H. A. MacKenzie

Measurement of the optical absorption coefficient of a liquid by use of a time-resolved photoacoustic technique

A time-resolved photoacoustic technique has been applied to the study of dissolved and dispersed absorbers in aqueous systems. The temporal pressure profiles generated from colloidal graphite and glucose solutions were measured, and it was found that the amplitude of the photoacoustic signal of both the glucose and the colloidal graphite solutions increase linearly with concentration and that acoustic signal time delay yields the acoustic velocity. The logarithm of the photoacoustic signal amplitude changes linearly with the time delay, with a slope that is proportional to the product of the acoustic velocity and the optical absorption that can thus be determined.

Application of pulsed laser photoacoustic sensors in monitoring oil contamination in water

Abstract A pulsed laser photoacoustic measurement technique, utilising laser diode sources, has been developed for the detection of oil contamination in water. The measurement of oil-contaminated water samples of oil concentration ranging from 0 to 900 mg 1−1 is presented. Also reported are results from a theoretical modelling, which includes several experimental factors important to the photoacoustic sensor design for this type of application.

Critical slowing-down phenomena in an InSb optically bistable étalon

We report the first known direct observation of the critical slowing-down effect and transient bimodality in an intrinsic semiconductor all-optically bistable device with a cw CO laser holding beam and a PbSSe diode laser switching beam. The measurements of critical slow-down effects show good agreement with the numerical model using a modified Debye dynamic equation. The switching profile has been shown to depend on both the size of the hysteresis loop and the diode pulse parameter. A scaling law for the delay time of the switching when external parameter P changes close to the critical value Pc has been indicated to fit (P − Pc)−1/2.

A pulsed photoacoustic system for the spectroscopy and monitoring of hydrocarbon liquids using stimulated Raman scattering in a silica fibre as a near-infrared source

Raman scattering in an optical fibre has been utilized as the near-infrared source for a pulsed photoacoustic spectroscopic study of liquid hydrocarbons. An open-cell system is described and the performance criteria for practical photoacoustic instrumentation are discussed. Results show that a typical signal-to-noise ratio of 50 may be achieved, which makes this system suitable for the detection of small concentrations of hydrocarbons in liquids. The photoacoustic system has been used to measure concentrations both of dissolved and of dispersed hydrocarbons and the results of this study indicate the practical potential for novel open-cell photoacoustic instrumentation for the detection of crude oil in water using near-infrared wavelengths in a spectral region for which suitable diode lasers are available.

Optical fiber interferometry for photoacoustic spectroscopy in liquids.

Optical fiber interferometry was used for measurement of the subnanometer surface and bulk displacements associated with photoacoustic pressure waves in methanol and water. The measurement system is both broadband and noncontacting, giving a pressure sensitivity of 0.1 Pa/ radicalHz.

Photoacoustic generation in liquids with low optical absorption

Abstract Short laser pulses (250 ns) from a 1064 nm Nd: YAG laser have been used to generate acoustic waves in distilled water and methanol. The mechanism of this process has been studied both experimentally and theoretically with good overall agreement. The measurement of the acoustic signal utilized a small wideband (0.1–20 MHz) piezoelectric hydrophone. The advantages of this detector in the measurement of photoacoustically induced ultrasonic waves in liquids with low optical absorption are discussed.

Time-resolved photoacoustics for glucose concentration measurement: theory and experiment

The capability of the time-resolved photoacoustic (PA) technique for glucose concentration measurement in the near-infrared range was demonstrated. Both experimental and theoretical studies shown that the concentration dependence of the sound velocity, optical absorption coefficient and thermal property of glucose solution can be simultaneously determined by the time-resolved PA techniques. Good agreement between theory and experiment has been obtained.

Detection of photoacoustic waves in liquids by fibre optic interferometry

Abstract A Michelson-based fibre optic interferometer was used to make absolute measurements of the sub-nanometer liquid surface displacements created by the photoacoustic process. This measurement system is both broad-band and non-contacting. Photoacoustic waves, generated using pulses from a Nd: YAG laser operating at 1.06 μm, were measured in methanol, water, and various concentration mixtures. The wave amplitude demonstrated a quadratic relationship with concentration.

The switching dynamics and noise limitations of indium antimonide bistable elements

The switching dynamics of an InSb optically bistable etalon are analyzed in terms of effective time constants which change dramatically near the switch points. Results are presented which show nanosecond switching, critical slowing down, optical biomodality, and noise filtration. The influence of noise levels on both the switching dynamics and the system stability close to switching are discussed. The InSb-CO system has the merits of being extremely stable and reproducible so that the detailed switching dynamics and statistics can be reliably assessed. In addition, it may be regarded as a model system so that the results presented can be scaled with appropriate material time constants to other bistable systems. >

Optical fibre interferometry for photoacoustic spectroscopy in liquids

The photoacoustic process has great potential as a spectroscopic technique appropriate for opaque, highly scattering or weakly absorbing samples, all unsuitable for use with standard absorption spectroscopy. In particular, it is suited to liquids with low optical absorptions, for example the sensing of hydrocarbon pollutants in water [1].