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Working Group 1.62

Optical measuring methods

The pressure variation in space and time of ultrasonic signals in water or other liquid media can be precisely determined also by optical measuring methods which have certain advantages over conventional hydrophones. When the pressure sensitivity is good, a higher spatial resolution can be achieved. At the same time, optical methods allow the bandwidth of pressure measurements to be extended, and a nondestructive measurement is possible.

PTB section 1.62 applies methods using sensing probes and non-contact methods. The sensing probes used are fibre-optic sensors whose modes of operation differ. They furnish an optical signal whose amplitude and phase variation is related to the sound field quantities and can be detected by means of a suitable optical set-up.

However, the displacement in the sound field can also be directly measured by interferometry. In this case, incidence of the sound field must be normal to the liquid surface which is covered by a foil about 2 micrometers in thickness. The backside of the foil is coated with aluminium to increase the optical reflection factor. The displacement of the foil is measured by means of an interferometer.

This method is also suited to calibrate transducers and hydrophones as the comparison with the wavelength makes direct reference to the SI definitions possible and the optical detection technique allows large bandwidths to be covered. The frequency range for the calibration of hydrophones and fibre-optic sensors could be extended to 50 MHz; this is of significance, for example, for shock wave measurements.

A novel technique for the determination of ultrasonic pressure uses a dielectric optical layer system which is evaporated on a glas substrate. Like thefiber-optic pressure sensors the deformation of the layer system generated by the ultrasound induces a change in the optical reflectivity which can be measured by a simple detection scheme. The measurement technique shows a nearly constant sensitivity in the frequency range 1 - 75 MHz and can be expanded to a two-dimensional optical ultrasound array. So it can be used as a standard for ultrasound detection even for complex valued calibration where the phase of the sensitivity is also determined for example using broad band pulses as a very simple technique. Phase information in addition to the amplitude measurement is necessary for the deconvolution of measured pulses.

Light diffraction tomography is a non-contact method. If a light beam with a radius greater than the sound wavelength passes through an ultrasonic wave, a diffraction effect appears in the path of the light field, which depends on the sound field properties. Use of a tomographic method allows the pressure distribution of the sound field to be determined from the intensity and the phase relationships of the individual diffraction orders.

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