FBG Demodulation With Enhanced Performance Based on Optical Fiber Relative Delay Measurement

Fiber Bragg grating (FBG) demodulation based on microwave photonics (MWP) would have a high resolution but the measurement range is usually limited. Here, we propose an FBG demodulation system based on wavelength to optical fiber relative delay (OFRD) mapping, where the signal reflected by the FBG is first modulated by an RF signal and then sent to a dispersion compensation fiber (DCF). The OFRD can be acquired by measuring the slope change of the phase response of the DCF. Thanks to the sub-picosecond resolution OFRD measurement of the phase-derived ranging, the FBG demodulation resolution would be very high. The integer ambiguity, which is the primary limitation for large measurement range, is removed by sweeping the frequency of the RF signal in a certain range. In addition, the relative slope change of the phase response can be obtained using a larger frequency sweep interval compared to the traditional phase-derived ranging method, leading to a significant reduction of the measurement time. An experiment is performed, which applies the system to measure the temperature. A measurement range of more than 300 degrees C (corresponding to a wavelength shift of 3 nm) and measurement precision of +/- 0.2 degrees C (corresponding to a wavelength shift of 2 pm) are achieved with a measurement speed of 20 ms.