Delay-Insensitive Time Stretch Interrogation of Fiber Bragg Grating Sensors

Photonic time stretch (PTS) approach using ultrashort optical pulses has enabled ultrafast real-time interrogation of wavelength-modulated optical fiber sensors. It has been always assumed that dispersion induced wavelength-to-time mapping is unique and determinic such that the wavelength information of the sensor can be determined from its temporal position. However, random time shifts caused by fibre length changes or distance variations in remote sensing have been overlooked. In this paper, a novel delay-insensitive PTS-assisted ultrafast interrogation technique for remote fiber Bragg grating (FBG) sensors has been proposed and demonstrated. It is made possible by applying chirped microwave frequency encoding such that optical wavelength can be detected from the instantaneous microwave frequency of the encoded stretched pulse, eliminating the effect of random time shifts. The performance of the proposed interrogation system is elaborated by numerical simulation studies. Ultrafast interrogation of remotely placed FBG sensors has been demonstrated. Results verified that the system is immune from distance changes between the sensor and the interrogation reader. A high wavelength resolution of 0.0016nm and a 50 MHz detecting speed with the superior time-delay insensitive characteristic is realized.