Low noise cavity enhanced absorption spectroscopy apparatus and method

Apparatus for performing low noise cavity enhanced absorption spectroscopy, the apparatus comprising:

• a resonant optical cavity configured to include a gas sample, wherein the resonant optical cavity supports TEMmnq modes having longitudinal index q and transverse indices m and n, wherein fmnq is a frequency of the TEMmnq mode;
• an optical source configured to provide light to the resonant optical cavity, wherein the light from the optical source has a full-width half-maximum line width ΔfL, and wherein the light from the optical source has a tunable output frequency;
• an optical detector configured to receive light from the resonant optical cavity;
• wherein light is coupled from the optical source to the optical cavity via first coupling optics including a first single mode optical fiber coupled to the resonant optical cavity;
• wherein the first coupling optics provides first mode selective coupling of the optical source to a selected TEMmnq mode (TEM00q0) of the resonant optical cavity having m=n=0 and q=q0, wherein the TEM00q0 mode has a frequency f00q0 and has a full-width half-maximum line width Δf00q0;
• wherein light is coupled from the resonant optical cavity to the optical detector via second coupling optics including a second single mode optical fiber coupled to the resonant optical cavity;
• wherein the second coupling optics provides second mode selective coupling of the TEM00q0 mode to the optical detector;
• wherein a round trip path length of the resonant optical cavity is selected such that the modes TEMmnq for 0<m+n<13 and for all q satisfy a design condition given by |fmnq−f00q0|>max (ΔfL, Δf00q0);
• and
• a processor configured to determine concentration of one or more gas analytes in the gas sample from measurements of loss in the resonant optical cavity.