Page 168 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 161

(a) (b) Discrete reference
beam-splitter
Single-mode
source fiber
Laser source
1mW
Multimode Object
return fiber
Fused fiber coupler
(single-, multimode) Moving
Signal
photodiode object
Fiber-end
reference reflection
Beat signal
Object reflection

Figure 7.19 By forming the interference with a reference reflection at the distal fiber end,
the delivery fiber length is unimportant; even multimode fibers work well. However, speckle
is still disturbing, so a singlemode fiber and tiny discrete beamsplitter can perform even
better.

cannot distinguish between particles moving up or down the page in Fig. 7.18;
they both give the same beat frequency. If this is a limitation, it can be removed
by frequency-shifting one arm of the interferometer using an acoustooptic mod-
ulator as in Fig. 7.17c. This has the effect of causing the fringe pattern to move
in one direction. Then particles moving with the pattern show a lower frequency
modulation; those moving against the fringe pattern motion show higher fre-
quency scattered light modulation. The differences can then be separated out
using electronic filtration techniques. This is called a heterodyne detection
system.
Simple homodyne interferometers lend themselves ideally to construction in
fiber-form. Figure 7.19a shows a fiber-coupler based interferometer, in which
the reference signal comes from the Fresnel reflection at the fiber end. This
amplitude is added to that from the object to give an interference beat which
can be detected via the beam-splitter. This geometry is very useful as it refers
the interferogram to a plane close to the moving object. However, if a multi-
mode fiber system is used there will still be signal fluctuations due to speckle
in the fiber. As the emitted light field is made from a large number of effective
modes, each of which may have a different optical phase, the detected intensity
depends on mechanical motion of the up-lead fiber. This can be alleviated
by using a single-mode fiber for the source fiber (Fig. 7.19b), an external beam-
splitter and a multimode receiving fiber. This leads to a much more stable
system in which vibration and movement in the fibers do not lead directly to
speckle-noise intensity fluctuations. Some intensity modulation caused by

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