Compact, narrow-linewidth, tunable ultraviolet laser source for detecting Hg emissions

摘要

Recent EPA regulations targeting mercury (Hg) emissions from utility coal boilers have prompted increased activity in the development of reliable chemical sensors for monitoring Hg emissions with high sensitivity, high specificity, and fast time response. We are developing a portable, laser-based instrument for real-time, stand-off detection of Hg emissions that involves exciting the Hg (6 ~3P_1←6~1S_0) transition at 253.7 nm and detecting the resulting resonant emission from Hg (6 ~3P_1). The laser for this approach must be tunable over the Hg absorption line at 253.7 nm, while system performance modeling has indicated a desired output pulse energy ≥0.1 μJ and linewidth ≤5 GHz (full width at half-maximum, FWHM). In addition, the laser must have the requisite physical characteristics for use in coal-fired power plants. To meet these criteria, we are pursing a multistage frequency-conversion scheme involving an optical parametric amplifier (OPA). The OPA is pumped by the frequency-doubled output of a passively Q-switched, monolithic Nd: YAG micro-laser operating at 10-Hz repetition rate and is seeded by a 761-nm, cw distributed-feedback diode laser. The resultant pulse-amplified seed beam is frequency tripled in two nonlinear frequency-conversion steps to generate 253.7-nm light. The laser system is mounted on a 45.7 cm × 30.5 cm breadboard and can be further condensed using custom optical mounts. Based on simulations of the nonlinear frequency-conversion processes and current results, we expect this laser architecture to exceed the desired pulse energy. Moreover, this approach provides a compact, all-solid-state source of tunable, narrow-linewidth visible and ultraviolet radiation, which is required for many chemical sensing applications.