The problem of obtaining vibratory information from remote, rotating, components within a machine has been overcome by means of a laser-Doppler fiber-optic probe. Laser light is transmitted down the fiber-optic and scattered light is returned for analysis in the same way. The Twyman-Green mode of operation of the laser-Doppler system has been shown to have serious shortcomings for this type of work and a new mode has been produced, and used successfully, in conjunction with Bragg cells for frequency shifting. The use of a single set of Bragg cells to compensate for the average velocity of a rotating system, limits the allowable speed of rotation to about 1000 rpm. However, other techniques for frequency shifting are available. An experimental investigation is reported in which a rotating system, consisting of a disk carrying flat cantilever blades, was rotated at about 1000 rpm while one of the blades was excited at 163 Hz. The measured frequency of the vibrating blade was 162.8 Hz and similar accuracy was obtained for the amplitude measurements.

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