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Nanopatterned evanescent-field fiber-optic interferometer as a versatile platform for gas sensing
Sensors & Actuators: B. Chemical 301 (2019) 127136
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nanopatterned evanescent-field fiber-optic interferometer as a versatile platform for gas sensing
T
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Yun Liua, , Ning Zhangb, Ping Lib, Sheng Bic, Xuhui Zhanga, Shimeng Chena, Wei Penga a
School of Physics, Dalian University of Technology, Dalian, 116024, China School of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology, Dalian, 116024, China c Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian, 116024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical fiber sensor Interferometer Evanescent field Hydrogen sensing Humidity measurement
We introduced a nanopatterned fiber-optic interferometer (FOI) based on an optical fiber line embedded with a short capillary which excites the evanescent field by total reflection of light in the wall waveguide interacting with the fluid and causing the optical signal change. As modal interference was generated with near-infrared light in the tube wall, we detected a reversible wavelength shifts of the proposed FOI in the interference fringe upon water vapor exposure by using an optical fiber interrogator. We also deposited a patterned palladium (Pd) nanoparticle film on the outer surface of the FOI by using self-assembly microspheres as a template and detected the reaction of the Pd membrane with hydrogen through monitoring the wavelength shifts of interference fringes. The Pd nanoparticle restrained the fiber stress response caused by hydrogen-induced expansion of the Pd film, and theoretically helped to improve the evaluation accuracy of the Pd permittivity in hydrogen. In addition, the nanoparticle-coated FOI with asymmetric structure could serve as a Mach-Zehnder interferometer (MZI) for fast hydrogen sensing and air refractive index measurement. Considering sensitivity and detector parameters, the FOI could provide a high resolution up to 7.57 × 10−7 RIU.
1. Introduction Fiber-optic (FO) evanescent-field technology has evolved significantly in the past few decades because of its advantages of high sensitivity, immunity to electromagnetic interference, all-fiber and hybrid integration capability [1–12]. A variety of processing techniques are used to modify optical fibers structure to excite strong evanescent field, such as core mismatched fibers [13,14], tapered fibers [15–17], D-shaped transmission fibers [18,19], S-shaped fibers [20], U-shaped fibers [21,22], microstructured fibers [23,24], micro/nanofibers [25,26] and fiber gratings [27,28]. However, these evanescent field sensor is struggling to balance the sensitivity, manufacturing process, cost and mechanical strength, which impose practical limitations to the development of practical FO evanescent field sensing technology. The core mismatched structure has a weak evanescent-field strength and a relatively low sensitivity [29], although it has a simple manufacturing process. Because of the difficult manufacturing of the tapered region, it is hard to ensure a high production consistency for tapered structure [30,31]. D-shaped fibers have a poor mechanical strength which restricts their performance [32]. S-shaped and U-shaped sensors are not easily packaged and a small bending radius leads to a high fragility
⁎
[33,34]. Micro/nanofibers have poor mechanical strength and experience a large scattering loss and transmission loss although their evanescent field is strong. [35]. Microstructured fibers have difficulties in terms of fabrication and fusion splicing [36]. Moreover, it is difficult to fill and clean the small hole where the evanescent wave is excited with a gas or liquid. The FBG has a weak mechanical strength and rough surface after the chemical etching or side-polishing cladding removal process [37]. The long-period fiber grating allow evanescent field penetration but its fabrication requires high-precision manufacturing and specialized inscription equipment [38–40]. As a result, complex techniques, expensive equipment, and special materials are often essential to fabricate fiber-optic evanescent-field devices. Herein, to improve the performance of the fiber-optic evanescentfield device, we proposed a novel structure which provides a robust, small size and high-efficient excitation evanescent-field element with simple and low-cost material and manufacture. The capillary wall acts as a curved slab waveguide without cladding, which can easily excite high-order modes and provide a strong evanescent field which is actually part of modal fields of high-order modes. The strength of the evanescent field increases with modal order. Due to the strong evanescent field and large coating surface area of the capillary wall, this
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.snb.2019.127136 Received 30 July 2019; Received in revised form 9 September 2019; Accepted 11 September 2019 Available online 13 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 301 (2019) 127136
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Fig. 1. Fabrication diagram of the FOI.
the FOI device was washed with ethanol to remove the PS microspheres and dried. The monodispersity affects the self-assembly of the PS microsphere, i.e., the quality of the microsphere array template [41]. Poor dispersibility may lead to an unordered large-area arrangement of the microspheres on capillary surface, which will result in palladium membrane rather than palladium nanoparticles on capillary surface. Because the void between the microspheres determined the shape of the Pd nanoparticle, the diameter of the PS microsphere affects the size of the Pd nanoparticles [42]. As we know, the size of the noble metal nanoparticles effect its optical properties of localized surface plasmon resonance significantly. Since the surface plasmon resonance effect of Pd nanostructures mainly occurs at visible wavelength [43], the effect of the size of the Pd nanoparticles on the spectral response of the sensor can be ignored in our experiments.
evanescent-field device can be used as a versatile optical platform for liquid, solid and gas sensing. Moreover, its near-infrared operating wavelength is in the usual communication band, which avoids the requirements for special instrument facilities. 2. Materials and methods 2.1. Fabrication of the sensing structure We used standard single-mode fiber and a capillary with an inner diameter and outer diameter of 250 μm and 350 μm to fabricate the FOI device, respectively. The process flow of the device is shown in Fig. 1. The fiber and capillary was spliced by a fusion splicer (FSM-60 s, Fujikura). A splicer mode was edited to manufacture the structure, which is usually used to fabricate fiber mismatch structure. We adjusted the parameter of lateral offset to make the fiber end to be spliced to the capillary wall. The core and cladding diameters of the SMFs are 8.2 μm and 125 μm, respectively. The inner and outside diameters of the fusedsilica capillary (FSC) in the fiber are 250 μm and 350 μm, respectively (TSP250/350, Polymicro Inc). First, we cleaved a single SMF into two sections with flat and smooth end interfaces. Second, we attached a cleaved end-face of SMF and one end face of capillary together with a large intentional lateral offset by fusion splicing. The current for the electric arc is 35 mA and the arc duration time is 700 ms. Third, we cleaved the spliced capillary to a designed length, as illustrated in Fig. 1. At last, we spliced another section of cleaved SMF to the cleaved end-face of capillary with the same value of intentional lateral offset and opposite offset direction compare with the first offset, as illustrated in Fig. 1. Thereafter, the capillary portion was immersed in piranha solution for 30 min to treat the surface of the capillary wall. After rinsing with water, we immersed the FOI device in a beaker containing deionized water. A polystyrene (PS) microsphere solution (monodispersity of 4%) with a diameter of 719.7 nm was dropped into a beaker. The PS microspheres formed a close packed monolayer on the surface of the deionized water. By using a vertical pulling method under constant temperature evaporation, PS microspheres attached to the surface of the capillary wall to form a mask layer. With the combination of vapor pressure and liquid surface tension, the PS microspheres self-assemble into colloidal crystals on the surface of the capillary wall. Then, the FOI device was fixed in a rotatable U-shaped slot of a magnetron sputtering machine to be coated with a Pd film of 12 nm. After the coating process,
2.2. Measurement setup Humidity sensing: We connected the sensor to a custom FP filterbased fiber-optic sensing interrogator with a built-in light source to detect output light. The interrogator has a resolution of up to 4 pm and a detection range from 1510 to 1590 nm. The lead-in fiber and lead-out fiber were connected to the output and input of the fiber-optic sensing interrogator, respectively. In the humidity sensing section, the fiberoptic sensor was placed in a humidity chamber with a constant temperature of 25 °C and humidity range from 25% RH to 80% RH. A computer equipped with a data acquisition card was connected to the interrogator for spectral analysis. The detected spectrum was recorded and processed through a program developed based on the Labview software. Hydrogen sensing: The optical system was slightly different for symmetrical and asymmetrical fiber-capillary structures. For the symmetrical structure, we used a fiber-optic sensing interrogator just like the humidity experiment. However, we used an optical spectrum analyzer and a broadband light source for the asymmetric structure. Highpurity nitrogen and high-purity hydrogen were used as gas sources to prepare different concentrations of hydrogen. We used two mass flow meters to control the respective flow rates of nitrogen and hydrogen, and used a Y-type connector to let nitrogen and hydrogen meet and mix. Flow Cell: A plastic tube with import and export interface was used as the flow cell. The sensor was packaged in the flow cell and held tight 2
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Fig. 2. (a) Schematic diagram of the FOI device with a symmetrical structure. (b) The simulation results show the electromagnetic field distribution of the crosssection of the capillary wall at four different distances. The light emitted from the single-mode fiber gradually spreads along the capillary wall. (c) As the light propagates forward, the optical field fills the entire cross-section, which means that the light propagates around the capillary wall and spirals forward.
capillary. Because the capillary without cladding results in transmission loss, the intensity of the transmitted light decreases with the increase in the capillary length. (Besides, since we used the lead-out fiber to receive the outgoing light, only a small amount of light wall was collected while most of the light was dissipated when they reached the capillary end face and enter into the air, which also influence intensity of the transmitted light.) In the experiment, we recorded transmissions of two FOI devices with capillary lengths of 0.8 mm and 1.6 mm. From the transmission spectra shown in Figs. 3a and 3b, we can see that the transmission intensity of the device with a short capillary is higher than that of the device with a long capillary. In addition, each valley in the transmission of the device with a short capillary is unique in shape, indicating that various modes are excited by and guided through the device with a short capillary and finally contribute to the interference spectrum. In contrast, the transmission of the device with a long capillary is regular and similar to Fabry-Perot interference fringes, which means that the device with a long capillary allows few modes to propagate. We also modeled and simulated the transmissions of the two devices, as shown in Figs. 3c and 3d. Compared with the experimental spectra, it can be found that the simulated spectrum results are close to the experimental spectra. As we know, because the optical fiber structure was designed to confine the light in core with low loss, it is difficult to obtain high-order modes and strong evanescent field, which leads to the conventional optical fiber sensor insensitive to humidity [44–46]. Thus, to experimentally investigate the properties of the evanescent field at the capillary surface, we applied the two devices to relative humidity sensing to evaluate the sensitivities of the evanescent field to the external surrounding environment by monitoring their spectral responses. To achieve this, the FOI devices were placed in a humidity chamber with a temperature at 25 °C in turn. We used an optical sensing interrogator with a resolution of 4 pm for spectral detection. Data acquisition and processing was performed by a computer equipped with a data acquisition card and Labview software. Fig. 4a shows a schematic diagram of the relative humidity and the tensile endurance test setup. First, we tested the robustness of the FOI device. As the axial tension is increased from 0 N to 2 N, the dip in the transmission of the FOI device exhibits a blueshift, as shown in the insert. Then, we used the FOI device with a short capillary to measure the relative humidity of 30% RH, 40%RH, 50%RH, 60%RH, and 70%RH. The recorded response spectra are shown in Fig. 4b, which illustrates that the wavelengths of the dips at 1538 nm and 1555 nm exhibit blueshifts with relative humidity. We
to avoid possible bending that would affect hydrogen response. Air refractive index measurement: In the optical system section, we still used the broadband light source and optical spectrum analyzer as light sources and detectors. We modified a large size syringe and used it as a gas chamber to create different air refractive index by moving the piston rod to change the volume of the gas chamber. 3. Experimental results and discussion 3.1. Humidity measurement Fig. 2a shows the scheme of the FOI device. Light propagated through a lead-in single-mode fiber as a fundamental mode until it reached the capillary wall. In the capillary wall, the light was no longer constrained by the fiber-optic structure, and multiple high-order modes were excited. To visually describe the propagation of light in the capillary wall, we used a commercial software and beam propagation method to simulate the optical behavior. The cross-section between the two single-mode fibers shown in Fig. 2b presents the path of the meridional rays. The high-order modes propagate forward through multiple reflections between the inner and outer surfaces of the capillary wall. From the illustration, we can see that the evanescent field penetrates into the external medium. The oblique rays shown in Fig. 2c spread along the capillary wall as high-order modes and coil forward around the capillary. Finally, part of the light propagates through the capillary wall and enters into the core of the lead-out fiber, while the rest of the light is dissipated in the cladding or air after propagating a few centimeters. The evanescent wave excited at the capillary surface can be expressed as: 2 ⎧ ⎤⎫ ⎡ 2π sini ⎞ 2π x sini1 ⎞ ⎤ ⋅ E ⃑ = E0⃑ exp ⎢− z ⎛ − 1 ⎥ ⋅exp ⎡−i ⎛ωt − ⎢ ⎝ ⎬ ⎨ λ n21 ⎠ λ n21 ⎠ ⎥ ⎝ ⎣ ⎦ ⎦⎭ ⎣ ⎩ ⎜
⎟
⎜
⎟
(1)
where λ is the incident wavelength. i and ω are the incident angle and angular frequency of the incident light. n21 is the ratio of the refractive indices of the capillary and the external environment. x is the distance along the surface of the tube wall, and z is the vertical distance from the surface of the capillary. E0⃑ is the incident light wave. By the amplitude
( )
2
2π sini − 1 ⎤, we can find that the enhancement factor E0⃑ exp ⎡− λ z n 21 ⎢ ⎥ ⎣ ⎦ intensity of the evanescent wave is related to the angle of incidence, the relative refractive index, and the distance from the surface of the
3
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Fig. 3. Fig. 3 (a) and (b) Experimental spectra of the FOI devices with a short capillary (0.8 mm) and a long capillary (1.6 mm) in air. (c) and (d) Simulated spectra of the FOI device with a short capillary and a long capillary in air.
graphically represent the relationship between the wavelength shifts of the dips and the relative humidity in Fig. 4c. The wavelength shifts of the dips increase with the relative humidity. The wavelength shifts of the dips can be attributed to the refractive index change of the air. When the relative humidity increased from 30%RH-70%RH, more water molecules attached to the surface of the capillary wall, which increased the refractive index of the environment surrounding the capillary, shifting the wavelength of the dips to longer wavelengths. Moreover, the wavelength shifts of the dips are linearly related to the relative humidity, and their sensitivities are estimated to be 0.03 nm/% RH and 0.12 nm/%RH, respectively. Thus, this device can excite an evanescent field that is strong enough to detect a relative humidity change without sensitive coating, which are indispensable for most fiber-optic humidity sensors [47–52]. Then, we also tested the relative humidity response of the FOI device with a long capillary. The experimental result is shown in Fig. 4d. We selected three sharp dips, and the data are plotted in Fig. 4e. Unlike the FOI device with a short capillary, as described above, the evanescent-field device with a long capillary has no significant response to a relative humidity variation. This can be attributed to the fact that the long capillary does not favor light propagation in the capillary wall with a small incident angle because of the lack of cladding. Therefore, in the FOI device with a long capillary, the low-order mode and high-order modes with large incident angles can guide through the capillary wall and enter the lead-out fiber, which cannot excite a strong evanescent field and results in a low relative humidity sensitivity.
εPd (c %) = h (c %) × εPd (0%) . εPd (0%) is the complex permittivity of the Pd film for a hydrogen concentration of 0%. To determine the complex permittivity of the Pd film at a certain concentration, we need to obtain the coefficient h (c %) , which is a decreasing nonlinear function. We assume that h (c %) is continuous in the interval of 0–4%. h (c %) is 1 and 0.8 for hydrogen concentrations of 0% and 4%, respectively.47 According to the general definition of the derivative of the function, lim Δ c %→ 0
Δh (c %) h (c %) − h (c0 %) = lim Δ c %→ 0 Δc % c %− c0 %
(2)
Therefore, when c0 %= 0 , we can obtain h (c %) = 1 + c % From the above formula, h (c0 %) can be expressed by Because
( ) and ( Δλ Δ c%
and simulation
(
Δλ Δ h (c %)
Δ h (c %) Δ c%
(
Δ h (c %) Δ c%
).
and c0 %.
)
Δλ can be obtained through the experimental Δ h (c %) Δ h (c %) results, can be calculated by the ratio of Δ h (c%) and Δ c% Δ c%
). Thus, the complex permittivity of Pd at a certain concentration
c0 % can be determined through the experimental and simulation results. However, the response of the fiber-optic hydrogen sensor generally results from a complex permittivity decrease and the compressive stress of the expansion of the Pd film due to hydrogen [55]. Furthermore, the response caused by the compressive stress of the Pd film may be dominant for stress-sensitive fiber-optic sensors, such as fiber grating sensors, SMS sensors, tapered fiber sensors and side polished D-shaped fiber sensors [56–59]. In this case, if we want to estimate the complex permittivity of the Pd film by comparing the experimental and simulation results, it is necessary to consider the response caused by the compressive stress in the simulation or to eliminate the influence of the expansion effect in the experiment. Thus, we used a nanopatterned Pd film covering the capillary wall to weaken the compressive stress caused by the Pd film expansion (Figs. 5a-5e). Pd nanoparticles have weaker expansion and shrinkage
3.2. Evaluation of the permittivity of palladium in hydrogen According to previous works [53,54], the complex permittivity of the Pd material can be estimated by comparing the experimental and simulation results of the hydrogen sensor. For a hydrogen concentration of c%, the complex permittivity of the Pd film can be expressed by 4
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Fig. 4. Fig. 4 (a) Humidity sensing and axial tensile testing setup. (b) Spectral response of the FOI device with short a capillary in different humidity environments. (c) Relationships between the wavelength shifts of the dips and the relative humidity. (d) Spectral response of the FOI device with a long capillary in different humidity environments. (e) Relationships between the wavelength shifts of the dips and the relative humidity.
effects than the Pd film because of their high plasticity. As shown in Figs. 5f and 5 g, the proposed hydrogen sensing device was manufactured by splicing two single-mode fibers symmetrically at the ends of a capillary covered with the nanopatterned Pd film. The Pd film was deposited on the capillary wall by magnetron sputtering coating, and its thickness was controlled by the sputtering current and time. As a comparative experiment, we examined the spectral response of a FOI device with a 0.8 mm capillary covered by a complete Pd film without using a PS microsphere array as a mask. We used the fiber-optic sensing demodulation system used in the humidity measurements to detect the spectrum. We tested hydrogen concentrations of 1%, 2%, and 4%. We recorded the spectral response during the experiment as shown in Figs. 6a-6c. With the introduction of hydrogen, the dip in the
spectrum at 1550 nm shifts to lower wavelengths, and the wavelength shift of the dip increases with the hydrogen concentration. The blueshift of the dip arises from two mechanisms: the hydrogen-induced permittivity decrease of the Pd film on the capillary surface and the stress applied to the surface of the capillary wall caused by the expansion of the Pd film. (The experimental result in Fig. 4a also shows that the wavelength of the dip exhibits a blueshift when the FOI device receives a tensile force). Thereafter, we tested a fiber-capillary FOI device with a nanopatterned Pd film in hydrogen. Figs. 6d-6f show the spectral responses of the device to hydrogen at different concentrations. We find that the transmitted light intensity increases and the wavelength shift of the dip is significantly smaller than that of the device with the complete Pd film 5
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Fig. 5. Surface of the capillary wall by Pd coating and preliminary cleaning. The nanopatterned Pd film and unremoved PS microspheres can be observed. (f)–(g) Schematic diagram of the hydrogen sensor based on a FOI device and its sensing mechanism.
relationship between the wavelength shift Δλ and the coefficient h (c %) . The functions are expressed as ΔλDip1 = 5 h (c %) -5 and ΔλDip2 = 25
shown in Figs. 6a-6c. The possible reason for this result is that the wavelength shift of the dip near 1550 nm is mainly caused by the compressive stress from the expansion of the Pd film. However, the stress applied to the capillary wall by the nanopatterned Pd film is small, and the dip in the transmission only exhibits a slight wavelength shift. Fig. 6g presents the relations between the hydrogen concentration and wavelength shifts of the two typical transmission dips of the device with the nanopatterned Pd film. The fitting curves show that the wavelength shifts of both dips are approximately linear with the hydrogen concentration. The functional relationships can be expressed as ΔλDip1
(
Δλ
)
h (c %) -25. Consequently, Δ h (c%) can be obtained by the slopes of the above functions. The complex permittivity of the Pd film can be calculated by using the above results. For example, because of
( ) = −127 Δλ Δ c% Δ h (c %) Δ c%
and
(
Δλ Δ h (c %)
) = 25
for dip2, it can be found that
= −5.08. Considering the εPd (0%) = 3.35 − i8.25 at 1550 nm [58], the complex permittivity of the Pd film is calculated as εPd (c %) = (1 − 5.08c %) εPd (0%) = (1 − 5.08c %)(3.35 − i8.25) .
( ) Δλ
= -0.16 c% -0.26 and ΔλDip2 = -1.27 c% -0.69. Δ c% can be obtained by multiplying the slopes of the above functions by 100. Because the area of the nanopatterned film is approximately a quarter of the complete film under ideal conditions, we substituted the nanopatterned Pd film with an equivalent area of the complete Pd film in the simulation to simplify the calculation. The simulation results in Fig. 6(h) show the simulated spectra of the sensor as a function of the coefficient h (c %) in the range of 1-0.96. The simulated spectrum continuously shifts to lower wavelengths as h (c %) decreases. The wavelength shifts of dip1 and dip2 are shown in Fig. 6i. There is a linear
3.3. Hydrogen sensing and air refractive index measurement In addition to the FOI device with a symmetrical structure, we also prepared a fiber-capillary device with an asymmetric structure that can be used as a MZI. The two arms of the MZI are the capillary wall covered by the nanopatterned Pd film on one side and the untreated capillary wall on the other side. The short diffusion length and large surface-to-volume ratio of the Pd nanoparticles are advantageous for the rapid absorption and desorption of hydrogen. As before, we also used a PS microsphere array as a mask and magnetron sputtering 6
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Fig. 6. Spectral response of the FOI device with the capillary completely covered by Pd film to different concentrations of hydrogen. (d)–(f) Spectral response of the FOI device with the capillary covered by a nanopatterned Pd film to different concentrations of hydrogen. (g) Wavelength shifts of the dips versus the hydrogen concentration. (h) Simulated spectral response of the FOI device to different values of h (c %) . (i) The wavelength shift of the dips increases linearly with h (c %) .
coating to coat a nanopatterned Pd film on the capillary wall. Because the complex permittivity of Pd changes with the hydrogen concentration, the capillary wall covered by the Pd film can be used as a sensing arm for hydrogen sensing. A reversible reaction between hydrogen and the Pd film influences the effective refractive index and optical path difference between the modes propagating in the capillary wall through the evanescent field. As a result, the hydrogen concentration can be monitored by detecting the transmission change of the MZI. Considering the transmission loss caused by the absence of cladding, we chose an MZI with a 0.5 mm long capillary in the sensing experiments. Figs. 7a and 7b are schematic diagrams of the fiber-capillary MZI. From the simulation results in Fig. 7c, it can be seen that the light from the single-mode fiber entered the capillary wall and was divided into two paths. One beam propagated along the capillary wall covered by the nanopatterned Pd film, and the other beam propagated along the raw capillary wall. Part of the electromagnetic field can enter the environment outside the capillary wall, as shown in the insets. When the two beams reached the core of the lead-out fiber, they interfered with each other due to the optical path difference and phase difference. We ignored the influence of the extinction effect of the Pd film to simplify the simulation. The interference of the MZI can be expressed as [60,61]:
Iout = Iout1 + Iout 2 + 2 Iout1 Iout 2 cos ⎛ ⎝
2π ΔnL + φ0⎞ λ ⎠
The hydrogen sensor test system consisted of hydrogen and gas sources, gas flow control systems, sensors, laser sources and spectrometers, as shown in Fig. 8a. High-purity hydrogen and nitrogen were used as the hydrogen gas source and diluent gas, respectively. Two mass flow controllers were used to control the flow of the hydrogen and nitrogen to produce a mixed gas. In the experiment, the MZI sensor was placed in a sealed gas chamber and connected to an optical spectrum analyzer and a 980 nm laser. During the test and recovery phase, we continuously introduced the hydrogen-nitrogen mixed gas until the sensor signal reached a steady state. Fig. 8b shows a four-cycle response of the MZI sensor at an ambient temperature of 20 ± 2 °C and hydrogen concentration of 0.25%. The signal changed immediately and reached a steady state at approximately 20 s when hydrogen was introduced into the flow cell. The light intensity can return to the initial level after continuously introducing nitrogen for 1 min. This means that there is a reversible reaction between the Pd film on the capillary wall and hydrogen. The test results show that the sensor has high sensitivity, a fast response and good recovery for hydrogen sensing. To further investigate the spectral response, we introduced hydrogen at concentrations of 0.5%, 1%, 2%, and 4%. By comparing Figs. 8c-8 g, we can see that the intensity was reduced as the hydrogen concentration increased. We plot the functional relationship between the light intensity and the concentration in Fig. 8h. The experimental data linearly fit well in the interval of 0.25%–2%. The relationship can be expressed as R = 0.095 C -0.028, where R and C are the light intensity and concentration, respectively. However, the quadratic fit is better for the 0.25%–4% interval. The relationship between the hydrogen concentration and the response is R = 0.013 C2 -0.095 C -0.028. The
(3)
where Iout1 and Iout2 are the light intensities of the two beams. Δn is the difference between the effective refractive indices of the two beams. The variables λ , φ0 and L represent the wavelength, initial phase and propagation distance, respectively. 7
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Fig. 7. (a)–(b) Schematic and working principle of the MZI for hydrogen sensing. (c) Simulation of the light propagation path in the capillary wall.
every 30 mL increment. We calculated the air refractive index and the peak intensity of the spectrum as shown in Fig. 9(c). Because the air pressure gradually increased with the decrease in the air volume, it caused a change in the air refractive index, which led to an increase in the peak intensity. We plot the experimental data and perform linear fitting to estimate the sensitivity, as shown in Fig. 9(d). The relationship between the peak intensity (I) and the refractive index of the air (RAir) is I = 2642RAir-2641. In this case, the ratio of signal to noise of the OSA is -80dbm. Considering the peak intensity of the spectrum is about 5uw, the noise level is better than 0.2%. Thus the sensor can detect a refractive index change with a resolution of 7.57 × 10−7 RIU.
reason for this result may be that the rate of response slowed down and gradually became saturated as the hydrogen concentration increased. Because the Pd film does not interact with air and can isolate the capillary wall from the air, the MZI can be applied to an air refractive index measurement by using the capillary wall covered by the Pd film as the reference arm (Fig. 9a). According to Ref. [62], the empirical formula of the air refractive index can be represented as:
n=1+
2.8793p × 10−3 1 + 0.003671t
(4)
The air refractive index is approximately n = 1 + 0.00273p under normal pressure and room temperature conditions. p is the air pressure. From the ideal gas equation pv = nRT , we can change the air pressure by adjusting the air volume at constant temperature. Therefore, different refractive indices of air can be achieved by changing the pressure. The air chamber in the sensing system is shown in Fig. 9(b), which is modified from the piston part of a 300 mL syringe. We fixed the syringe on a bracket and enveloped the sensor in the chamber. The front end of the syringe was sealed with glue. We slowly pushed the piston to compress the chamber volume to increase the air pressure. The chamber volume can be determined by the scale on the syringe. V Therefore, the air pressure can be calculated as p1 = V0 p0 , where V1, 1 V0, and p0 are the current volume of air, the initial volume and the initial pressure, respectively. Substituting p1 in the formula of
4. Conclusions In summary, we demonstrated a simple and low-cost strategy to create a highly sensitive and versatile optical sensing platform. Through theoretical analysis and a numerical simulation, we described the sensing mechanism of the FOI device based on the modal interference and the interaction between the evanescent field and external environment. The FOI device with a symmetric structure detected a humidity variation without humidity sensitive coating and was used to evaluate the complex permittivity of the Pd film, while the partially nanoparticlecoated FOI with asymmetric structure could serve as a MZI with economical intensity demodulation for fast, reversible hydrogen sensing and air refractive index measurement. The transmittance of MZI decreased by 20% for a hydrogen concentration range from 0% to 4%. With the air refractive index increasing from 1.0027 to 1.0004, the transmittance of MZI increased by 35%, and the refractive index resolution was estimated to be 7.57 × 10−7 RIU. Because the proposed FOI in essential is an optical fiber evanescent field device, such an optical device will have potential to be applied to carbon dioxide and ammonia after coating a functional film layer, such as ZIF-8 and polyaniline carbon nanotubes. The FOI device has the advantages of
V p
n = 1 + 0.00273p gives n = 1 + 0.00273 V0 0 . 1 Before conducting the air refractive index measurement, we tested the air tightness of the chamber. We pushed the piston to a certain position and compressed the volume of the air. The peak intensity of the light increased and gradually tended to be stable, which proved that the chamber was airtight. The initial pressure of the chamber was atmospheric pressure. Thereafter, the piston was slowly pushed forward to maintain a quasi-equilibrium state within the chamber. Compressing the air volume from 300 mL to 180 mL, the spectrum was recorded at 8
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Fig. 8. (a) Hydrogen sensing setup. (b) Real-time reversible response of the MZI hydrogen sensor by cycling between nitrogen and hydrogen. (c)–(g) Spectrum comparison of the sensor at different concentrations of hydrogen and pure nitrogen. (h) Light intensity as a function of the hydrogen concentration.
Fig. 9. Air sensing principle of the MZI and (b) air refractive index sensing test setup. (c) Response of the fiber-capillary MZI and (d) peak intensity as a function of the air refractive index. 9
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low cost, simplicity, and versatility, making it an efficient, extensible and flexible platform for optical sensing.
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Sheng Bi received the Ph.D. degrees in department of electrical and computer engineering in University of Alabama, United States. He has been a Lecturer with the school of mechanical engineering, Dalian University of Technology. His research interests include High performance organic triode, organic solar cells and high brightness flexible light-emitting diode (OLED). Xuhui Zhang received his B.S. degree in Optical information science and technology from the Northwest University, China, in 2016. He is currently working toward the Ph.D. degree with Dalian University of Technology. His research interests are fiber-optic sensors and surface plasmon resonance sensors. Shimeng Chen received a B.S. degree in Dalian University of Technology Dalian, China, in 2014. She is currently pursuing her Ph. D. degree in optical engineering in Dalian University of Technology, Dalian, China. Her research interests include biochemical fiber sensors, nano-reinforcing SPR sensors.
Yun Liu received the B.S. and Ph.D. degrees in optical engineering in Dalian University of Technology Dalian, China. He has been a Lecturer with the school of physics and optoelectronic engineering, Dalian University of Technology. His research interests include optical fiber sensor, surface Plasmon resonance sensor.
Wei Peng received doctoral degree in optical engineering from Dalian University of Technology, Dalian, China in 1999. Then she joined Center for Photonics Technology within Department of Electrical Engineering at Virginia Polytechnic Institute and State University as worked as a Postdoctoral Associate. From 2004–2007, she worked as a Research Assistant Professor at Department of Chemistry and Biochemistry, Arizona State University. During 2007–2009, she worked as a Senior Research Scientist in Applied Technologies Division at Physical Optics Corporation, Inc. She took a professor faculty position in College of Physics and Optoelectronic Engineering at Dalian University of Technology. Her research interests include subwavelength optics, surface plasmonics, and surface plasmon resonance.
Ning Zhang is currently working toward the master degree with Dalian University of Technology. Her research interests include the optical fiber hydrogen sensor and its application in the field of industry. Ping Li received the BS degrees in practical chemistry from the Taiyuan normal university in 2018. She is currently working toward the MS degree with Dalian university of tchnology. Her research interests in optical fiber sensing.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nanopatterned evanescent-field fiber-optic interferometer as a versatile platform for gas sensing
T
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Yun Liua, , Ning Zhangb, Ping Lib, Sheng Bic, Xuhui Zhanga, Shimeng Chena, Wei Penga a
School of Physics, Dalian University of Technology, Dalian, 116024, China School of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology, Dalian, 116024, China c Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian, 116024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical fiber sensor Interferometer Evanescent field Hydrogen sensing Humidity measurement
We introduced a nanopatterned fiber-optic interferometer (FOI) based on an optical fiber line embedded with a short capillary which excites the evanescent field by total reflection of light in the wall waveguide interacting with the fluid and causing the optical signal change. As modal interference was generated with near-infrared light in the tube wall, we detected a reversible wavelength shifts of the proposed FOI in the interference fringe upon water vapor exposure by using an optical fiber interrogator. We also deposited a patterned palladium (Pd) nanoparticle film on the outer surface of the FOI by using self-assembly microspheres as a template and detected the reaction of the Pd membrane with hydrogen through monitoring the wavelength shifts of interference fringes. The Pd nanoparticle restrained the fiber stress response caused by hydrogen-induced expansion of the Pd film, and theoretically helped to improve the evaluation accuracy of the Pd permittivity in hydrogen. In addition, the nanoparticle-coated FOI with asymmetric structure could serve as a Mach-Zehnder interferometer (MZI) for fast hydrogen sensing and air refractive index measurement. Considering sensitivity and detector parameters, the FOI could provide a high resolution up to 7.57 × 10−7 RIU.
1. Introduction Fiber-optic (FO) evanescent-field technology has evolved significantly in the past few decades because of its advantages of high sensitivity, immunity to electromagnetic interference, all-fiber and hybrid integration capability [1–12]. A variety of processing techniques are used to modify optical fibers structure to excite strong evanescent field, such as core mismatched fibers [13,14], tapered fibers [15–17], D-shaped transmission fibers [18,19], S-shaped fibers [20], U-shaped fibers [21,22], microstructured fibers [23,24], micro/nanofibers [25,26] and fiber gratings [27,28]. However, these evanescent field sensor is struggling to balance the sensitivity, manufacturing process, cost and mechanical strength, which impose practical limitations to the development of practical FO evanescent field sensing technology. The core mismatched structure has a weak evanescent-field strength and a relatively low sensitivity [29], although it has a simple manufacturing process. Because of the difficult manufacturing of the tapered region, it is hard to ensure a high production consistency for tapered structure [30,31]. D-shaped fibers have a poor mechanical strength which restricts their performance [32]. S-shaped and U-shaped sensors are not easily packaged and a small bending radius leads to a high fragility
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[33,34]. Micro/nanofibers have poor mechanical strength and experience a large scattering loss and transmission loss although their evanescent field is strong. [35]. Microstructured fibers have difficulties in terms of fabrication and fusion splicing [36]. Moreover, it is difficult to fill and clean the small hole where the evanescent wave is excited with a gas or liquid. The FBG has a weak mechanical strength and rough surface after the chemical etching or side-polishing cladding removal process [37]. The long-period fiber grating allow evanescent field penetration but its fabrication requires high-precision manufacturing and specialized inscription equipment [38–40]. As a result, complex techniques, expensive equipment, and special materials are often essential to fabricate fiber-optic evanescent-field devices. Herein, to improve the performance of the fiber-optic evanescentfield device, we proposed a novel structure which provides a robust, small size and high-efficient excitation evanescent-field element with simple and low-cost material and manufacture. The capillary wall acts as a curved slab waveguide without cladding, which can easily excite high-order modes and provide a strong evanescent field which is actually part of modal fields of high-order modes. The strength of the evanescent field increases with modal order. Due to the strong evanescent field and large coating surface area of the capillary wall, this
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.snb.2019.127136 Received 30 July 2019; Received in revised form 9 September 2019; Accepted 11 September 2019 Available online 13 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Fabrication diagram of the FOI.
the FOI device was washed with ethanol to remove the PS microspheres and dried. The monodispersity affects the self-assembly of the PS microsphere, i.e., the quality of the microsphere array template [41]. Poor dispersibility may lead to an unordered large-area arrangement of the microspheres on capillary surface, which will result in palladium membrane rather than palladium nanoparticles on capillary surface. Because the void between the microspheres determined the shape of the Pd nanoparticle, the diameter of the PS microsphere affects the size of the Pd nanoparticles [42]. As we know, the size of the noble metal nanoparticles effect its optical properties of localized surface plasmon resonance significantly. Since the surface plasmon resonance effect of Pd nanostructures mainly occurs at visible wavelength [43], the effect of the size of the Pd nanoparticles on the spectral response of the sensor can be ignored in our experiments.
evanescent-field device can be used as a versatile optical platform for liquid, solid and gas sensing. Moreover, its near-infrared operating wavelength is in the usual communication band, which avoids the requirements for special instrument facilities. 2. Materials and methods 2.1. Fabrication of the sensing structure We used standard single-mode fiber and a capillary with an inner diameter and outer diameter of 250 μm and 350 μm to fabricate the FOI device, respectively. The process flow of the device is shown in Fig. 1. The fiber and capillary was spliced by a fusion splicer (FSM-60 s, Fujikura). A splicer mode was edited to manufacture the structure, which is usually used to fabricate fiber mismatch structure. We adjusted the parameter of lateral offset to make the fiber end to be spliced to the capillary wall. The core and cladding diameters of the SMFs are 8.2 μm and 125 μm, respectively. The inner and outside diameters of the fusedsilica capillary (FSC) in the fiber are 250 μm and 350 μm, respectively (TSP250/350, Polymicro Inc). First, we cleaved a single SMF into two sections with flat and smooth end interfaces. Second, we attached a cleaved end-face of SMF and one end face of capillary together with a large intentional lateral offset by fusion splicing. The current for the electric arc is 35 mA and the arc duration time is 700 ms. Third, we cleaved the spliced capillary to a designed length, as illustrated in Fig. 1. At last, we spliced another section of cleaved SMF to the cleaved end-face of capillary with the same value of intentional lateral offset and opposite offset direction compare with the first offset, as illustrated in Fig. 1. Thereafter, the capillary portion was immersed in piranha solution for 30 min to treat the surface of the capillary wall. After rinsing with water, we immersed the FOI device in a beaker containing deionized water. A polystyrene (PS) microsphere solution (monodispersity of 4%) with a diameter of 719.7 nm was dropped into a beaker. The PS microspheres formed a close packed monolayer on the surface of the deionized water. By using a vertical pulling method under constant temperature evaporation, PS microspheres attached to the surface of the capillary wall to form a mask layer. With the combination of vapor pressure and liquid surface tension, the PS microspheres self-assemble into colloidal crystals on the surface of the capillary wall. Then, the FOI device was fixed in a rotatable U-shaped slot of a magnetron sputtering machine to be coated with a Pd film of 12 nm. After the coating process,
2.2. Measurement setup Humidity sensing: We connected the sensor to a custom FP filterbased fiber-optic sensing interrogator with a built-in light source to detect output light. The interrogator has a resolution of up to 4 pm and a detection range from 1510 to 1590 nm. The lead-in fiber and lead-out fiber were connected to the output and input of the fiber-optic sensing interrogator, respectively. In the humidity sensing section, the fiberoptic sensor was placed in a humidity chamber with a constant temperature of 25 °C and humidity range from 25% RH to 80% RH. A computer equipped with a data acquisition card was connected to the interrogator for spectral analysis. The detected spectrum was recorded and processed through a program developed based on the Labview software. Hydrogen sensing: The optical system was slightly different for symmetrical and asymmetrical fiber-capillary structures. For the symmetrical structure, we used a fiber-optic sensing interrogator just like the humidity experiment. However, we used an optical spectrum analyzer and a broadband light source for the asymmetric structure. Highpurity nitrogen and high-purity hydrogen were used as gas sources to prepare different concentrations of hydrogen. We used two mass flow meters to control the respective flow rates of nitrogen and hydrogen, and used a Y-type connector to let nitrogen and hydrogen meet and mix. Flow Cell: A plastic tube with import and export interface was used as the flow cell. The sensor was packaged in the flow cell and held tight 2
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Fig. 2. (a) Schematic diagram of the FOI device with a symmetrical structure. (b) The simulation results show the electromagnetic field distribution of the crosssection of the capillary wall at four different distances. The light emitted from the single-mode fiber gradually spreads along the capillary wall. (c) As the light propagates forward, the optical field fills the entire cross-section, which means that the light propagates around the capillary wall and spirals forward.
capillary. Because the capillary without cladding results in transmission loss, the intensity of the transmitted light decreases with the increase in the capillary length. (Besides, since we used the lead-out fiber to receive the outgoing light, only a small amount of light wall was collected while most of the light was dissipated when they reached the capillary end face and enter into the air, which also influence intensity of the transmitted light.) In the experiment, we recorded transmissions of two FOI devices with capillary lengths of 0.8 mm and 1.6 mm. From the transmission spectra shown in Figs. 3a and 3b, we can see that the transmission intensity of the device with a short capillary is higher than that of the device with a long capillary. In addition, each valley in the transmission of the device with a short capillary is unique in shape, indicating that various modes are excited by and guided through the device with a short capillary and finally contribute to the interference spectrum. In contrast, the transmission of the device with a long capillary is regular and similar to Fabry-Perot interference fringes, which means that the device with a long capillary allows few modes to propagate. We also modeled and simulated the transmissions of the two devices, as shown in Figs. 3c and 3d. Compared with the experimental spectra, it can be found that the simulated spectrum results are close to the experimental spectra. As we know, because the optical fiber structure was designed to confine the light in core with low loss, it is difficult to obtain high-order modes and strong evanescent field, which leads to the conventional optical fiber sensor insensitive to humidity [44–46]. Thus, to experimentally investigate the properties of the evanescent field at the capillary surface, we applied the two devices to relative humidity sensing to evaluate the sensitivities of the evanescent field to the external surrounding environment by monitoring their spectral responses. To achieve this, the FOI devices were placed in a humidity chamber with a temperature at 25 °C in turn. We used an optical sensing interrogator with a resolution of 4 pm for spectral detection. Data acquisition and processing was performed by a computer equipped with a data acquisition card and Labview software. Fig. 4a shows a schematic diagram of the relative humidity and the tensile endurance test setup. First, we tested the robustness of the FOI device. As the axial tension is increased from 0 N to 2 N, the dip in the transmission of the FOI device exhibits a blueshift, as shown in the insert. Then, we used the FOI device with a short capillary to measure the relative humidity of 30% RH, 40%RH, 50%RH, 60%RH, and 70%RH. The recorded response spectra are shown in Fig. 4b, which illustrates that the wavelengths of the dips at 1538 nm and 1555 nm exhibit blueshifts with relative humidity. We
to avoid possible bending that would affect hydrogen response. Air refractive index measurement: In the optical system section, we still used the broadband light source and optical spectrum analyzer as light sources and detectors. We modified a large size syringe and used it as a gas chamber to create different air refractive index by moving the piston rod to change the volume of the gas chamber. 3. Experimental results and discussion 3.1. Humidity measurement Fig. 2a shows the scheme of the FOI device. Light propagated through a lead-in single-mode fiber as a fundamental mode until it reached the capillary wall. In the capillary wall, the light was no longer constrained by the fiber-optic structure, and multiple high-order modes were excited. To visually describe the propagation of light in the capillary wall, we used a commercial software and beam propagation method to simulate the optical behavior. The cross-section between the two single-mode fibers shown in Fig. 2b presents the path of the meridional rays. The high-order modes propagate forward through multiple reflections between the inner and outer surfaces of the capillary wall. From the illustration, we can see that the evanescent field penetrates into the external medium. The oblique rays shown in Fig. 2c spread along the capillary wall as high-order modes and coil forward around the capillary. Finally, part of the light propagates through the capillary wall and enters into the core of the lead-out fiber, while the rest of the light is dissipated in the cladding or air after propagating a few centimeters. The evanescent wave excited at the capillary surface can be expressed as: 2 ⎧ ⎤⎫ ⎡ 2π sini ⎞ 2π x sini1 ⎞ ⎤ ⋅ E ⃑ = E0⃑ exp ⎢− z ⎛ − 1 ⎥ ⋅exp ⎡−i ⎛ωt − ⎢ ⎝ ⎬ ⎨ λ n21 ⎠ λ n21 ⎠ ⎥ ⎝ ⎣ ⎦ ⎦⎭ ⎣ ⎩ ⎜
⎟
⎜
⎟
(1)
where λ is the incident wavelength. i and ω are the incident angle and angular frequency of the incident light. n21 is the ratio of the refractive indices of the capillary and the external environment. x is the distance along the surface of the tube wall, and z is the vertical distance from the surface of the capillary. E0⃑ is the incident light wave. By the amplitude
( )
2
2π sini − 1 ⎤, we can find that the enhancement factor E0⃑ exp ⎡− λ z n 21 ⎢ ⎥ ⎣ ⎦ intensity of the evanescent wave is related to the angle of incidence, the relative refractive index, and the distance from the surface of the
3
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Fig. 3. Fig. 3 (a) and (b) Experimental spectra of the FOI devices with a short capillary (0.8 mm) and a long capillary (1.6 mm) in air. (c) and (d) Simulated spectra of the FOI device with a short capillary and a long capillary in air.
graphically represent the relationship between the wavelength shifts of the dips and the relative humidity in Fig. 4c. The wavelength shifts of the dips increase with the relative humidity. The wavelength shifts of the dips can be attributed to the refractive index change of the air. When the relative humidity increased from 30%RH-70%RH, more water molecules attached to the surface of the capillary wall, which increased the refractive index of the environment surrounding the capillary, shifting the wavelength of the dips to longer wavelengths. Moreover, the wavelength shifts of the dips are linearly related to the relative humidity, and their sensitivities are estimated to be 0.03 nm/% RH and 0.12 nm/%RH, respectively. Thus, this device can excite an evanescent field that is strong enough to detect a relative humidity change without sensitive coating, which are indispensable for most fiber-optic humidity sensors [47–52]. Then, we also tested the relative humidity response of the FOI device with a long capillary. The experimental result is shown in Fig. 4d. We selected three sharp dips, and the data are plotted in Fig. 4e. Unlike the FOI device with a short capillary, as described above, the evanescent-field device with a long capillary has no significant response to a relative humidity variation. This can be attributed to the fact that the long capillary does not favor light propagation in the capillary wall with a small incident angle because of the lack of cladding. Therefore, in the FOI device with a long capillary, the low-order mode and high-order modes with large incident angles can guide through the capillary wall and enter the lead-out fiber, which cannot excite a strong evanescent field and results in a low relative humidity sensitivity.
εPd (c %) = h (c %) × εPd (0%) . εPd (0%) is the complex permittivity of the Pd film for a hydrogen concentration of 0%. To determine the complex permittivity of the Pd film at a certain concentration, we need to obtain the coefficient h (c %) , which is a decreasing nonlinear function. We assume that h (c %) is continuous in the interval of 0–4%. h (c %) is 1 and 0.8 for hydrogen concentrations of 0% and 4%, respectively.47 According to the general definition of the derivative of the function, lim Δ c %→ 0
Δh (c %) h (c %) − h (c0 %) = lim Δ c %→ 0 Δc % c %− c0 %
(2)
Therefore, when c0 %= 0 , we can obtain h (c %) = 1 + c % From the above formula, h (c0 %) can be expressed by Because
( ) and ( Δλ Δ c%
and simulation
(
Δλ Δ h (c %)
Δ h (c %) Δ c%
(
Δ h (c %) Δ c%
).
and c0 %.
)
Δλ can be obtained through the experimental Δ h (c %) Δ h (c %) results, can be calculated by the ratio of Δ h (c%) and Δ c% Δ c%
). Thus, the complex permittivity of Pd at a certain concentration
c0 % can be determined through the experimental and simulation results. However, the response of the fiber-optic hydrogen sensor generally results from a complex permittivity decrease and the compressive stress of the expansion of the Pd film due to hydrogen [55]. Furthermore, the response caused by the compressive stress of the Pd film may be dominant for stress-sensitive fiber-optic sensors, such as fiber grating sensors, SMS sensors, tapered fiber sensors and side polished D-shaped fiber sensors [56–59]. In this case, if we want to estimate the complex permittivity of the Pd film by comparing the experimental and simulation results, it is necessary to consider the response caused by the compressive stress in the simulation or to eliminate the influence of the expansion effect in the experiment. Thus, we used a nanopatterned Pd film covering the capillary wall to weaken the compressive stress caused by the Pd film expansion (Figs. 5a-5e). Pd nanoparticles have weaker expansion and shrinkage
3.2. Evaluation of the permittivity of palladium in hydrogen According to previous works [53,54], the complex permittivity of the Pd material can be estimated by comparing the experimental and simulation results of the hydrogen sensor. For a hydrogen concentration of c%, the complex permittivity of the Pd film can be expressed by 4
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Fig. 4. Fig. 4 (a) Humidity sensing and axial tensile testing setup. (b) Spectral response of the FOI device with short a capillary in different humidity environments. (c) Relationships between the wavelength shifts of the dips and the relative humidity. (d) Spectral response of the FOI device with a long capillary in different humidity environments. (e) Relationships between the wavelength shifts of the dips and the relative humidity.
effects than the Pd film because of their high plasticity. As shown in Figs. 5f and 5 g, the proposed hydrogen sensing device was manufactured by splicing two single-mode fibers symmetrically at the ends of a capillary covered with the nanopatterned Pd film. The Pd film was deposited on the capillary wall by magnetron sputtering coating, and its thickness was controlled by the sputtering current and time. As a comparative experiment, we examined the spectral response of a FOI device with a 0.8 mm capillary covered by a complete Pd film without using a PS microsphere array as a mask. We used the fiber-optic sensing demodulation system used in the humidity measurements to detect the spectrum. We tested hydrogen concentrations of 1%, 2%, and 4%. We recorded the spectral response during the experiment as shown in Figs. 6a-6c. With the introduction of hydrogen, the dip in the
spectrum at 1550 nm shifts to lower wavelengths, and the wavelength shift of the dip increases with the hydrogen concentration. The blueshift of the dip arises from two mechanisms: the hydrogen-induced permittivity decrease of the Pd film on the capillary surface and the stress applied to the surface of the capillary wall caused by the expansion of the Pd film. (The experimental result in Fig. 4a also shows that the wavelength of the dip exhibits a blueshift when the FOI device receives a tensile force). Thereafter, we tested a fiber-capillary FOI device with a nanopatterned Pd film in hydrogen. Figs. 6d-6f show the spectral responses of the device to hydrogen at different concentrations. We find that the transmitted light intensity increases and the wavelength shift of the dip is significantly smaller than that of the device with the complete Pd film 5
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Fig. 5. Surface of the capillary wall by Pd coating and preliminary cleaning. The nanopatterned Pd film and unremoved PS microspheres can be observed. (f)–(g) Schematic diagram of the hydrogen sensor based on a FOI device and its sensing mechanism.
relationship between the wavelength shift Δλ and the coefficient h (c %) . The functions are expressed as ΔλDip1 = 5 h (c %) -5 and ΔλDip2 = 25
shown in Figs. 6a-6c. The possible reason for this result is that the wavelength shift of the dip near 1550 nm is mainly caused by the compressive stress from the expansion of the Pd film. However, the stress applied to the capillary wall by the nanopatterned Pd film is small, and the dip in the transmission only exhibits a slight wavelength shift. Fig. 6g presents the relations between the hydrogen concentration and wavelength shifts of the two typical transmission dips of the device with the nanopatterned Pd film. The fitting curves show that the wavelength shifts of both dips are approximately linear with the hydrogen concentration. The functional relationships can be expressed as ΔλDip1
(
Δλ
)
h (c %) -25. Consequently, Δ h (c%) can be obtained by the slopes of the above functions. The complex permittivity of the Pd film can be calculated by using the above results. For example, because of
( ) = −127 Δλ Δ c% Δ h (c %) Δ c%
and
(
Δλ Δ h (c %)
) = 25
for dip2, it can be found that
= −5.08. Considering the εPd (0%) = 3.35 − i8.25 at 1550 nm [58], the complex permittivity of the Pd film is calculated as εPd (c %) = (1 − 5.08c %) εPd (0%) = (1 − 5.08c %)(3.35 − i8.25) .
( ) Δλ
= -0.16 c% -0.26 and ΔλDip2 = -1.27 c% -0.69. Δ c% can be obtained by multiplying the slopes of the above functions by 100. Because the area of the nanopatterned film is approximately a quarter of the complete film under ideal conditions, we substituted the nanopatterned Pd film with an equivalent area of the complete Pd film in the simulation to simplify the calculation. The simulation results in Fig. 6(h) show the simulated spectra of the sensor as a function of the coefficient h (c %) in the range of 1-0.96. The simulated spectrum continuously shifts to lower wavelengths as h (c %) decreases. The wavelength shifts of dip1 and dip2 are shown in Fig. 6i. There is a linear
3.3. Hydrogen sensing and air refractive index measurement In addition to the FOI device with a symmetrical structure, we also prepared a fiber-capillary device with an asymmetric structure that can be used as a MZI. The two arms of the MZI are the capillary wall covered by the nanopatterned Pd film on one side and the untreated capillary wall on the other side. The short diffusion length and large surface-to-volume ratio of the Pd nanoparticles are advantageous for the rapid absorption and desorption of hydrogen. As before, we also used a PS microsphere array as a mask and magnetron sputtering 6
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Fig. 6. Spectral response of the FOI device with the capillary completely covered by Pd film to different concentrations of hydrogen. (d)–(f) Spectral response of the FOI device with the capillary covered by a nanopatterned Pd film to different concentrations of hydrogen. (g) Wavelength shifts of the dips versus the hydrogen concentration. (h) Simulated spectral response of the FOI device to different values of h (c %) . (i) The wavelength shift of the dips increases linearly with h (c %) .
coating to coat a nanopatterned Pd film on the capillary wall. Because the complex permittivity of Pd changes with the hydrogen concentration, the capillary wall covered by the Pd film can be used as a sensing arm for hydrogen sensing. A reversible reaction between hydrogen and the Pd film influences the effective refractive index and optical path difference between the modes propagating in the capillary wall through the evanescent field. As a result, the hydrogen concentration can be monitored by detecting the transmission change of the MZI. Considering the transmission loss caused by the absence of cladding, we chose an MZI with a 0.5 mm long capillary in the sensing experiments. Figs. 7a and 7b are schematic diagrams of the fiber-capillary MZI. From the simulation results in Fig. 7c, it can be seen that the light from the single-mode fiber entered the capillary wall and was divided into two paths. One beam propagated along the capillary wall covered by the nanopatterned Pd film, and the other beam propagated along the raw capillary wall. Part of the electromagnetic field can enter the environment outside the capillary wall, as shown in the insets. When the two beams reached the core of the lead-out fiber, they interfered with each other due to the optical path difference and phase difference. We ignored the influence of the extinction effect of the Pd film to simplify the simulation. The interference of the MZI can be expressed as [60,61]:
Iout = Iout1 + Iout 2 + 2 Iout1 Iout 2 cos ⎛ ⎝
2π ΔnL + φ0⎞ λ ⎠
The hydrogen sensor test system consisted of hydrogen and gas sources, gas flow control systems, sensors, laser sources and spectrometers, as shown in Fig. 8a. High-purity hydrogen and nitrogen were used as the hydrogen gas source and diluent gas, respectively. Two mass flow controllers were used to control the flow of the hydrogen and nitrogen to produce a mixed gas. In the experiment, the MZI sensor was placed in a sealed gas chamber and connected to an optical spectrum analyzer and a 980 nm laser. During the test and recovery phase, we continuously introduced the hydrogen-nitrogen mixed gas until the sensor signal reached a steady state. Fig. 8b shows a four-cycle response of the MZI sensor at an ambient temperature of 20 ± 2 °C and hydrogen concentration of 0.25%. The signal changed immediately and reached a steady state at approximately 20 s when hydrogen was introduced into the flow cell. The light intensity can return to the initial level after continuously introducing nitrogen for 1 min. This means that there is a reversible reaction between the Pd film on the capillary wall and hydrogen. The test results show that the sensor has high sensitivity, a fast response and good recovery for hydrogen sensing. To further investigate the spectral response, we introduced hydrogen at concentrations of 0.5%, 1%, 2%, and 4%. By comparing Figs. 8c-8 g, we can see that the intensity was reduced as the hydrogen concentration increased. We plot the functional relationship between the light intensity and the concentration in Fig. 8h. The experimental data linearly fit well in the interval of 0.25%–2%. The relationship can be expressed as R = 0.095 C -0.028, where R and C are the light intensity and concentration, respectively. However, the quadratic fit is better for the 0.25%–4% interval. The relationship between the hydrogen concentration and the response is R = 0.013 C2 -0.095 C -0.028. The
(3)
where Iout1 and Iout2 are the light intensities of the two beams. Δn is the difference between the effective refractive indices of the two beams. The variables λ , φ0 and L represent the wavelength, initial phase and propagation distance, respectively. 7
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Fig. 7. (a)–(b) Schematic and working principle of the MZI for hydrogen sensing. (c) Simulation of the light propagation path in the capillary wall.
every 30 mL increment. We calculated the air refractive index and the peak intensity of the spectrum as shown in Fig. 9(c). Because the air pressure gradually increased with the decrease in the air volume, it caused a change in the air refractive index, which led to an increase in the peak intensity. We plot the experimental data and perform linear fitting to estimate the sensitivity, as shown in Fig. 9(d). The relationship between the peak intensity (I) and the refractive index of the air (RAir) is I = 2642RAir-2641. In this case, the ratio of signal to noise of the OSA is -80dbm. Considering the peak intensity of the spectrum is about 5uw, the noise level is better than 0.2%. Thus the sensor can detect a refractive index change with a resolution of 7.57 × 10−7 RIU.
reason for this result may be that the rate of response slowed down and gradually became saturated as the hydrogen concentration increased. Because the Pd film does not interact with air and can isolate the capillary wall from the air, the MZI can be applied to an air refractive index measurement by using the capillary wall covered by the Pd film as the reference arm (Fig. 9a). According to Ref. [62], the empirical formula of the air refractive index can be represented as:
n=1+
2.8793p × 10−3 1 + 0.003671t
(4)
The air refractive index is approximately n = 1 + 0.00273p under normal pressure and room temperature conditions. p is the air pressure. From the ideal gas equation pv = nRT , we can change the air pressure by adjusting the air volume at constant temperature. Therefore, different refractive indices of air can be achieved by changing the pressure. The air chamber in the sensing system is shown in Fig. 9(b), which is modified from the piston part of a 300 mL syringe. We fixed the syringe on a bracket and enveloped the sensor in the chamber. The front end of the syringe was sealed with glue. We slowly pushed the piston to compress the chamber volume to increase the air pressure. The chamber volume can be determined by the scale on the syringe. V Therefore, the air pressure can be calculated as p1 = V0 p0 , where V1, 1 V0, and p0 are the current volume of air, the initial volume and the initial pressure, respectively. Substituting p1 in the formula of
4. Conclusions In summary, we demonstrated a simple and low-cost strategy to create a highly sensitive and versatile optical sensing platform. Through theoretical analysis and a numerical simulation, we described the sensing mechanism of the FOI device based on the modal interference and the interaction between the evanescent field and external environment. The FOI device with a symmetric structure detected a humidity variation without humidity sensitive coating and was used to evaluate the complex permittivity of the Pd film, while the partially nanoparticlecoated FOI with asymmetric structure could serve as a MZI with economical intensity demodulation for fast, reversible hydrogen sensing and air refractive index measurement. The transmittance of MZI decreased by 20% for a hydrogen concentration range from 0% to 4%. With the air refractive index increasing from 1.0027 to 1.0004, the transmittance of MZI increased by 35%, and the refractive index resolution was estimated to be 7.57 × 10−7 RIU. Because the proposed FOI in essential is an optical fiber evanescent field device, such an optical device will have potential to be applied to carbon dioxide and ammonia after coating a functional film layer, such as ZIF-8 and polyaniline carbon nanotubes. The FOI device has the advantages of
V p
n = 1 + 0.00273p gives n = 1 + 0.00273 V0 0 . 1 Before conducting the air refractive index measurement, we tested the air tightness of the chamber. We pushed the piston to a certain position and compressed the volume of the air. The peak intensity of the light increased and gradually tended to be stable, which proved that the chamber was airtight. The initial pressure of the chamber was atmospheric pressure. Thereafter, the piston was slowly pushed forward to maintain a quasi-equilibrium state within the chamber. Compressing the air volume from 300 mL to 180 mL, the spectrum was recorded at 8
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Fig. 8. (a) Hydrogen sensing setup. (b) Real-time reversible response of the MZI hydrogen sensor by cycling between nitrogen and hydrogen. (c)–(g) Spectrum comparison of the sensor at different concentrations of hydrogen and pure nitrogen. (h) Light intensity as a function of the hydrogen concentration.
Fig. 9. Air sensing principle of the MZI and (b) air refractive index sensing test setup. (c) Response of the fiber-capillary MZI and (d) peak intensity as a function of the air refractive index. 9
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low cost, simplicity, and versatility, making it an efficient, extensible and flexible platform for optical sensing.
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Sheng Bi received the Ph.D. degrees in department of electrical and computer engineering in University of Alabama, United States. He has been a Lecturer with the school of mechanical engineering, Dalian University of Technology. His research interests include High performance organic triode, organic solar cells and high brightness flexible light-emitting diode (OLED). Xuhui Zhang received his B.S. degree in Optical information science and technology from the Northwest University, China, in 2016. He is currently working toward the Ph.D. degree with Dalian University of Technology. His research interests are fiber-optic sensors and surface plasmon resonance sensors. Shimeng Chen received a B.S. degree in Dalian University of Technology Dalian, China, in 2014. She is currently pursuing her Ph. D. degree in optical engineering in Dalian University of Technology, Dalian, China. Her research interests include biochemical fiber sensors, nano-reinforcing SPR sensors.
Yun Liu received the B.S. and Ph.D. degrees in optical engineering in Dalian University of Technology Dalian, China. He has been a Lecturer with the school of physics and optoelectronic engineering, Dalian University of Technology. His research interests include optical fiber sensor, surface Plasmon resonance sensor.
Wei Peng received doctoral degree in optical engineering from Dalian University of Technology, Dalian, China in 1999. Then she joined Center for Photonics Technology within Department of Electrical Engineering at Virginia Polytechnic Institute and State University as worked as a Postdoctoral Associate. From 2004–2007, she worked as a Research Assistant Professor at Department of Chemistry and Biochemistry, Arizona State University. During 2007–2009, she worked as a Senior Research Scientist in Applied Technologies Division at Physical Optics Corporation, Inc. She took a professor faculty position in College of Physics and Optoelectronic Engineering at Dalian University of Technology. Her research interests include subwavelength optics, surface plasmonics, and surface plasmon resonance.
Ning Zhang is currently working toward the master degree with Dalian University of Technology. Her research interests include the optical fiber hydrogen sensor and its application in the field of industry. Ping Li received the BS degrees in practical chemistry from the Taiyuan normal university in 2018. She is currently working toward the MS degree with Dalian university of tchnology. Her research interests in optical fiber sensing.
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