The reflective FFP sensor is suited for time-division multiplexing using a pulsed light source such as semiconductor LD .
Figure 5.20 (a) Reflective signal from two CWDM channels and (b) FFT spectrum of four FFP sensors with different cavity lengths.
Figure 5.21 Schematic diagram of TDM system for FFP sensors.
A schematic diagram is given in Figure 5.21. In such a system, fiber delay lines with different lengths are provided between the transmitter and receiver such that, for each laser pulse, the photodetector receives reflected pulses from each of the sensors in different time slots. The signals can be processed by digital means under microprocessor control. In such a scheme, the reflected waveforms are sampled for ana- log-to-digital conversion at fixed time delays relative to the start of the pulse, and the samples are averaged digitally. In-phase and quadrature signals from the interferometers needed for high-sensitivity sensing can be obtained by adjusting the dc bias current of the laser between pulses. A reference FFP time multiplexed with the sensors can be used to correct the fluctuations of the light source and drift of the laser wavelength.
To minimize cross talk between sensors, they are arranged in a single fiber. The reflectivity of the sensor should be controlled at a low level (typical <4%). However, cross-talk effects are limited as the transmissions vary at the individual sensors. This cross talk can be further eliminated by measuring the Rayleigh back-scattered level just in front of the sensor, which can be used as the reference for calculating the reflectivity of a particular sensor in the network . The alternative method for cross-talk elimination can be summing of the losses on all preceding sensors, while calculating the reflectivity of the observed one.
A fatal drawback of this technique is that it puts forward a tight requirement on the consistency of different sensors, for achieving the same quadrature points.