Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy

A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-in rare diode lasers and absorption-spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of approximately 1.4 x 10(-5) over an effective optical path of 4.2km was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of approximately 3.1 x 10(-11) cm(-1) Hz(-1/2). The instrument has been used for sensitive measurements of CO, CH4, C2H2 and NH3