Page 166 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 159

Moving
(a) Laser particle

1nW
1μW Diffuse
Detector
scatter
-
+
Moving
Beam-splitter
(b) Laser particle

Detector -

+
Beam-splitter Moving
(c) Fringe-spacing =l 2 sinq particle
Laser

2q

Folding Detector
mirror Optional -
frequency- Intensity +
shifter

Time
Figure 7.17 Where intense scatter is unavoidable, source modulation is little help (a).
Coherent detection effectively modulates the received signal depending on the particle’s
velocity (b). (c) An alternative configuration which allows more flexible choice of the
modulation frequency.

I = A + B + 2 AB cos D (7.9)
2
2
where D= f 2 - f 1 is the phase difference. As the particle moves toward the beam-
splitter the scattered signal phase will change with time, and with it the
detected intensity. In this case we do not need to modulate the laser to obtain
an intensity modulation. If the particle is moving with a constant v m/s, then
we will detect a sinusoidal beat. With the almost collinear geometry shown, the
beat signal will have a frequency of 2v/l (Hz). For a 0.633mm wavelength helium
neon laser and a velocity of 1m/s this is about 3.2MHz. Hence the particle has
provided its own unique modulation signal, coded to represent its velocity. All

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