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Abstract
We propose and experimentally demonstrate an airflow velocity sensing method based on a 45° tilt fiber grating (TFG) that is combined with a single-walled carbon nanotube (SWCNT) coated fiber Bragg grating (FBG). The principle behind which is to produce a dynamic thermal equilibrium between the light heating and the airflow cooling. For the first time, to the best of our knowledge, a 45°-TFG is used as the heating element for the hot-wire anemometer. By diagnosing the Bragg wavelength of the SWCNT coated FBG, the temperature variations of the sensing fiber are measured with respect to the airflow velocities, which vary from 0 to 1 m/s. Moreover, under low light power consumption of 20 mW, the proposed sensor is shown to have good performance. Experimental results reveal that the sensitivity of the sensor increases with the heating pump. Due to the advantages of its simplicity and reliability, alongside its high photo-thermal conversion efficiency, this technique has excellent potential for future use in remote monitoring with airflow velocity sensing.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the past decade and beyond, all-optical fiber airflow sensors have become the subject of much attention. This is because they exhibit great potential in next-generation industrial applications, such as process control systems, environment monitoring, fossil fuel and nuclear electric power generation, and natural gas transportation. Typically, commercial sensors mainly involve differential pressure, Coriolis, cantilever, electromagnetic, thermal, and microelectromechanical system (MEMS) flowmeters [1]. Although they have previously been widely adopted, and prove to be reliable in practice, fiber-optic sensors can provide enormous advances compared with flowmeters of these types, which include high stability and sensitivity, remote sensing, and resilience in various harsh environments such as high-temperature, highly corrosive conditions and areas with significant electromagnetic interference. Recently, many researchers have devoted to the development of various all-optical fiber anemometers that involve different principles and structures [2–5]. As an alternative, the fiber-optic hot-wire anemometry (HWA) is currently one of the most extensively investigated methods, owing to its simplicity and reliability. HWA based sensors can determine the airflow velocity by monitoring the heat transfer from the sensors to the surrounding environment. In the fiber-optic heating unit, many of the sensors mainly adopt high attenuation fiber [6,7], Co2+ doped fiber [8], and heat-generating material coated sensing fiber [9–11], for enabling spectrum controllable fiber Bragg gratings (FBGs) or long-period gratings. In addition, thermo-optic based Fabry-Perot interferential HWA are also explored to improve the measurement sensitivity and range [12–14]. However, the fabrication and use of compact silicon films, pillars, and Sn-microspheres greatly increase the cost and complexity of the sensor.
According to the mode coupling mechanism, the tilted fiber grating (TFG) is simply divided into tilted fiber Bragg grating (TFBG), 45°-TFG and excessively TFG [15]. The TFBG can couple the core mode in the direction of the forward propagating into the backward propagating cladding modes, which results in a dense backward propagating cladding mode resonance in the transmission spectrum [16]. Combined with a single-walled carbon nanotubes (SWCNTs) coating, TFBG has been successfully used in fiber-optic HWA [17]. Although this scheme achieves a high-efficiency and compact all-optical fiber HWA, it lacks both robustness and stability. Noteworthy here is that, with a gradually increasing title angle and coat thickness of the TFBG, the photo-thermal heating efficiency is improved. However, this produces a lower signal-to-noise ratio in the resonance spectrum, which ultimately causes difficulties in the demodulation of the resonance spectrum. More recently, it has been shown that this HWA can also be achieved through a measure of surface plasma resonance (SPR) waves, which involves adding a gold layer between the TFBG and the SWCNTs [18]. But, an additional optical pump at a wavelength of 1311 nm and a special tilted angle of the TFBG is needed to excite the SPR. Sensing systems of this type are relatively complex and have a high-cost.
Alternatively, the 45°-TFG displays an extra-high radial coupling efficiency and a polarization extinction ratio, coupling the s-polarized light of the forward propagating core mode into the radiation modes. On this basis, the 45°-TFG is popularly regarded as an ideal in-fiber polarizer and diffraction grating [19–21], which has excellent potential for applications in polarimeter, filter, spectrometer, imaging, optical communication, and laser [22–27]. It is worthy of note that the coupling efficiency can reach as high as 97% when fully s-polarized light is injected [28]. As a light heating component, the latter is the configuration of choice that also meet the requirement of a low-power consumption HWA. Compared with a typical TFBG, the 45°-TFG contains no cladding resonance mode. Consequently, fewer reports appear in the literature on 45°-TFG based sensing technology, particularly, in connection with fiber-optic HWA.
In this paper, for the first time (to the best of our knowledge), the utilization of 45°-TFG in combination with a SWCNT coated FBG for airflow velocity measurement is proposed; this has the advantage of low-cost and easy fabrication. In the following, initially we present the principles of 45°-TFG and then discuss the sensing mechanism that is involved in the fabrication process. Next, we introduce the theoretical model for this scheme, and finally, the proof-of-principles sensor is verified by experiment.
2. Sensing mechanism and theoretical model
The 45°-TFG is UV-inscribed in SMF-28e by using a method of a rotating phase mask, which requires the same refractive index modulation as general FBGs. However, its refractive index fringes are tilted (i.e., by 45°) with respect to the fiber axis instead of perpendicular to it [29]. Figure 1 depicts the structure and the polarization-dependent transmission characteristics of the 45°-TFG. According to Brewster’s theorem [30], a unique feature of the 45°-TFG is that it can significantly radiate the s-polarization component of the incident unpolarized light out of the cladding, in the vertical direction. This leaves the p-polarization component to travel along the optical fiber core with near zero transmission losses. This fascinating peculiarity enables the development of such a process for high optical heating elements in fiber-optic HWA.
Fig. 1. Schematic diagram showing the polarization-dependent radiation and the transmission characteristics of the 45°-TFG
The layout of the heating and sensing fiber are exhibited in Fig. 2(a). Here, a 45°-TFG with a grating length of 4 cm was, at multiple points, tightly glued alongside the sensing optical fiber. This obtained uniform physical contact between the two components that, when exposed to a moving airflow through the pipeline, can perform airflow velocity sensing. In this configuration, the sensing fiber is a SWCNT coated FBG. When the heating pump light is injected, the s-polarized light is significantly coupled into the radiation mode and then passes through the cladding, radiating into free-space in a direction perpendicular to the fiber axis. The SWCNT is an important component because, as a single-wall cylindrical nanostructured material, it preserves excellent light absorption over a wide range of infrared wavelengths and has high-thermal conductivity [31]. Hence, the effective heating pump light, that is, the radiation mode exposed to the surface of the sensing fiber, can be significantly absorbed by the SWCNTs film, leading eventually to a raised the local temperature. This process can be regarded as a hot-wire heating. Although the radiation mode decays exponentially along the fiber axis direction [32], the temperature of the sensing fiber can still be increase rapidly and uniformly due to the natural peculiarity of the high-thermal conductivity of the SWCNTs film. When no airflow is applied across the senor, the sensing fiber maintains a constant highest temperature. However, in the presence of airflow, the heat is dissipated by the airflow, which results in a drop in temperature. In this scenario, the heat transfer induced by the airflow can be measured by an interrogation of the Bragg wavelength of the sensing fiber. Figure 2(b) shows the assembled sensor and its geometric dimensions. The anemometer integrates the airflow temperature monitoring function by the utilize of a FBGT, which is used for real-time temperature calibration. The gap between the two sensing units enables a reduction in the temperature crosstalk. These sensing fibers were fixed, with pre-stressed, by a 1064 nm laser welding platform. Then, the grating parts are completely exposed outside the housing.
Polydimethylsiloxane (PDMS) is a type of hydrophobic material that has a versatility in its characteristics, such as high degree of light transmittance, high viscosity, resistance to various chemicals, large elasticity, and low density; it is also non-toxic and inexpensive. [33]. Utilizing these features, the sensing fiber adopts a PDMS-assisted curing to produce the SWCNT coating. The diameter of the SWCNTs varies from 1 to 2 nm, and they have a length of ∼20 nm. The fabrication process of SWCNTs film, in brief, is as follows. Firstly, we prepared PDMS with a 10:1 mixing ratio of the precursor of the elastic material (Sylgard 184-A) to the hardener (Sylgard 184-B), and then placed the PDMS mixture onto a stirring machine for 40 minutes. This ensured that any bubbles would disappear completely. After that 10 mg of SWCNTs powder was added to the PDMS mixture and dispersed by stirring for 60 minutes. The FBG was encapsulated in a Teflon capillary tube (inside diameter of 0.8 mm and length of 5 cm), and the SWCNT/PDMS coating mixture was filled into the tube by using a vacuum pump [34,35]. The position of the FBG was carefully aligned to the center of the SWCNT/PDMS coating mixture. The FBGSWCNT was then cured by heating up the sample to 120 °C for 20 minutes. Finally, the SWCNT coated sensing fiber was obtained by removal of the tube and the excessively cured material. The thickness of the SWCNTs film used in our experiment was ∼ 300 μm. In this scenario, the 45°-TFG is only used as a highly efficient heating element. The temperature variation was obtained by diagnosing Bragg wavelength of the SWCNT coated FBG rather than via the film guided modes [16], cladding resonance modes [17] or SPR [18] of the TFBG. Further research is required to study the dependence of the thermal response time and the measurement sensitivity on the coating thickness.
Fig. 2. Diagram depicting the sensor design. (a) Principles of the proposed HWA. (b) Assembled sensor.
We now examine the theoretical description of the sensing mechanism. Based on King’s Law, for a HWA, the dependence on temperature change in the heating element, due to an airflow on the airflow velocity v through the system, can be expressed as [7]
where Hloss represents the heat loss, Ta(v) and T0 are the temperature of the heated anemometer and the airflow, respectively. Furthermore, A and B are the empirical calibration constants. Due to the principle of energy conservation, the total heat loss is equal to the power consumption of the FBGSWCNT, which can be described in the following form where Pinput is the total pump power, η is the coupling coefficient between the core mode and the radiation mode, β is the effective illumination coefficient between the lateral emitted light beam of the 45°-TFG and the FBGSWCNT, and γ is the thermo-optical coefficient that relates to the inherent characteristics of the SWCNTs. Moreover, the Bragg wavelength λ is sensitive to ambient temperature. The relative Bragg wavelength shift of the FBGSWCNT (i.e., Δλ(v)), which is determined from the temperature change ΔT(v) = Ta(v) – T0, is given by where α and ξ are the thermal-expansion coefficient and thermos-optic coefficient of optical fiber, respectively.From above equations, under the condition that the power is constant, we can obtain the dependence of the wavelength of FBGSWCNT on the airflow velocity, λ(v) = Δλ(v) + λ, which is also expressed as
For simplicity, and without a loss of generality, in the theoretical model we employ only considers airflows that have the same temperature. It is important to note that the proposed anemometer can also achieve an accurate measure of airflow velocity under different airflow temperatures, provided that a necessary calibration measurement is applied [6].
3. Experimental results and discussion
Figure 3(a) shows the experiment setup that characterizes the airflow sensing performance for the proof-of-principle demonstration. A low-cost distributed feedback (DFB) laser operating at 1550 nm with a linewidth of less than 2 MHz, and an output power of ∼22 mW, acts as a light heating source. It passes through a 3-paddle polarization controller (PC) that is connected to a MEMS variable optical attenuator (VOA). The output light is divided into two branches via the optical coupler (OC), in which 99% is directly injected into the 45°-TFG, and the remaining 1% is used as the servo-optical signal. The MEMS VOA acts a kind of fundamental electrically controlled tunable component with a capacity to control the propagation level of the light power [36]. In this setup, we also utilize a reflection-type MEMS VOA, which possesses a precise and a rapid attenuation response from 0 to 25 dB on application of a modulation voltage between 0 and 5V. The 1% branch of the OC is collected at a photodiode (PD) for the self-feedback control via a MEMS VOA. We synchronously control the photoelectric data acquisition and the output modulation voltage based on a STM32 chip. Thus, the self-feedback loop control unit enables a stable optical power output; and in addition, achieves the desired steady-state optical heating of the FBGSWCNT, with different heating powers. Furthermore, a high radial luminous efficiency is guaranteed by the utilization of PC to control the polarization of the input core mode of the 45°- TFG. For airflow velocity measurement, the proposed fiber-optic HWA is fixed into the middle of a metal pipeline. Airflow velocities are provided by a variable frequency fan with velocities ranging from 0 to 1 m/s, and their values are monitored with a flow meter. The temperature and airflow velocities in the pipeline are determined by the wavelength response of the FBGT and the FBGSWCNT, respectively, using a commercial interrogator (HD808N) characterized over the spectrum range 1510 nm to 1590 nm; for which there is a wavelength stability and accuracy of 1 pm, and a scanning frequency of 1 Hz [37]. Finally, the experimental data are collected and processed by a computer. Figure 3(b) is a representation of the home-made printed circuit board that involves the STM32 chip.
Fig. 3. Diagram illustrating the experimental procedure. (a) Experimental setup. (b) Physical drawing of the home-made PCB. DFB Laser, distributed feedback laser; PC, polarization controller; VOA, variable optical attenuator; OC, optical circulator; PD, photodiode; PCB, printed circuit board; VF, variable frequency.
First, the temperature sensitivities of the FBGT and FBGSWCNT are initially calibrated, in which the proposed sensor was placed in an oven, with a temperature changing from 20 to 120 °C. The dependence of the wavelength on temperature of the FBGT and the FBGSWCNT are plotted in Figs. 4(a) and 4(b), respectively. It is determined that a linear fitting relationship arises for both. The results indicate that a linearly fitted temperature coefficient for the FBGSWCNT of ∼ 10.26 pm/°C, which is almost consistent with a general FBGT (i. e., temperature sensitivity of ∼10.02 pm/°C), is achieved. On combining the calibration results, the real-time airflow temperature surrounding the heating element can be obtained by analysing the wavelength shifts of the FBGT. More importantly, the temperature variation for the ambient background of the proposed HWA is induced by an isothermal flow field, which can be determined by detecting the wavelength response of the FBGSWCNT.
Next, to evaluate the optical heating efficiency of the anemometer, we quantify the wavelength shifts of the sensor by the injection of different optical pumping powers. The stable output power was increased gradually from 0 to 20 mW, under the precise control of a MEMS VOA. Figure 5 illustrates the measured wavelength response of the FBGSWCNT. It shows that the wavelength increases as the heating power (i. e., the radiation mode of the 45°-TFG) increases. From which a good linear relationship arises between the heating power and the wavelength shift with a slope of 9.26 pm/mW is obtained.
Finally, the performance of the HWA between the airflow velocity and the wavelength shift under different heating powers was studied by experiments, as shown in Fig. 6. The wavelength response, as a nonlinear function with respect to the airflow velocity, is reasonably consistent with the theoretical analysis from Eq. (4). It is found that the sensing sensitivity increases with the heating power, which indicates that heating power is a key factor in influencing the sensing performance. Besides, it can be seen that the wavelength of the temperature monitoring unit FBGT at the front of the anemometer remains almost unchanged, which is consistent with the steady indoor temperature. From an application point of view, the proposed sensing device can produce synchronous real-time sensing of the airflow temperature and the velocity, following the calibration measurements.
Fig. 6. Wavelength shifts of FBGT and FBGSWCNT versus the airflow velocity, including the condition with various pumping powers of P = 20 mW, 15 mW and 10 mW, respectively.
4. Conclusion
In conclusion, we propose a simple and low power consumption all fiber-optic airflow velocity sensing method, which is based on a 45°-TFG and a SWCNTs coated FBG; it is experimentally demonstrated, for the first time. The HWA is heated by a thermos-optic conversion medium (i. e., SWCNTs) via the output radiation mode of the 45°-TFG. The proof-of-principle sensor also incorporates a temperature measurement function. Through a further optimization of the power of the heating light and an adaption of the coating thickness of the sensing fiber, a controllable measurement range (with moderate sensitivity) can be expected, for a variety of industrial application scenarios.
Funding
Department of Education, Heilongjiang Province (RCCXYJ201901); Heilongjiang Provincial Science and Technology Department (GZ11A306).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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