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Instrumentation for Sensitive Gas Measurements

Measurements are based on cavity-enhanced absorption.

An improved instrument for optical absorption spectroscopy utilizes off-axis paths in an optical cavity in order to increase detection sensitivity while suppressing resonance effects. The instrument is well suited for use in either cavity ring-down spectroscopy (CRDS) [in which one pulses an incident light beam and measures the rate of decay of light in the cavity] or integrated cavity output spectroscopy (ICOS) [in which one uses a continuous-wave incident light beam and measures the power of light in the cavity as a function of wavelength].

Typically, in optical absorption spectroscopy, one seeks to measure absorption of a beam of light in a substance (usually a gas or liquid) in a sample cell. In CRDS or ICOS, the sample cell is placed in (or consists of) an optical cavity, so that one can utilize multiple reflections of the beam to increase the effective optical path length through the absorbing substance and thereby increase the sensitivity for measuring absorption. If an absorbing substance is not present in the optical cavity, one can utilize the multiple passes of the light beam to increase the sensitivity for measuring absorption and scattering by components of the optical cavity itself.

It is desirable to suppress the effects of resonances in the cavity in order to make the spectral response of the cavity itself as nearly constant as possible over the entire wavelength range of interest. In the present instrument, the desired flattening of the spectral response is accomplished by utilizing an off-axis beam geometry to effectively decrease the frequency interval between longitudinal electromagnetic modes of the cavity, such that the resulting transmission spectrum of the cavity is nearly continuous: in other words, the cavity becomes a broad-band optical device.

The instrument (see figure) includes an external-cavity diode laser, the output of which is directed by folding mirrors into an optical cavity. Optionally, the electromagnetic modes of the laser and the cavity can be matched by use of two telescope lenses. Also, optionally, the laser beam can be chopped for (CRDS) or not chopped (for ICOS). The optical cavity is bounded by two identical mirrors, typically having a diameter of 25 mm and a radius of curvature of 6 m (and, hence, a focal length of 3 m). Typically, the mirrors are longitudinally separated by a distance of 67 cm. Preferably, the mirrors both have reflectivities >0.99, though reflectivities as low as 0.5 could be acceptable in some applications.

The aforementioned choice of typical parameters — and in particular, the choice of a mirror focal length greater than the cavity length — was made on the basis of numerical simulations of a variety of cavity designs that yield dense mode spectra. The simulations showed that the long-focal-length condition minimizes the sensitivity of performance to changes in the alignment and
spacing of the mirrors. In addition, as a result of this choice of parameters, the light emerges from the cavity in a pattern that facilitates focusing of the light onto a possibly small photodetector.

The greatest advantage afforded by this design, relative to an on-axis design, is that one can use a narrow-band laser (bandwidth < 100 MHz), without need for complex, expensive equipment for precisely controlling the cavity length to manipulate or suppress resonances. Another advantage is reduction of optical feedback from the cavity to the laser and consequent elimination of need for optical components to suppress such feedback.

Yet another advantage is removal of a major constraint on the alignment of the overall optical system: Instead of only one possible alignment geometry (the laser beam aligned with the optical axis of the cavity), one can choose any alignment for which there is a stable optical path through the cavity. As a result, alignment routine can be simplified and the instrument is less sensitive to vibration.

Currently, a product line of instruments is being developed based on the technology described above for highsensitivity, precision, and accurate gas measurements. Some applications that will benefit from this technology include atmospheric monitoring, chemical and biological agent detection, industrial process control, and medical diagnostics.

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This work was done by Anthony O’Keefe of Los Gatos Research for Ames Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to the Patent Counsel, Ames Research Center, (650) 604-5104. Refer to ARC-14690-1.