in intensity between the oscillating two axial modes with orthogonal linear polarizations is detected by modulating the azimuths of polarization of the modes by means of a highly efficient Faraday rotator. The electric voltage proportional to the difference signal is fed to a d.c. motor which rotates a cooling fan placed near the laser tube. This suppresses the thermal expansion of the cavity length due to the discharge current, consequently the laser frequency is stabilized. The relative frequency stability is obtained as 1.7×10-8 over a period of 1 min or longer.