In this work, a novel and simple optical fiber hot-wire anemometer based on single-walled carbon nanotubes (SWCNTs) coated tilted fiber Bragg grating (TFBG) is proposed and demonstrated. For the hot-wire wind speed sensor design, TFBG is an ideal in-fiber sensing structure due to its unique features. It is utilized as both light coupling and temperature sensing element without using any geometry-modified or uncommon fiber, which simplifies the sensor structure. To further enhance the thermal conversion capability, SWCNTs are coated on the surface of the TFBG instead of traditional metallic materials, which have excellent thermal characteristics. When a laser light is pumped into the sensor, the pump light propagating in the core will be easily coupled into cladding of the fiber via the TFBG and strongly absorbed by the SWCNTs thin film. This absorption acts like a hot-wire raising the local temperature of the fiber, which is accurately detected by the TFBG resonance shift. In the experiments, the sensor’s performances were investigated and controlled by adjusting the inherent angle of the TFBG, the thickness of SWCNTs film, and the input power of the pump laser. It was demonstrated that the developed anemometer exhibited significant light absorption efficiency up to 93%, and the maximum temperature of the local area on the fiber was heated up to 146.1°C under the relatively low pump power of 97.76 mW. The sensitivity of −0.3667 nm/(m/s) at wind speed of 1.0 m/s was measured with the selected 12° TFBG and 1.6 μm film.
© 2017 Optical Society of America
Measurement of the wind speed is of great practical importance in a variety of industries, such as coal mine, power transmission, and environmental monitoring. Traditional anemometers are mainly based on mechanics, thermology and acoustics, including differential pressure, ultrasonic and electromagnetic anemometers [1–4]. Nowadays, fiber optic sensors have proven to be a new kind of promising sensors due to their unique advantages, such as high accuracy, long term stability, immunity to electromagnetic interference (EMI), compact size and simple fabrication [5–8]. Wind speed sensors with different structures or mechanisms based on optical fibers have been reported. Among them, hot-wire fiber anemometer is an important type that has been widely investigated and exhibits great value. Hot wire anemometer (HWA) is a well-known technique for wind speed measurement which is based on the heat transfer from sensor to the surrounding environment [9,10]. In general, when the sensor is heated up, the speed of wind flow around the HWA determines the cooling rate of the sensor head which is usually made of the materials with good heat conversion coefficient. By measuring the cooling rate or the temperature variation, the wind flow measurement is achieved. For fiber optic hot-wire anemometer, fiber Bragg grating (FBG) is widely used as the temperature measuring element which is a mature and standard in-fiber sensor in the industry. When the wind takes away the heat on the sensor, the FBG’s wavelength will shift according to the temperature which can be accurately detected by the optical spectra. To effectively heat the sensing area near the FBG, the main methods reported are guiding the input pump laser to the cladding to interact with metallic thin film. Geometry-modified fiber structures like bending, tapper, making bubble in the fiber, mismatch fusing of multimode fiber with single mode fiber, or uncommon fiber like photonic crystal fiber (PCF) were applied to coupling the light to the film [11–13]. Alternatively, some researchers have used special high absorption fiber like Co2+-doped fiber to heat the FBG [14,15]. However, none of them is ideal. The physical-modifying methods will definitely weaken the structure strength of sensor and the methods using special fiber like PCF or rare earth doped fiber will complicate the sensor fabrication process. Furthermore, most fiber HWAs use traditional metallic materials like Au or Ag as an exothermic material converting optical power to heat on the sensor surface [13–17]. These metal films show good performance, but they cannot exceed the limitations of optical power absorption capability and thermal conductivity of the metals. Thus, simple and efficient all fiber optic HWAs are still needed.
Tilted fiber Bragg grating (TFBG) is one kind of FBGs, in which the index variation in the fiber core is at an angle to the fiber axis [18,19]. On the one hand, it possesses unique and inherent ability of stably coupling most of the light in the core to the cladding of the fiber without any physical structure changes to the fiber [20–23]. The light will then interact with the surrounding materials on the fiber surface. This specific feature has been widely used in TFBG based SPR sensors for promising bio-chemical measurements [24,25]. On the other hand, similar to FBG, TFBG has the same temperature sensing capability by detecting the wavelength shift of the spectrum . This particular combination of the two crucial functions of coupling and sensing makes the TFBG an ideal candidate for a simple and stable in-fiber HWA. Besides, instead of metal films, single-walled carbon nanotube (SWCNT) is one kind of innovative nanomaterials showing competitive thermal characters as thermal conversion medium. For HWA design, the most interesting thing is that the SWCNT is a strong infrared light absorber superior to the traditional metal materials and the thermal conductivity of SWCNTs is almost ten times higher than that of silver.
In this paper, taking the unique advantages of both TFBG and SWCNTs, we proposed and demonstrated a simple and efficient wind speed sensor based on SWCNTs coated TFBG. A 15 mm TFBG was inscribed using standard single mode fiber and then immobilized with SWCNTs on the fiber surface. In the experiments, the properties of proposed anemometer were fully investigated through the transmission spectrum. By adjusting the key parameters of the TFBG and SWCNTs, the response of the sensor could be easily controlled and optimized. It was demonstrated that the developed hot wire wind sensor exhibited significant light absorption efficiency up to 93%, and the maximum temperature of the local area on the heated fiber was up to 146.1°C with a relatively low pump power. The sensitivity at wind speed of 1.0 m/s was measured to be −0.3667 nm/(m/s). The developed anemometer based on SWCNTs and TFBG shows promising future with good efficiency and simple structure.
2. Experimental system and methods
Figure 1(a) shows the experimental setup of the proposed sensing system. A C + L band broadband source (BBS) and a pump laser working at 1550 nm were combined to illuminate the sensing head through a 3 dB optical coupler. The BBS was used to get the transmission spectrum of TFBG and the laser was used to heat the sensor. The output spectrum was monitored by an optical spectrum analyzer (OSA) with minimum wavelength resolution of 0.02 nm. Figure 1(b) shows the sensing principle of the anemometer. When the laser is pumped into the device, the pump light in the core will be coupled into the high order cladding mods and the radiation modes via the TFBG and interact with the SWCNTs deposited on the surface of the fiber. Then the pump light will be largely absorbed by the thin film of SWCNTs. This absorption acts like a hot wire raising the fiber temperature locally, which is detected by the TFBG resonance peak shift. In this experiment, TFBGs were inscribed in standard single mode fiber (SMF-28 Corning telecom fiber, with high pressure hydrogen loading for two weeks) using a 248 nm UV excimer laser and phase-mask method. During the fabrication, the laser pulse was set to 6 mJ/150Hz and 15 mm long TFBG was made by scanning technique. By adjusting the angle or the period of the TFBG, we made the wavelength of the deepest cladding mode near the input laser’s working wavelength around 1550 nm to obtain the optimal coupling efficiency.
Carbon nanotubes (CNTs) are a group of one-dimensional nanoscale materials composed of carbon atoms with fullerene structure, in which each carbon atom is sp2 hybrid and every carbon atom is covalently bonded to another three adjacent carbon atoms . They play a very important role in nanotechnology which greatly influence many different disciplines including biology, chemistry, physics, medicine, engineering, electronics and material science. To deposit SWCNTs on the surface of fiber with controllable process and low cost, chemical solution method was introduced. 10 mg of carboxylic SWCNTs was dispersed in DMF (N, N-dimethylformamide) solution via sonication. Then the TFBG was dipped in 0.1wt% aqueous APTES (3-Aminopropyltriethoxysilane) solution for 1 minute to produce amino-terminated (silanized positively charged surface). After that the functionalized TFBG was submerged in the DMF-CNT suspension for 30 seconds. The positively charged fiber surface will readily immobilize the negatively charged and functionalized SWCNTs. Finally, to increase the thickness of the nanotube layer, the dipping cycle were repeated. Figures 2(a) and 2(b) presents the output spectrum of the sensor and the simulation of the interactions between incident light and single layer carbon nanotubes using Comsol Multiphysics software. The SWCNT was set to 2 nm for inner diameter and 3 nm for outer diameter with 30nm length. It can be seen that the plane waves of 1550 nm propagate through the bottom of silica structure and strongly interacts with the SWCNT on the surface. Most of the lights interacts with the tube inside or near the two ends of the tube converting most of the power of incident light to the thermal energy. Furthermore, The SEM images were also obtained to verify the distribution and the uniformity of the SWCNTs film, as can be seen in Fig. 2(c). The film is uniform in general with small cluster effect on some local area, which might be due to the chemical solution method itself that we used compared to evaporation method.
To characterize the temperature response of the sensor, a calibration measurement was conducted. The temperature sensitivity of the SWCNTs film-coated TFBG is shown in Fig. 3(a). The thermal response of coated TFBG was measured by putting it into an electric furnace as the temperature cooled down from 140 to 30°C. The cladding mode near 1550 nm was monitored to measure the temperature. It can be seen that the temperature sensitivity of the SWCNTs coated TFBG is ~10.33 pm/°C which is almost the same as standard FBG’s temperature response (~10 pm/°C). By using the calibration value, the exact temperature variation of the heated TFBG could be obtained by detecting the wavelength shift. To evaluate the practicability of the anemometer, the wavelength shifts of the sensor with respect to different pump powers were detected. The output power of ~1550 nm laser was increased gradually, and the wavelength of the cladding mode showed obvious red shift. As shown in Fig. 3(b), the wavelength increases linearly as the pumping power grows with a slope of ~7 pm/mW. It is worth to note that the transmission spectrum was observed to be broadened under higher pumping power, which might be resulted from the temperature gradient induced chirp effect in the TFBG.
3. Results and discussion
To further understand the principle of the sensor, the theory of TFBG based hot-wire anemometry was deduced [9,10]. The relationship between the heat loss and the wind speed, ν, is showed as belowEq. (1) in Eq. (2), the cladding mode wavelength shift of TFBG can be expressed as a function of wind speed,
To experimentally verify the efficiency of proposed wind speed sensor, a 1.5 cm long TFBG with tilt angle of 12° was fabricated (the maximum angle that can be made in our lab) and coated with a 1.6 μm SWCNTs film at first. The wind speed in the small wind tunnel can be manually adjusted over the range of 0-2.1 m/s and recorded by an electrical anemometer (TESTO405V1). An on-line thermal imager (MAG30) was used to catch the real-time temperature variation and distribution image of the fiber during the heating process. Figure 4 presents the spectral response of the sensor and temperature distribution along the fiber via the on-line thermal imager. The maximum power of the laser was set to be 97.76 mW. The pump laser was launched into the TFBG and coupled into the cladding modes and the radiation modes. When the lights were strongly absorbed by the SWCNTs, the whole TFBG region was heated over 100°C as can be seen from the image of Fig. 4(b). It can also be observed that the maximum temperature was occurred at laser input side of the TFBG and was measured up to 146.1°C with the pump power of only 97.76 mW. This power consumption is much lower compared with the previously reported systems usually up to 200~500 mW [15,16]. When this “hot” sensor is put into wind field, it will be significantly cooled down resulting in wavelength shift of the TFBG. As shown in Fig. 4(a), the wavelength changed as much as 1.050 nm when the wind speed increased from 0 to 2.1 m/s. As to this 12° TFBG with 1.6 μm film, the transmission spectrum was observed to be broadened again, and temperature gradient along the fiber was clearly observed from the thermal imager. This temperature distribution will affect the wavelength demodulation to some extent and could be avoided by two side pumping method. Considering the complexity and the cost efficiency of the sensing system, we kept this system simple.
The input power of the pump laser and the inherent angle of the TFBG are the two key parameters that determine the performance of the anemometer. In the experiments, we accurately investigated and optimized the key parameters with the same coating thickness of 1.3μm. Besides, although the TFBG has polarization dependence to the input light, if we use polarized light to heat the sensor head, the insertion loss resulted from the polarization control device will largely reduce the light power (more than 3dB) injected into the sensor and obviously lower the light-heat conversion efficiency of the whole system. Thus to simplify the system structure and keep the efficiency, we did not control the polarization state of the input laser. As the power of the pump laser was increased from 42.37 mW to 97.76 mW, the anemometer of 12° TFBG with 1.3 μm film exhibited similar behavior but higher sensitivity as shown in Fig. 5. The wavelength showed a nonlinear response with the wind speed which is reasonable according to the Eq. (4). Furthermore, it is worthy to note that the monitored dip’s wavelengths under the same 2.1 m/s wind speed are different. This could be explained by the differences of the heat balance temperature of the sensor head under different pumping power. Generally speaking, a simple solution to improve the sensitivity of the anemometer at high wind-speed condition is to increase the pump power but there should be a limitation of the pump power if we consider the chirped effect of FBG/TFBG that might weaken the sensor response.
Besides, the inner angle of the TFBG will obviously affect the light coupling efficiency. In principle, the larger angle of TFBG applied, the more high order of cladding modes or radiation modes can be excited and more lights can be extracted to heat the sensor. The coupling effect could be partly observed from full spectrum of SWCNTs coated TFBG. In the experiments, several TFBGs with different angles were fabricated with the consistent thickness of 1.3 μm. Figure 6 shows that the compression of the transmission spectrum (measured within the BBS wavelength band of 1520-1590nm) of the sensor becomes stronger with the increasing angle of TFBG. This compression results from the strong coupling and absorbing of the light in a broadband wavelength range. The strongest compression occurred around 1550 nm was 6.758 dB which measured by the upper envelope of the cladding modes resonance when the angle is 12° . Four different TFBGs with different angles were selected to compare the wind speed response at the same pump power of 97.76 mW. It can be seen in Fig. 7 that the wavelength shifts are 0.214 nm, 0.406 nm, 0.482 nm and 0.620 nm respectively as the angle of the TFBGs are 6°, 8°, 10°, and 12°. Therefore, with gradually increasing angles, the interactions between lights and SWCNTs become more and more intense. Based on the results showed above, it can be predicted that larger angle of the TFBG would give a significant enhancement to the sensitivity of our sensors.
Another key parameter controlling the sensing ability is the thickness of the SWCNTs film on the fiber, which will directly affect the light absorption and heat conversion process. Thicker films should be preferred but considering the sensing element of TFBG, the thickness should be optimized to a certain point. On the one hand, when the thickness increases, the heat generated inside the SWCNTs will be harder to dissipate to the air resulting in lower wind speed sensitivity. On the other hand, the TFBG’s output spectrum will be compressed as the thickness goes up. This will broaden the cladding modes and affect the sensing resolution in some extent. In the experiments, for 12° TFBG, the light absorption capability of the TFBG with different coating thickness was first investigated. As showed in Fig. 8(a), with the increase of the thickness, the upper envelope of the resonance is compressed to be 5.312 dB, 6.758 dB, 7.372 dB and 7.930 dB, respectively. The thickness of the film is obtained by the SEM images and the thickness of the film shows good linear relationship with respect to the deposition cycle within the range of 1.0 to 1.6 μm as showed in Fig. 8(b). It is notable that when the thickness increases to 1.6 μm, the compression is very strong. If the thickness grows further, the spectrum might be hard to use for wavelength demodulation. Furthermore, the output power with and without the sensor in the system were also measured by a power meter (FPM-600). The absorption efficiency of SWCNTs coated TFBG is calculated up to ~93% (12.14dB) for 1.6 μm coating. It can be seen in Fig. 9(a) that the wavelength shifts are 0.544nm, 0.620 nm and 1.050 nm respectively as the thickness of the SWCNTs film are 1.2μm, 1.3μm and 1.6μm. And Fig. 9(b) shows the sensitivity of the sensor as function of the wind speed with different coating thickness. Taking the wind speed of 1m/s as an example, the sensitivities of the sensors with the SWCNTs’ thickness from 1.2 μm to 1.6 μm are −0.0713 nm/(m/s), −0.1081 nm/(m/s), and −0.3667 nm/(m/s) under the same pumping power, as shown in Fig. 9(b). If a good interrogation system with 1 pm wavelength resolution is given, the sensing resolution of proposed anemometer can be calculated up to 0.0027 m/s under the wind speed of 1 m/s.
In this paper, a novel fiber optic anemometer based on tilted fiber Bragg grating and single-walled carbon nanotube has been proposed. On behalf of the unique characteristics of TFBG, the pumping light propagating in the fiber core can be effectively coupled into the cladding and strongly interacted with immobilized SWCNTs on the fiber surface. Meanwhile, it also acts as a local temperature sensing element. SWCNT was chosen as a competitive heat-absorbing material. As the SWCNTs film is heated up, the speed of wind flow around the HWA determines the temperature change of the film which can be deduced from the TFBG spectrum. Aimed at the novel fiber optic anemometer, we fully investigated and optimized the key parameters of the sensor, including the inherent angle of the TFBG, the SWCNTs coating thickness, and the input power of the pump laser. It is demonstrated that the proposed anemometer exhibits good performance with relatively low input power (under 100 mW). Under 97.76 mW pumping power, with the selected 12° TFBG and 1.6 μm coating, the temperature of the local area on the fiber can be heated up to 141.6°C and the sensitivity at wind speed ν = 1.0 m/s has been measured of −0.3667 nm/(m/s) with the reachable resolution of 0.0027 m/s. During the experiments, there are still some valuable issues that have to be dealt with. For example, the cladding modes were broadened by the temperature gradient and wind direction measurement was not realized by current mechanism. Thus, our future works will focus on sensitivity/resolution enhancement, temperature/strain self-compensation and wind vector sensing. As to the miniaturization or simplification of the system, combination of the two light sources into one light as both sensing and heating source is a possible direction to go. Our research has provided a simple and low-cost platform for all-optical fiber anemometer device that has potential application value for remote long term flow monitoring and on-chip sensing in the future.
National Natural Science Foundation of China (NSFC) (61505019, 61520106013, 61605023); Natural Science Foundation of Liaoning (2015020666); Fundamental Research Funds for Dalian University of technology (DUT16RC (4)52); Fundamental Research Funds for Central Universities (DUT16TD17); PetroChina Innovation Foundation (2016D-5007-0603).
The thanks are given to the anonymous reviewers for their valuable suggestions and comments.
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