Page 194 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 187

Sources Detectors
L
S 1 11 D 1
L 12
S 2 D 2
L 13
S 3 D 3
a L 14
S 4 D 4

L
Figure 8.22 More sources, detectors, and
path permutations can improve absorption
resolution.

Modulated Monitor
LED source photodiode
1mW 0.5mW
To ref. channel
demodulation
3.6% reflection
(18μW)
0.5mW
Polished fiber Liquid
coupler under test
Signal 50μW
photodiode
Sensor fiber:
10dB one-way loss
To signal channel
demodulation n:1.33-1.40
0.06–0.25% reflection
(30nW-125nW)
Figure 8.23 Measurement of changes in power reflected back from a distant fiber end
is attractive for a wide variety of sensing applications. However, performance is severely
compromised by changes in fiber loss, or by the more intense reflection from a monitor-
photodiode branch. The simple configuration is unusable except in the most benign
environments.

8.7 Design Example: Multimode
Fiber Refractometer

Figure 8.23 shows a basic reflection-mode refractometer constructed using mul-
timode fibers. This is an initially attractive system for detecting and measur-
ing small refractive index changes in a liquid, brought about by chemical
concentration changes. Similar systems have been used in applications such as
a remote saccharimeter for sugar concentrations and sensors to detect the state
of charge of secondary batteries. The idea is to use the variation of power reflec-
tion coefficient at the bare-fiber/liquid interface to give an intensity change
related to the liquid refractive index. The reflection coefficient is given by the
Fresnel relations.
The source chosen is a high intensity near-infrared LED which couples 1mW

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