Coarse Wavelength-Division Multiplexing
A multiplexing technique based on coarse wavelength-division multiplexing (CWDM) could be used to separate different
Figure 5.17 Example of multiplexing four sensors using CWDM.
sensors into their individual wavelength bands. These sensors share a broadband light source and a spectrometer, but do not interfere with each other. As shown in Figure 5.17, an example of multiplexing four sensors is given by a 1 X 4 CWDM coupler. It can be seen clearly from Figure 5.18 that the four signals corresponding to the four FFP sensors are in different wavelength regions and can be demodulated with similar ways as a single FFP sensor.
SFDM/CWDM of FFP Sensors
Furthermore, CWDM and SFDM are combined to enhance the multiplexing capability considerably. The schematic diagram is given by Figure 5.19. Multiplexing of more than 10 FFP sensors was experimentally demonstrated using SFDM in each wavelength channel predetermined by CWDM. This method can multiplex a large number of sensors of up to 100 in principle and hence can reduce the cost of
Figure 5.19 Schematic diagram of hybrid SFDM/CWDM multiplexing.
the whole sensor system greatly. Also, such a hybrid method can take advantages of both SFDM and CWDM when applied to the multiplexing of a large number of sensors, that is, excellent multiplexing capability when using SFDM, and good efficiency in using every part of the optical power of the broadband source with CWDM.
As a verification example, the mixed spectrum of four FFP strain sensors is shown in Figure 5.20a. By using filtering and FFT, the waveform of the mixed signal is shown in Figure 5.20b . It can be seen that the spatial-frequency component corresponding to each cavity length of these four FFP sensors can be distinguished clearly.