Cavity ringdown spectroscopy: New approaches and outcomes

Cavity ringdown (CRD) absorption spectroscopy provides high sensitivity and photometric precision in the measurement of weak optical absorption spectr a. We report several innovations in CRD technique involving either pulsed o r continuous-wave (cw) tunable coherent sources. In the former case, we use narrowband, injection-seeded optical parametric oscillator (OPO) devices b ased on quasi phase-matched nonlinear-optical media and a novel intensity-d ip cavity control scheme; one such pulsed OPO is pumped by a compact, inexp ensive multimode laser but still delivers single-mode OPO signal output. Th e latter area concerns cw-CRD spectroscopy with a single-mode tunable diode laser (TDL) and a rapidly swept ringdown cavity, most recently enhanced by a novel optical heterodyne approach that yields a noise-limited absorption sensitivity of 3 x 10(-9) cm(-1). Such pulsed and cw CRD methods are used to record weak rovibrational. spectra of carbon dioxide gas (CO2) with high resolution in the 1.5-mum near-infrared region. Emphasis is on high-perfor mance spectroscopic systems with relatively simple, inexpensive, compact in strumentation.