Leland Stanford Junior University

Juergen Martin

Richard Neil Zare

Barbara A Paldus

Jinchun Xie

Ring-shaped resonant cavities for spectroscopy allow a reduction in optical feedback to the light source, and provide information on the interaction of both s- and p-polarized light with samples. A laser light source is locked to a single cavity mode. An intracavity acousto-optic modulator may be used to couple light into the cavity. The cavity geometry is particularly useful for Cavity Ring-Down Spectroscopy (CRDS).

Ring resonant cavities for spectroscopy

A cavity ring down spectroscopy (CRDS) system uses a free-running continuous wave (c.w.) diode laser stabilized by frequency-shifted optical feedback in the presence of strong reflections from a high-finesse Fabry-Perot resonator. The frequency-shifted feedback stabilization eliminates the need for tightly controlling the relative positions of the laser and resonator. Non-frequency-shifted feedback is used for linewidth broadening. An acousto-optic modulator placed between the diode laser output and the resonator input frequency-shifts light reflected by the resonator input, causing the laser to cycle in phase with a period equal to the inverse of the frequency-shift. The laser diode linewidth can be stabilized from several MHz for high resolution spectroscopy of species at low pressures, to several hundred MHz for lower resolution spectroscopy of species at atmospheric pressures.

Spectroscopy using active diode laser stabilization by optical feedback

Light is coupled into a cavity ring down spectroscopy (CRDS) resonant cavity using an acousto-optic modulator. The AOM allows in-coupling efficiencies in excess of 40%, which is two to three orders of magnitude higher than in conventional systems using a cavity mirror for in-coupling. The AOM shutoff time is shorter than the roundtrip time of the cavity. The higher light intensities lead to a reduction in shot noise, and allow the use of relatively insensitive but fast-responding detectors such as photovoltaic detectors. Other deflection devices such as electro-optic modulators or elements used in conventional Q-switching may be used instead of the AOM. The method is particularly useful in the mid-infrared, far-infrared, and ultraviolet wavelength ranges, for which moderately reflecting input mirrors are not widely available.

Growing community of inventors

Harxheim, Germany

Juergen Martin