Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment

A simple and easy to use method that allows high-finesse optical cavities t o be used as absorption cells for spectroscopic purposes is presented. This method introduces a single-mode continuous-wave laser into the cavity by u se of an off-axis cavity alignment geometry to eliminate systematically the resonances commonly associated with optical cavities, while preserving the absorption signal amplifying properties of such cavities. This considerabl y reduces the complexity of the apparatus compared with other high-resoluti on cavity-based absorption methods. Application of this technique in conjun ction with either cavity ringdown spectroscopy or integrated cavity output spectroscopy produced absorption sensitivities of 1.5 x 10(-9) cm(-1) Hz(-1 /2) and 1.8 x 10(-10) cm(-1) Hz(-1/2), respectively. (C) 2001 Optical Socie ty of America.