A demodulation method of high-speed fiber Bragg grating based on dispersion-compensating fiber

Fiber Bragg grating sensing is one of the most attractive researches in the field of optical fiber sensing. It has made considerable progress due to its advantages in high multiplexing, high precision, small size, light weight, good corrosion resistance and immunity to electromagnetic interference. However, the traditional fiber Bragg grating demodulation technology can hardly achieve high-speed demodulation of multiplexing gratings, which seriously limits its extensive application. A novel high-speed fiber Bragg grating demodulation method is proposed and demonstrated in this paper. Large dispersion will be generated when light going through the long-distance dispersion compensation fiber. Based on the dispersion effect of dispersion compensation fiber, a light beam of different wavelength will generate different time delay, and the wavelength shift of the fiber Bragg grating sensor is then transformed into time domain, and ultimately the fiber Bragg grating wavelength demodulation can be realized by measuring the delay of grating reflective light pulse. The reflective light pulse train of all the cascade fiber Bragg grating sensors can be obtained only through one pulse of light source. This method can be applied in all-fiber structure without wavelength scanning so that it can promote the demodulation speed greatly and can be applied to the demodulation of quasi-distributed fiber Bragg grating sensor network. Disturbing influence of dispersion compensating fibers can be eliminated by introducing the reference grating, and the demodulation process is immune to light intensity disturbance. A test system is set up to demodulate a quasi-distributed sensor network which is made up of three fiber Bragg grating sensors. Results show that the linearity of the demodulated wavelength is good and the demodulation speed can be up to 1 MHz. The demodulation linearity is about 0.9969, and the error is about 27.8 pm after 10 times average. The novel demodulation method proposed in this paper has been tested through theoretical analysis and experimental demonstration, its feasibility to realize high-speed demodulation of fiber grating has been proved, but significant improvements still can be made in the demodulation system. The next step of research work will focus on how to realize decoupling between the location information and the wavelength, to avoid the influence of temperature disturbance on wavelength demodulation, so as to further improve the wavelength resolution and demodulation accuracy.