THE INFLUENCE OF â FREE-RUNNINGâ FP-QCL FREQUENCY JITTER ON CAVITY RINGDOWN SPECTROSCOPY OF C$_{60}$
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Date
2009
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Publisher
Ohio State University
Abstract
Our group is engaged in an effort to acquire a high-resolution gas phase spectrum of buckminsterfullerene in the mid-IR. To achieve this goal, we have constructed a continuous-wave cavity ringdown spectrometer (cw-CRDS) using a Fabry-Perot quantum cascade laser (FP-QCL) as the principal light source. The cw-CRDS technique has the beneficial aspects of both high sensitivity and high resolution. When aligning the high finesse cavity of the spectrometer a light source with a stable narrow linewidth is preferable as this leads to a regular occurrence of build-up events correlated to the sweep of the piezoelectric transducer. The regular interval during the sweep of the cavity resonances over the frequency of the laser source can be a useful aid by providing insight into the quality of the mode-matching. The instantaneous linewidth of a free-running quantum cascade laser operating in cw mode is sub-MHz, but frequency jitter from mechanical, optical, and/or electrical perturbations evidently yields a time averaged effective linewidth that is substantially broader. This frequency jitter, and the resulting time averaged linewidth, will therefore have an impact on the cw-CRDS system we are using for C$_{60}$ spectroscopy. To characterize the effective linewidth, we have conducted self-pressure broadening studies of select SO$_{2}$ rovibrational lines in the $\nu _{1}$ vibrational band via direct absorption spectroscopy using FP-QCLs intended for C$_{60}$ spectroscopy, and have measured a time-averaged effective linewidth of $\sim$120 MHz. We will discuss the self-pressure broadening coefficients measured for select rovibrational lines of SO$_{2}$ between 1194-1198 cm$^{-1}$, the implications of the measured frequency jitter of our laser source on cw-CRDS data collection, and a progress report on our C$_{60}$ spectroscopy project.
Description
Author Institution: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Department of Electrical Engineering, Princeton University, Princeton Institute for the Science and Technology of Materials, Princeton, NJ 08544; Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801