With the development of science and technology, humidity detection is required in more and more fields, such as semiconductors, medical treatment, biochemistry, agricultural production, and so on. In the field of environmental health, the analysis of maximum absolute humidity can be used to estimate the duration of COVID-19 epidemic transmission, which is helpful to provide a reference for intervention planning during future potential virus pandemics [1]. People have developed flexible intelligent wearable fabrics that can realize humidity detection for respiratory detection in mobile medical and personal medical use [2]. In the food industry, new intelligent equipment with various sensors is used to record the evolution of humidity to evaluate the quality and safety of food [3].

The traditional dry-wet bulb humidity sensor is not satisfactory for humidity detection now [4, 5]. Resistance and capacitance humidity sensors [6–10] have a long response time and low sensitivity. Optical fiber humidity sensors are attracting the attention of researchers more and more due to their soft texture, strong anti-interference ability, fast response, high sensitivity, and high temperature and pressure resistance [11–13].

Typical optical fiber humidity sensors include interferometric sensors such as the Mach-Zehnder sensor, Fabry-Perot sensor, Michelson sensor and Sagnack sensor [14]. These sensors usually have large volume or a relatively loose structure, and are not suitable for many environmental conditions. The Mach-Zehnder fiber optic humidity sensor has received wide attention due to its stable structure, small size and low manufacturing cost. Many researchers have investigated Mach-Zehnder fiber optic humidity sensors with different special structures. In 2018, Tong et al. [15] made an optical fiber humidity sensor with two convex cone single-mode fibers and photonic crystal fibers, using graphene quantum dots (GQDs)-polyvinyl alcohol (PVA) composite as humidity-sensitive material. Sensitivity of −0.0901 nm/% RH is achieved, and it varies with sensitive film thickness. In 2014, Shao et al. [16] proposed a fiber optic Mach Zehnder humidity sensor based on a lumbar amplification fiber structure with a single mode and multimode fibers. In the range of relative humidity 35–90% RH, the humidity response sensitivity reaches 0.119 dB/% RH with good linear response. In 2013, Zhang et al. [17] fused single-mode fiber and photonic crystal fiber, and cascaded the Mach-Zehnder interferometer and fiber Bragg grating, realizing the synchronous measurement of relative humidity and temperature.

The fabrication of optical fiber humidity sensors often uses humidity-sensitive materials. To select these materials, in addition to its own humidity-sensitive characteristics, it is also necessary to consider whether the humidity-sensitive materials can be easily obtained and combined with the optical fiber. Graphene is a kind of two-dimensional carbon nano-material that has attracted wide attention because of its unique optical and electrical properties [18–21]. Graphene and a variety of graphene derivatives are often used in the manufacture of humidity sensors because of their good humidity sensitivity. In 2021, Chaloeipote et al. [22] designed a high-performance resistance humidity sensor with a composite of graphene quantum dots and silver nano-materials as a humidity-sensitive material. The sensor has a good response to humidity in the humidity range of 25–95% RH. In 2021, Liu et al. [23] proposed a Mach-Zehnder humidity sensor based on fine-core optical fiber coated with carboxymethyl cellulose/graphene oxide composite film. In the range of 35–80% RH, its humidity sensitivity can reach −75.6 pm/% RH. In 2019, Liu et al. [24] proposed a Mach-Zehnder humidity sensor based on a spiral structure and coated with graphene oxide (GO), which has a high humidity response sensitivity of −0.885 dB/% RH in the range of 70–80% RH. In 2019, Liu et al. [25] investigated a high sensitivity evanescent field humidity sensor based on microcapillaries and GO. In 2016, Wang et al. [26] proposed an optical fiber humidity sensor based on a GO-inclined fiber Bragg grating structure. The humidity response sensitivity can reach 0.129 dB/% RH in the range of 10–80% RH.

Many reports showed that the optical fiber humidity sensor has a complicated fabrication process, high-cost materials, low sensitivity, and narrow humidity measurement range. Therefore, how to design and manufacture an optical fiber humidity sensor with a simpler structure, lower production cost, and high sensitivity is important. In addition, optical fiber humidity sensors often have cross-sensitivity of temperature and humidity, such as the optical fiber Fabry-Perot humidity sensor filled with Polyvinyl alcohol [27]. Temperature and humidity often change simultaneously, which leads to the limitation of the sensor in practical application. It is very important to study this cross response. One solution is that the large sensitivity difference between humidity and temperature response is helpful to realize the cross-response separation of humidity and temperature.

We successfully fabricated an optical fiber humidity sensor with a stable structure, simple fabrication, and high sensitivity by combining graphene quantum dots with an optical fiber dislocation fusion structure. Furthermore, it is easy for the sensor to achieve temperature and humidity cross-response separation because of its high humidity response sensitivity and low temperature response sensitivity.

2.1. Sensor Fabrication and Principle

The structure of the optical fiber sensor with dislocation fusion is shown in Fig. 1. The sensor is composed of three offset-fused single-mode optical fibers, which can form Mach-Zehnder interference. At the first dislocation fusion joint, there is an offset between SMF1 and SMF2. The magnitude of dislocation can be controlled manually by the fusion process. Similarly, SMF2 and SMF3 are offset-fused to form the second dislocation fusion joint. Light travels along the core of SMF1 to the first dislocation fusion joint. Some light is transmitted to the core of SMF2, while the rest is transmitted to the cladding of SMF2, which excites the cladding mode. When two beams reach the second dislocation fusion point, some of the light is coupled into the core of SMF3. Because of the refractive index difference between the core and the cladding, the optical path difference between the light transmitted in the core and the cladding is achieved. Then, interference occurs after the two beams meet. SMF2 fiber is coated with graphene quantum dots as humidity-sensitive materials. When the ambient humidity changes, the effective refractive index of the graphene quantum dots (GQDs) changes, and the optical signal leaked from the optical fiber cladding to the outside also varies, which affects the interference effect and causes the output light intensity change. By demodulating the output light intensity signal, we can obtain humidity change information, which is the basic working principle of the offset fusion optical fiber humidity sensor.

Figure 1. Dislocation fusion sensor structure.

The light at fiber 1 is transmitted into the core and cladding of SMF2 at the dislocation fusion point, and propagated as two beams at the fiber core and cladding, respectively, in SMF2. At the second fusion point, the cladding mode and core mode are simultaneously transmitted into the core of fiber 3, which causes optical interference. The intensity of interference light can be described as:

where I₁ and I₂ are the intensity of light propagating along the core and cladding, respectively. ∆φ is the phase difference of the two beams, and can be expressed as [28–30]:

where L is the length of SMF2, λ is the light wavelength, and represent the effective refractive index of the core and the m-order cladding mode, respectively, is the effective refractive index difference.

The corresponding transmitted light intensity is expressed as [31]:

where E is the amplitude of the electric field on the SMF1, and R is a function related to the refractive index of the external environment. The change of the refractive index of the humidity-sensitive material in this structure will directly affect the R value, and the transmission light intensity I is obviously affected by the change of the R value. Since graphene quantum dots coated on optical fibers are humidity-sensitive materials, their refractive index changes with humidity. Then the R value changes, which finally causes the change of spectral intensity.

The dislocation fusion structure is made of three single-mode optical fibers by fiber staggered fusion. In experiments, we used single-mode optical fiber (SMF-G657A1; Changfei fiber optic cable Co., Shanghai, China). A Fujikura 80S+ fusion splicer (Fujikura, Tokyo, Japan) is used for fiber splicing. We adjusted the relative position of the optical fiber with the manual splicing mode of the splicer to get enough fiber misalignment. Due to the special dislocation structure, the selection of welding parameters is particularly important when welding optical fiber. Repeated adjustments of fusion parameters and fusion experiments provide optimization of fiber misalignment fusion parameters. The sensor structure is successfully fabricated. GQDs are made by the citric acid thermal decomposition method.

2.2. Experimental System

The experimental system of the humidity response experiment is mainly composed of a Micron Optics SM125 optical fiber sensor demodulator (Micron Optics, GA, USA), MOI-ENLIGHT computer demodulation software (Micron Optics, GA, USA), dislocation fusion optical fiber sensor coated with graphene quantum dots and many humidity bottles with different relative humidity, as shown in Fig. 2.

Figure 2. Humidity response experiment system.

The SM125 can provide light in a wavelength range of 1,510–1,590 nm as the light source. At the same time, its demodulation module also enables the demodulator to have full spectrum scanning and data collection functions. The light emitted by the SM125 propagates along the optical fiber to the optical fiber sensor with the dislocation fusion structure. Due to the existence of the dislocation structure, the transmission light in the sensor produces Mach-Zehnder interference. When the external humidity changes, the refractive index of graphene quantum dots coated on the optical fiber also changes, which leads to changes in the leakage light at the interface between the cladding and graphene quantum dots, thereby affecting the interference spectrum. The interference spectrum is received by SM125, and MOI-ENLIGFT computer demodulation software showed spectral intensity changes with the external humidity.

3.1. Humidity Response Experiments

The humidity response characteristics of the sensor are experimentally investigated by a humidity response experimental system. A typical sensor with 3 cm length of SMF2, 5 μm fiber misalignment and 200 mg/mL GQDs material is investigated. The initial environmental temperature is 22.6 ℃ with 26.7% RH initial humidity. The humidity testing environment is provided by eight humidity bottles with 22, 33, 40, 51, 68, 75, 83, and 98% RH. Place the sensor in a humidity bottle and wait for its spectrum to stabilize. Use SM125 to record the spectral data in different humidity conditions, as shown in Fig. 3(a).

Figure 3. Sensor spectrum under different relative humidity, (a) spectrum of different relative humidity, (b) enlarged spectrum of valley 3.

As Fig. 3 shows, the spectrum significantly changed with different humidity, which indicated that the sensor has good humidity response characteristics. As humidity increased, the spectral intensity significantly increased. This is caused by graphene quantum dots, because their refractive index increased with increasing humidity. Correspondingly, the refractive index difference between the quantum dot film and the fiber cladding increased, which caused the decreased leakage of light at the interface between the cladding and the quantum dot film. Then the intensity of the interference spectrum ultimately changed. In order to analyze the humidity response characteristics more clearly, Fig. 3(b) shows an enlarged figure of typical wave trough 3. The humidity increased from 22% RH to 98% RH, and the spectral intensity of wave trough 3 increased significantly, from 41.96 dBm to 61.5 dBm. According to these spectral intensity response values under different humidity conditions, the fitting response curve of the spectral intensity value with relative humidity is achieved as shown in Fig. 4. It can be seen that there is a good linear relationship between the intensity of trough 3 and relative humidity, with 0.9825 linearity. The humidity response sensitivity of the sensor is 0.24 dB/% RH.

Figure 4. The humidity response experimental curve.

Table 1 compares the humidity sensing characteristics of this sensor with other recently reported optic fiber humidity sensors. It can be found that the sensor proposed in this paper has some advantages such as simple fabrication, low cost, firm structure, wide humidity detection range, and high response sensitivity. The humidity response sensitivity of references [32, 33] is close to that of this sensor, but the humidity response range is relatively small. Moreover, reference [32] requires special processes to get S-shaped structures, which means complex fabrication and a weak structure. Reference [33] uses high-cost special optical fibers. The response sensitivity of reference [34] is high, but the humidity response range is very small, only from 60% to 62.1% RH, which limits the practical application range. In addition, reference [34] also requires a complex side polishing technology and special twin-core optical fibers. The response sensitivity of references [35–37] is lower than that of the sensor in this paper. They also have high production costs and complex fabrication.

TABLE 1. Comparison of humidity sensing characteristics between our work and recently reported optical fiber humidity sensors.

Reference	Type	Humidity-sensitive Material	RH (%)	Sensitivity (dB/% RH)
[32]	S Fiber Taper	Graphene-oxide	84–95	0.36
[33]	Thin-core Fiber	Methylcellulose	55–85	0.224
[34]	Side-polished Symmetrical Twin-core Fiber	Graphene-oxide	60–62.1	3.76
[35]	Tilted Fiber Bragg Grating	Polyvinyl Alcohol	45–75	0.045
[36]	Light-sheet Skew-ray Probed Multimode Fiber	Diallyldimethylammonium/styrenesulfonate	10–94	0.14
[37]	Twin-hole Fiber	Black Phosphorus	20–80	0.028
Our work	Dislocation Fusion Fiber	GQDs	22–98	0.24

3.2. Temperature Response Experiments and Cross-response Discussion

An experimental temperature response system is similar to an experimental humidity system. The temperature-controlled environment is achieved by a THP-100 constant temperature and humidity test chamber. Its working temperature range is 5–150 ℃, with accuracy of ±0.1 ℃.

We placed the sensor in the test chamber with 26.7% RH initial humidity and 30 ℃ initial temperature. After reaching the set humidity and temperature, the humidity and temperature inside the test chamber are relatively stable, and the sensor spectrum is also stable. Spectral data can be gathered by MOI SM125 and computer. Then we change the temperature and repeat the above operation to record the spectral data at 40, 50, 60, and 70 ℃, respectively. The corresponding spectrum is shown in Fig. 5.

Figure 5. Sensor spectrum under different temperatures.

As Fig. 5 showes, with increased temperature, the spectral intensity also changed. According to the spectrum data of typical trough 3, we achieved the fitting experimental curve, which showed the corresponding relationship between spectral intensity and temperature, as shown in Fig. 6. The temperature sensitivity of the sensor is 0.02 dB/℃, with 0.9824 linearity.

Figure 6. Experimental data of humidity and temperature cross response.

Many have reported that optical fiber humidity sensors have a temperature and humidity cross-response effect. Polyelectrolyte humidity sensors are often greatly affected by temperature crosstalk [38]. Temperature has a great impact on the conductivity of polyelectrolyte. Reference [39] showed a resistance humidity sensor based on cross-linked polyelectrolyte prepared by ultraviolet irradiation. At high temperature, the ion mobility of polyelectrolyte increases, and the impedance of the sensitive membrane decreases with the increase in temperature. Therefore, the sensor requires additional temperature compensation processing with a complex manufacturing process. In addition, FBG optic fiber humidity sensors also exhibit a temperature and humidity cross response [40]. The fiber Bragg grating humidity sensor with thermoplastic polyimide in reference [41] is sensitive to humidity and temperature, and showed obvious cross interference between temperature and humidity.

To evaluate the cross-response of this sensor, the spectrum data of temperature and humidity response are compared in Fig. 6. The temperature sensitivity of the sensor is 0.02 dB/℃, and the humidity response sensitivity of the sensor is 0.24 dB/% RH. Obviously, the humidity response sensitivity is higher than the temperature response sensitivity with one order of magnitude difference. The impact of temperature change on spectral intensity is much smaller than that of humidity change.

Due to the significant difference in sensitivity, the cross-response of temperature and humidity is not obvious. So, the influence of temperature fluctuations in relative humidity measurements is very small. In practical applications, it is very conducive to the separation of temperature and humidity cross response.

An optical fiber dislocation fusion humidity sensor coated with graphene quantum dots is fabricated and experimentally investigated. The typical sensor is investigated with 3 cm single-mode fiber, 5 μm fiber misalignment and 200 mg/mL GQDs material. From 22% RH to 98% RH, the humidity response sensitivity of the sensor is 0.24 dB/% RH, with 0.9825 linearity of the fitting curve. The results showed that this sensor has high humidity response sensitivity with good linearity, easy fabrication and low cost. Furthermore, the temperature response was also investigated from 30 ℃ to 70 ℃, with 0.02 dB/℃ sensitivity and 0.9824 linearity. The linear response performance is also good. To investigate its cross-response characteristic between temperature and humidity, the response curves are compared with each other. The experimental results showed that the humidity response sensitivity is higher than the temperature response sensitivity. For this sensor, the humidity measurements are not obviously affected by temperature fluctuations. The big difference between humidity response and temperature response is very helpful in solving the cross-response between humidity and temperature. The research results provide a solution for the cross response.

The Key Deployment Project of the Ocean Science Research Center of the Chinese Academy of Sciences (COMS2020J11); National Training Program of Innovation and Entrepreneurship for Undergraduates (No. 202211033).

The authors declare no conflicts of interest.

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.