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Abstract
An intensity-interrogated optical fiber hot-wire anemometer based on the chirp effect of fiber Bragg grating (FBG) is presented. The FBG is coated with a silver film and heated optically by a 1480 nm laser beam, which is coupled into the fiber cladding by a long-period grating (LPG) and absorbed by the silver film to convert to thermal heat. Due to the gradual decrease of laser power along the length of the FBG, a temperature gradient is formed that induces a chirp effect to the FBG. Bandwidth of the FBG’s reflection spectrum is therefore broadened that increases its reflected light power. The chirp rate of the FBG reduces with airflow velocity since the temperature gradient is weakened under the cooling effect of the airflow, resulting in a certain relationship between the reflected power of the FBG and airflow velocity. In the experiment, by detecting the reflected power of the FBG, airflow velocity measurement is achieved successfully with a high sensitivity up to −28.60 µW/(m·s−1) at airflow velocity of 0.1 m/s and a dynamic response time of under one second. The measurement range is up to 0 to 11 m/s. The intensity interrogation scheme of the FBG hot-wire anemometer reduces its cost greatly and makes it a promising solution for airflow velocity measurement in practical applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical fiber thermal anemometers (OFTAs) have become a competitive candidate for airflow velocity measurement in the past decade due to their miniature size, high sensitivity, anti-electromagnetic interference, and long-distance measurement capability. OFTAs have been proposed based on various optical fiber devices, such as fiber gratings including fiber Bragg gratings (FBGs) [1–3], long-period gratings (LPGs) [4], and tilted fiber gratings (TFBGs) [5–9]; and optical fiber interferometers including Fabry-Pérot Interferometers (FPI) [10–13] and Michelson interferometers (MI) [14]. However, most of the reported OFTAs are demodulated by the optical wavelength interrogation method, which requires a bulky and high-cost optical spectrometer or wavelength interrogator to read the output signal. It greatly limits the practical application of OFTAs. Optical intensity interrogation method has been proposed to solve this problem in recent years [7,8,15]. For example, R. Gao et al. reported an intensity-interrogated solution based on laser-heated few-layer graphene in aligned graded-index fibers (GIFs). Airflow velocity measurement was realized since the transmitted signal light power is modulated through the temperature-dependent transmission loss of the heated graphene films. Thanks to the excellent heat dissipation of the graphene films [16] and its open structure, a fast response time of 64 ms was achieved. However, the closely attached V-grooved glass substrate for collimation inevitably disturbs the flow field around the sensor and affects the measurement accuracy [13]. Intensity-interrogated OFTAs by using TFBGs coated with carbon nanotubes (CNTs) were also obtained by measuring the transmitted intensity of the resonant peak of the cladding mode and the SPR mode, respectively. The high absorption rate of CNTs enabled the anemometers to achieve efficient heating even at relatively low pump powers. However, the measurement range was limited due to the intrinsic narrow bandwidth of their resonance peaks. Besides, the transmitted light measuring schemes led to inconvenient operation and more signal disturbs [17].
In this paper, we propose an intensity-interrogated OFTA based on the chirp effect of a silver film-coated FBG, which is optically heated with a 1480 nm pump laser via a long-period grating. The FBG is chirped by the non-uniform laser heating with its chirp rate and, consequently the reflected light power, being modulated by airflow velocity. Airflow velocity measurement with high sensitivity of −28.60 µW/(m·s−1) at airflow velocity of 0.1 m/s is achieved. The measurement range is 0 to 11 m/s and the dynamic response time is less than one second. The low-cost intensity interrogation method together with other merits of the proposed OFTA makes it a promising solution for airflow velocity measurement in practical applications.
2. Sensor structure and principle
The proposed optical fiber anemometer probe is schematically shown in Fig. 1(a). It is formed by a LPG cascading a silver film-coated FBG. The former serves as a coupling element to the pump light so the pump light can be coupled into cladding of the FBG from the core, while the latter is the sensing element. The FBG, 15-mm long with high reflectivity of 99.9% at the wavelength of 1550 nm, was photo-inscribed in a segment of single-mode fiber by using an excimer laser through a phase mask. The LPG, with a 12-dB resonant dip (corresponding to a coupling ratio of ∼93.7%) at 1480 nm, was fabricated using a CO2 laser with grating pitch of 510 µm. The two fiber gratings were fusion spliced with a separation of 3 mm. The FBG’s fiber pigtail was angel-cleaved to minimize the Fresnel reflection. Then a ∼260 nm thick silver film layer was deposited uniformly on the surface of the FBG through vacuum magnetron sputtering. A layer of aluminum-doped zinc oxide (AZO) with thickness of ∼80 nm was finally deposited outside the silver film to prevent it from being oxidized in the air [18]. Figure 1(b) shows the microscopic images of the FBG before and after coating. The magnified scanning electron microscope (SEM) image in Fig. 1(c) shows that the total thickness of the coating films is ∼340 nm.
Fig. 1. (a) Schematic diagram of the FBG hot-wire anemometer probe. (b) Microscopic images of the bare FBG and coated FBG. (c) SEM image of the fiber cross-section with the measured thickness of the coating films.
When the pump laser beam is coupled by the LPG into the cladding of the FBG, it will be absorbed gradually by the silver film with its power reducing exponentially along the distance [19]. It can be expressed as
where Pinput is the input pump laser power, φ is the coupling coefficient of LPG, α is the absorption coefficient of the silver film to the pump laser, and z is defined as the distance starting from the enter side of the silver film.The silver film absorbs the pump light and converts to heat, which raises the temperature of the FBG to become an optical fiber hot wire. When the airflow blows through the anemometer probe, the heat loss speeds up that reduces temperature of the FBG until new thermal equilibrium is reached. Assuming that the temperature of a certain point of the FBG under airflow velocity v is $T_{(v)}^{(z)}$, the following relationship can be achieved [20]
where Ta is the airflow temperature, β is the converting coefficient of the absorbed optical power to heat by the silver film, A, B, and n are empirical coefficients.A temperature gradient along the grating axis is therefore formed with the maximum temperature difference described as
Given that the reflectivity of the FBG, R, remains stable while the spectrum is broadening, the increase of reflected optical power can be given as [22,23]
where ρ is the power spectral density of the broadband optical source (BBS).Finally, the reflected power of the chirped FBG can be expressed as a function of the airflow velocity
where $P_{r}^{0}$ is the reflected power of the FBG without heating, X = ρ·R·k·Pinput·φ·α·β·(1−e−α·l)/B, Y = A/B. According to Eq. (6), the airflow velocity can be obtained by measuring the reflected power of the signal light modulated through the chirped FBG.3. Experimental results and discussion
The experimental setup is shown in Fig. 2. The wind tunnel with tunable airflow velocity ranging from 0 to 11 m/s was calibrated by using a commercial anemometer (Testo 425). The pump laser at 1480 nm with a maximum output power of 530mW was launched into the anemometer through a 1550/1480 nm wavelength division multiplexer (WDM). The broadband light source (BBS) provided a flattened optical spectrum with the maximum intensity variation less than 0.4 dB in the wavelength range of 1545-1555 nm. Reflected light from the anemometer was measured by using a handheld optical power meter (OPM) via the optical circulator. The anemometer probe were placed vertically to the wind direction and fixed tightly at the both ends by using two fiber clamps to prevent undesirable fiber bending.
The heating effect of the pump laser on the anemometer was first investigated under different power levels when there was no airflow. We gradually increased the power of the pump laser from 0 to 530 mW with a step of 50 mW and recorded the spectra using an optical spectral analyzer (OSA, Anritsu MS9740A). The results, as shown in Fig. 3(a), show that bandwidth of the reflection spectrum was broadened gradually with pump power, accompanied by a redshift of the center wavelength. When the pump laser power changed from 0 to 530 mW, the 3 dB bandwidth of the anemometer increased by over one time from 576 to 1195 pm.
Fig. 3. (a) Reflection spectra of the anemometer at different pump laser powers. (b) 3 dB bandwidth and reflected power as functions of the pump laser power.
Figure 3(b) shows that the broadening rate of 3 dB bandwidth is 1.087 pm/mW with a linearity of 0.9961. Evidently, the non-uniform heating of the pump laser can effectively enhance the temperature gradient of the FBG, resulting in a broadening of the reflection spectrum. To check the optical power variation against pump laser power, we measured the reflected optical power of the anemometer by using a hand-held OPM. The measured results, as presented in Fig. 3(b), show that the reflected light power increased from 36.59 to 59.40 µW with a growth rate of 0.042 µW/mW when the pump laser power was increased from 0 to 530 mW. Obviously, a higher pump laser power leads to a stronger chirp effect of the FBG, which will give a more sensitive response to the sensor. However, an overly high power would decrease the reflectivity of the FBG when it was seriously chirped and affects the growth of the reflective power to a certain extent [22]. Thus, the maximum pump laser power was set to 530 mW in this experiment.
The airflow velocity measurement was conducted with the airflow temperature kept at 24°C. The response of the reflected spectra of the anemometer to airflow velocity was recorded first by the OSA. The measured results are shown in Fig. 4(a). When the airflow velocity was increased, the whole reflection spectrum of the anemometer shifted towards the shorter wavelengths, together with an obvious reduction in bandwidth. This agrees well with the theoretical predictions. The blueshift of the spectrum was caused by the decreased temperature of the FBG due to the airflow-induced faster heat loss, and the reduction in bandwidth was because the temperature gradient was reduced with airflow velocity too. When the airflow velocity reached 11 m/s from 0 m/s, the center wavelength and the 3 dB bandwidth were changed by −0.773 and −0.586 nm, respectively.
Fig. 4. (a) Reflection spectra of the anemometer at different airflow velocities. (b) Reflected power of the anemometer as a function of the airflow velocity.
The reflected optical power of the anemometer was measured at different airflow velocities between 0 to 11 m/s. The results presented in Fig. 4(b) show a nonlinear relationship with the airflow velocity. The maximum variation in reflected power was 19.77 µW, achieved at the airflow velocity of 11 m/s. By data fitting using Eq. (6), we achieved a close-fitting curve with an R-square value of 0.9894. Equation (6) is then rewritten as
Based on this equation, airflow velocity measurement can be realized by detecting the reflected optical power of the anemometer.By differentiating Eq. (7) with respect to airflow velocity, sensitivity of the anemometer can be given asThe dynamic response of the anemometer was tested by alternately switching the airflow velocity between 0.3 m/s and 7 m/s. In the experiment, we recorded the reflected power of the anemometer over a period of 50 s using a benchtop OPM (Thorlabs PM320E). The results, as shown in Fig. 5, indicate the average response times of the anemometer were ∼0.3 s when the airflow velocity was switched from 0.3 to 7 m/s and ∼0.9 s for the reverse switching. It took less time for the anemometer to drop from high temperature than to rise back to high temperature, indicating that the heat generation efficiency is lower than the cooling efficiency of the airflow [24]. Of course, the actual response time of the anemometer should be shorter than the measured ones due to the inherent switching time between different airflow velocities. Besides, the reflected power can retrace to the initial values after each cycle, indicating good repeatability of the proposed anemometer.
Table 1 summarizes the key parameters of some reported OFTAs. It can be seen that our OFTA offers a competitive performance compared with the wavelength-interrogated OFTAs while using a more convenient interrogation method. Compared with the previously reported intensity-interrogated OFTAs, the change rate of the signal light power from 0 to 0.5 m/s of the proposed anemometer is calculated as 19.3%, which is 3.52 times that reported in [15], indicating a more sensitive response to the airflow velocity. In addition, the measurement range is much larger than that of the others, making it more capable for different tasks. It should be mentioned that since the proposed anemometer’s response is based on the temperature gradient of the FBG rather than its absolute temperature, the cross-sensitivity issue to the ambient temperature can be prevented to a great extent [25].
Table 1. Performance comparison of some reported OFTAsa
4. Conclusions
In this study, an intensity-interrogated OFTA based on the chirp effect of FBG has been proposed and demonstrated experimentally. The 15 mm silver film-coated FBG was non-uniformly heated by a 1480 nm pump laser such that a large temperature gradient was formed along the grating, inducing an obvious chirp effect. The chirp rate of the FBG reduces with airflow velocity since the temperature gradient is weakened under the cooling effect of the airflow, resulting in a certain relationship between the reflected power of the FBG and airflow velocity. Experimental results show that the reflected light power of the anemometer changed by 33.3% in the measurement range of 0 to 11 m/s. High sensitivity of −28.60 µW/(m·s−1) has been achieved at airflow velocity of 0.1 m/s. The dynamic response time was less than 1 s (∼0.3 and ∼0.9 s for airflow velocity changed from 0.3 to 7.0 m/s and vice versa, respectively), which is sufficient to meet most of the actual demands. Moreover, by using the intensity-interrogation method, the volume, complexity, and cost of the proposed FBG hot-wire anemometer can be greatly reduced, that paves the way for practical uses.
Funding
National Key Research and Development Program of China (2020YFB1805804); National Natural Science Foundation of China (11974083); Open Projects Foundation of State Key Laboratory of Optical Fiber and Cable Manufacture Technology (YOFC) (SKLD1905); Program for Guangdong Introducing Innovative and Entrepreneurial Teams (2019ZT08X340).
Acknowledgments
The authors would like to thank Ms. Li Zhang, Mr. Junpo Dang, and Mr. Zhaoyuan Fan for technical guidance and support on the vacuum magnetron sputtering device and the fiber rotation component.
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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