Page 186 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 179

pellicles, formed from pure or doped silicon and other semiconductors, as less
compliant pellicles for near infrared beam-splitters.
The optical manufacturers also offer glass plates polished into a wedge with
a small angle, (for example 5 minutes of arc). In this case the two reflected
beams are separated in angle, as well as in offset. The angle gives interferences
with much finer fringes which can average out over a large detector’s area, and
makes it easier to block and absorb the unwanted beam at a convenient dis-
tance from the beam-splitter. The main disadvantages are the shift in the trans-
mitted beam optical axis and the aberrations.
For fiber-connected sensor systems, fiber-couplers are very attractive as
beam-splitters for optical referencing. The primary, and large, problem with
multimode fiber couplers is that they are notoriously sensitive to the distribu-
tion of modes entering the coupler. In some types the low-order modes prefer-
entially continue on along the input fiber strand, while higher order modes
couple across to the second fiber. Unless effective mode mixing and power-
equalization is in place large fluctuations in split-ratio are to be expected, except
in the most benign of environments. This is the case for both incoherent and
laser sources. One big advantage of fiber splitters is their completely closed path
and hence freedom from contamination.
Where a coherent (laser) source is used, speckle effects will also lead to split-
ratio fluctuations (modal noise). The speckle pattern present at the point of cou-
pling will be sensitive to even small movements of the input fiber. The solutions
here are to either substitute a source of lower coherence length (e.g., an LED),
or to dynamically vary the speckle pattern though the use of a fiber shaker or
moving diffuser at the point of coupling into the fiber. These potential errors
are in addition to the severe intensity fluctuations seen if a multimode fiber
output is vignetted before detection. As speckles move on and off the detector
with fiber movements, large intensity fluctuations can be caused. In my expe-
rience, the loss of signal intensity in changing from a laser to an LED in fiber-
based systems can often be more than compensated for in the reduced speckle
interference effects. This even includes the use of low-power visible LEDs
coupled to single-mode fibers (see the TRY IT! in Chap. 3). Last, both single-
mode and multimode types are available, in silica, glass, and plastic. All fiber
components exhibit polarization sensitivity, which are more pronounced due to
little averaging in single-mode fibers.
Where small fractional power tapping is adequate, fiber-bend taps can be very
convenient. Here the fiber is subjected to a small bend radius by bending or
wrapping it around a small mandrel. It is generally not necessary or desirable
to strip the fiber, but to clamp onto the primary or secondary coating. The radius
of curvature may be changed to adjust the output coupling efficiency. However,
strong out-coupling can sometimes take the fiber close to its minimum allowed
bend radius, so care is needed. Clamp-on sources and detectors are available
commercially, which are very convenient for a range of optical test and meas-
urement applications. One home-made design which is easy to make is shown
in Fig. 8.15. This uses a plastic heat-shrink coated steel mandrel to bend the

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