Measurements in periodic motions
If a flow is periodic, it is necessary to correlate the measured velocity with the phase of the periodic phenomenon, e. g. in measurements in turbo engines, the measurement volume sweeps a path in the circumferential row of rotating blades, thus the flow between two blades is present cyclically with a frequency that depends on the speed of rotation and the number of blades. A synchronizer, working like an electronic shaft position encoder detects a pulse generated from the shaft itself. The frequency of the input signal (one lap) is multiplied by the number of blades to generate a pulse per blade (frequency of the blades).
If measurements are made between the blades of a turbo engine in motion, the laser beam must be instantly shut down, in order to prevent strong reflections of the laser beam on the blades. This mode of operation improves the signal to noise ratio as the photocathodes of the photomultiplier are restored during periods of absence of laser light.
If a turbine disk with 30 blades is running at 10,000 rpm, laser beams encounter a blade 5,000 times a second: no mechanical shutter is capable of such a performance. A Bragg cell is then used which, as already stated, consists of a transparent crystal which is excited by a piezoelectric transducer connected to a high frequency generator; by applying an alternate current to the transducer, a train of acoustic waves is generated inside the crystal, that produces a periodic variation of the refractive index of the crystal which, in turn, generates the periodic diffraction of the laser beam. The outgoing beam oscillates with the frequency of the transducer in turn generating a beam of zero order (non-deviated), beams of the first order, second, and so on (Figure 4.30).
If the piezoelectric transducer is not excited, the incoming beam undergoes no deviation (zero-order beam only exiting the crystal), and is thus interrupted by the optical stop; the Bragg cell and the stop are adjusted so that, with the transducer excited, the beam of the first order is able to exit the hole in the optical stop with a frequency equal to that of the blades.