Method and apparatus for enhancing the accuracy of CRDS measurements
A method and apparatus for enhancing the accuracy of spectroscopic measurements using a cavity ringdown spectrometer (CRDS) is provided. A first aspect of the invention consists of a novel algorithm for the processing of ring-down data that significantly reduces the amplitude of an exponential fitting artifact, and thereby gives a better estimate of the actual loss. The primary cause of the artifact is the presence of an unwanted backwards-traveling wave that counter-propagates within the ringdown cavity. Scattering due to small imperfections at the cavity mirrors produces this wave and its intensity may be minimized by adjustment of the mirror positions during cavity construction. A second aspect of the invention consists of an apparatus for measuring the backscattered wave within a cavity to allow such cavity mirror adjustments to be made.
1. A cavity ringdown spectrometer comprising:
i) a laser light source;
ii) an optical cavity comprising at least three mirrors, wherein the optical cavity forms a ring resonator having a clockwise propagating mode and a counter-clockwise propagating mode;
iii) at least two photodetectors, at least one of said photodetectors being placed at an output mirror of said cavity for detecting the clockwise propagating mode generated by light input into said cavity from said light source; and
iv) at least a second of said detectors being placed at the same or a different output mirror for detecting the counter-clockwise propagating mode generated by light input into said cavity from said light source.
2. A cavity ringdown spectrometer in accordance with claim 1 wherein each of said photodetectors is independently a photodiode, a photomultiplier, or an avalanche photodiode.