Objective The monitoring of relative humidity is very important in the fields of agriculture, medical treatment and biochemical research, which urges scholars to develop various humidity sensors. Among many humidity sensors, optical fiber humidity sensor has become a research hotspot owing to their unique advantages such as high sensitivity, anti-electromagnetic interference, compact structure and other unique advantages. There are many kinds of optical fiber humidity sensors, and the interferometric humidity sensor has attracted wide attention because of its advantages of simple preparation and high sensitivity, among which the humidity sensor based on Mach-Zehnder interferometer (MZI) is widely used. However, the sensitivity and stability of these sensors still need to be further improved. In order to improve the sensitivity of the sensor and maintain its stability, a humidity sensor based on seven core tapered fiber is proposed and demonstrated. The sensor consists of a short section of seven core fiber between two single-mode fibers, in which the seven core fiber is fused and tapered by a hydrogen oxygen flame to form a tapered structure. Mach-Zehnder interferometer is formed by interference between the base mode of the cladding and the higher mode excited after the fiber is tapered. The effective refractive index of the cladding mode is easily affected by the external environment parameters. Therefore, the structure is very sensitive to the changes of the external environment parameters. The proposed structure is particularly suitable for the situations where high measurement sensitivity and high stability are required.

Methods Firstly, the seven core fiber with a length of about 1 cm is fused between two single-mode fibers. Then, the seven core fiber in the sensing structure is fused and tapered by a fiber taper machine (Kaipule Co. Ltd. AFBT-8000MX-H). Finally, GO film is coated on the surface of the tapered area by photothermal method. The coating process is as follows: the GO prepared by the improved Hummers method is mixed into a solution with the concentration of 1 mg/mL, and the GO solution is dropped on the surface of the fiber. In the coating process, the SLED broadband light source is used to transmit the light in the sensing structure. When the exciting light passes through the tapered fiber, a part of the light enters the cladding and generates a lot of heat in the cladding. GO molecules can be firmly adsorbed on the fiber surface by using the photothermal effect of laser, and thus the film is uniform and firm.

Results and Discussions Firstly, the wavelength scanning function of Rsoft software is used to calculate the sensor uncoated with diameter of 10 μm. The refractive index sensitivity of sensor is about 1200 nm/RIU (Fig. 4), which proves the feasibility of the experiment and provide a theoretical basis for humidity measurement. In order to provide experimental basis for subsequent humidity test, the refractive index response of samples with different diameters of uncoated GO was tested. The refractive index sensitivities of the samples with the diameter of 15 μm (s-1), 12 μm (s-2) and 10 μm (s-3) were 685 nm/RIU, 753 nm/RIU and 1123 nm/RIU, respectively (Fig. 7). Thus, the refractive index sensitivity of the sample can be greatly improved by increasing the stretching length to reduce the tapered diameter of the seven core fiber. Then, the s-4 was prepared under the same parameters as the sample with the highest sensitivity. And the GO film was coated on the surface of s-4 to prepare a humidity sensor. The experimental results show that the humidity sensitivity of sample s-4 is the highest at 1533 nm, and the maximum sensitivity is -0.0535 nm/(%RH) (Fig. 10). And the humidity sensitivity of the sensor with diameter of 14 μm (s-5) is -0.0173 nm/(%RH) (Fig. 11). Thus, the humidity sensitivity can be increased by reducing the sample diameter. In addition, we also evaluated the stability of s-4. When the relative humidity is 34.8%RH, 45.0%RH and 60.3%RH, the maximum error of the sensor wavelength is 0.03 nm, 0.04 nm and 0.04 nm, respectively, which indicates that the proposed sensor has good stability (Fig. 12).

Conclusions In summary, we propose and demonstrate a humidity sensor based on tapered seven core fiber, and the seven core fiber is fused and tapered by a hydrogen oxygen flame to form a tapered structure. The experimental results show that the refractive index sensitivity is up to 1123 nm/RIU for the sensor uncoated with taper waist diameter of 10 μm, which is consistent with the simulation results. Then the humidity sensor was fabricated by coating a layer of GO film on the surface of fiber. The maximum humidity sensitivity of the sensor was -0.0535 nm/(%RH), and the linearity was 98.5%. The sensor has the advantages of high sensitivity, simple preparation and good stability, which can be used in the field of humidity sensing.