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This feature issue of Applied Optics contains 31 research papers on photon correlation and scattering, many of which were presented at an OSA Topical Meeting that was held 21-23 August 2000 in Whistler, British Columbia, Canada. These papers focus on research in dynamic light scattering, surface light scattering, photon correlation, and laser velocimetry and their applications to physical, chemical, and biological processes.

Optical fiber homodyne dynamic light scattering employs both the advantage of a sensitivity improvement over the standard self-beating technique and the inherent self-aligning simplicity of the optics. Ultralow concentrations of approximately nanometer-sized particles become accessible by dynamic light-scattering techniques.

This feature issue of Applied Optics contains 25 research papers on photon correlation and scattering. Many of the papers in this volume were presented at an OSA Topical Meeting that was held 21-24 August 1996 in Capri, Italy. The focus of these papers is research in dynamic light scattering, surface light scattering, photon correlation, laser velocimetry, and their applications to biological, chemical, and physical processes.

Obtaining true and beautiful data from any photon correlation experiment demands serious attention to optimizing both the measuring system and experimental conditions. The laser must have sufficient power, be stable under all likely conditions, and usually be restricted to a single transverse mode. The beam-delivery optics must be carefully designed, built, and verified. The scattering medium must contain a proper concentration of suitably sized scatterers with appropriate characteristics. Surfaces surrounding the point of measurement must not introduce optical noise. Flare reduction in the receiver optics may be improved with ghost analysis, spatial and spectral filtering, and careful choice of stops, baffles, and surface coatings. The photon detector must have adequate speed and sensitivity with suitably low internal correlations and noise. The choice of correlator is crucial. Sometimes the equipment must be small to reach inaccessible places. Performance may be compromised by thermal, mechanical, or electrical instabilities caused by exposure to environmental excesses. Errors may even be introduced by preprocessing hardware and software before proper information is extracted. With so many conditions and potential problems, how does one obtain beautiful data, leading to correct results and enlightening information? That is the focus of our work.

We describe the design, construction, and testing of a miniature, all-solid state laser light scattering instrument for determination of particle sizes and distributions using photon correlation spectroscopy techniques (i.e., quasielastic or dynamic light scattering). Detailed comparative tests with standard photon correlation spectroscopy equipment are presented.

We describe the design, construction and testing of a miniature, solid state laser Doppler velocimeter (LDV) for measurement of the velocities of gases, liquids and solid surfaces, especially using photon correlation techniques. Detailed comparative tests with standard LDV equipment are presented.

We continue examination of the photon correlation properties of silicon avalanche photodiodes operated in photon-counting mode by extending their operation from that of passive quenching(1) to active quenching, yielding shorter dead time and higher frequency operation.

We describe the use of monomode optical fibers in dynamic light scattering, i.e., photon correlation spectroscopy or quasi-elastic light scattering.

We continue examination of the photon correlation properties of silicon avalanche photodiodes operated in the single-photon counting mode by extending their operation from that of passive [Appl. Opt. 25, 4122-4126 (1986)] and active [Appl. Opt. 26, 2383-2389 (1987)] quenching to the sub-Geiger mode, with potential for high quantum efficiency and very low afterpulsing.