The reference beam LDA
In this system, two laser beams converge on the area of measurement (Figure 4.10) and a photodetector is placed on the extension of beam 1. The particle, when crossing the measurement zone, scatters the light from both beams and the photomultiplier receives the beam 2 spread in the direction of beam 1, in addition to the direct beam 1 (the Doppler effect for beam 1 is null since the direction of observation and that of the incident beam coincide). The intensity of beam 1 must be strongly attenuated by a suitable filter to be made comparable with the faintest light scattered by the particle.
Schematic of a reference beam LDA
The frequencies of the rays that reach the photomultiplier are:
fi = f f2 = f U • ( Єу2 )
From Equation (4.5), the output signal from the photomultiplier has the frequency
f2 – fo = ‘U • ( – e,2 ) = fD
In this system, the scattered light must be captured in a very small solid angle, as near as possible to zero, since the relationship between velocity and the Doppler frequency depends on the direction of observation. This feature is a major drawback since it results in a limitation of the intensity of the detected optical signal and therefore requires a high-power laser. The system is also sensitive to vibrations because it requires perfect alignment between the photomultiplier and the laser beam.