Page 245 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Multiple Channel Detection

238 Chapter Eleven

imperfect phase adjustment. Nevertheless, the lock-in realization of this
approach makes a highly didactic demonstration, which for moderate isolation
is effective.

11.4.2 Multimode fiber refractometer
revisited—again
In the design of a simple refractometer for remote liquid refractive index meas-
urement shown in Chapter 8 we wanted to measure two signals, one a refer-
ence intensity representing the power coupled into the fiber system, the other
a variable intensity reflected from the sensing fiber end. One problem was how
to suppress the reflection from the monitor fiber arm to very low levels (at least
-60dB), such that it does not perturb the weak reflection from the sensing
probe. One way to do this is to treat it as a two-channel measurement system,
and to apply the two-phase detection technique.
If the light source of Fig. 11.6 is square-wave modulated at f mod, the interfer-
ence and signal intensities will be superposed on the signal photodiode, but due
to group delays they are not necessarily aligned in time. So far we have only
considered group-delays caused by RC-networks in the photodiode and receiver
components. However, with the long transmission lengths typical of fiber

Modulated LED Monitor
source (f mod =100kHz) L= 0 photodiode
To ref. channel
demodulation
3.6% reflection
Polished fiber Liquid
coupler under test
Signal
photodiode
Sensor fiber:
To signal channel 10dB one-way loss
demodulation L = 255m n:1.33-1.40
s
0.06–0.25%
reflection
1/f mod

Signal from monitor
arm reflection
Signal from sensor
arm reflection

d t = 1 / 4 *1/f mod (quadrature)
= 2.5μs
Figure 11.6 Phase-coding applied to a long fiber reflectometer. The main reflections
from the sensor and from the reference arm can be arranged to be in quadrature
through correct choice of the source modulation frequency.

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