Modifications to a Cavity Ringdown Spectrometer to Improve Data Acquisition Rates

Abstract

Cavity ringdown spectroscopy (CRDS) makes use of light retention in an optical cavity to enhance the sensitivity to absorption or extinction of light from a sample inside the cavity. When light entering the cavity is stopped, the output is an exponential decay with a decay constant that can be used to determine the quantity of the analyte if the extinction or absorption coefficient is known. The precision of the CRDS is dependent on the rate at which the system it acquires and processes ringdowns, assuming randomly distributed errors. We have demonstrated a CRDS system with a ringdown acquisition rate of 1.5 kHz, extendable to a maximum of 3.5 kHz, using new techniques that significantly changed the way in which the ringdowns are both initiated and processed.On the initiation side, we combined a custom high-resolution laser controller with a linear optical feedback configuration and a novel optical technique for initiating a ringdown.Our optical injection "unlock" method switches the laser off-resonance, while allowing the laser to immediately return to resonance, after terminating the unlock, to allow for another ringdown (on the same cavity resonance mode). This part of the system had a demonstrated ringdown initiation rate of 3.5 kHz. To take advantage of this rate, we developed an optimized cost-effective FGPA-based data acquisition and processing system for CRDS, capable of determining decay constants at a maximum rate of 4.4 kHz, by modifying a commercial ADC-FPGA evaluation board and programming it to apply a discrete Fourier transform-based algorithm for determining decay constants. The entire system shows promise with a demonstrated ability to determine gas concentrations for H 2 O with a measured concentration accuracy of ±3.3%. The system achieved an ii absorption coefficient precision of 0.1% (95% confidence interval). It also exhibited a linear response for varying H 2 O concentrations, a 2.2% variation (1σ) for repeated measurements at the same H 2 O concentration, and a corresponding precision of 0.6% (standard error of the mean). The absorption coefficient limit of detection was determined to be 1.6 x 10 -8 cm -1 (root mean square of the baseline residual). Proposed modifications to our prototype system offer the promise of more substantial gains in both precision and limit of detection. The system components developed here for faster ringdown acquisition and processing have broader applications for CRDS in atmospheric science and other fields that need fast response systems operating at high-precision.iii DedicationTo my wife, Alison, without whose continual support and encouragement I could never have come close to accomplishing this.To my daughter, Naomi, who has only ever known me to be a graduate student-may she always appreciate physics the way she does now.To my Dad, whom I think would have really liked to have seen this, and to myMom, who has been proud enough of me for both of them.iv

The discrete Fourier transform algorithm for determining decay constants—Implementation using a field programmable gate array

“…In the figure, the large residuals around the peak are a result of issues with the CRDS system performance resulting from inconsistent cavity locking at higher absorption and not an artifact of the DFT algorithm. 22 Our overall system was designed for high precision measurements at low concentrations of analyte. Therefore, absorption coefficients much larger than the cavity background losses were not expected, which led us to allocate the output bits towards precision and not for maximum detectable absorption.…”