WO3 -Pd composite films were deposited on the side-face of side-polished fiber Bragg grating as sensing elements by magnetron sputtering process. XRD result indicates that the WO3 -Pd composite films are mainly amorphous. Compared to standard FBG coated with same hydrogen sensitive film, side-polished FBG significantly increase the sensor’s sensitivity. When hydrogen concentrations are 4% and 8% in volume percentage, maximum wavelength shifts of side-polished FBG are 25 and 55 pm respectively. The experimental results show the sensor’s hydrogen response is reversible, and side-polished FBG hydrogen sensor has great potential in hydrogen’s measurement.
© 2011 OSA
Optical fiber hydrogen sensor has attracted many research interests due to its potential as next generation of hydrogen sensor. At present there are several kinds of optical fiber hydrogen sensors, such as evanescent sensor [1-6], micro-mirror sensor [7-8], surface plasmon resonance (SPR) sensor , acoustic resonator sensor  and fiber Bragg grating (FBG) sensor [11-14]. Although the evanescent and micro-mirror sensors are simple and inexpensive, their multiplexing capability is quite limited. Another disadvantage for the evanescent and micro-mirror sensors is that the sensor’s accuracy is susceptible to the effect of light source’s intensity. Compared to SPR and acoustic resonator sensor, FBG is more suitable for real-time and distributed measurement. FBG sensor has been widely used in many industry applications due to its anti-electromagnetic interference, excellent distributed sensing possibilities, easy for temperature compensation [15-17]. Therefore utilizing FBG to monitor hydrogen leakage is very meaningful.
At present the most sensitive FBG hydrogen sensor is based on WO3 doped with Pt on which hydrogen can undergo an exothermic reaction . The sensor has fast response and high sensitivity, but it still has the potential to explode in high hydrogen concentration. Sutapun and Tabib-Azar  proposed a FBG hydrogen sensor coated with 560nm Pd film, which showed a linear sensitivity for 0.3-1.8% H2. However the pure Pd film is so thick that the sensor’s hydrogen response is almost irreversible. In their research, it is also reported that reducing the FBG diameter can enhance sensitivity. M. Aleixandrea  demonstrated etched FBG with sputtered Pd thin film has a higher sensitivity under different hydrogen concentration, but etched FBG is so fragile that it is not suitable for practical application. Side-polished FBG (SP-FBG) has an interesting structure that can increase sensitivity of FBG sensors. Research works on side-polished FBG are mainly concentrated on its sensitivity to external refractive index, while its bending sensitivity is rarely investigated and used. J. Zhou  proposed that D-shaped FBG had an intrinsic sensitivity to curvature, a characteristic that does not happen with standard FBG. It has been demonstrated that SP-FBG coated with 20nm Pd film shows sensitive to hydrogen , but pure Pd film has poor adhesion to FBG duo to their different physical properties. It is found that WO3/Pd composite film has good mechanical property .
With the aim to achieve high sensitivity and robust mechanical property, a novel hydrogen sensor combined SP-FBG structure and WO3 -Pd composite film is proposed in this paper. WO3 -Pd composite films with different thicknesses were deposited on SP-FBG by magnetron sputtering technology, and SP-FBG hydrogen sensor coated with composite WO3 -Pd thin film as sensing media has been developed and its sensing characteristics have been investigated.
WO3 -Pd composite film consists of WO3, WO3/Pd composite film and Pd film. WO3 is deposited the basal layer for it has good adhesion toward fiber, and Pd film is used as hydrogen sensitive film duo to its high sensitivity and selectivity to hydrogen. WO3/Pd composite film is set as intermediate layer to overcome material properties’ mismatch of WO3 and Pd. When the thickness of pure Pd film is more than 40nm, it is easy to crack because of the accumulation of lattice dislocations caused by its volume expansion. By adding the intermediate layer, the accumulation of lattice dislocations of single Pd layer can be reduced, therefore the repeatability of WO3 -Pd composite film can be enhanced. FBG sensing head with 40nm WO3 -Pd composite film is firstly prepared by a 5nm WO3 coating realized with radio frequency (RF) sputtering process, followed by depositing 5 nm WO3/Pd composite film with co-sputtering process, and then finished with 30nm Pd film by direct-current (DC) sputtering method. Meanwhile FBG and SP-FBG coated with 110nm WO3 -Pd composite film was prepared by alternately adding 5nm WO3/Pd composite film and 30nmPd film in two times for comparison. The schematic illustration of WO3 -Pd composite film is shown in Fig. 1 .
As it is well known, the reflected wavelength (λB) of FBG has correlation with its effective refractive index (neff) and grating pitch (Λ).The equation can be expressed as :
In order to increase the hydrogen sensor’s sensitivity, FBG was side-polished in our experiments. WO3 -Pd coating will expand when it absorbs hydrogen, which will induce strain in the underlying FBG. The grating pitch of FBG will be changed by the expansion of WO3 -Pd composite film, therefore FBG wavelength is drifted. In this way FBG wavelength shift has correlation with hydrogen concentration. By measuring the wavelength shift of SP-FBG, the hydrogen concentration can be deduced.
A Lambda Physik excimer laser (COMPex-150T) operating at 248nm was used as UV light source to write FBG in SMF-28 by phase mask method . The optical cladding of FBG was mechanically ground by a motor-driven polishing wheel  and polished to about several micro-meter of minimal residual thickness to the fiber core. During the side-polishing process, the FBG fiber was connected to light source and optic power meter to enable in-line monitoring of the polishing process. The optical loss ratio has relationship with the side-polishing depth . By controlling side-polishing time and the optical power loss, FBG was polished to different depths in our experiment. After the side-polishing process, the width of side face of SP-FBG was measured by VHX-100 digital microscope. The microphotography SP-FBG1 are shown in Fig. 2 : (a) side view before sputtering, (b) top view before sputtering and (c) top view after sputtering. Considering the diameter of the SMF-28 as 125μm, the polishing depth of SP-FBG1 is 58.2μm by subtracting the residual diameter of 66.8μm. The polishing depth also can be calculated by measuring the side-face width of SP-FBG. As shown in Fig. 2, (d), (e) and (f), the widths of SP-FBG2, SP-FBG3 and SP-FBG4 are 124.8, 124.5, 124.3μm respectively, which corresponds to polishing depth of 59, 56.9 and 55.9μm respectively. The SP-FBG2 has the largest side-polishing depth since it has the biggest width.
Then WO3 -Pd composite thin film were deposited on the side-polished FBG fiber by using a BESTECH sputtering system. The system is equipped with DC and RF sputtering sources. 3 inch Pd and WO3 targets are installed to DC and RF sources, and the distance between the FBG samples and target is about 150 mm. Meanwhile several 10 mm×10 mm Si pieces are set in the chamber for further characterization of the deposited films. Under 0.5 Pa sputtering pressure of Ar, deposition power for Pd and WO3 targets are 100 and 150W respectively, which corresponds to deposition rate of 0.14 and 0.04 nm/s respectively. During the sputtering process, the thickness of the WO3 -Pd film was monitored by quartz crystal method.
Figure 3 is the schematic diagram of hydrogen concentration sensing system. The sensing FBG and temperature compensating FBG are connected to a SLED light with maximum power of 85 μW by a 3 dB coupler. The sensing FBG represents FBGs coated with WO3 -Pd composite film, and a standard non-coated FBG with the similar wavelength is used as the temperature compensating element. The varying hydrogen concentrations are provided by changing flowing rate of H2 and N2. A commercially available MIC-500 hydrogen concentration meter is connected to the gas chamber for calibration. During hydrogen concentration characterization, the reflected wavelength is collected with a BCD-100 FBG demodulator. The BCD-100 FBG demodulator is equipped with a Fiber Fabry-Perot Tunable Filter (FFP-TF) from Micron Optics Inc. USA, which can provide high precision (1pm) to detect a slight wavelength shift of FBG. The measured data is recorded by a computer connecting with the FBG demodulator for further data treatment.
4. Result and discussion
XRD patterns reveal the presence of the WO3 -Pd composite film crystalline phase. From the Joint Committee on Powder Diffraction Standards cards (65-6174, 41-0905), crystalline Pd has relatively sharp peaks at 2θ=40°, 46.7°, 68.2°, and 82.2°, while for crystalline WO3 they locates at 24°, 34°, 42°, 49°, 55.3° and 61.1°. Only broad peaks can be observed when the film is amorphous [24-26]. Figure 4 shows the XRD result for WO3 -Pd composite film on Si piece prepared by BESTECH sputtering system. There is no sharp peak in the spectrum of 110 nm WO3 -Pd composite film. A relatively sharp peak which disperses from 2θ=40°can be observed in the XRD pattern of 40 nm WO3 -Pd composite film, but the peak intensity is not high enough compared to that of background. So the XRD result exhibits that the 40 and 110 nm composite films are mainly amorphous.
The relative wavelength is derived by subtracting the central wavelength of reference FBG from that of sensing FBG. In this method, the sensor’s accuracy can be improved by reducing ambient temperature’s impact. Figure 5 shows two cycle’s of hydrogen response of the FBG and SP-FBG coated with 40 and 110nm WO3 -Pd composite film. All the FBGs shift to longer with the increase of hydrogen concentration. During the two cycle’s response, the sensor has good repeatability to different hydrogen concentration. But the FBG cannot restore to previous wavelength perfectly, the reason for this phenomenon may be due to the defaults in the amorphous WO3 -Pd composite film.
Figure 6 illustrates the central wavelength shift of FBG coated with 40 and 110 nm WO3 -Pd composite film under different hydrogen concentration. It can be seen that all FBG coated with hydrogen sensitive film shift to longer wavelength with the increase of hydrogen concentration, and the FBG wavelength shift has a nonlinear relationship on hydrogen concentration. When the hydrogen concentration is 6% in volume percentage, 40nm WO3 -Pd composite film saturate and the wavelength shifts of FBG and SP-FBG1 are 5 and 15 pm respectively. Standard FBG, SP-FBG2, SP-FBG3 and SP-FBG4 coated with 110nm WO3 -Pd composite film saturate at 8% hydrogen, and corresponding wavelength shifts are 16, 40, 45 and 55pm respectively. Because of the different polishing depth of the fiber, SP-FBG2, SP-FBG3 and SP-FBG4 have different sensitivity to hydrogen. FBG coated with thicker film has more wavelength shift, but the response rate will be sacrificed. FBG coated with 40nm WO3 -Pd composite film is about 40 s, and that for FBG sputtered with 110nm WO3 -Pd composite film is about 90 s.
It is interesting to find that SP-FBG greatly increase sensitivity compared to standard FBG coated with the same film. For 40nm WO3-Pd composite film, SP-FBG’s wavelength shift is three times as that of standard FBG under the same hydrogen concentration. Compared to standard FBG, the sensitivity of SP-FBG2, SP-FBG3 and SP-FBG4 coated with 110nm WO3-Pd composite film are increased by 244%, 181% and 150% respectively. These results can be explained by the deduction of the effective cross-area and high bending sensitivity of SP-FBG. It is reported  that D-shaped FBG’s bending sensitivity is about 80 times higher than standard FBG. When FBG is side-polished, it will form D-shaped cross section, and there is slight outward curvature on the fiber. This is why SP-FBG can increase the sensitivity of the sensor more than 100%.
Owing to the highest sensitivity of SP-FBG2, further investigation was carried out in our experiment. Figure 7 displays wavelength response of SP-FBG2 under different hydrogen concentration. From Fig. 7 we can conclude that the SP-FBG2 has reversible response under different hydrogen concentration. When hydrogen concentration is 4% in volume percentage, the wavelength shift of SP-FBG2 is about 25 pm, whereas it is only 5 pm in case of 1%. The central wavelength of SP-FBG2 cannot restore to its initial value, this could be attributed to the imperfect restore of sensitive films. The hydrogen sensitive material’s microstructure and the design of the sensor’s structure will be investigated later. Furthermore, temperature and humidity can affect the sensor’s performance , this should also be investigated in future.
SP-FBG Hydrogen sensor based on WO3 -Pd composite films has been developed in this paper. XRD results demonstrate the WO3 -Pd composite films are mainly amorphous. FBG coated with thicker WO3 -Pd composite films has higher sensitivity but slower response rate. Compared to standard FBG, SP-FBG can significantly improve the sensor’s sensitivity. For 40nm WO3 -Pd composite film, the wavelength shift of SP-FBG1 is three times as that of standard FBG when the saturated hydrogen is 6%. SP-FBG2, SP-FBG3 and SP-FBG4 coated with 110nm WO3 -Pd composite film saturate at 8% hydrogen, and the corresponding wavelength shifts are 40, 45 and 55pm respectively, which corresponds to the increase of sensitivity of 244%, 181% and 150% respectively compared to standard FBG. By optimizing the structure of the sensor, the SP-FBG is very promising for distributed measurement of hydrogen concentration.
This work is finically supported by the Project of National Science Foundation of China, NSFC (Number: 60908020) and the Research Grant of the Key Laboratory for Surface Physics and Chemises (Number: SPC 201005).
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