1. Introduction

Humidity control is of great importance in a wide range of fields including environment monitoring, food processing and storage, medicine, paper and semiconductor industries. For example, in semiconductor or medical manufacturing, precise control and fast detection of humidity are necessary for reliable manufactured products, and there is a substantial necessary to develop humidity sensors with high sensitivity and fast response [1].

The development of optical fiber for humidity sensing offers several advantages: possibility of remote sensing and continuous monitoring in confined or hazardous environments, immunity to electromagnetic interference and safe operation in explosive or combustive atmosphere, as well as small size and the feasibility of multiplexing the information from different sensors in one optical fiber [2]. Most of the optical fiber humidity sensors are based on the change of optical properties of a certain kind of sensitive layer coated on the fiber. For example, polymer layers immobilized with cobalt chloride salt are used due to the RH-dependent absorption [3–4], and hydrophilic gel layers are used due to their RH-dependent refractive indices [5–7]. Other types of the optical fiber humidity sensors are also well studied. For example, the humidity sensors based on the changes in fluorescence intensity and/or lifetime of indicator dyes have also been reported [8, 9].

All the above-mentioned RH fiber sensors are based on fibers or fiber tapers with diameters much larger than the wavelength of the probing light. Recently demonstrated low-loss optical wave guiding in a subwavelength-diameter fiber brings new opportunities for fiber optic sensing [10, 11]. Due to the small dimension of the fiber and high fraction of the evanescent fields, this kind of fiber is promising for high-sensitivity sensing with small footprint and fast response [12–22]. Here we show that, using a subwavelength-diameter fiber coated with a thin layer of gelatin as the sensing element, it is possible to detect humidity with a response time as fast as 70 ms. Also, the sensor shows high sensitivity and good reversibility within a wide RH range.

2. Sensor configurations

In this work, the subwavelength-diameter fiber was drawn from a standard single-mode fiber (SMF-28, Corning) using a flame-heated taper-drawing technique. The as-drawn fiber taper, with a uniform waist of 680 nm diameter and 8 mm length, connected to standard fibers at both ends through tapering regions of about 20-mm length, as schematically illustrated in Fig. 1. Powdered gelatin (5% by weight) was dissolved in distilled water and was heated to 65 ° for 10 minutes to form an aqueous solution. The tapered region was then dipped into a small drop of the solution and left out carefully by micromanipulation under an optical microscope. After drying on its own in air at room temperature, a 80-nm thickness gelatin layer was formed on the surface of the taper. The gelatin coated taper shows smooth surface and uniform diameter (see inset of Fig. 2).

 

Fig. 1. Schematic diagram of a single-mode tapered fiber for RH sensing.

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Calculated Ponyting vector of a 80-nm-thickness gelatin (refractive index n is assumed to be 1.50) coated 680-nm-diameter silica fiber (n=1.44) operating at 1550-nm wavelength, by solving Maxwell’s equations of a 3-layer-structured cylindrical waveguide numerically [23, 24], is plotted in Fig. 2. It shows that, a considerable amount of optical power (about 10%) is guided inside the gelatin layer. When the RH increases, the diffusion of water molecules into the gelatin layer causes reduction in refractive index of the gelatin layer [5], which changes the mode field of the guided mode of the coated fiber, and converts a portion of power from guided mode to radiation mode, resulting in RH-dependent loss measured at the output end.

 

Fig. 2. Calculated Ponyting vector of a 80-nm-thickness gelatin (n=1.50) coated 680-nm-diameter silica fiber (n=1.44) operating at 1550-nm wavelength. The calculation is performed in a cylindrical coordinate, in which abscissa r represents the radial direction on the cross section of the fiber. Inset: a SEM image of the gelatin coated tapered fiber.

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3. Experimental results and discussion

The humidity sensing experiments are carried out by placing the sensing element in a sealed glass chamber with a gas flowing system. A hygrometer was used for monitoring the temperature and humidity. The analyte humid gas flows through the chamber at a flow rate of about 200 mL/min. The mass flow rate and concentration of the analyte humid gas were controlled by mass flow controllers. A 1550-nm-wavelength light from a laser diode was launched into the fiber from the input side (see Fig. 1), and the output from the other side was sent to an optical power meter. All experiments were carried out at room temperature and atmospheric pressure.

 

Fig. 3. The transmitted light intensity of the sensor at 1550 nm wavelength in the range of 9–94 % RH.

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Figure 3 gives the RH-dependent optical transmittance of the sensing element with RH ranging from 9 to 94%. It shows that the transmittance of the gelatin coated fiber taper decreases monotonously with the increasing RH. The sensor exhibits high sensitivity as a function of RH for approximately 10 dBm change of the optical transmittance in the range from 9 to 94% RH. In addition, it can be seen that at high RH level (e.g., RH>50%) the transmittance decreases in a steeper way, which can be explained as aggregation of water molecules and formation of clusters on the gelatin layer.

To estimate the response time of the sensor, the coated fiber was exposed to an environment with rapid changes of the RH. First the RH in the chamber was maintained at 75%, and then the cover of the chamber was opened suddenly to expose the sensor to the environmental RH of 88%. Typical time-dependent response of the sensor is shown in Fig. 4(a). The estimated response time (baseline to 90% signal saturation) of the sensor is about 70 ms when RH jumps from 75 to 88% which are one or two orders of magnitude faster than other types of RH sensors relying on conventional optical fibers or films (usually on an order of 1 s) [3–9]. We believe that the ultra fast response of the sensor can be attributed to the small diameter of the fiber taper and the thin gelatin layer, which allows rapid diffusion (or evaporation) of water molecules. In addition, the reversible response of the sensor was tested by alternately cycling 75%- and 88%-RH air inside the chamber. Typical response is given in Fig. 4(b), showing excellent reversibility of this kind of RH sensor.

 

Fig. 4. (a). Typical time-dependent transmittance of the sensor reveals the response time of about 70 ms when RH jumps from 75 to 88%. (b) Reversible response of the sensor tested by alternately cycling 75%- and 88%- RH airs.

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4. Conclusion

In conclusion, a subwavelength-diameter tapered optical fiber coated with polymer (gelatin) layer for humidity sensing was demonstrated. The sensor is operated within a wide humidity range (9–94% RH) with high sensitivity and good reversibility. Measured response time is as fast as 70 ms. Since polymers are good hosts for a wide range of dopants and are easily functionalized by a number of techniques such as ionized gas treatments and UV irradiation [25], polymer coated subwavelength-diameter fiber tapers for sensing a variety of specimens could be realized with fast response, high sensitivity and small footprint.