A demodulation scheme for a hybrid fiber sensor system for composite materials

A demodulation scheme is presented for a hybrid sensing system based on a polarimetric fiber sensor and a fiber Bragg grating (FBG) for composite structural health monitoring (SHM). The demodulation module is comprised of a Thin Film Filter Wavelength Division Demultiplexer (TFF WDM- Demux) and an Electro-optic (EO) modulator. Unlike "laboratory-use" demodulation systems which typically do not need a compact form factor, the proposed miniaturized demodulation system is compact, lightweight and has low power consumption. The bandpass responses of the TFF WDM- Demux are designed to match the peak reflected wavelengths of the FBGs so that the differential wavelength information can be converted to intensity variations recorded by the array of detectors connected to the output channels of the TFF WDM- Demux. In the polarimetric sensor demodulation section of the system, an electrical control voltage is applied to the electro-optic modulator in order to shift the polarimetric sensor output to the maximum sensitive linear response region. Two types of polarimetric fiber sensors are used; a Panda fiber and a polarization maintaining photonic crystal fiber. The polarimetric strain sensors provide the average strain and temperature information, while the fiber Bragg grating sensors give localized strain information. The demodulation system uniquely allows for the multiple outputs of FBG and polarimetric sensors to be converted to a common optical intensity domain, for strain and temperature measurements.