Laser Doppler instrument for measurement of vibration of moving turbine blades

A laser Doppler instrument using a 1 mW laser has been developed for the measurement of the vibration of moving objects. Particular interest has been taken in rotating turbine blades. The design and optimisation of the instrument are described, and details given of its performance and resolution. It is shown that resolution is limited by signal/noise ratio and by Doppler broadening, both of which can be predicted. A coherence loss factor is established which enables the expected signal/noise ratio to be estimated from simple `noncoherent¿ measurements of light scattered from a target. Experiments are described which compare the performance of photomultipliers and avalanche photodiodes. The instrument has been used successfully on a turbine having blade-tip velocities up to 300 m/s, equivalent to Doppler shifts up to 1000 MHz. At the lower frequencies, photomultipliers and avalanche photodiodes are comparable in performance, but at the top end of the range only the latter is satisfactory. Resolution in velocity of about one part in 104 is achieved with an output bandwidth of 1 kHz.