Triantafillos Koukoulas

Gated photon correlation spectroscopy for acoustical particle velocity measurements in free-field conditions.

The measurement of acoustic pressure at a point in space using optical methods has been the subject of extensive research in airborne acoustics over the last four decades. The main driver is to reliably establish the acoustic pascal, thus allowing the calibration of microphones with standard and non-standard dimensions to be realized in an absolute and direct manner. However, the research work so far has mostly been limited to standing wave tubes. This Letter reports on the development of an optical system capable of measuring acoustic particle velocities in free-field conditions; agreement within less than 0.6 dB was obtained with standard microphone measurements during these initial experiments.

Particle velocity measurements using heterodyne interferometry and Doppler shift demodulation for absolute calibration of hydrophones

Underwater hydrophones are calibrated using the three-transducer reciprocity method as primary standard, where their sensitivities are obtained with traceability to electrical standards. However, there are some disadvantages associated with this technique, one of which being the lack of direct traceability to the unit of sound pressure. In the current technique, one of the three transducers needs to be reciprocal to be used as transmitter/receiver, whereas a transmitter (sound source) and receiver (hydrophone under calibration) configuration with no need for reciprocal response would be more straightforward. Optical interferometry provides an alternative method, potentially overcoming these limitations. In this case, a transducer provides the sound excitation and a pellicle strip is placed in the far field. The interferometer uses a frequency-shifted reference beam and also provides a measurement beam probing a fixed point on the pellicle. By analysing the reflected signal mixed with the reference, the Doppler shift is calculated and the acoustic velocity field is measured in a direct and absolute way. This paper presents the results of a hydrophone calibration comparison between reciprocity and interferometry with reasonable agreement between the two methods.

Characterization of solid complex multiphase systems based on oscillatory photon correlation spectroscopy

The particle loading and dispersion profiles are two significant properties directly affecting the engineering properties and performance characteristics of polymer nanocomposites. Current measurement techniques are often destructive, require special sample preparation, are limited to small, unrepresentative sample size, and/or cannot discriminate between the two aforementioned parameters. This Letter demonstrates the application of photon correlation spectroscopy on mechanically oscillated solids; experimental results show that this technique can discriminate between different grades of such materials.

Novel non-contact optical characterisation methods of polymeric nanocomposite structures based on their particle loading and dispersion profile

Current methods to characterise specific properties of polymeric nanocomposites (PNCs), such as particle loading and dispersion profile, rely on a number of techniques that require special sample preparation and treatment, are very expensive, require long measurement times and quite often produce ambiguous results that are difficult to evaluate and interpret. In addition, given their complexity, they are not entirely suited for in-situ industrial environments. This paper presents alternative techniques based on optical diffraction and diffusion mechanisms combined with signal processing that can successfully discriminate between different particle loadings and levels of dispersion. The techniques discussed in this paper are Fourier-domain optical coherence tomography in the infra-red, Fraunhofer wavefront correlation in the visible red and oscillatory photon correlation spectroscopy in the visible green parts of the spectrum. Most importantly, they are non-invasive, are compact, fast and efficient, can potentially analyse large areas of the material and therefore suited for a wide variety of research and industrial situations.

Towards direct realisation of the SI unit of sound pressure in the audible hearing range based on optical free-field acoustic particle measurements

Since the introduction of the International System of Units (the SI system) in 1960, weights, measures, standardised approaches, procedures, and protocols have been introduced, adapted, and extensively used. A major international effort and activity concentrate on the definition and traceability of the seven base SI units in terms of fundamental constants, and consequently those units that are derived from the base units. In airborne acoustical metrology and for the audible range of frequencies up to 20 kHz, the SI unit of sound pressure, the pascal, is realised indirectly and without any knowledge or measurement of the sound field. Though the principle of reciprocity was originally formulated by Lord Rayleigh nearly two centuries ago, it was devised in the 1940s and eventually became a calibration standard in the 1960s; however, it can only accommodate a limited number of acoustic sensors of specific types and dimensions. International standards determine the device sensitivity either through coupler or through free-field reciprocity but rely on the continuous availability of specific acoustical artefacts. Here, we show an optical method based on gated photon correlation spectroscopy that can measure sound pressures directly and absolutely in fully anechoic conditions, remotely, and without disturbing the propagating sound field. It neither relies on the availability or performance of any measurement artefact nor makes any assumptions of the device geometry and sound field characteristics. Most importantly, the required units of sound pressure and microphone sensitivity may now be experimentally realised, thus providing direct traceability to SI base units.

Primary ultrasonic interferometer photodiode characterization using frequency-modulated laser wavefront radiation

Photodiodes play an important role in many optical systems and, as such, their stability and performance are of great importance. Manufacturers of such detectors typically only provide information regarding their sensitivity as a function of the optical wavelength and this is often sufficient for applications such as telecommunications. However, in certain applications such as primary ultrasonic standards based on interferometry, the frequency response of the photodiodes is of critical importance because, for accurate calibration, a correction factor must be applied, which contributes a major source of measurement uncertainty. Most optical calibration systems reported in the literature operate either in the GHz range or a very limited MHz range. This work reports on the development of a system based on rotational optical components that produces light patterns scanned through a slit at varying speeds using different deflection mechanisms. This results in the generation of spatially dependent interference fringes in the range 1 Hz up to 115 MHz with an expanded uncertainty of less than 5% for the majority of its frequency range of operation.

A comparison between heterodyne and homodyne interferometry to realise the SI unit of acoustic pressure in water

Optical approaches for hydrophone calibrations offer significant advantages over existing methods based on reciprocity. In particular, heterodyne and homodyne interferometry can accurately measure particle velocity and displacements at a specific point in space thus enabling the acoustical pressure to be measured in an absolute, direct, assumption-free manner, with traceability through the SI definition of the metre. The calibration of a hydrophone can then be performed by placing the active element of the sensor at the point where the acoustic pressure field was measured and monitoring its electrical output. However, it is crucial to validate the performance and accuracy of such optical methods by direct comparison rather than through device calibration. Here we report on the direct comparison of two such optical interferometers used in underwater acoustics and ultrasonics in terms of acoustic pressure estimation and their associated uncertainties in the frequency range 200 kHz–3.5 MHz, with results showing agreement better than 1% in terms of pressure and typical expanded uncertainties better than 3% for both reported methods.

Utilization of carbon nanofibers for airborne ultrasonic acoustic field detection using heterodyne interferometry

Carbon nanofibers and nanotubes are currently being utilized as active elements in acoustic sensors for emerging microelectromechanical systems and nanoelectromechanical systems technologies. A methodology for measuring the displacement of carbon nanofibers in combination with heterodyne interferometry is reported here. Experimental results show that ultrasonic field detection is possible using this technique, and results are presented for measurements in the ultrasonic frequency range. This approach could potentially lead to new calibration methods for ultrasonic sensors. A different approach to that of interferometry is also reported for future investigation.

Measurement and imaging of high‐frequency sonar fields using acousto‐optic tomography.

The acoustic near‐field of a high‐frequency sonar array transducer has been measured using a laser scanning acousto‐optic method and reconstructed using a tomographic technique similar to that used for x‐ray computed tomography. This measurement, using a laser interferometer to scan a plane in front of the transducer for a series of transducer rotations, provides an image of the acoustic field for a plane in front of the transducer array. This method allows measurement of the acoustic field without perturbing the field being measured, which can occur when using the planar hydrophone scanning method, provides high spatial resolution with minimal phase averaging across the measurement aperture and provides potential for significantly decreasing the scan time. Measurements have been performed on two 1–3 composite transducer arrays at frequencies between 330 and 500 kHz. These are compared with planar hydrophone scans obtained using a 1.5‐mm probe hydrophone. Both parallel and fan beam tomographic scan geomet...

Acousto‐optic tomography for mapping of high‐frequency sonar fields

This paper presents an acousto‐optic method for tomographic mapping of acoustic fields generated by high‐frequency sonar array transducers. The method uses a laser interferometer to measure the integrated refractive index change across the propagating acoustic wave generated by the transducer. An interferometer being a displacement or velocity measuring device, interprets this rate of change of optical path length as a displacement or velocity. Obtaining a series of these projections by scanning the laser beam or the transducer for a number of rotation angles of the transducer allow a two‐dimensional plane of the acoustic field to be reconstructed using the techniques commonly used in X‐ray Computed Tomography. Acousto‐optic tomographic measurement results are presented for a 95 by 95 mm, 400 kHz 1‐3 composite, 4 element sonar array transducer and are compared to conventional planar hydrophone scans obtained using a 1.5 mm probe hydrophone. The optical method allows measurement of the acoustic field witho...