Open Access
Add to BibSonomy
Abstract
The ambient temperature fluctuation is a long-standing challenge for the hot wire anemometer due to the strong cross-talk. Here, a graphene-coated elliptical core micro-fiber Bragg grating has been proposed for the detection of flow rate with the temperature compensation. With the strong interaction between the graphene layers and the heating light, the graphene coat on the surface of the microfiber can be heated, acting as a heater. The flow rate can be measured through the different responses of two orthogonal polarization modes to the refractive index change of the graphene layer induced by the heat transfer. More importantly, due to the identical response of two polarization modes to the ambient temperature, the fiber-optic anemometer could compensate the temperature cross-talk effectively. The experimental results show that the sensitivity of the 0.42 nm/(m/s) for the fiber anemometer can be achieved, and the temperature standard variation is only 0.084 nm with the range from 20°C to 50°C. The proposed fiber-optic anemometer is very attractive in the fields of various industries for the temperature self-compensation detection of gas flow.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Precise detection of gas flow has been extensively investigating and developing in various areas of gas transportation, solar generation, and environmental monitoring, etc [1]. Conventional anemometers are mainly based on mechanical or electronic methods [2–4]. In recent, fiber-optic anemometers have attracted much attention due to their unique merits, such as high sensitivity, low cost, and immunity to electromagnetic interference [5].
Hot wire anemometer (HWA) is a well-known technique for the measurement of wind speed according to the principle of heat transfer from heat source in the sensor to the surrounding environment [6]. Among them, fiber optic hot-wire anemometers have been widely investigated. Based on the principle of the sensor, the fiber-optic hot wire anemometers can approximately be classified into two types: fiber gratings and interferometers. For the fiber grating, such as fiber Bragg grating (FBG) [7], long period fiber grating (LPFG) [8], and tilted fiber Bragg grating (TFBG) [9], the temperature change induced by the heat transfer modulates the effective refractive index and period of the grating due to the thermo-optic effect and thermal expansion, making a drift for the resonant wavelength. Various fiber gratings based fiber-optic anemometers have been researched. Liu’s group demonstrated a highly sensitive fluidic flowmeter based on an FBG heated by Co2+-doped fiber [7]. Caldas fabricated a metallic coated hybrid LPFG [8]. Zhang presented a carbon nanotube coated TFBG [9]. For the “hot-wire” interferometer, different Fabry-Pérot fiber interferometers have been constructed. The temperature change of the heat source, such as silicon [10] or Co2+-doped fiber [11], could modulates the optical path difference (OPD), making a shift of the interference fringes. Both grating and interferometer based self-heated fiber “hot-wire” anemometers are promising approaches with high integration, fast response, and high resolution.
However, the ambient temperature may influence the fiber-optic anemometers seriously. Thus it is essential to eliminate the temperature cross-talk for hot-wire anemometers, including a Fabry-Pérot silicon interferometer or a pair of FBGs [10,12]. However, turning on and off the pump laser or fitting the envelope of the spectrum increase the complexity of the measurement significantly. Actually, few researches have been investigated the temperature-compensation of fiber anemometers because it is very hard to separate the temperature change from the heat transfer of the wind flow or the ambient environment.
In this paper, we propose a graphene coated elliptical core micro-FBG. A section of elliptical core fiber was tapered to form a microfiber, in which a FBG was inscribed. The graphene layer coated on the surface of the elliptical core micro-FBG was heated by the visible laser beam of a 532 nm laser. Two reflection peaks of the micro-FBG induced by the high geometrical birefringence were shifted differently due to the refractive index change induced by the heat transfer of the graphene layers. However, two reflection peaks exhibit the same performance to the ambient temperature. Hence the temperature-compensated flow rate measurement can be developed by interrogating the wavelength interval of two different birefringence peaks.
2. Fabrication of the graphene coated elliptical core micro-FBG
In the proposed sensor, an elliptical core optical fiber was employed. The fiber has a silica cladding with the diameter of d = 125 µm, and an elliptical core with the long axis of a = 7 µm and short axis of b = 5 µm, as shown in Fig. 1(a). The elliptical core fiber was immersed into the hydrofluoric (HF) acid in order to taper the fiber. The microfiber with the diameter of 9 µm were formed by immersing into the HF with 80 min, as shown in Fig. 1(b). Then the microfibers were washed with distill water for three times to clean the remained HF. The mode field distribution is calculated using COMSOL. The field distribution of the elliptical core microfiber is obviously elliptical, indicating a large fiber birefringence.
Fig. 1. (a) Cross section of the elliptical core fiber. The microfiber with the diameter of (b) 9, (c) 11, and (d) 13 µm. (e) Mode field distribution. (f) SEM of the graphene coated microfiber.
Then the experimental setup and fabrication process of the elliptical core micro-FBG is similar with that of the single mode fibers (SMFs) [13]. The elliptical core fiber was placed under an ultraviolet (UV) light ArF laser with the wavelength of 193 nm, the energy of 0.3 mJ, and the repetition rate of 5 Hz, respectively. Between the elliptical core fiber and the laser, the UV light was illuminated to a phase mask with a period of 1078 nm through a cylindrical lens in order to shape the UV light and enhance the energy density. After the inscription, the effective refractive index for both the elliptical core and the silica cladding was modulated due to the two photon absorption at the wavelength of 193 nm [13]. The reflection spectrum of the elliptical core micro-FBG is shown in Fig. 2(a). Obviously, two reflection peaks with the wavelength separation of 3.4 nm are exhibited in the reflection spectrum corresponding to the two polarization axis, and the reflectivity contrast between the two peaks can be adjusted by using a polarization controller. During the entire fabrication process, the elliptical core was illuminated through a super-wideband light source and the characteristic of the FBG was monitored by using an optical spectrum analyzer (OSA).
After that the surface of the elliptical core micro-FBG was coated with the graphene. The microfiber was placed on a substrate. A commercial graphene was transferred to cover the microfiber by using the method in [14]. The graphene film was then cut to a 10 µm width alongside the microfiber by using the Focused Ion beam (FIB). Finally, the microfiber and the graphene were lifted spontaneously, and the graphene was wrapped around the micro-FBG, as shown in Fig. 1(f). The wavelength interval was decreased as 2.1nm due to the graphene layer, as shown in Fig. 2(a). Two other graphene coated micro-FBGs with the diameter of 11, and 13 µm were also fabricated, as shown in Figs. 1(c) and 1(d). The wavelength interval of the graphene coated micro-FBGs was decreased with the increasing of the diameter, as shown in Fig. 2(b). Finally, a hollow square hole with the side length of 1.5mm was fabricated in the center of a steel square plane. In the middle of each side, a rectangle groove with the width of 150 um was fabricated by using laser micromachining fabrication. After inscription of the microFBG in the elliptical core fiber, the fiber was fixed into the rectangle groove by using the polyamide to keep the strain of the fiber during the experiment. The wind can pass the micro-FBG through the hollow square hole.
3. The principle of the proposed sensor
In the proposed sensor, the few layers graphene was used as a heat source to generate amount of heat due to the absorption of the light with the wavelength of 532 nm. As a result, the temperature of the graphene is increased [15]. The conductance $\sigma (\omega ,T)$ of the graphene can be expressed as [16]
Fig. 3. Simulation of (a) the relationship between the real part of the refractive index for the graphene and the temperature and the refractive index sensitivity of the graphene. (b) the effective refractive index change of two polarizations with different refractive index of graphene. Insert shows the mode field distribution by using COMSOL. (c) the wavelength shifts of two polarizations and the wavelength interval change response for the flow rate. (d) the wavelength shifts of two polarizations and the wavelength interval change at the temperature range from 0 to 80°C. (e) the wavelength shifts of two polarizations and the wavelength interval change with different refractive index change of graphene. (f) the entire wavelength shifts of two polarizations and wavelength interval change for temperature cross-sensitivity. Insert shows the wavelength interval change at the temperature range from 0 to 80°C.
The refractive index of graphene is very sensitive to the temperature change due to the modulation of the conductance. During the flow rate sensing process, the graphene was heated through a 532 nm laser. When the wind pass through the graphene, the heat generated by the graphene is carried away, which modulate the refractive index for the graphene. The heat transfer between the heat source and the wind speed can be expressed as [9]
where ${H_{power}}$ and $\Delta {T_h}$ are the heat loss and temperature change, A and $B$ are empirical constants, and $\upsilon$ is the wind speed.For the elliptical core micro-FBG, the evanescent field of the guided light is interacted with the graphene, modulating the effective refractive indexes of the guided light at two polarizations simultaneously through the heated graphene. Hence two reflection peaks was shifted due to the wind. The wavelength interval change of two reflection peaks can be expressed as
From Eq. (4) we can see that the wavelength interval of two reflection peaks is modulated by both the temperature change and refractive index change of the graphene. We simulated the wavelength interval response of the elliptical core micro-FBG with the grating pitch of 1078 nm for the flow rate. In Eq. (4), ${H_{power}}$, A, and B were set as $\textrm{200}J$, 0.8, and 0.4, respectively. The flow rate $\upsilon$ is ranged from 0 to 1.0 m/s. The thermal-expansion coefficients ($\alpha$) is set as 8.3×10−7/°C, and the refractive index sensitivity of the graphene ($dn\textrm{/}dT$) is set as the calculated result shown in Fig. 3(a) [10]. The effective refractive index change ($d{n_{eff}}\textrm{/}dT$) of two polarizations at different temperature can be simulated by using COMSOL. Also, the effective refractive index change ($d{n_{eff}}\textrm{/}dn$) of two polarizations with different refractive index of graphene is also simulated by using COMSOL, as shown in Fig. 3(b). Therefore, all of calculated results are substituted into Eq. (4) to simulate the wavelength interval response for the flow rate, as shown in Fig. 3(c). The wavelength interval changes 0.392 nm with the flow rate range from 0 to 1.0 m/s. Besides, the wavelength shifts of two polarizations are also shown in Fig. 3(c). The wavelength shifts of the fast axis and slow axis are 1.164 and 0.772 nm with the flow rate range from 0 to 1.0 m/s, respectively.
Generally, the temperature cross-sensitivity for the fiber optic anemometer contains two parts: the fiber device and heating materials. For the fiber device, the ambient temperature could change the geometry size and refractive index of the device, making a wavelength shift of the reflection peak. Hence the temperature cross-sensitivity may influence the grating based fiber-optic anemometers seriously. For the heating materials, the ambient temperature may change the refractive index of the graphene, resulting in a similar wavelength shift of the reflection peak. We simulated the wavelength interval response for the temperature change by using the same method mentioned above. Figure 3(d) shows the wavelength shifts of two polarizations and the wavelength interval change at the temperature range from 0 to 80°C. Although the wavelength shifts of two polarizations is relative large at different temperature (0.842 nm for both two polarizations), the wavelength interval is only changed as 0.0017nm due to the very close temperature response of two polarizations. Figure 3(e) shows the wavelength shifts of two polarizations and the wavelength interval change with the different refractive index change of the graphene (induced by the temperature change ranged from 0 to 80°C). The wavelength shifts of two polarizations is 0.244nm and 0.117nm, while the wavelength interval is changed as 0.127nm. The wavelength shifts of two polarizations and the wavelength interval change for temperature and refractive index of the graphene are summed to simulate the entire temperature cross-sensitivity of the proposed fiber optic anemometer, as shown in Fig. 3(f). The wavelength shift for fast axis and slow axis, and the wavelength interval change are 1.087, 0.981, 0.126 nm, respectively. The simulated wavelength shifts show that the temperature cross-sensitivity from the fiber device (0.842 nm for both two polarizations) is much larger than that from heating materials (0.244nm and 0.117nm) greatly, which is the primary factor for the temperature cross-sensitivity. However, the temperature cross-sensitivity from the fiber device can be eliminated effectively by using the interrogation of wavelength interval (0.0017nm). The cross-sensitivity is defined as $C = \Delta {\lambda _t}/\Delta \lambda$, where $\Delta {\lambda _t}$ is the wavelength shift or wavelength interval change at different ambient temperature, $\Delta \lambda$ is the wind flow response simulated in Fig. 3(c). The cross-sensitivity for wavelength shift for fast axis and slow axis, and the wavelength interval change are 93.38%, 127.07%, 32.14%, respectively. It can be seen that the temperature cross-sensitivity of the wavelength shift is very large, which is an conventional interrogation in most of grating based fiber optic anemometer (such as FBG, LPFG, or TFBG) [7–9]). However, although the refractive index change of the heating material induced by the temperature change also affect the sensor response, the temperature cross-sensitivity from the fiber device can be eliminated significantly through the interrogation of wavelength interval due to the temperature compensation of the fiber device itself. In generally. the ambient temperature fluctuation is always slow in a small range, which only induces a small refractive index change of the graphene compared with that of the heat transfer induced by wind. For instants, the wavelength interval is only changed as 0.052nm in room temperature (from 20°C to 50°C) with the flow rate range from 0 to 1.0 m/s. In this way, the entire temperature cross-sensitivity can be self-compensated greatly by the graphene coated elliptical core micro-FBG compared with other conventional grating based fiber optic anemometers, such as FBG, LPFG, or TFBG.
4. Experiment and discussion
The steel square plane with the graphene coated elliptical core micro-FBG is shown in Fig. 4(a). The experimental setup of the temperature-compensated fiber-optic anemometer is shown in Fig. 4(d). An amplified spontaneous emission (ASE) with the wavelength from 1525 to 1565 nm and output power of 0.25 mW is used to illuminate the graphene coated elliptical core micro-FBG through the lead-in single mode fiber (SMF). The reflection spectrum was measured by using a high-speed spectrometer (BaySpec FBGA-F-1525-1565) through a coupler. A pump laser with the wavelength of 532 nm was also illuminated into the elliptical core micro-FBG through a fiber coupler in order to heat the graphene. Both a commercial anemometer and the graphene coated elliptical core micro-FBG were placed in a home-made tunnel in order to calibrate the measurement result. The wind can be generated and adjusted by using a blower driven by the applied current. In order to test the characteristic of the graphene film, we measure the Raman spectrum (AvaSpec-3648TEC) and the transmittance in visible range (ShimadzuModel UV-2600) of the graphene, respectively, as shown in Figs. 4(f). The graphene shows the characteristic of few layers due to the strong intensity of the G band compared with that of the 2D band in the Raman spectrum [18], and there are five layers graphene due to the transmittance of 87.5% (2.3% of absorbance per graphene layer) [19].
Fig. 4. (a) A steel square plane with a hollow square hole. (b) evanescent field of the 532 nm laser. (c) Thermal image. (d) Experimental setup. (e) Temperature change of graphene with different pump laser. (e) Raman spectrum of the graphene and transmittance of the graphene in visible range.
Figure 4(b) shows the micro-FBG illuminated with 532 nm laser. Obvious the graphene was visible from the scattered light. Hence the evanescent field of the 532 nm laser could also interact with graphene film as the heating light. Figure 4(c) shows the thermal image of the graphene coated elliptical core micro-FBG heated by using evanescent field of the 532 nm laser by using the thermographic camera. The temperature is increased as a function of the heating laser intensity, indicating the absorption of the heating light. The linear fitting curve of the temperature is $y = 0.31x + 2\textrm{1}.\textrm{8}.$, as shown in Fig. 4(e). Noted that the power of the heating light is hard to be measured due the multimode at the wavelength of 532nm. Instead, the intensity at the output of the pump laser is used to replace that of the evanescent field.
The wind speed measurement of the proposed graphene coated elliptical core micro-FBG was carried out with the pump laser intensity of 400 mW. Figure 5(a) shows the measured reflected spectra with the wind speed range from 0-1.0 m/s. Once the graphene coated micro-FBG was illuminated with 532nm laser, the wavelength interval is reduced as 1.48nm due to the refractive index change of the graphene. Both two peaks are blue shifted with the increasing wind speed, as shown in Fig. 5(b). However, the sensitivity of the fast axis is higher than that of the slow axis due the stronger evanescent-field interaction with the graphene, indicating the quite different sensitivities of two polarizations due to the different evanescent-field for two orthogonal modes. At the same time, Fig. 5(b) shows the wavelength interval of two peaks. The wavelength interval with different wind speed can be fitted by a quadratic polynomial regression expressed as $y = 0.\textrm{15}{x^2} + 0.12x + 1.48.$. The maximum sensitivity of 0.42 nm/(m/s) can be achieved with the wind speed of 1.0 m/s. Moreover, Fig. 5(c) shows response time when the wind speed is changed from 0 to 0.2 m/s. The response time of 0.44 s was achieved, indicating the fast modulation of the graphene. Compared with the stable signal without the wind, there are some fluctuation in wavelength interval under strong wind flow, which is attributed to the locally instable wind speed. Three graphene coated micro-FBGs with the diameter of 9, 11, and 13 µm were also investigated for the wind speed response, as shown in Fig. 5(d). Obviously, the sensitivity of the micro-FBG is decreased with the increase of the microfiber diameter due to the few evanescent field interacted with the graphene when the cladding is thick. In general, the sensitivity of the proposed graphene coated elliptical core micro-FBG at wind speed of 1.0 m/s (0.42 nm/(m/s)) is higher than that of −0.3667 nm/(m/s) based on the carbon nanotube coated TFBG [9] and 45.3 pm/(m/s) based on the silver-coated FBG [20]. Besides, the time response of the proposed graphene coated elliptical core micro-FBG (0.44 s) is faster than that of 4.0s based on the single-walled carbon nanotubes coated TFBG [21], but slower than that of 0.14s based on the silicon Fabry-Pérot interferometer [10]. Meanwhile, the measurement range of the proposed graphene coated elliptical core micro-FBG (0.0 to 1.0 m/s) is larger than that from 0.05 to 0.65 m/s based on carbon nanotubes coated the fiber surface plasmon resonance [22], but smaller than that from 0.0 to 2.0 m/s based on the carbon nanotube coated TFBG [9].
Fig. 5. (a) Reflected spectra of the micro-FBG with different wind speeds. (b)Wavelength shift of two peaks and wavelength interval between two peaks. (c) Response time from 0–0.2 m/s. (d) Wavelength interval between two peaks with different diameter.
Many research have been already confirmed that fiber optic hot-wire anemometers are highly depend on the different ambient temperature. The proposed sensor was fixed into an environmental chamber in order to adjust the ambient temperature from 20°C to 80°C with the interval of 5°C. Figure 6(a) shows the wavelength shifts of two peaks with slow axis and fast axis under the flow rate of 1.0 m/s with the temperature range from 20°C to 50°C. It can be seen that both two peaks shift to the longer wavelength with the increasing of the temperature. Figure 6(b) presents wavelength interval between two peaks at different temperature. In the temperature range from 20°C to 50°C, the wavelength interval between two peaks was almost fixed at each temperature compared with the wavelength shifts of two peaks. The maximum standard variation of the wavelength interval is only 0.084 nm under the flow rate of 1.0 m/s, indicating that the proposed fiber anemometer could compensate the ambient temperature cross-talk with the range from 20°C to 50°C through the interrogation of the wavelength interval effectively, which is suitable in many practical applications in room temperature. However, when the temperature exceeds 50°C, the variation of the wavelength interval becomes greater due to the large refractive index change of the graphene (the maximum standard variation of the wavelength interval is 0.78 nm under the flow rate of 1.0 m/s in the temperature range from 50°C to 80°C). Thus the temperature compensation range of the proposed sensor is from 20°C to 50°C. Compared with other conventional grating based fiber optic anemometer (FBG, LPFG, or TFBG), the interrogation of wavelength interval in the graphene coated elliptical core micro- FBG is low cost and effectiveness.
Fig. 6. (a) Reflected spectra of the micro-FBG at different temperature with the wind speed of 1.0m/s. (b) Wavelength interval between two peaks with different wind speed at different temperature.
The diameter of the elliptical core micro-FBG is a key parameter for the proposed anemometer. Generally, there are two factors for the diameter of the microfiber that can influence on the performance of the sensor: (i) The sensitivity: the intensity of the evanescent field for the microfiber is increased with the decrease of the diameter [23]. Figures 7(a)–6(c) show the intensities of the evanescent field for three microfibers with different diameters. With the decrease of the diameter, more evanescent field can be formed on the surface of the microfiber, which can enhance the sensitivity of the sensor significantly. However, when the diameter of the microfiber is much smaller than the wavelength of the guide light (<1/10 $\lambda$), the attenuation loss would be increased significantly due to the nonuniform geometric size on the surface of the microfiber [24,25]. Hence for the guide light at the wavelength of 1550nm, the diameter of the microfiber should be larger than 0.3 µm for the low transmission loss. (ii) The strength: the structure strength of the microfiber becomes weak when the diameter is reduced. Especially for the fiber anemometer, the microfiber is very fragile with the deformation induced by the wind flow. Hence the diameter of the microfiber should be large in order to avoid damage. Based on the two factors, the diameter of the elliptical core micro-FBG is optimized as 9 µm in the proposed sensor in order to keep both the sensitivity and strength of the microfiber.
Fig. 7. The mode field distribution of the elliptical core micro-FBG with the diameter of (a) 7.6 µm, (b) 9 µm, and (c) 13 µm. Figures below show the enlarge views of the evanescent field three microfibers with different diameter.
5. Conclusion
In conclusion, we proposed and experimentally demonstrated a temperature-compensated all-fiber hot-wire anemometer based on graphene coated elliptical core micro-FBG. A few layer graphene was coated on the surface of the elliptical core micro-FBG, acting as a “hot-wire” anemometer. Meanwhile, due to the identical response of two polarization mode to the temperature, the proposed fiber-optic anemometer could compensate the temperature cross-talk. The experimental results show that the maximum sensitivity of 0.42 nm /(m/s) can be achieved, and the temperature standard variation is only 0.084 nm with the range from 20°C to 50°C. The proposed fiber-optic anemometer is very attractive in the fields of various industries for the temperature self-compensation detection of gas flow.
Funding
National Natural Science Foundation of China (61601436).
Disclosures
The authors declare no conflicts of interest.
References
1. D. W. Lamb and A. Hooper, “Laser-optical fiber Bragg grating anemometer for measuring gas flows: application to measuring the electric wind,” Opt. Lett. 31(8), 1035–1037 (2006). [CrossRef]
2. L. H. Piao, T. Zhang, Y. F. Ma, and J. P. Wang, “Structural optimization of mental cone rotameter based on CFD,” Transduc. Microsyst. Technol. 30(3), 90–93 (2011).
3. J. Firth, F. Ladouceur, Z. Brodzeli, M. Wyres, and L. Silvestri, “A novel optical telemetry system applied to flowmeter networks,” Flow Meas. Instrum. 48, 15–19 (2016). [CrossRef]
4. J. Wu and W. Sansen, “Electrochemical time of flight flow sensor,” Sens. Actuators, A 97-98, 68–74 (2002). [CrossRef]
5. O. Frazão, P. Caldas, F. M. Araújo, L. A. Ferreira, and J. L. Santos, “Optical flowmeter using a modal interferometer based on a single nonadiabatic fiber taper,” Opt. Lett. 32(14), 1974–1976 (2007). [CrossRef]
6. S. Gao, A. P. Zhang, H.-Y. Tam, L. H. Cho, and C. Lu, “All-optical fiber anemometer based on laser heated fiber Bragg gratings,” Opt. Express 19(11), 10124–10130 (2011). [CrossRef]
7. Z. Y. Liu, L. Htein, L. K. Cheng, Q. Martina, R. Jansen, and H. Y. Tam, “Highly sensitive miniature fluidic flowmeter based on an FBG heated by Co2+-doped fiber,” Opt. Express 25(4), 4393–4402 (2017). [CrossRef]
8. P. Caldas, P. A. Jorge, G. Rego, O. Frazão, J. L. Santos, L. A. Ferreira, and F. Araújo, “Fiber optic hot-wire flowmeter based on a metallic coated hybrid long period grating/fiber Bragg grating structure,” Appl. Opt. 50(17), 2738–2743 (2011). [CrossRef]
9. Y. Zhang, F. Wang, Z. G. Liu, Z. H. Duan, W. L. Cui, J. Han, Y. Y. Gu, Z. L. Wu, Z. G. Jing, C. S. Sun, and W. Peng, “Fiber-optic anemometer based on single walled carbon nanotube coated tilted fiber Bragg grating,” Opt. Express 25(20), 24521–24530 (2017). [CrossRef]
10. G. Liu, W. Hou, W. Qiao, and M. Han, “Fast-response fiber-optic anemometer with temperature self-compensation,” Opt. Express 23(10), 13562–13570 (2015). [CrossRef]
11. Y. Li, G. F. Yan, L. Zhang, and S. L. He, “Microfluidic flowmeter based on micro “hotwire” sandwiched Fabry-Perot interferometer,” Opt. Express 23(7), 9483–9493 (2015). [CrossRef]
12. B. Zhou, H. H. Jiang, C. T. Lu, and S. L. He, “Hot Cavity Optical Fiber Fabry–Perot Interferometer as a Flow Sensor With Temperature Self-Calibrated,” J. Lightwave Technol. 34(21), 5044–5048 (2016). [CrossRef]
13. Y. Ran, Y. N. Tan, L. P. Sun, S. Gao, J. Li, L. Jin, and B. O. Guan, “193 nm excimer laser inscribed Bragg gratings in microfibers for refractive index sensing,” Opt. Express 19(19), 18577–18583 (2011). [CrossRef]
14. W. Li, B. G. Chen, C. Meng, W. Fang, Y. Xiao, X. Y. Li, Z. F. Hu, Y. X. Xu, L. M. Tong, H. Q. Wang, W. T. Liu, J. M. Bao, and Y. R. Shen, “Ultrafast All-Optical Graphene Modulator,” Nano Lett. 14(2), 955–959 (2014). [CrossRef]
15. J. L. Benítez, J. H. cordero, S. Muhl, and D. Mendoza, “Few layers graphene as thermally activatedoptical modulator in the visible-near IR spectral range,” Opt. Lett. 41(1), 167–170 (2016). [CrossRef]
16. G. W. Hanson, “Dyadic Green’s functions and guided surface waves for a surface conductivity model of graphene,” J. Appl. Phys. 103(6), 064302 (2008). [CrossRef]
17. A. Vakil and N. Engheta, “Transformation optics using graphene,” Science 332(6035), 1291–1294 (2011). [CrossRef]
18. L. M. Malard, M. A. Pimenta, G. Dresselhaus, and M. S. Dresselhaus, “Raman spectroscopy in graphene,” Phys. Rep. 473(5-6), 51–87 (2009). [CrossRef]
19. S. Bae, H. Kim, Y. Lee, X. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. Song II, Y.-J. Kim, K. S. Kim, B. Özyilmaz, J.-H. Ahn, B. H. Hong, and S. Iijima, “Roll-to-roll production of 30-inch graphene films for transparent electrodes,” Nat. Nanotechnol. 5(8), 574–578 (2010). [CrossRef]
20. X. Y. Dong, Y. Zhou, W. J. Zhou, J. Cheng, and Z. D. Su, “Compact Anemometer Using Silver-Coated Fiber Bragg Grating,” IEEE Photonics J. 4(5), 1381–1386 (2012). [CrossRef]
21. Y. Zhang, F. Wang, Z. H. Duan, Z. X. Liu, Z. G. Liu, Z. L. Wu, Y. Y. Gu, C. S. Sun, and W. Peng, “Plasmonic Fiber-Optic Photothermal Anemometers With Carbon Nanotube Coatings,” Sensors 17(9), 2107 (2017). [CrossRef]
22. Y. K. Liu, B. H. Liang, X. J. Zhang, N. Hu, K. W. Li, F. Chiavaioli, X. C. Gui, and T. Guo, “Plasmonic Fiber-Optic Photothermal Anemometers With Carbon Nanotube Coatings,” J. Lightwave Technol. 37(13), 3373–3380 (2019). [CrossRef]
23. M. Sumetsky, “How thin can a microfiber be and still guide light?” Opt. Lett. 31(7), 870–872 (2006). [CrossRef]
24. M. Sumetsky, Y. Dulashko, P. Domachuk, and B. J. Eggleton, “Thinnest optical waveguide: experimental test,” Opt. Lett. 32(7), 754–756 (2007). [CrossRef]
25. L. M. Tong, J. Y. Lou, and E. Mazur, “Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides,” Opt. Express 12(6), 1025–1035 (2004). [CrossRef]
