CW-OPO-based cavity-leak-out spectrometer for ultrasensitive and selective mid infrared trace gas analysis

An all-solid-state infrared trace gas sensor is presented combining a continuous-wave optical parametric oscillator (OPO) with Cavity Leak-Out spectroscopy (CALOS), a cw version of Cavity Ring Down spectroscopy. The PPLN based pump resonant, singly resonant OPO is pumped at 1064 nm (2 W). Dual cavity design allows to select any desired wavelength within the emission range of the OPO (3.1 - 3.8 μm) and to use different tuning schemes in order to scan absorption features. To detect the CALOS signals the OPO frequency is scanned over the cavity resonance at kHz rates. The high power of the OPO (up to 100 mW at each end of the cavity) allows a strong excitation of the TEM00 mode of the cavity, yielding large detector signals. A noise-equivalent absorption coefficient of 1.6*10-10cm-1/√Hz is reached for integration times up to 180 sec. This corresponds to a detection limit for ethane at sub-ppt level. Measurements at reduced pressure (100 mbar) combined with a scanning of the OPO over cm-1 wide regions allows a multi-gas analysis of ambient air and human breath samples without a cooling-trap.