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An infrared laser spectrometer based on difference-frequency generation in LiI[O.sub.3] is described. The spectrometer has a frequency uncertainty of less than 1 MHz and a signal-to-noise ratio between 3000:1 and 10,000:1. These properties allow the spectrometer to be used for studies of the non-Lorentzian and non-Voigt character of absorption line shapes in atmospheric trace gases. OCIS codes: 300.1030, 300.6190, 300.6320, 300.6340, 300.6360, 300.6390.

We show that aperiodic coupled cavities may be designed to have a frequency-dependent complex reflectance that is anisotropic in phase (birefringent) and in amplitude (dichroic). The complex mean or average reflectivity may also be frequency dependent. As an illustration we show how such multicavity effects influence the stability of the polarization of light in quasi-isotropic lasers. Possible applications of the properties of aperiodic coupled cavities are suggested.

An intracavity polarimeter is described that is capable of measuring both weak birefringence and dichroism simultaneously. It uses, for the first time to our knowledge, polarization flips in a quasi-isotropic He-Ne laser. The laser operates at 3.39 micrometers. Calibration of the polarimeter is carried out by using the anisotropic properties of a tilted plate. As an illustration of the sensitivity, the diffractive anisotropy of an intracavity slit is measured.

Precise spectral line measurements of carbon monoxide in an operating pressure modulator cell permit independent determinations of the instantaneous gas temperature and pressure.

A digital divider circuit is used in a feedback circuit that is designed to stabilize laser-beam pointing. Key words: Digital divider, beam-pointing stability.