Note: A latched comparator circuit for triggering continuous-wave cavity ring-down spectroscopy

Abstract

Continuous-wave cavity ring-down spectroscopy offers several advantages over cavity ring-down spectroscopy with a pulsed laser, such as a higher repetition rate and decreased cost. However, the continuous-wave technique requires a more complicated experimental setup because the laser must be switched off rapidly when the intensity is high in order to observe a ring-down event. This note describes an inexpensive and simple latched comparator circuit that can be used to detect light intensity above a threshold value and send a signal to rapidly steer the beam out of the cavity and initiate a ring-down event. The latch eliminates switching noise by preventing the comparator from switching during the ring-down event. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4811846] Cavity ring-down spectroscopy (CRDS), first developed by O'Keefe and Deacon, 1 has become increasingly used to measure the absorption and extinction properties of gases, suspended aerosol particles, and liquids. CRDS measures the decay lifetime of photons in an optical cavity composed of highly reflective mirrors. Because the mirrors are highly reflective, the path length of the light in the cavity is typically on the order of kilometers for a cavity with a length of approximately one meter, resulting in very high sensitivity. CRDS has been reviewed extensively in the literature 2-7 and its principles will only briefly be discussed here.Continuous-wave cavity ring-down spectroscopy (CW-CRDS) using a diode laser, developed by Romanini et al., 8 offers several advantages over the initially developed CRDS using a pulsed laser. 7-9 In general, CW-CRDS uses a less expensive and smaller continuous-wave (CW) laser and offers faster sampling rates leading to a more sensitive measurement. Continuous-wave lasers have a more narrow linewidth than pulsed lasers (typically much more narrow than the cavity free spectral range) and the wavelength of some CW lasers can be scanned across a small wavelength range allowing spectral information to be measured. However, CW-CRDS has several disadvantages when compared to pulsed laser CRDS. 7 Most notably, the experimental setup for CW-CRDS is more complicated requiring a method to quickly turn off the laser in order to observe the ring-down event. Commonly, an acousto-optic modulator (AOM) is used to steer the beam into the cavity. When the AOM is triggered off, the beam rapidly steers away from the cavity allowing the intensity of light in the cavity to decay and the ring-down event to be observed.Most CW-CRDS systems use mode-matching lenses 10 to trigger the AOM when the Gaussian TEM 00 transverse mode of the cavity is matched to the laser and the intensity exiting the cavity is highest. Overlap of the laser mode and cavity a)

Analytical chemistry research at primarily undergraduate institutions: training tomorrow's investigators

“…18 In another study, a neural network was used to optimize conditions for a uorescence-based binding assay. 22 In addition to these fundamental advances, several publications present a range of applications-driven advances in spectroscopic methods. For example, high resolution, coherent, threedimensional spectroscopic studies of gas phase bromine demonstrated how higher dimensionality methods reduce spectral congestion and aid in accurate peak assignments, 20 and a recently published chemometric method makes multivariate calibration of spectral data less demanding.…”

“…21 Another paper demonstrated that a relatively inexpensive circuit could facilitate continuous-wave cavity ring-down spectroscopy (CW-CRDS) measurements. 22 In addition to these fundamental advances, several publications present a range of applications-driven advances in spectroscopic methods. Recent biomedical applications include a molecular beacon for a cancer biomarker, 23 a surface plasmon resonance method for drug screening, 24 and a dynamic light scattering assay for protein-aptamer binding.…”