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Abstract
A compact, humidity-insensitive, fiber optic temperature sensor based on a quartz capillary, antiresonant reflecting optical waveguide was proposed and experimentally demonstrated. The transmission spectral responses of the proposed sensor were experimentally investigated regarding temperature variation and environmental humidity. Resonant dips exhibited temperature sensitivity as large as 201 pm/°C from −30 to 45 °C in a humid environment. By coating a sufficiently thick gold film onto the sensor surface, the humidity cross-sensitivity issue was effectively resolved. This proposed sensor was anticipated to find potential applications in humid environments, and moreover, immunity to humidity-sensitivity ensures its applicability in marine environments.
© 2017 Optical Society of America
1. Introduction
An optical fiber sensor is a type of optical sensing technology for detecting light transmission within a fiber as it changes with the external environment. It has many outstanding advantages over conventional electrical or chemical sensors, such as low cost, compact size, high sensitivity, high flexibility, corrosion resistance, and good electromagnetic interference immunity [1]. These features render it useful for measuring many kinds of parameters, such as temperature [2], humidity [3], refractive index [4], strain [5], curvature [6], acceleration [7], and gases [8]. Consequently, various types of optical fiber sensors have received extensive research and development in recent decades [9–11]. However, for temperature sensing, the practicality of such devices has been limited by the ambient humidity among other factors, such as polymer optical fiber (POF) and fiber bragg gratings (FBGs) humidity-insensitive temperature sensor with high material loss [12, 13].
Anti-resonant reflecting fiber is a kind of waveguide whose spectral characteristics are governed by the thickness of the first high refractive index layer rather than the lattice constant, and has been identified to play an important role in near-infrared and THz optical signal transmission [14]. The cladding of this capillary waveguide is considered a Fabry-Perot (F-P) cavity, in which wavelengths, which satisfy the resonant conditions of no reflection at the core-cladding interface, radiate through the cladding (leaky modes) and wavelengths under the cladding’s antiresonant conditions are confined in the air core (guided core modes) [15]. The presence of leaky modes in the cladding makes these waveguides sensitive to the surrounding environments, and various sensing applications have been proposed, including biosensing [16], film sensing [17], and magnetic field sensing [18]. Various structures of anti-resonant reflecting fibers have been reported, such as hollow core photonic crystal fibers that are infiltrated with magnetic fluid and ethanol [19], reduced graphene oxide (rGO) coated hollow core fiber humidity sensors based on anti-resonant reflecting guidance [20], and a metal surface optical antireflection coating [21]. A quartz capillary is a type of cylindrical, hollow waveguide with an air core and low visible and infrared transmission loss [22]. The unique medium structure and transmission characteristics make this capillary appropriate as an anti-resonant reflecting optical waveguide (ARROW) to be used widely for its extraordinary advantages in the sensing fields [23]. For the ARROW temperature sensor structure, it is liable affected by surrounding environment particularly humidity. Thus, it is value to overcome influence of humidity for temperature.
In this paper, the mechanism and sensing applications of an anti-resonant reflecting guide in polydimethylsiloxane (PDMS)-coated quartz capillary is reported. The transmission spectral responses of a proposed temperature sensor to temperature variation as well as environmental humidity were investigated. The experimental results indicated that the proposed temperature sensor exhibited insensitivity to humidity in a humid environment. In addition, the influence of the gold film thickness on light propagation was discussed using theoretical simulations. The proposed sensor possessed many distinctive advantages over other temperature sensors based on fiber-capillaries and films, such as humidity-insensitivity, easy fabrication, and simple configuration and lower transmission loss. This sensor showed sensitivity of 201 pm/°C in a humid environment.
2. Sensor fabrication
The sensor was fabricated by splicing a section of the quartz capillary to single mode fibers (SMFs). In this experiment, as the sensing portion of the device, two hollow quartz capillaries were used that had inner diameters of 76 and 100 μm and ring cladding with a thickness of 36 and 33.5 μm, respectively. Optical images for the cross-section of the quartz capillaries are shown in Figs. 1(a) and 1(b), respectively. Then, well-cleaved capillary sections, with lengths of 0.4, 0.8, and 1 cm, were spliced with SMFs by using a conventional fiber splicer [Fig. 1(c)]. Collapse of the hollow core in the capillary was avoided by controlling the arc parameters, which were optimized for the process (arc power of 25 mA, a gap of 10 μm which refer the distance between end face of SMF and capillary, arc fusion time of 300 ms), and the splicing loss was ~0.2 dB.
Fig. 1 The optical image of the cross-section of the quartz capillary (a) with inner and outer diameter of 76 μm and 148 μm (b) with inner and outer diameter of 100 μm and 167 μm, respectively, (c) Fiber splicing with capillary.
PDMS was then coated on the outer surface of the quartz capillary by immersing a section of capillary into a PDMS chloroform solution (10% PDMS and 90% chloroform) for 2 h. After holding the capillary at room temperature for 24 h to thoroughly evaporate the chloroform, the capillary surface was covered by a PDMS film. The PDMS thickness can be control by controlling time and deep of capillary immersing in PDMS chloroform solution. For both end of capillary spliced with SMF, such that only the capillary outer surface was coated with PDMS and the inner surface kept clean. Finally, the PDMS surface was coated with a >100 nm gold film using a vacuum sputtering coating machine. It is noted that when coating gold film the sample should be turn over in the interval of vacuum sputtering coating machine work. By strictly control the time of spraying gold, the gold film thickness can be precisely controlled. In order to robust the sensor device, we packed it by glass dish and silica gel [24].
3. Operation principle
The principle of a PDMS-based temperature sensor can be described as an ARROW [25]. The quartz capillary can be briefly as a pipe waveguide and cladding modes propagate through the cladding region [Fig. 2 (a)]. As the refractive index of the core was less than that of the cladding, the core modes oscillated and radiated through the cladding and reflected on the outer gold layer. The capillary’s cladding region can be described as an F-P resonator [Fig. 2 (b)] [26]. When wavelengths cannot satisfy the resonant conditions of the resonator, the guided light is reflected back by the resonator and confined in the capillary’s air core. In contrast, through the F-P resonator and leaks out of the capillary cladding. Thus, periodic and narrow lossy dips corresponding to the resonant conditions of the F-P resonator occurred in the transmission spectrum.
Fig. 2 (a) Schematic diagram of the cross-section of pipe waveguide with silica cladding of thickness d1 and index of refraction n1. PDMS film of thickness d2 and index of refraction n2 and hollow core index of refraction n0, (b) Guiding mechanism of quartz capillary coated with PDMS and gold. (nau>n1>n0 and n1~n2, nau is the refractive index of the gold layer).
The two partial beams reflected at the two interfaces of the layer is given by [27]:
Where △ is optical path of I2 express as:
where α is incidence angle of I0(λ), In the regime where the anti-resonant are with glancing reflections, α is nearly 90°. Applying Snell’s law and after some manipulations we can obtain the wavelengths of lossy dips:where d1 and d2 are the thickness of the quartz capillary cladding and PDMS film, respectively; n0, n1, and n2 the refractive indices of the air, capillary cladding, and PDMS, respectively; and m the resonance order. The light intensity corresponding to the resonant conditions can be expressed as:where Tresonant is the transmission power at the resonant wavelengths, r0 and r1 the reflection coefficient of the incident light at the air core-cladding interface and PDMS cladding-surrounding gold interface, respectively, and the input light intensity at the resonant wavelength. The model considers negligible reflectance at the silica-PDMS interface due to small difference in refractive index. As can be derived from Eq. (2), if the r1 unaffected by surrounding media, the sensor will meet resonance condition in water or marine environment.According to Eq. (3), we can derive the free spectral range (FSR)
where λ1 and λ2 are the adjacent valleys with phase difference of 2π. According to Eq. (5), the FSR depends on the RI of the air cavity, capillary and PDMS as well as thickness of capillary and PDMS.By taking Eq. (3) we can derive wavelength shift with temperature. In consideration of the variation range of temperature from only −30 to 45°C, the thermal expansion of the quartz capillary can be considered negligible [28]. In addition, the RI variation of the air cavity with temperature is only 10−5 [29, 30], which can be considered negligible here. Therefore, the wavelength shift can be approximately given out as follows:
It can be derived that the temperature sensitivity dependence on thermo-optical and thermal expansion coefficient of PDMS and glass capillary. There is no relation between sensor length (L) and temperature sensitivity.
According to the above formula, the guided light leaked out of the air core and radiated into the silica cladding and PDMS film. However, the guided light only radiated into the silica cladding and PDMS film at the corresponding resonant wavelengths of the silica or PDMS resonator. When changes in ambient temperature affected the proposed sensor, the resonant conditions of the coated-with-PDMS F-P resonator changed due to the tunable RI of PDMS, and the resonance wavelengths thus shifted.
For the proposed sensor, the primary role of the gold film coating on the PDMS was to enhance the PDMS interface reflection coefficient. In addition, as an isolation layer, the gold film was thick enough to eliminate the influence of humidity from the proposed sensor. For illustration, the optical transmittance of the gold film with different thicknesses and angles of incidence were simulated. All temperature of simulation models were set as room temperature in this paper.
As shown in Figs. 3(a) and 3(c), a beam of light irradiated onto the gold film with angle of 0° and 45°. In this simulation model, the ‘transition boundary condition’ was used as the gold film in the optical waveguide with diameter of 10 μm, which effectively saved the computing space and improved the computing speed. As the gold film thickness increased, the electric field intensity gradually decreased and the time averaged power outflow exhibited significant attenuation. The time averaged power outflow decreased to zero when the gold film thickness was close to 60 nm [Figs. 3(b) and 3(d)]. The light incident on gold film is grazing in our sensor structure. To match the condition of grazing incidence, we simulated a similar model. A beam of light propagates in the waveguide with diameter of 10 μm. The outside of waveguide with shells are gold film. As shown in Fig. 3(e), we simulated the case of light grazing incidence on gold film. It is observed an obviously decrease of evanescent field on both sides of the waveguide which the gold film as waveguide boundary. From Fig. 3(f) we can observe that evanescent field exhibited significant attenuation (shown in both sides of the dotted line) as the gold film thickness increased. In other words, the light propagation in waveguide unaffected by changing of surrounding refractive. Thus, this was regarded as a sensible method for excluding humidity influence from the sensor. Meanwhile, this thickness also minimized and even avoided the surface plasmon resonance effect in this sensor [31, 32]. Notably, the gold film did not reduce PDMS’s sensitivity to temperature because of gold’s good temperature conductivity [33].
Fig. 3 (a) The mode distribution in the waveguide with gold thicknesses of 10 and 60 nm (vertical incidence). (b) The corresponding transmitted power with gold film thickness changes. (c) The mode distribution in the waveguide with gold thicknesses of 10 and 60 nm (incident angle, 45°). (d) The corresponding transmitted power with gold film thickness changes. (e) The mode distribution in the waveguide with gold thicknesses of 10 and 60 nm (grazing incidence). (f) The corresponding transmitted power with gold film thickness changes (the dash present gold film boundary).
4. Experiments and discussion
In the experimental setup, a super-wideband light source covering 1450–1650 nm was used as a broadband source to illuminate the PDMS-gold-coated capillary [Fig. 4]. The sensor’s transmission spectrum was interrogated using an optical spectrum analyzer. The sensor was put inside the environment chamber and the chamber’s humidity and temperature adjusted and monitored using a microprocessor-based control unit and feedback sensors. The polarization controller was used to eliminate the effects of polarized light.
Fig. 4 The experimental setup of the proposed sensor. (OSA: optical spectrum analyzer, SLED: self-scanning light emitting device, TC: temperature cabinet).
A scanning electron microscopic (SEM) image and transmission spectrum of the PDMS-coated quartz capillary clearly showed that the PDMS film, (a) is 20 μm and (c) is 18 μm, coated the capillary outer surface, with the film thickness appearing not strictly uniform [Fig. 5]. Thus, the transmission spectrum was not very uniform, there being many other frequency components. Obviously, there were several dips in the transmission spectrum, which corresponded to resonant conditions. Furthermore, the thickness of the cladding region in the quartz capillary was believed to vary a little at different locations, as was observed from the capillary cross-section view [Fig. 5]. The different resonant dips in transmission spectra for the PDMS coating-gold film before and after are shown in [Fig. 5(b) and 5(d)].
Fig. 5 (a) SEM image of capillary (with inner and outer diameters of 76 and 148 μm, respectively). (b) Transmission spectra of 1 cm length capillary with and without PDMS-Gold film. (c) SEM image of capillary (with inner and outer diameters of 100 and 167 μm, respectively). (d) Transmission spectra of 1 cm length capillaries with and without PDMS-gold film.
For the two samples with length of 1cm, transmission spectra differences in density and free spectral range (FSR) were clearly observed between before and after coating with PDMS-gold film. In addition, the intensity in the transmission spectrum of the capillary without PDMS is higher than that with the PDMS (the intensity difference between two transmission spectra is ≈4.6 dB), indicating the higher surface roughness and absorption of the PDMS film. It can be observed that the extinction ratio of resonant wavelength present obvious difference for PDMS-gold film and only PDMS film. However, the positions of resonant wavelength present slight difference. According to Eq. (3), there are no relationships exist between resonant wavelength and gold film. The gold film affects the transmission power at the resonant wavelengths by changing the reflectivity of PDMS gold interface derived from Eq. (4).
Using a full-vector finite element method (FEM) [34], so-called commercial COMSOL software, the transmission spectrum and mode field of a PDMS-coated quartz capillary was simulated. Given that the guided light was assumed as a Gaussian beam with a 10-μm diameter, for the two different sizes capillaries, the theoretical resonant wavelengths were calculated as 1485.56, 1518.13, 1572.64, and 1620.07 nm, and 1470.21, 1512.78, 1556.32, and 1597.61 nm, respectively. Moreover, transmission spectra of the three sensor specimens coating PDMS-gold film with lengths (L) of 0.4, 0.8, and 1 cm showed that the locations of the sharp transmission lossy dips exhibited slight deviations from theoretical predictions [Fig. 6]. This might have been related to nonuniform thickness in the silica cladding and PDMS film. The different visibilities of the samples of three lengths indicated that resonant effects accumulated along the length of the capillary at resonant wavelengths. Therefore, there was a tradeoff between the visibilities and transmission lossy dips. It was noted that the transmission spectra were not particularly uniform but appeared to include other periodic components, distributed throughout the anti-resonance regions of the transmission spectra. The guided light was also reflected by the silica-PDMS interface, but the reflectivity was believed to be relatively low, which resulted in ripples in the transmission spectra [35]. Figures 6(b) and 6(e) show the mode field for resonant wavelengths and, thus, this guided light was transmitted through the resonator. Figure 6(c) and 6(f) show the mode field for the anti-resonant wavelengths, which were confined in the fiber’s hollow core, in the guided-core mode.
Fig. 6 (a) Simulation and measured transmission spectra of sensor with inner and outer diameters of 76 and 148 μm respectively. Simulation of mode field distribution (capillary with inner and outer diameters of 76 and 148 μm, PDMS thickness is 20um, gold film thickness is 100nm). (b) The mode field at anti-resonant wavelength. (c) The mode field at resonant wavelength. (d) Simulation and measured transmission spectra of sensor with inner and outer diameters of 100 and 167 μm respectively. Simulation of mode field distribution (capillary with inner and outer diameters of 100 and 167 μm, PDMS thickness is 20um, gold film thickness is 100nm). (e) The mode field at anti-resonant wavelength. (f) The mode field at resonant wavelength. (n0 = 1, n1 = 1.45, n2 = 1.42, n3 = ngold, where the ngold is refractive index of gold).
To illustrate that the gold film plays a key role in this humidity-insensitivity temperature measurement, a comparison study was performed of humidity sensing between the sensors with and without gold film [Fig. 7]. The length of these sensor smaples are 1 cm. Before coating with gold film, the transmission spectra of a sensor showed great fluctuations induced by ambient humidity [Fig. 7(a) and 7(c)]. After the sensor coating with gold film, there were almost no fluctuations in the transmission spectra with humidity changes [Fig. 7(b) and 7(d)]. The temperature was controlled strictly at 28.5 °C in the variation of humidity. By comparison of experimental results, it was concluded that the gold film thickness was sufficient to eliminate the influence of of humidity atmosphere, concluding the water vapour.
Fig. 7 Humidity response of sensor sample with inner and outer diameters of 76 and 148 μm, and length of 1cm. (a) capillary coated with only PDMS film, (b) capillary coated with only PDMS-gold film. Humidity response of sensor sample with inner and outer diameters of 100 and 167 μm, and length of 1 cm. (c) capillary coated with only PDMS film, (d) capillary coated with only PDMS-gold film.
In addition, the resonant wavelength extinction ratio of the capillary with gold film was higher than that without gold film, indicating high reflectivity of the interface between the PDMS and gold films. In addition, this result also reveal a posibility of temperature sening application in water or marine. The gold film completely isolated from the humidity influence on proposed sensor, which will further be improved and consummated if the gold film fall off in liquid environment is resolved.
The proposed PDMS-gold-coated quartz capillary with length of 1 cm was tested over a range from −30 to 45°C. The corresponding transmission spectra for the two different sensor sizes are shown in Figs. 8(a) and 8(c). The resonant wavelengths shifted to shorter wavelengths, which was induced by the refractive index sensitivity of PDMS to variations in temperature [36]. Temperature-fitting curves obtained from the two sensors showed that all curves showed linear relationships (R2>0.997) between wavelength shifts and temperature, as shown in Figs. 8(b) and 8(d), indicating that the sensors present linear responses over the studied temperature range. The linear fitting curve of the two sensors was expressed as y1 = −0.201x - 6.088 and y2 = −0.176x - 5.817, for the sensor 1 and sensor 2 (sensor 1 with inner and outer diameters of 76 and 148 μm, sensor 2 with inner and outer diameters of 100 and 167 μm) which indicated the temperature sensitivity of the sensors were 201 and 176 pm/°C, respectively. According to Eq. (6), The slightly different sensitivity between the two sensor samples was caused by different thickness of the PDMS film and capillary walls. In addition, we implemented temeprature down cycle after the temprature up. The quite similar temperature response indicate the proposed sensor have good stablility and repeatbility. It is note that the sensor samples were damaged in various degrees after many repeatitive tests.
Fig. 8 (a) Transmission spectra of capillary coated with PDMS-gold film. (b) The temperature response. (capillary inner diameter 76 μm, outer diameter 148 μm, PDMS thickness 20 μm). (c) Transmission spectra of capillary coated with PDMS-gold film. (d) The temperature response (capillary inner diameter 100 μm, outer diameter 167 μm, PDMS thickness 18μm).
6. Conclusion
In conclusion, a compact humidity-insensitive fiber optic temperature sensor based on a capillary coated with a composite PDMS-gold film ARROW was proposed and experimentally demonstrated. The ARROW was fabricated by splicing a quartz capillary section with a segment of SMF at each end and then coating PDMS onto the capillary’s outer surface. Humidity influences were eliminated by coating the PDMS layer with a sufficient layer of gold. A verification experiment regarding humidity insensitive was conducted with the humidity range from 55% to 70%. In addition, the extinction ratio of the transmission spectrum was experimentally verified proportional to the length of capillary. The temperature sensitivity was experimentally proved proportional to the thickness of PDMS. Owing to PDMS’s optical properties and it’s isolation by the gold film from humidity, wavelength sensitivities as large as 201 and 176 pm/°C were experimentally acquired with the two selected sensor samples for a temperature range of −30 to 45 °C in a changing humidity environment. Our proposed temperature sensor design possessed desirable merits, such as humidity-insensitivity, compactness, simple structure, ease of fabrication, and low cost of manufacturing. It is anticipated that this sensor will find many potential applications in temperature monitoring in fluctuating-temperature environments and, particularly, in marine environmental temperature measurements.
Funding
National Natural Science Foundation of China (NSFC) (61605031, 61422505, 61635007); National Key Scientific Instrument and Equipment Development Project (2013YQ040815); The National Key Research and Development Program of China (2016YFF0200700); Harbin Science and Technology Innovative Talents Project of Special Fund (2015RAYXJ009); Program for New Century Excellent Talents in University (NCET) (NCET-12-0623). Fundamental Research Funds for the Central Universities.
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