Multiplexing Methods of FFP Sensors

Spatial Frequency-Division Multiplexing

A multiplexing technique that allows for FFP sensors with different lengths to be arranged parallely by using an optical coupler is given in Figure 5.15. These sensors share a broadband light source and a

(a) Mixed signal of 11 FFP sensors

Figure 5.16 (a) Mixed signal of 11 FFP sensors

and (b) FFT spectrum of 11 signals.

spectrometer. Their mixed reflective optical signals are received by the spectrometer, then processed using FFT. These sensors with different cavity lengths are located at different frequency bands in the FFT spectrum, which could be clearly differentiated. Therefore, they could be individually demodulated using FFT-based OPD demodulation methods.

A series of 11 FFP sensors is multiplexed in 1550-nm band. Their mixed optical signal and FFT spectrum are given in Figure 5.16. It can be seen that the spatial-frequency component corresponding to each cavity length can be distinguished clearly so that multiplexing of up to 11 FFP sensors is achievable.

Such a spatial frequency-division multiplexing (SFDM) s ensor system with the advanced demodulation algorithm can be used for a wide range of applications, in particular, for those applications where remote monitoring with a large number of sensors is essential. By combining with optical switches, the total sensor number, which could be multiplexed, may be up to several hundreds. Also, it can reduce the cost of the whole sensor system to a very large extent and have excellent performance-to-price ratio due to the significant improvement of the multiplexing capability over conventional FFP sensor systems.

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