UV cavity-enhanced absorption spectroscopy detects 2 ps transients

In cavity-enhanced absorption spectroscopy (CEAS), the many round trips of a probe beam in a specimen-filled optical cavity increase the sensitivity to measurements of optical absorption. But the higher the finesse of the optical cavity and thus the number of round trips for the light beam, the less able the setup is to measure fast, transient events. A group at Stanford University (Palo Alto, CA) is creating CEAS setups that reduce the cavity's finesse while maintaining sensitivity, thus enabling the ability to measure transients. Now, the researchers have extended their work to the UV region using a picosecond-pulsed laser with quasi-continuous-wave (QCW) detection, and tested the setup in a shock tube (which produces highly transient events).

In the technique, picosecond pulses from a wavelength-tunable Ti:sapphire mode-locked laser are frequency-quadrupled to a wavelength adjustable from 206 to 245 nm and coupled into a low-finesse CEAS cavity without sweeping the laser wavelength or trying to match the laser mode to the cavity. A 15.24 cm cavity was used with 2 ps pulses in an on-axis configuration. The pulses, which arrived at intervals of 12.8 ns, do not interact with each other in the cavity. The system's photodetector has a 150 kHz bandwidth, meaning that the detector time-averages about 500 pulses at a time. Measurements of ethyl formate were compared with a single-pass and a CEAS setup—the minimum detectable absorbance and noise-equivalent absorption sensitivity of the CEAS setup were improved by a factor of 20 over those for the single-pass setup. Time resolution of the shock-tube measurements was 2 ps. Reference: S. Wang et al., Opt. Express (Jan. 11, 2016); doi:10.1364/oe.24.000308.