Continuous Wave Optical Parametric Oscillators Break New Spectral Ground

摘要

New mid-infrared spectroscopic sources, based upon advances in fiber lasers and in nonlinear frequency conversion, are now enabling high-resolution laser spectroscopy in the 2-4 (mu)m wavelength region and beyond. With this in mind, the authors discuss continuous wave (CW) optical parametric oscillators (OPOs) in particular. Sensors based upon laser absorption spectroscopy enable sensitive and selective detection of gas phase species in a wide range of applications (1). Concentrations in the parts-per-million (ppm) to parts-per-trillion (ppt) ranges (2) can be detected, using relatively compact laser-based devices. Applications include detection of trace moisture in semiconductor process gases, monitoring of atmospheric pollutants with significance in the occurrence of disease, breath analysis for diagnosis of asthma, and measurement of hydrocarbons for combustion diagnostics. A variety of techniques can be used to implement the required detection capability. For fence-line monitoring applications, such as for emission monitoring at chemical plants, long-path absorption measurements can provide concentration integrated over distances of hundreds of meters -- in this case a laser beam is directed at a retroreflector and the return signal is measured by a detector adjacent to the source. For in situ sensing, where measurement of the species is performed in proximity to the optical source, prominent absorption techniques are photoacoustic detection, tunable diode laser absorption spectroscopy (TDLAS) using a multipass cell, and cavity ring-down spectroscopy (CRDS).