Ultrafast Humidity Interrogation Based on Agarose-Coated Fiber Bragg Grating With Time-Stretching Method

Ultrafast humidity interrogation is valuable in industrial production, healthcare, and environmental monitoring due to its fast response and high sensitivity. We present an ultrafast humidity sensor based on an agarose-coated fiber Bragg grating (AG-FBG) structure combined with a time-stretching method. Leveraging AG’s ability to swell or contract in response to humidity variations results in changes to the effective grating period, which in turn causes a shift in the Bragg wavelength. Using a homemade high-power femtosecond all-polarization-maintaining fiber laser, the time-stretching technique can be achieved through a long dispersion-compensating fiber. According to wavelength-to-time mapping, the wavelength shift associated with humidity response can be converted into the time domain, resulting in a humidity sensitivity of −0.017 ns/% relative humidity (RH) at an interrogation speed of 76.75 MHz and a humidity resolution of 0.007%RH. The proposed humidity monitoring system has demonstrated high stability and repeatability. Therefore, with the advantages of high sensitivity, high accuracy, and ultrafast response time, our proposed humidity sensor shows significant potential in the field of real-time monitoring.