Frequency stabilisation of He-Ne lasers

Use of the beat frequency between two modes for frequency stabilization of internal-mirror lasers

We present a simple method of frequency stabilization of a laser with two orthogonal modes that uses the effects of anomalous dispersion at the active line. The result of this effect is that the frequency difference between oscillation modes varies during the frequency tuning along the Doppler line of the transition and has its minimum or maximum when the modes are symmetrically placed around the central part of the gain curve. We applied the semiclassical approach to describe the laser, and we established the rules for laser parameters to obtain the most convenient conditions for stabilization. We stabilized the thermally compensated experimental laser tube and reached frequency stabilities of 3 × 10(-9) and 3 × 10(-10) for integrating times of 1 and 134 s, respectively.

Polarization-stabilized 1.15- and 3.39-microm He-Ne lasers

Two methods for polarization stabilization of an internal-mirror 3.39-microm He-Ne laser are reported. The first relies on a concurrently lasing 1.15-microm transition by fixing the relative amplitude of two orthogonally polarized longitudinal modes that are split by a Rochon prism and detected with separate Si photodiodes. In the second method, two spatially separated orthogonally polarized adjacent 3.39-microm modes are optically balanced, differentially chopped, and recombined on a single InSb photodiode for phase-sensitive detection. The dual-wavelength scheme has been tested by beating against a methane-stabilized 3.39-microm He-Ne laser, which yields maximum excursions of < 0.5 MHz over several hours and comparable reproducibility. The polarization-stabilized He-Ne laser has been used as a reference for a tunable color-center laser molecular-beam optothermal spectrometer and provides a precision of better than 2 MHz.

Frequency stabilisation of multimode helium-neon lasers in laser Doppler flowmetry

Low-frequency noise in laser Doppler recordings can be generated because of thermal instabilities in the gas laser cavity. Mode partition noise can be eliminated by thermal stabilisation of the laser cavity. The paper describes a method for closed loop temperature control of the laser cavity. The method uses orthogonal properties of longitudinal laser modes for the temperature control. The signal-to-noise ratio is considerably improved by the procedure. It is recommended that all gas laser equipped Doppler instruments used for precision blood flow studies should be equipped with stabilised laser sources.