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The red side (lower-frequency) mode of a two-mode stabilized 633-nm He–Ne laser has been locked to the hyperfine structure of the P(33) line of ^127I_2 by means of frequency modulation spectroscopy enhanced by an external optical cavity. Both the red side and blue side (higher-frequency) modes of the laser exhibit a frequency stability of 2.3 × 10^-11 τ^-1/2. In addition, the frequency fluctuations of the blue side mode are detected by a Fabry–Perot cavity and compensated through an acousto-optic frequency shifter. The short-term stability of better than 3 × 10^-11 is attained for integration times of between 2 × 10^-3 and 2 × 10^-1 s.

A noncontact interferometric thermometry method based on phase-compensating interferometry is described. Direct frequency monitoring by the reference interferometer connected in parallel to the sensing interferometer provides temperature measurement of high accuracy.