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Fiber-optical microprobe for ultrasound and temperature

The ultrasonic pressure and the temperature in liquids can now be detected simultaneously by means of a fiber-optical microprobe. The properties of the microprobe, e. g. extreme miniaturization and high temporal and spatial resolution, open up novel applications in medicine and technology.

Arrangement for shock wave and temperature measurements in the vitreous body of a pig's eye during exposure to infrared laser pulses. Sound pressures of several MPa and temperature increases up to 15 K were measured. Photo taken at the Physics Department

Appropriately prepared optical fiber ends constitute an efficient alternative to conventional electrical sensor systems. In particular, for measurement tasks requiring high temporal and spatial resolution, minimal invasiveness, high shock resistance or small heat conduction, very small optical sensors offer important advantages.

In the sensor developed at PTB, the sensitive element is the front face of an optical fiber coated with a system of optical interference layers. Ultrasound detection is based on the elastic deformation caused by an incident sound wave in the layer system which itself acts as a microinterferometer. The total thickness of the layer system is less than 2 µm. It is designed to yield a sensitive change in the optical reflectance as a function of the deformation at the operational light wavelength. The change in optical reflectivity is detected with a simple optical arrangement. As the refractive indices and the thicknesses of the layers also depend on the ambient temperature - and thus the resonator length of the microinterferometer, too -, the same sensor can be used simultaneously for temperature measurements. The time scales on which the acoustic and thermal measurands change are significantly different and the different signals can be separated by frequency filtering.

In a first application, the new measuring technique was applied in cooperation with the University of Kaiserslautern to investigate shock wave propagation and heat development in the vitreous body of an eye during tissue removal by infrared laser pulses. Such insitu measurements are important to assess the potential damage vitreous-body laser surgery may cause in the surrounding tissue, in particular, in the retina. Some other examples where ultrasonic pressure and temperature are to be measured simultaneously are ultrasound exposimetry and applications in chemical process technology, sonochemistry and ultrasound cleaning technology, where the entire ultrasonic pressure range up to 100 MPa and temperature increases by more than 100 K can be covered

Contact at PTB:

C. Koch,
fax: +49 (0) 531 592-1405