Frequency domain digital signal processing for cavity ringdown spectroscopy

摘要

In this thesis, we demonstrate, through simulation and experiment, new digital signal processing techniques that increase the throughput, and thus the sensitivity, of Cavity Ringdown Spectroscopy (CRDS). Almost all of the experiments in the literature are designed to generate single decay transients, and achieve sensitivity improvements by increasing the reflectance of the cavity mirrors. We have designed and built a laser-locked cavity ringdown spectrometer using modest reflectance mirrors where the sensitivity is achieved by increasing the data throughput and processing the ringdown data more efficiently.We report on the development of a Fourier transform based signal processing method for laser-locked Continuous Wave CRDS. Rather than analysing single ringdowns, as is the norm in traditional methods, we analyse the entire waveform output of the locked optical cavity. We have compared our method to Levenburg-Marquardt non linear least squares fitting, and have found that our method has a comparable accuracy and comparable or higher precision; moreover, the analysis time is approximately 500 times faster.We demonstrate the first laser-locked Cavity Attenuated Phase Shift measurement: we have adapted this technique, replacing the lock in amplifier and ratiometer, which were previously essential, with digital signal processing. Rather than using a traditional unstabilised cavity, we have used our laser-locked instrument to generate data with a high signal-to-noise ratio (SNR): this improvement in the SNR, along with our rapid signal processing, results in a sensitivity two orders of magnitude higher than any other CAPS experiment in the literature. We show that the sampling rate may be reduced to a point where the data processing runs faster than data is generated: this offers the prospect of real time analysis.We show that our Fourier transform based signal processing method may also be applied to the analysis of single exponential decays: we have implemented the data processing on a field programmable gate array that extracts, in real time, the ringdown time from a Pulsed CRD Spectrometer.