Acoustic interferometer

Measurements of sound velocity at ultrasonic frequencies are usually made by an acoustic interferometer. An example of this apparatus11 is shown in Fig. 2. An optically flat piezo-quartz crystal is set into oscillation by an appropriate electrical circuit, which is coupled to an accurate means of measuring electrical power consumption. A reflector, consisting of a bronze piston with an optically flat head parallel to the oscillating face of the quartz, is moved slowly towards or away from the quartz by a micrometer screw. The electrical power consumption shows successive fluctuations as the distance between quartz and reflector varies between positions of resonance and non-resonance of the gas column. Measurement of the distance between resonance positions gives a value for A/2, and if /... 

Fig. 2. 4 Mc.sec-1 acoustic interferometer. (Reproduced from Lambert and Rowlinson11 by kind permission of the Council of the Royal Society)...

Figure 3.11 An acoustic interferometer of the type used in the author s laboratory (from Nethery [104]). A X-cut quartz crystal, 100-600 KHz B crystal support mount and aligning screws. Optical flat E is attached to a movable reflector D for generation of ultrasonic standing waves. Invar rod F position is read from precision micrometer slide L.

Using the method of Pierce s acoustic interferometer, Meyer [3.52] measured the speed of sound in liquid Freon-21 and approximated the experimental data with an equation of type (3.1) having the coefficients... 

The interferometer assembly has been modified to provide acoustic isolation from both building vibrations and airborne noise (7). These improvements have greatly enhanced the signal to noise. The spectra presented here show a signal to noise ratio in excess of 500 for silica samples and in excess of 100 for the alumina samples the differences are due to different sample porosities. No smoothing of the spectra has been performed, and all spectra reported are direct reproductions of the plotter output from the spectrometer. 

In addition to interference filters, NIR manufacturers use holographic gratings (moving and fixed), interferometers, polarization interferometers, diode arrays, acoustic-optic tunable filters, as well as some specialty types. 

A review of the common methods of measuring acoustic absorption and dispersion is presented by Cottrell and McCoubrey [9], The ultrasonic interferometer, the absorption tube, the condenser transducer, and the reverberation chamber are the standard types of apparatus. 

The addition of mass provides the means of transduction for many chemical sensors, including surface acoustic wave (SAW) devices, quartz crystal microbalances (QCM), and microcantilevers. In all these devices, the mass addition either perturbs the vibration, oscillations, or deflection within the transducer. The mode of transduction in an optical interferometer can also be linked to mass addition the sensor s response is altered by refractive index changes in the material being monitored. It is possible that this change can be elicited solely from refractive index changes without the addition of mass, although in sensing a particular... 

Worthy of speoial mention is the use of laser for phonon detection, as in Brillouin speotrosoopy which is especially suitable for deteoting surface acoustic waves (SAWs). Generalized SAWs can be used to obtain Brillouin speotra with a 5-pass Fabry-Perot interferometer [29] on the other hand, pseudo-SAWs, which are weaker than the previous ones, require a tandem (3 -r 3)-pass Fabry-Perot interferometer system [30]. 

This uitrasonio-opticai technique (or haif-opticai technique [89]) was aiso a hyphenated technique in terms of energy sources viz. thermai and opticai for phonon and photon production, respectiveiy). Thermai surface phonons restrict practical application of the technique owing to their iow scattering efficiency, which results in overly long data collection times (typicaiiy severai hours for a singie spectrum, even with advanced multipass interferometers). Similar to active Raman spectroscopy, coherent acoustic phonons are assumed to be excited by two narrow-line frequency tunable laser beams at different frequencies or by laser pulses of short duration compared to the acoustic period. 

Ultrasound-based sensors for metal-coated fiber optic measurements based on interferometric determination of the displacement using a Michelson interferometer have also been designed. The input acoustic field can be detected by using two reference methods, namely (a) time-delay spectroscopy with a calibrated hydrophone (a hydrophone with known frequency response determining the sound pressure, the input displacement being obtained by simple algebra) and (b) the interferometric foil technique (the displacement of a metallized foil situated at the surface of the fluid measured by the interferometer used for fibre tip measurements). The frequency dependence of the transfer function compared well with the theoretical models [51]. 

With a sensitive pump-probe technique, possibly within a common-path interferometer, one can detect the acoustic vibrations of an individual gold nanoparticle [36]. This measurement directly gives the vibration s damping time, a parameter inaccessible to measurements on ensembles of nanoparticles, because of the inhomogeneity in sizes and shapes of populations of nanoparticles. The damping of vibrations of a nanoparticle depends critically on the acoustic impedance mismatch between particle and substrate materials, as well as on the mechanical contact area between them. Acoustic damping is therefore a probe of this contact, which may often be limited to a few nanometers only in diameter. 

The ablation depths are measured by profilometry (optical interferometer, mechanical stylus [59], atomic force microscopy [60]) and starts sharply at the threshold fluence. Similar conclusions can be drawn from reflectivity [61] or acoustic measurements [62]. The problem with these measurements is that either single- or multi-pulse experiments are used to determine the ablation depths and threshold which might give different results. 

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