Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 168

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60 m 60 m

Sensor

FIGURE 4.13
Moving vehicle target.

4.14 FiberSensorSignalProcessing
The fiber sensor is analogous to the classical phase-locked-loop (PLL) circuit
used in a phase demodulator application. As a target's signature influences
the sensor, it causes a phase shift in one of two coherent signals applied to
a phase comparator network. A voltage proportional to the induced phase
shift is generated and applied to an internal phase shifter on the reference
leg. The signal is then high-pass filtered to eliminate external noise-induced
bias below around 0.05 Hz in a detection loop. This action forces phase track-
ing between the two signals; as one signal's phase is modulated by the tar-
get, the reference signal's phase is controlled to track the effect, just as in a
conventional PLL. The output of the system is the control voltage driving the
internal phase shifter, which directly represents the phase shifts due to the
target's influence on the sensor transducer; thus, the phase shifts are demod-
ulated. Identifying the classical blocks in a PLL, the combining directional
coupler and photodiode represent a phase detector, the signal fiber produces
the phase-modulated signal, and the piezoelectric phase shifter and refer-
ence fiber are analogous to a voltage-controlled oscillator (VCO). It is worth
noting that the phase modulator is not precisely identical to a VCO in that
the loop filter cannot induce a change in the frequency of the source. This
difference becomes significant since a frequency drift of the source remains
uncorrected and manifests itself as a loop bias (i.e., is tracked). Loop filter
transfer functions are selected to provide the desired steady-state and tran-
sient responses (phase domain only) in accordance with the classical PLL
demodulator.

4.14.1 Reference Phase Modulation

A phase modulator is required in the reference arm to maintain phase lock
at the quadrature condition and to correct for thermal drift. Such a modu-
lator is formed by winding a portion of the reference arm fiber around a