Humidity is an important factor affecting industrial production, crop planting, food processing, microbial culture, human health, and cultural relics preservation because it is easy for bacteria, fungi, and viruses to grow and reproduce under appropriate humidity. Especially in the high humidity environment, the surface of organic cultural relics (such as leather, bamboo, paper, and textiles) is prone to breed mold. However, in a dry (low humidity) environment, it is subject to cause dry cracking of cultural relics, and even result in oxidation and deterioration of cultural relics. Therefore, accurate in-situ real-time detection of environmental humidity of cultural relics preservation is vital for effective preventive protection of cultural relics. At present, the main sensors adopted for on-line humidity detection are electrochemical humidity sensor, surface acoustic wave humidity sensor, and fiber optic humidity sensor. Optical fiber sensors feature small size, high temperature resistance, corrosion resistance, electromagnetic interference resistance, and quasi-distributed measurement, and have become one of the most promising sensors for online relative humidity detection. However, it still faces the problems of low sensitivity, long response time, and low accuracy. Therefore, it is necessary to develop a fiber optic humidity sensor with high sensitivity, short response time, and high accuracy.

To improve the performance of fiber Bragg grating (FBG) humidity sensors, firstly, we construct a new humidity-sensitive material composed of chitosan, polyvinyl alcohol, and nanocarbon powder. Secondly, the FBG humidity sensor is made. Thirdly, the principle of humidity detection by sensor is analyzed. Fourthly, the humidity measurement system is set up. Fifthly, the surface morphology and composition of the samples are characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Additionally, the influence of the preparation conditions and the environment on the sensor performance is studied experimentally, and the output spectrum, sensitivity, response time, and accuracy of the sensor are tested.

We develop an FBG humidity sensor based on nanochitosan/polyvinyl alcohol/nanocarbon powder composite organic film. The research results show that the humidity sensor yields the best performance when the nanocarbon powder doping mass fraction in the humidity sensitive film is 10% (Fig. 4) and the thickness of the humidity sensitive film is 185 μm (Fig. 5). The best performance sensor can perform highly sensitive, fast, and accurate detection of relative humidity (20%RH-90% RH) in the temperature range of 5-65 ℃,when the wavelength of the optical radiation source is 220-1200 nm and the light irradiation intensity is 50 mW/cm². Sensor sensitivity humidity reaches 57.7 pm/(%RH) [Fig. 5(d)], response time is 420 s, and recovery time is 540 s [Fig. 5(b)].

We develop a new FBG humidity sensor based on nanochitosan/polyvinyl alcohol/nanocarbon powder composite organic film. The presence of a large number of hydroxyl and amino groups in the chitosan/polyvinyl alcohol complex enhances the swelling effect of the polymer. Additionally, the nanocarbon powder with a larger surface area to the chitosan/polyvinyl alcohol complex enhances the water adsorption, greatly improving the sensitivity and response rate of the sensor to humidity. The reference grating is employed to decouple the temperature. The creative utilization of black PTFE capillary packaging structure eliminates the light interference on the FBG humidity sensor coated with humidity sensitive materials. Our research has application significance in in-situ, real-time, and online humidity detection, and also provides a new solution for humidity detection and sensing.