This paper contains an analysis of the behavior of saturable filters, based on models with simple energy level structures. In this analysis, the effects of possible excited state absorption at the irradiating wavelength are considered and are shown to give rise to a nonsaturable component of the absorption coefficient. Expressions for the steady-state transmission of an optically thick saturable absorber, and for the temporal response of an optically thin absorber, are derived. The question of homogeneous vs inhomogeneous broadening of the absorption line is discussed, and it is suggested that spectral hole burning may occur in a thermally broadened line at a sufficiently high irradiance level. The desirable features of a saturable absorber for use in a Q-switched laser system are briefly described, and it is shown how the usual rate equations for a laser system may be amended to include the effects of an intracavity saturable filter.

The spectrum of resonantly scattered light at right angles to a sodium atomic beam is reported. The light source was a cw dye laser tuned to resonance with a hyperfine component of the Dz line, and incident at right angles to the atomic beam. The spectrum, with the Stark effect sidebands, was recorded as a function of both the laser intensity and its detuning from resonance. The overall resolution is better than 20 MHz.

A new flow cytometer with a very small observation volume has been developed to detect individual viruses with good resolution, and has been used to discriminate between two types of viral particles based on differences in their light scattering. Measurements of light scattering and fluorescence made with such an instrument can provide a basis for quantitative analysis and sorting of viruses and other particles in the micron and submicron size range.

The theory, design, and use of the confocal spherical mirror Fabry-Perot interferometer (FPS) is described in detail. Topics covered include performance of an FPS for small departures from the confocal mirror separation, optimization of the (resolution) x (light gathering power) product, factors limiting realizable finesse, mode matching considerations, alignment procedures, and general design considerations. Two specific instruments are described. One is a versatile spectrum analyzer with piezo-electric scanning; the other is a highly stable etalon with fixed spacing. Examples of the performance of these instruments are given.

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