Robert J. Holdsworth

Phase-shift off-axis cavity-enhanced absorption detector of nitrogen dioxide

Phase-sensitive, off-axis cavity-enhanced absorption spectroscopy techniques with a rectangular pulse current modulated violet diode laser (~404 nm) were applied to measurement of nitrogen dioxide (NO2) in diluted standard mixture and laboratory air. An optical cell of an NO2 detector was bounded by two plano-concave mirrors with a diameter of 7.75 mm and a reflectivity of 0.999, separated by a distance of 43 cm. The phase shift measurements due to Rayleigh scattering extinction were used for calibration of the detector. NO2 sensitivities were extracted from synchronous measurements of either the cavity output light intensity or the phase shift induced by the cavity. NO2 noise equivalent detection limit (S/N = 1) of 1–2 ppbv was demonstrated for the intensity measurements with a lock-in amplifier time constant of 1 s. From the Allan plot of the intensity measurements the NO2 minimum concentration was estimated to be 0.24 ppbv in 80 s at atmospheric pressure.

In situ vehicle engine exhaust measurements of nitric oxide with a thermoelectrically cooled, cw DFB quantum cascade laser

This paper describes the application of a thermoelectrically cooled, continuous wave (cw), single mode distributed feedback (DFB) quantum cascade laser (QCL) to the real-time, in situ vehicle engine exhaust monitoring of nitric oxide. Experiments have been carried out on a static gasoline engine test bed where the engine operating conditions can be varied in a controlled manner. Results show the response of the nitric oxide concentration to engine conditions. The prospects for further multispecies in situ measurements are discussed.

Multispecies in situ monitoring of a static internal combustion engine by near-infrared diode laser sensors

A multispecies near-infrared diode laser spectrometer has been constructed for measurements of carbon monoxide, carbon dioxide, and methane directly in the exhaust of a static internal combustion engine. A wavelength modulation-division multiplexing scheme was implemented for the two distributed feedback diode lasers. Gas concentration variations were observed for changes in operating conditions such as increasing and decreasing the throttle, adjusting the air-fuel ratio, and engine start-up.

Time-Resolved In-Situ Spectroscopic Monitoring of the CVD of Tin Oxide onto a Glass Substrate

Near-infrared diode laser absorption spectroscopy has been demonstrated as an in-situ, non-invasive probe for use with a CVD reactor. The technique has been applied to the CVD of tin oxide onto a glass substrate, and by monitoring the evolution of methane in the reactor, it has been shown that the concentration of methane is correlated with the deposition rate of the tin oxide film. This illustrates the powerful possibilities for monitoring thin film production and properties, in-situ, during deposition.

2D Species Concentration Mapping of Thermal AP-CVD Reactors for Monitoring, Control and Design

This paper describes progress towards the two-dimensional mapping of chemical species in an atmospheric pressure chemical vapour deposition (AP-CVD) reactor during the deposition process. The approach uses tunable infrared laser absorption spectroscopy to measure line-of-sight optical absorbance data for specific chemical species. 2D distributions can then be obtained by tomographic inversion techniques. The developed techniques will have advantages for monitoring, control and design of reactors.

Near - Infrared Diode Laser Air Monitoring

Inexpensive near-infrared diode lasers are now being used to enable high sensitivity, real time monitoring of gases both in open path measurements of urban air quality and in industrial environments for stack gas emission monitoring. Individual species are detected in a highly selective manner via overtone and combination bands of their vibrational spectra. Operating at room temperature and with simple optical components they can be made into portable instruments, ideal for field measurements. Combined with optical fibres they can be used for accessing remote and possibly dangerous locations. This paper presents current progress on the development of such a system illustrating recent results on ammonia monitoring at 1540 nm and acetylene at 780 nm. Sensitivities of the order of parts per million and below have been attained. Advantages and disadvantages of this approach to air pollution monitoring will be described. Pressure broadening results are also given for the 2v1 band of ammonia.