Using an optical probe to investigate pulsed ultrasonic fields.
Michael Johansen, Kathy J. Hays-Stang, Bill D. Cook · The Journal of the Acoustical Society of America · 1991
For continuous waves, a laser-beam probe can be configured with a photodetector to be a line detector of pressure in an ultrasonic field. It is important that this detector behaves in a linear manner. For this linearity, the experimental configuration includes filtering the electrical signal from the photodiode. This filtering eliminates the second and higher harmonics caused primarily by the optics and secondarily by the harmonics in the sound field. This convenience is not available for pulsed ultrasonic waves where the spectrum is wide. Such a filter would remove the part of the pulse. Moreover, the size of the fast-responding photodetector becomes important at the higher frequencies cutting off where the wavelength of the sound and the size of the photodetector are comparable. These and other limitations of the laser optical probe define a limited range where such a laser-beam probe can be used.