Fiber Optic Ultrasonic Sensor Using Raman-Nath Light Diffraction

A novel fiber optic ultrasonic sensor using the principle of Raman-Nath light diffraction has been developed. The sensor does not perturb the acoustic field and exhibits a wideband frequency response. In addition to the remote sensing of the field, it is suitable for measurements of both continuous and pulsed ultrasonic waves. The experimental results obtained with the sensor were compared to those measured using a calibrated PVDF needle hydrophone, showing excellent agreement. The sensor's frequency response in the range from 3 to 15 MHz, typical of that used in medical ultrasound imaging, was determined using the time delay spectrometry (TDS) technique. It appears that the fiber optic sensor provides a useful alternative to the widely used PVDF ultrasonic probes in specific applications where any perturbation between acoustic field and sensor is undesirable. Also, since the active element diameter of the sensor can be made comparable to the core diameter of an optical fiber, the fiber optic sensor minimizes the spatial averaging effects and of fers significant improvement in comparison with the present state-of-the-art hydrophones which have a minimum diameter on the order of 300 µm.