Page 197 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

190 Chapter Eight

Small core Small core Power
Source Mode-mixing monitor
region

Receiver Large core Large core Sensor
Figure 8.25 Asymmetric power splitters, for example formed using
two types of optical fiber, can reduce the effects of reflections in
the power monitor branch.

to use different fiber types. A small core and a large core fiber fused together
to effectively mix modes can help (Fig. 8.25). Here the input light passes pri-
marily to the sensor fiber, but returning light passes primarily to the signal
detector. Only a small fraction is tapped off to the monitor photodiode. This
sounds perfect, except that the small source fiber also reduces the power coupled
from a multimode source such as the LED. If we allow ourselves loss, then a
simpler approach is to carefully splice a long, lossy length of fiber between
coupler and monitor photodiode. With 20dB one-way loss the monitor signal
will still be adequate for low-noise detection, but the 40dB two-way loss will
significantly reduce spurious reflections, especially if combined with coatings,
gels or angle-polishing.
In Chap. 11 we will discuss additional electronic measures which can make
further improvements. In the interim the simple design points out the very
great difficulties of intensity-based measurements of this sort. While it is not
impossible to build the fiber reflectometer for refractive index measurements
great care is needed to make it effective even in a benign environment. For the
“real world” there are probably much better solutions.

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