Toby K. Boyson

Pulsed quantum cascade laser-based CRDS substance detection: real-time detection of TNT

This paper presents experimental results from a pulsed quantum cascade laser based cavity ringdown spectrometer used as a high-throughput detection system. The results were obtained from an optical cavity with 99.8% input and output coupling mirrors that was rapidly swept (0.2s to 7s sweep times) between 1582.25 cm(-1) (6.3201μm) and 1697.00 cm(-1) (5.8928μm). The spectrometer was able to monitor gas species over the pressure range 585 torr to 1μtorr, and the analysis involves a new digital data processing system that optimises the processing speed and minimises the data storage requirements. In this approach we show that is it not necessary to make direct measurements of the ringdown time of the cavity to obtain the system dynamics. Furthermore, we show that correct data processing is crucial for the ultimate implementation of a wideband IR spectrometer that covers a range similar to that of commercial Fourier transform infrared instruments.

Frequency domain analysis for laser-locked cavity ringdown spectroscopy

In this paper we report on the development of a Fourier-transform based signal processing method for laser-locked Continuous Wave Cavity Ringdown Spectroscopy (CWCRDS). Rather than analysing single ringdowns, as is the norm in traditional methods, we amplitude modulate the incident light, and analyse the entire waveform output of the optical cavity; our method has more in common with Cavity Attenuated Phase Shift Spectroscopy than with traditional data analysis methods. We have compared our method to Levenburg-Marquardt non linear least squares fitting, and have found that, for signals with a noise level typical of that from a locked CWCRDS instrument, our method has a comparable accuracy and comparable or higher precision. Moreover, the analysis time is approximately 500 times faster (normalised to the same number of time domain points). Our method allows us to analyse any number of periods of the ringdown waveform at once: this allows the method to be optimised for speed and precision for a given spectrometer.

Nonlinear estimation of ring-down time for a Fabry-Perot optical cavity

This paper discusses the application of a discrete-time extended Kalman filter (EKF) to the problem of estimating the decay time constant for a Fabry-Perot optical cavity for cavity ring-down spectroscopy (CRDS). The data for the estimation process is obtained from a CRDS experimental setup in terms of the light intensity at the output of the cavity. The cavity is held in lock with the input laser frequency by controlling the distance between the mirrors within the cavity by means of a proportional-integral (PI) controller. The cavity is purged with nitrogen and placed under vacuum before chopping the incident light at 25 KHz and recording the light intensity at its output. In spite of beginning the EKF estimation process with uncertainties in the initial value for the decay time constant, its estimates converge well within a small neighborhood of the expected value for the decay time constant of the cavity within a few ring-down cycles. Also, the EKF estimation results for the decay time constant are compared to those obtained using the Levenberg-Marquardt estimation scheme.

Real-time FPGA data collection of pulsed-laser cavity ringdown signals

This paper presents results from a pulsed-laser cavity ring-down spectrometer with novel field programable gate array real-time data collection. We show both theoretically and experimentally that the data extraction can be achieved from a single cavity ringdown event, and that the absorbance can be determined without the need to fit the ringdown time explicitly. This methodology could potentially provide data acquisition rate up to 1 MHz, with the accuracy and precision comparable to nonlinear least squares fitting algorithms.

Nonlinear estimation of a Fabry-Perot optical cavity for cavity ring-down spectroscopy

This paper considers the application of a discrete-time extended Kalman filter (EKF) to a problem of estimating the ring-down time constant of a Fabry-Perot optical cavity for the purpose of cavity ring-down spectroscopy (CRDS). The ring-down time corresponds to the time taken by the light inside an optical cavity to decay to 1/e of its initial intensity. The online estimation of ring-down time (or decay time) for a cavity is a direct indication of the absorbing species contained in it and can be used to detect improvised explosive devices and concealed explosives. Two cases are considered for EKF design depending on whether the resonant frequency of the optical cavity is perfectly locked to the input laser frequency, or not. In the case of a perfect lock, a three-state EKF is designed to estimate the magnitude and phase quadrature components of the cavity and the ring-down time. In the other case where there is not a perfect lock between the two frequencies, leading to non-zero detuning, a four-state EKF is designed to estimate the detuning parameter in addition to the magnitude and phase quadrature components of the cavity and the ring-down time. The detuning parameter gives an indication of the deviation of laser frequency from the resonant frequency of the cavity, which can then be used by a controller to maintain detuning at zero.

Pulsed quantum cascade laser based hypertemporal real-time headspace measurements

Optical cavity enhancement is a highly desirable process to make sensitive direct-absorption spectroscopic measurements of unknown substances, such as explosives, illicit material, or other species of interest. This paper reports advancements in the development of real-time cavity ringdown spectroscopy over a wide-bandwidth, with the aim to make headspace measurements of molecules at trace levels. We report results of two pulsed quantum cascade systems operating between (1200 to 1320)cm(-1) and (1316 to 1613)cm(-1) that measure the headspace of nitromethane, acetonitrile, acetone, and nitroglycerin, where the spectra are obtained in less than four seconds and contain at least 150,000 spectral wavelength datapoints.

Offline estimation of ring-down time for an experimental Fabry-Perot optical cavity

This paper presents an application of the discrete-time extended Kalman filter (EKF) for offline estimation of the ring-down time constant of an experimental Fabry-Perot optical cavity for the purpose of cavity ring-down spectroscopy (CRDS). The ring-down time corresponds to the time taken by the light inside an optical cavity to decay to 1/e of its initial intensity. The estimation of ring-down time (or decay time) for a cavity is a direct indication of the absorbing species contained in it and can be used to detect improvised explosive devices and concealed explosives. The data for the estimation process is obtained in the form of light intensity at the output of the optical cavity from a CRDS experimental setup. During the process of data collection, the resonant frequency of the optical cavity is held in lock with the input laser frequency with the aid of a proportional-integral (PI) controller in order to allow for maximum constructive interference of light within the cavity.

Digital locking of a three-mirror ring cavity

This paper presents results for digitally locking an experimental optical cavity. Locking an optical cavity refers to the process of matching the resonant frequency of the cavity with the input laser frequency in order to facilitate maximum energy build-up inside the cavity. The cavity used in this paper is a three-mirror ring cavity with a piezo actuator mounted on one of the mirrors, which is digitally controlled to hold the cavity in lock. An integral LQG controller is designed and digitally implemented in order to achieve cavity lock and the closed-loop step response is presented. This is compared with the closed-loop step response using a digital proportional-integral controller for the three-mirror cavity.

Real-time multiplexed digital cavity-enhanced spectroscopy

Cavity-enhanced spectroscopy is a sensitive optical absorption technique but one where the practical applications have been limited to studying small wavelength ranges. This Letter shows that wideband operation can be achieved by combining techniques usually reserved for the communications community with that of cavity-enhanced spectroscopy, producing a multiplexed real-time cavity-enhanced spectrometer. We use multiple collinear laser sources operating asynchronously and simultaneously while being detected on a single photodetector. This is synonymous with radio frequency (RF) cellular systems in which signals are detected on a single antenna but decoded uniquely. Here, we demonstrate results with spectra of methyl salicylate and show parts-per-billion per root hertz sensitivity measured in real-time.

Rapid, wideband cavity ringdown spectroscopy for the detection of explosives

We present results from a variant of CRDS that allows large spectral bandwidths to be analysed in real time. We have applied the technique to the analysis and detection of explosives and related compounds.