Optical fibre hydrophone

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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 optical fibre hydrophone system, based upon the use of a Fabry Perot polymer film sensing interferometer [4,5] has been developed in collaboration with Precision Acoustics Ltd. under the EPSRC/DTI LINK Photonics programme. Applications include the assessment of the output of diagnostic and therapeutic medical ultrasound equipment, measurements of in vivo ultrasound exposure and characterisation of sources used in non destructive testing apparatus and ultrasonic industrial processes.

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System description

Figure 1 Schematic and photograph of the fibre optic hydrophone system.

The system, shown in figures 1 and 2, comprises an optics unit containing a tuneable DBR laser diode source, coupling optics and detection optoelectronics. The output of the laser is launched via a 2x2 fibre optic coupler into a connectorised single mode optical fibre sensor downlead. The acoustically sensitive element comprises a 25μm thick Parylene film that is vacuum deposited [4] on to the tip of the fibre and acts as a Fabry Perot interferometer - the mirrors of which are formed by the deposition of optically reflective coatings. An incident acoustic wave modulates the thickness of the film and hence the optical phase difference between the light reflected from the two sides of the film. This produces a corresponding reflected intensity modulation which is then transmitted back along the fibre to the optics unit where it is directed, via the fibre coupler, to the photodiode. To set the phase bias of the interferometer to the optimum working point, the laser wavelength is thermally tuned by adjusting the temperature of the laser diode (for more on the transduction mechanisms [3], click here).

The advantages of this approach 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 for an omnidirectional response and to 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 interferometer using 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 phase bias and detecting the corresponding output of the interferometer enables changes in sensitivity to be continuously monitored.
• Simultaneous temperature and pressure measurement - Thermally induced changes in the optical thickness of the interferometer enable temperature changes to be measured for investigating effects such as ultrasound induced heating.
• 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.