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A cavity-enhanced absorption spectroscopy instrument has an optical cavity with two or more cavity mirrors, one mirror of which having a hole or other aperture for injecting a light beam, and the same or another mirror of which being partially transmissive to allow exit of light to a detector. A sph

A cavity-enhanced absorption spectroscopy instrument has an optical cavity with two or more cavity mirrors, one mirror of which having a hole or other aperture for injecting a light beam, and the same or another mirror of which being partially transmissive to allow exit of light to a detector. A spherical-spherical configuration with at least one astigmatic mirror or a spherical-cylindrical configuration where the spherical mirror could also be astigmatic prevents a reentrant condition wherein the injected beam would prematurely exit the cavity through the aperture. This combination substantially increases the number of passes of the injected beam through a sample volume for sensitive detection of chemical species even in less than ideal conditions including low power laser or LED sources, poor mirror reflectivity or detector noise at the wavelengths of interest, or cavity alignment issues such as vibration or temperature and pressure changes.

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1. A cavity-enhanced absorption spectroscopy instrument, comprising: an optical cavity defined by at least two cavity mirrors and containing a sample volume, a first one of the cavity mirrors having an optical aperture formed therein, a same or different one of the cavity mirrors being partially tra

1. A cavity-enhanced absorption spectroscopy instrument, comprising: an optical cavity defined by at least two cavity mirrors and containing a sample volume, a first one of the cavity mirrors having an optical aperture formed therein, a same or different one of the cavity mirrors being partially transmissive of light;a tunable-wavelength light source supplying a light beam directed through the aperture so as to be injected into the optical cavity, at least one of the cavity mirrors being characterized by having an astigmatism such that respective longest and shortest radii of curvature through orthogonal axial planes of that astigmatic mirror differ by at least 1% and selected such that the light beam in the cavity is prevented from exiting the cavity through the aperture until passing in excess of 30 times through the sample volume;a detector positioned to collect light passing through the partially transmissive cavity mirror; anda data acquisition and analysis system coupled to the detector and configured to at least determine wavelength-dependent absorption of a sample in the sample volume an integrated cavity output spectroscopy (ICOS) mode or cavity ring-down spectroscopy (CRDS) mode, identify one or more component species present in that sample, and determine a concentration level of the identified component species. 2. The spectroscopy instrument as in claim 1, wherein the optical cavity comprises two mirrors with substantially spherical curvature. 3. The spectroscopy instrument as in claim 1, wherein the optical cavity comprises a first mirror with a substantially spherical curvature and a second mirror with a cylindrical curvature. 4. The spectroscopy instrument as in claim 1, wherein at least one mirror is aspheric and/or conic such that the cavity is characterized by a different optical mode density at the center and periphery of the cavity. 5. The spectroscopy instrument as in claim 1, wherein the aperture is a partially reflective and partially transmissive region of an otherwise highly reflective cavity mirror. 6. The spectroscopy instrument as in claim 1, wherein the first cavity mirror having the aperture is a highly reflective dielectric-coated mirror with a transmissivity less than 10 parts per million except at the aperture. 7. The spectroscopy instrument as in claim 1, wherein the partially transmissive mirror has a reflectivity of at least 98% and a transmissivity of at most 2%. 8. The spectroscopy instrument as in claim 1, wherein the light beam is directed through the aperture at a tilt angle of greater than 0.5° that is selected to maximize number of passes through the sample volume. 9. The spectroscopy instrument as in claim 1, wherein the aperture is located off of a central axis of the optical cavity for off-axis injection of the light beam. 10. The spectroscopy instrument as in claim 1, wherein the optical cavity has mirror curvatures and separation selected such that the optical cavity is a stable cavity. 11. The spectroscopy instrument as in claim 1, wherein the number of passes of the light beam through the sample volume is in excess of 300. 12. The spectroscopy instrument as in claim 1, wherein any one or more of a bare optical fiber, a bare fiber with a collimating element, or a GRIN lens is inserted into the aperture. 13. The spectroscopy instrument as in claim 1, wherein the light source is selected from any one of a quantum cascade laser, an inter-band cascade laser, a supercontinuum laser, and a laser coupled to a difference frequency generator. 14. The spectroscopy instrument in claim 1, wherein the light source is selected from any one of a light emitting diode, super-luminescent diode, thermal bar and broad band light source. 15. The spectroscopy instrument in claim 14, wherein the aperture is removed and the light emitting diode is placed inside the cavity or on the surface of one mirror. 16. The spectroscopy instrument in claim 14, wherein more than one light emitting diode or other broad-band source is used. 17. The spectroscopy instrument as in claim 1, wherein the light beam has a tunable wavelength in a 2 μm to 4 μm range. 18. The spectroscopy instrument as in claim 1, wherein the light beam has a tunable wavelength in a 4 μm to 10 μm range. 19. The spectroscopy instrument as in claim 1, wherein the light beam has a tunable wavelength longer than 10 μm. 20. The spectroscopy instrument as in claim 1, wherein the light beam has a tunable ultraviolet wavelength shorter than 0.46 μm. 21. The spectroscopy instrument as in claim 1, wherein the detector is a mid-infrared detector coupled to a thermo-electric cooler. 22. The spectroscopy instrument as in claim 1, having a pulsed light source and a cavity ring-down mode of operation. 23. The spectroscopy instrument as in claim 1, wherein an angle of light beam injection into the optical cavity, injection off-axis position, mirror rotation, mirror tilt and mirror separation have been computed using a ray tracing model based on measured mirror topography.