Page 247 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Multiple Channel Detection

240 Chapter Eleven

development and use. Of course, with a two-channel measurement system like
this, you will be asking “Why not measure the power of the monitor reflection
in the same way, and do without the monitor photodiode entirely?” This is
certainly an attractive approach. We have then reduced the system to a single
photoreceiver, although we have a true signal plus reference measurement. The
two-phase lock-in amplifier can do it all.
There are many variations on this theme which have been published and even
used in real applications. We could, for instance, use a partial reflector half-way
along the sensor fiber as the power monitoring reflection. This might reduce
errors due to split ratio in the fiber coupler.

11.5 Spectral Frequency Analysis
Frequency coding is clearly capable of multichannel operation, but where more
than two or three channels are required, the fabrication and adjustment of one
synchronous detector per channel becomes inconvenient, even in the lab. We
could of course use a single synchronous detector with a frequency synthesizer
or just the individual source modulation generators connected to the reference
input. In this way we could sequentially select the different frequency compo-
nents. Alternatively, we might move from dedicated fixed-frequency synchro-
nous detectors to full spectral analysis. After digitizing and storing a series of
time-samples, the magnitudes of each frequency-coded channel can be calcu-
lated in software. Figure 11.7 shows a system in which the separation of dif-
ferent frequency signals is performed digitally, for example using an FFT (Fast
Fourier Transform) process. The FFT algorithm has become so ubiquitous that
it is easy to think that it is the only spectral analysis technique to use. For a
few, known spectral components, however, the FFT is overkill, and a (slow)
Fourier transform performed in the classic way may be faster. Just as in the
case of lock-in detection, we can operate either with or without a phase refer-

f 1
R L
PD
LED1 + A
l
-
f 2 1
l μProc. 123 .. . 8
LED2 2 + FFT
Spectrum
μProc.
+ FT A(f1)
A(f2)
A(f8)
Figure 11.7 Extension to multiple channels is
straightforward, although electronic frequency
separation may require (slow) or fast Fourier
transform analysis.

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