INTERFEROMETRIC CALORIMETER FOR THE MEASUREMENT OF WATER-VAPOR ABSORPTION

An interferometric calorimeter has been developed to measure the optical-absorption coefficient of gases at the level of 10(-10) cm-1. The system operates by measuring the phase shift on a cw He-Ne laser probe beam, which is caused by the expansion of the gas after it has been heated in the focal volume of a pulsed excitation laser beam. The minimum detectable fringe shift is approximately 2 x 10(-5) waves. For a 1-J excitation laser this shift corresponds to an absorption coefficient of 2 x 10(-10) cm-1. The continuum absorption of water vapor at 9466 cm-1 has been measured to be 6 x 10(-10) cm-1 at a 16.5-Torr partial pressure in a 1-atm (760 Torr) N2 buffer at 30-degrees-C.