Ultrasonic hydrophones based upon piezoelectric PVDF sensing elements are widely used for characterising medical and industrial ultrasound fields. However, a fundamental difficulty arises in obtaining adequate detection sensitivity with the small (<100µm) element sizes required for an omnidirectional response at MHz frequencies. Additionally, the sensitivity of piezoelectric hydrophones to EMI can present difficulties in the measurement of CW fields and their fragility and expense can preclude their use for the measurement of potentially damaging high amplitude fields and in hostile environments. With a view to overcoming these limitations, a prototype 50MHz optical fibre hydrophone system, based upon the use of a Fabry Perot polymer film sensing interferometer has been developed in collaboration with Precision Acoustics Ltd, originally under the EPSRC/DTI LINK Photonics programme and most recently under an EPSRC CASE PhD studentship. Applications include the assessment of the output of diagnostic and therapeutic medical ultrasound equipment, in vivo measurements of ultrasound exposure and the characterisation of sources used in industrial non destructive testing apparatus. A recent development has been to extend the capability of the system so that it can make measurements of temperature as well as pressure. This offers the prospect of characterising ultrasound-induced heating, for example that produced during HIFU therapy.

System description

Figure 1 Schematic (left) of ultrasonic fibre optic hydrophone system (uFOHS) and prototype demonstrator instrument (right).

The system is shown in figure 1. It comprises a detachable fibre optic sensor downlead that is inserted into a sensor interrogation unit. The latter contains a fibre-coupled tuneable 1550nm laser source, a 2x2 single mode fibre coupler, an InGAs photodiode-transimpedance amplifier detector unit and associated control and data acquisition hardware. The output of the laser is launched via the fibre optic coupler into the single mode optical fibre sensor downlead. The acoustically sensitive element comprises a 10μm thick polymer (Parylene C) film that is vacuum deposited on to the tip of the fibre and acts as a Fabry Perot interferometer (FPI) - the mirrors of which are formed by the deposition of gold coatings. An incident acoustic wave modulates the optical thickness of the polymer film and hence the optical phase difference between the light reflected from its two sides. This produces a corresponding reflected intensity modulation which is transmitted back along the fibre to the optics unit where it is directed, via the fibre coupler, to the photodiode unit. In order to optimally bias the interferometer for maximum sensitivity and linearity, the laser wavelength is adjusted so that is corresponds to the point of maximum slope on the interferometer transfer function. The system can also be used to measure temperature by monitoring thermally-induced changes in the optical thickness of the FPI. This is achieved by employing a different interrogation scheme to that referred to above, one that is based upon tracking shifts in the reflectance minimum of the interferometer transfer function. The system has now been engineered into a self contained portable instrument (shown right in figure 1) with all control and diagnostic functions under computer control via a single USB connection.

The advantages of the concept over piezoelectric based methods are:

• Small effective radius - The use of a single mode fibre (core diameter <10µm) to illuminate the sensing film enables acoustically small element sizes to be achieved. These are required to provide an omnidirectional response and avoid errors due to spatial averaging. Unlike piezoelectric detection, this does not come at the expense of detection sensitivity as element size and sensitivity are independent.
• Inexpensive fabrication - Fabrication of the sensor element using all-vacuum deposition methods enables a rugged sensor head to be batch fabricated with good repeatability at low unit cost for single use applications.
• Self – calibration - Modulating the laser wavelength and recording the corresponding output of the sensor enables changes in sensitivity to be continuously monitored.
• Simultaneous temperature and pressure measurement – Detection of thermally induced changes in the optical thickness of the polymer film enables temperature changes to be measured for investigating effects such as ultrasound induced heating produced by HIFU fields.
• Electrical passivity and immunity to EMI - Measurements of cw fields and measurements in electrically noisy environments can be made without the "breakthrough" experienced with piezoelectric detectors.

Performance

The system provides a 50 MHz acoustic bandwidth, an optically defined element size of 10µm, and an upper limit of linear detection of 11MPa (to within 10%). A notable feature of this type of sensor is its high acoustic detection sensitivity: it can provide a noise-equivalent-pressure (NEP) of approximately 10kPa (over a 25MHz measurement bandwidth). This is comparable to the NEP of a 200µm diameter piezoelectric PVDF receiver and several orders of magnitude higher than fibre optic hydrophones based upon monitoring acoustically-induced changes in optical reflectance at the cleaved tip of a fibre. In addition to measuring acoustic pressure, the system can simultaneously measure temperature with a resolution of 0.1oC and an acquisition rate of 300 samples per second.

Applications

In addition to general field measurement and characterisation, a specific application is the in situ measurement of ultrasound fields produced by diagnostic and therapeutic medical ultrasound equipment. For this, the disposable nature of the hydrophone, its miniature, flexible probe-type configuration, low directional sensitivity, electrical passivity and ability to measure temperature are of benefit. An important emerging potential application is the characterisation of HIFU (high intensity focussed ultrasound) fields used to destroy tissue pathologies such as tumours. The fibre optic hydrophone offers the prospect of being able to measure both the acoustic pressure and induced temperature rise simultaneously at the same spatial point in the HIFU field. Furthermore, the low cost of the sensor is an advantage given the risk of damage in the harsh environment of a HIFU field.
Industrial applications include characterising transducers used for ultrasonic non destructive testing (NDT) of engineering materials or as an embedded sensor in ultrasonic NDE. The system may also be suited to the measurement and monitoring of cw low frequency high power industrial ultrasound such as that employed in ultrasonic cleaning which requires small physical size to avoid disturbing standing wave patterns, the ability to withstand hostile environments and low sensitivity to electrical interference. Other applications include its use as a specialist measurement tool for basic research in ultrasound metrology, sonochemistry, and laser generated ultrasound.

References

Morris, PM, Hurrell, A, Zhang, E, Rajagopal, S, and Beard, PC (2006): A Fabry-Perot fibre-optic hydrophone for the measurement of ultrasound induced temperature changes, Proc. IEEE Ultrasonics Symposium, 536-539. Download PDF file.

Morris, P, Hurrell, A, and Beard, P (2006): Development of a 50MHz Fabry-Perot type fibre-optic hydrophone for the characterisation of medical ultrasound fields. Proc. Institute of Acoustics 28, 717-725. Download PDF file

Cox, BT, Zhang, EZ, Laufer, JG, and Beard, PC (2004): Fabry Perot polymer film fibre-optic hydrophones and arrays for ultrasound field characterisation, Journal of Physics: Conference Series, Advanced Metrology for Ultrasound in Medicine 2004 (AMUM 2004), 32-37. Download PDF file

Beard, PC, Hurrell, A, and Mills, TN (2000): Characterisation of a polymer film optical fibre hydrophone for the measurement of ultrasound fields for use in the range 1-30MHz: a comparison with PVDF needle and membrane hydrophones, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency control 47(1), 256-264. Download PDF file

Beard, PC, and Mills, TN (1997): A miniature optical fibre ultrasonic hydrophone using a Fabry Perot polymer film interferometer, Electronics Letters 33(9), 801-803. Download PDF file