Advanced Phase-Contrast Method for Measuring Ultrasonic Fields
MEC 2006: Beste Mechatronik-Diplomarbeit an der Johannes Kepler Universität
Erwin K. Reichel
In this thesis an optical imaging technique was developed for the visualization and the quantitative measurement of acoustic fields generated by ultrasonic transducers with power levels in the medical diagnostic range. The acoustically distorted light wavefront of short laser pulses synchronized to the ultrasound source is analyzed by a two lens optical system with a pixel addressable liquid crystal spatial light modulator as the filtering device in the focal plane of the first lens. Focused phase-contrast images are recorded by a CCD camera for further processing. It is shown that by applying multiple optical phase-only filtering functions the phase shift of the light wavefront caused by the acoustically induced density variations can be acquired. The measured phase shift is proportional, according to the piezo-optical effect, to the local pressure deviation projected along the line of sight.
Based on the theory of coherent light by Fourier optical means an analytical model is derived, algorithms for phase retrieval were developed and simulated numerically and measurements with pulsed and continuous wave ultrasonic fields were carried out. The results were compared to measurements with hydrophones to evaluate the accuracy of the system and to proof agreement with the theory.
Using this technique it is possible to measure and characterize the ultrasonic field in minutes time, which would otherwise last hours or days using the standard method of measuring the acoustic intensity on a fine three dimensional grid with a hydrophone. The developed method is an advancement of the phase--contrast method which is only applicable for the quantitative measurement of low pressure amplitudes and it is superior to schlieren techniques as the true time-resolved phase distribution is preserved in the measured signal.
Fig. 1 shows the measurement principle, where F10 is the plane where the acoustic field causes optical phase shifts to the incident laser beam. The lens L1 acts as a Fourier transformer. The spatial light modulator in plane F11=F20 carries out the filtering operation and lens L2 generates the image in plane F21 where it is recorded by a CCD camera.
Fig. 2 shows the measurement result for a transducer operated in continuous wave mode. In the upper image the momentary field is depicted, below the intensity profile calculated from many such momentary measurements is shown.