Measuring and testing – Vibration – Sensing apparatus
Patent
1992-08-18
1995-10-24
Williams, Hezron E.
Measuring and testing
Vibration
Sensing apparatus
25022717, 385 13, H01J 516, G01N 2100
Patent
active
054600480
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
Ultrasonic sensors have application in the characterisation of high frequency ultrasonic transducers and transducer arrays. This characterisation takes the form of output power measurements and beam profile measurements which determine the spatial distribution of output power and radiation pattern measurement which determine the angular distribution of output power. Such measurements also allow the calculation of other required parameters of ultrasonic devices. These measurements are particularly important in relation to ultrasonic devices for medical applications since the total output power and beam profiles must be accurately measured to ensure the safety of patients.
BACKGROUND ART
Hitherto the sensing of ultrasound has commonly been based on the use of a piezoelectric active element, such as a polyvinylidene-fluoride film. When the ultrasonic power is very high, this material can lose sensitivity or even become damaged. Additionally, the currently available size of these active elements is too large to resolve very narrow ultrasonic beams generated by highly focusing transducers. For these reasons, conventional hydrophones are not very satisfactory for characterising medical ultrasound that is of high power and highly focused.
A fibre optic ultrasonic sensor based on the use of a single-mode optic fibre has been proposed to alleviate some of these disadvantages. These devices operate on the principle that when an ultrasonic wave in the megahertz range is incident normally upon a single-mode fibre, the fibre becomes anisotropic, and consequently, the polarisation of the light at the output end of the fibre is modulated by the ultrasonic pressure along the fibre and thus varies at the ultrasonic frequency. The difficulty with sensors based on a single-mode fibre is that means must be provided to ensure that the light entering the region of interaction is circularly polarised, and that the optimum bias phase is also maintained at the same time. In practice this requires massive feedback control on optical components at both the input and output ends of the fibre to avoid signal fading due to environment disturbances to the fibre. This requirement has limited the application of ultrasonic sensors based on a single mode fibre.
Furthermore, the prior art does not provide an effective method for measuring the radiation pattern (or directivity) of the ultrasonic wave.
DISCLOSURE OF THE INVENTION
It is an object of this invention to provide an ultrasonic sensor which will overcome, or at least ameliorate, one or more of the abovementioned disadvantages.
Accordingly, an ultrasonic sensor according to the invention includes a polarisation maintaining fibre having two orthogonal principal axes, an input end and an output end, input means to launch into said fibre a light beam aligned with at least one of said principal axes, and output means to determine a degree of mode coupling induced in said fibre by incident ultrasonic waves.
The polarisation maintaining fibre is an anisotropic, or birefringent, medium including two orthogonal or mutually perpendicular principal axes. The light beams may be linearly polarised along these two axes and may encounter slightly different refractive indices Accordingly, the light beams may propagate at slightly different velocities. Since these two linear polarisations of light do not easily couple to each other, the fibre is said to maintain these two polarisations. The two light waves are termed the polarised modes of the fibre. A single-mode fibre does not possess this property since there is no preferred axis associated with the fibre. The polarisation maintaining fibre used in this invention can be of any known type as long as it maintains linear polarisations. Examples of suitable known polarisation maintaining fibres are the "bow-tie" fibre, PANDA fibre and elliptically clad fibre. The section of the fibre exposed to the ultrasonic field can be jacket-free or coated with any kind of material, depending on specific applications. The overall fibre diame
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Chan Helen L. W.
Chiang Kin S.
Finley Rose M.
Williams Hezron E.
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