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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 15 — Jul. 28, 2014
  • pp: 18556–18563
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A hydrogen curing effect on surface plasmon resonance fiber optic hydrogen sensors using an annealed Au/Ta2O5/Pd multi-layers film

Ai Hosoki, Michiko Nishiyama, Hirotaka Igawa, Atsushi Seki, and Kazuhiro Watanabe  »View Author Affiliations


Optics Express, Vol. 22, Issue 15, pp. 18556-18563 (2014)
http://dx.doi.org/10.1364/OE.22.018556


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Abstract

In this paper, a response time of the surface plasmon resonance fiber optic hydrogen sensor has successfully improved with keeping sensor sensitivity high by means of hydrogen curing (immersing) process of annealed Au / Ta2O5 / Pd multi-layers film. The hydrogen curing effect on the response time and sensitivity has been experimentally revealed by changing the annealing temperatures of 400, 600, 800°C and through observing the optical loss change in the H2 curing process. When the 25-nm Au / 60-nm Ta2O5 / 10-nm Pd multi-layers film annealed at 600°C is cured with 4% H2 / N2 mixture, it is found that a lot of nano-sized cracks were produced on the Pd surface. After H2 curing process, the response time is improved to be 8 s, which is two times faster than previous reported one in the case of the 25-nm Au / 60-nm Ta2O5 / 3-nm Pd multi-layers film with keeping the sensor sensitivity of 0.27 dB for 4% hydrogen adding. Discussions most likely responsible for this effect are given by introducing the α-β transition Pd structure in the H2 curing process.

© 2014 Optical Society of America

1. Introduction

In recent years, hydrogen has attracted much attention as a clean, sustainable, and abundant energy source and would be widely used in the automotive industry and in rocket engines. On the other hand, hydrogen has flammable and highly explosive properties when mixed with oxygen. A leakage of hydrogen in air would lead to an explosive atmosphere of easy ignition if hydrogen concentration exceeds more than 4% (the lower explosive limit: LEL). Therefore, the development of rapid and accurate hydrogen sensors is essential to monitor the hydrogen leakages. In potentially explosive atmospheres optical fiber sensors are preferable to be used because of no electrical contacts in the sensing point and transmission line. Moreover, they are compact, light in weight, immune to electromagnetic interference and resistant to corrosion. Therefore, a number of optical fiber hydrogen sensors have been attractively proposed [1

1. J. Villatoro, D. Luna-Moreno, and D. Monzon-Hernandez, “Optical fiber hydrogen sensor for concentrations below the lower explosive limit,” Sens. Actuators B Chem. 110(1), 23–27 (2005). [CrossRef]

7

7. A. Trouillet, E. Marin, and C. Veillas, “Fibre gratings for hydrogen sensing,” Meas. Sci. Technol. 17(5), 1124–1128 (2006). [CrossRef]

]. In the most of optical hydrogen sensors a thin palladium layer is usually employed as the sensitive material because palladium allows the selective absorbability for hydrogen. At present, there are mainly several kinds of optical fiber hydrogen sensors, such as tapered fibers [1

1. J. Villatoro, D. Luna-Moreno, and D. Monzon-Hernandez, “Optical fiber hydrogen sensor for concentrations below the lower explosive limit,” Sens. Actuators B Chem. 110(1), 23–27 (2005). [CrossRef]

,2

2. J. Villatoro and D. Monzón-Hernández, “Fast detection of hydrogen with nano fiber tapers coated with ultra thin palladium layers,” Opt. Express 13(13), 5087–5092 (2005). [CrossRef] [PubMed]

], unclad fibers [3

3. M. Tabib-Azar, B. Sutapun, R. Petrick, and A. Kazemi, “Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions,” Sens. Actuators B Chem. 56(1-2), 158–163 (1999). [CrossRef]

5

5. P. Bhatia and B. D. Gupta, “Surface plasmon resonance based fiber optic hydrogen sensor utilizing wavelength interrogation,” Proc. SPIE 8351, 83511V(2012). [CrossRef]

], Fiber Bragg grating [6

6. C. L. Tien, H. W. Chen, W. F. Liu, S. S. Jyu, S. W. Lin, and Y. S. Lin, “Hydrogen sensor based on side-polished fiber Bragg gratings coated with thin palladium film,” Thin Solid Films 516(16), 5360–5363 (2008). [CrossRef]

,7

7. A. Trouillet, E. Marin, and C. Veillas, “Fibre gratings for hydrogen sensing,” Meas. Sci. Technol. 17(5), 1124–1128 (2006). [CrossRef]

]. These tapered or unclad fibers has been required to eliminate thick cladding layers to access transmitted light in core through evanescent wave, resulting in a loss of mechanical strength, while FBG sensors need to be compensated because of their temperature dependency.

Since the first demonstration of the hetero-core technique [11

11. K. Watanabe, K. Tajima, and Y. Kubota, “Macrobending Characteristics of a Hetero-Core Splice Fiber Optic Sensor for Displacement and Liquid Detection,” IEICE Trans. Electron. 83(3), 309–314 (2000).

] by the present authors, the hetero-core fiber sensor [12

12. M. Iga, A. Seki, and K. Watanabe, “Hetero-core structured fiber optic surface plasmon resonance sensor with silver film,” Sens. Actuators B Chem. 101(3), 368–372 (2004). [CrossRef]

14

14. K. Takagi and K. Watanabe, “Near Infrared Characterization of Hetero-Core Optical Fiber SPR Sensors Coated with Ta2O5 Film and Their Applications,” Sensors (Basel) 12(12), 2208–2218 (2012). [CrossRef] [PubMed]

] has become well known SPR devices because of the practical advantages in terms of the simplicity of structure and the interesting sensing principles. The structure consists of a peace of single-mode optical fiber between two multi-mode fibers. Due to the core diameter mismatch the transmitted light is guided to the cladding of the single-mode fiber, which works as a sensing region.

Recently, in order to improve the response time, the hydrogen sensors have been reported based on the Pd/Au alloy [15

15. D. Monzón-Hernández, D. Luna-Moreno, and D. Martinez-Escobar, “Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers,” Sens. Actuators B Chem. 136(2), 562–566 (2009). [CrossRef]

] or nano-sized cracks [16

16. Y. Tack Lee, J. M. Lee, Y. J. Kim, J. H. Joe, and W. Lee, “Hydrogen gas sensing properties of PdO thin films with nano-sized cracks,” Nanotechnology 21, 165503 (2010).

] on the Pd surface. For instance, D. Monzon-Hernandez et al. [15

15. D. Monzón-Hernández, D. Luna-Moreno, and D. Martinez-Escobar, “Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers,” Sens. Actuators B Chem. 136(2), 562–566 (2009). [CrossRef]

] made an effort to produce a non-SPR sensor with a faster response by annealing Pd / Au alloy films using the hetero-core technique [11

11. K. Watanabe, K. Tajima, and Y. Kubota, “Macrobending Characteristics of a Hetero-Core Splice Fiber Optic Sensor for Displacement and Liquid Detection,” IEICE Trans. Electron. 83(3), 309–314 (2000).

13

13. M. Iga, A. Seki, and K. Watanabe, “Gold thickness dependence of SPR-based hetero-core structured optical fiber sensor,” Sens. Actuators B Chem. 106(1), 363–368 (2005). [CrossRef]

] and the finding by Z. Zhao et al. [17

17. Z. Zhao and M. A. Carpenter, “Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films,” J. Appl. Phys. 97(12), 124301 (2005). [CrossRef]

], where annealing and curing effect to response time was interestingly reported. The Pd / Au alloy films annealed at temperatures of 200°C for 1h in an Ar atmosphere. Their experimental results showed a response time of 5 s for 4% hydrogen with the 4-nm Pd / Au alloy films. However, the sensitivity of the sensor for hydrogen is very low because of the usage of thin Pd films and of non-SPR operation. On the other hand, Y. Tack Lee [16

16. Y. Tack Lee, J. M. Lee, Y. J. Kim, J. H. Joe, and W. Lee, “Hydrogen gas sensing properties of PdO thin films with nano-sized cracks,” Nanotechnology 21, 165503 (2010).

] proposed an electrical hydrogen sensor with nano-sized cracks on the Pd surface onto the Si substrates, which was generated by the reduction process of palladim oxide (PdO) films with hydrogen. The electrical resistance on Si substrates is changed for 4% hydrogen concentration with a response time of about 15 s.

2. Experimental set up

Several samples of hetero-core sensors were fabricated by using a multi-mode GI fiber as the transmission line whose core diameter is 50 µm and the inserted SI fiber with 3 µm of core diameter. The cladding diameters of both fibers were 125 µm. Au, Ta2O5 and Pd were cylindrically coated on the fabricated hetero-core region with thicknesses of 25 nm, 60 nm and 10 nm, respectively, by means of an RF sputtering machine (CFS-4ES-231, Shibaura Mechatronics Co. Ltd.), which was specially designed to introduce a rotating mechanism to realize axial symmetrically uniform deposition on the cladding surface. After the coating process, the sensors were annealed at temperatures of 400, 600, and 800°C in an air atmosphere for 1h. In order to investigate H2 response properties of the annealed films, an experimental setup was used to measure the optical loss change of the sensors, as illustrated in Fig. 1. It consists of a set of flow meters and a 15-ml volume acrylic gas chamber which has an inlet and an outlet to allow a continuous flow of gas. N2 or 4% H2 in N2 was alternatively introduced by controlling the two flow meters. The flow rate of nitrogen was kept at 1000 ml / min and the flow rate of hydrogen was adjusted to given the required concentration. The gas filling time for the chamber is calculated to be approximately 1 s from the chamber volume and the flow rate. The fiber is illuminated by an LED light source whose wavelength is 850 nm and the optical loss is detected using an optical power meter.

3. Results and discussions

Curing [17

17. Z. Zhao and M. A. Carpenter, “Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films,” J. Appl. Phys. 97(12), 124301 (2005). [CrossRef]

] is reported as an initial process to stabilize hydrogen sensor performance by exposing an annealed layer to hydrogen atmosphere to absorb it soon after annealing is completed. Morphological findings are described in this section in more details than other previously reported works [4

4. C. Perrotton, R. J. Westerwaal, N. Javahiraly, M. Slaman, H. Schreuders, B. Dam, and P. Meyrueis, “A reliable, sensitive and fast optical fiber hydrogen sensor based on surface plasmon resonance,” Opt. Express 21(1), 382–390 (2013). [CrossRef] [PubMed]

,5

5. P. Bhatia and B. D. Gupta, “Surface plasmon resonance based fiber optic hydrogen sensor utilizing wavelength interrogation,” Proc. SPIE 8351, 83511V(2012). [CrossRef]

,15

15. D. Monzón-Hernández, D. Luna-Moreno, and D. Martinez-Escobar, “Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers,” Sens. Actuators B Chem. 136(2), 562–566 (2009). [CrossRef]

17

17. Z. Zhao and M. A. Carpenter, “Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films,” J. Appl. Phys. 97(12), 124301 (2005). [CrossRef]

]. One of the purposes of this work is to confirm whether the cylindrically produced Pd /multi-layers is strong enough to stably operate during the experiment and other usage after high temperature annealing. Comparing with a single layer case [15

15. D. Monzón-Hernández, D. Luna-Moreno, and D. Martinez-Escobar, “Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers,” Sens. Actuators B Chem. 136(2), 562–566 (2009). [CrossRef]

], multi-layer was fraught with film deterioration due to the internal stresses in the different material coatings. It has been found that annealing and curing shows no defect on the cylindrical three-layer-structure which was successfully coated using a specially designed rotating RF sputtering machine [17

17. Z. Zhao and M. A. Carpenter, “Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films,” J. Appl. Phys. 97(12), 124301 (2005). [CrossRef]

] in our work. During a series of our repeated experiment, the hydrogen detecting performance is reproducibly made with a reliable lifetime.

Figure 2
Fig. 2 H2 response properties in the light intensity change of annealed Au 25 / Ta2O5 60 / Pd 10 nm annealed at a temperature of 600°C when exposed to pure N2 and 4% H2 / N2 mixture.
shows the light intensity change during the H2 curing detected at the optical power meter for the case of 25-nm Au / 60-nm Ta2O5 / 10-nm Pd multi-layers film annealed at a temperature of 600°C when the sensor is exposed to pure N2 and 4% H2 / N2 mixture. When the annealed multi-layers film was exposed to 4% H2 diluted with N2, the transmitted light intensity largely reduced and then showed a stable state in 12 min soon after hydrogen was initially introduced into the chamber. In addition, it is observed that the light intensity of the annealed film after H2 curing is different from that of the annealed film before H2 curing. It is generally known that when Pd is exposed to hydrogen, hydrogen molecules are first adsorbed onto the Pd surface and then dissociate into atomic hydrogen that diffuses into the interstitial sites in the bulk Pd, leading to formation palladium hydride (PdH). In the α-β phase transition of Pd structure, the drastic lattice expansion is occurred [18

18. F. A. Lewis, “The Palladium Hydrogen System, (Academic Press, London and New York, 1967).

,19

19. X. Bévenot, A. Trouillet, C. Veillas, H. Gagnaire, and M. Clement, “Hydrogen leak detection using an optical fibre sensor for aerospace applications,” Sens. Actuators B Chem. 67(1-2), 57–67 (2000). [CrossRef]

].

In order to investigate the H2 curing effect on the H2 sensor characteristics, the real-time response was obtained as Fig. 4
Fig. 4 Optical loss changes of the hetero-core hydrogen SPR sensor with H2-cured multi-layers film after 600°C annealing process.
in the optical loss change of multi-layers film, which was cured with H2 as mentioned in the above. It is found from Fig. 4 that the H2 cured film at the stable state can absorb and desorb the H2 gas rapidly with a high sensitivity for 4% H2 in N2. The rise time and recovery time are defined as the period of the time for a loss change from 10% above ground to 90% of the maximum value and as the time required to fall from 90% to 10%, respectively. The rise time was 8 s at 4% H2 in N2 with a recovery time of 20 s. The experimental result showed that both of the response and recovery times for the proposed sensor are two times faster than those for the 3-nm Pd sensor [8

8. A. Hosoki, M. Nishiyama, H. Igawa, A. Seki, Y. Choi, and K. Watanabe, “A surface plasmon resonance hydrogen sensor using Au / Ta2O5 / Pd multi-layers on hetero-core optical fiber structures,” Sens. Actuators B Chem. 185, 53–58 (2013). [CrossRef]

] which was subjected to no H2 curing after annealing, with keeping the sensor sensitivity as high as 0.27 dB for 4% hydrogen adding, because a lot of nano-sized cracks on the Pd surface helps the rapid absorption of hydrogen. In addition, the sensor with forming the nano-cracks on Pd surface showed excellent reproducibility for the pure N2 and 4% H2 in N2. As a result, it is confirmed that the nano-cracks make it possible to improve the response time. The optical losses as a function of hydrogen concentrations of the H2-cured sensor (square plots) and 3-nm Pd sensor (circle plots) are shown in Fig. 5
Fig. 5 Optical loss as a function of hydrogen concentration for the H2 cured sensor (squares) and 3-nm Pd sensor (circles).
. It can be seen from Fig. 5 that the optical losses of both sensors increase with the hydrogen concentration and saturate near 4% hydrogen concentration. The maximum optical loss change of the H2-cured film is slightly larger than that of the 3-nm Pd film. This difference could be attributed to the expansion of the Pd surface caused by a lot of nano-sized cracks.

Figure 6
Fig. 6 Real-time responses in the optical loss changes of the hetero-core hydrogen SPR sensor with multi-layers film after the H2 curing process of 25 nm Au / 60 nm Ta2O5 / 10 nm Pd annealed at 400°C.
shows the optical loss changes in the real-time response of multi-layers film, which was cured with H2 after 400°C annealing process. From Fig. 6, the response time of this film was 17 s when the sensor was exposed to 4% H2 in N2 and recovery time is 35 s. The optical loss change is indicated to be about 0.45 dB for the H2-cured film after 400°C annealing process, which shows that the sensitivity at 4% H2 of the H2-cured film after 400°C annealing process is higher than that of the H2-cured film after 600°C annealing process. The light intensity change of the multi-layers film annealed at a temperature of 400°C for exposure to 4% H2 in N2 is shown during the H2 curing in Fig. 7
Fig. 7 The change of transmitted light intensity of multi-layers film annealed at a temperature of 400°C for exposure to 4% H2 in N2 during the H2 curing process.
. When the annealed multi-layers film was exposed to 4% H2 in N2 in the H2 curing process, the transmitted light intensity slightly reduced and then reached back to a stable state in 3 min after initial exposure of hydrogen with the light intensity change of 0.7 dB, which was observed differently from the case of 600°C annealing temperature as shown in Fig. 2. This is probably because the deformed Pd layer differently shows the dielectric functions, depending on the crack formation condition. Comparing with 600°C annealing, the 800°C annealing films showed a very small response to 4% H2 / N2 mixture during the H2 curing process, more likely because a very small amount of grain aggregates would be produced on the Pd surface.

4. Conclusion

References and links

1.

J. Villatoro, D. Luna-Moreno, and D. Monzon-Hernandez, “Optical fiber hydrogen sensor for concentrations below the lower explosive limit,” Sens. Actuators B Chem. 110(1), 23–27 (2005). [CrossRef]

2.

J. Villatoro and D. Monzón-Hernández, “Fast detection of hydrogen with nano fiber tapers coated with ultra thin palladium layers,” Opt. Express 13(13), 5087–5092 (2005). [CrossRef] [PubMed]

3.

M. Tabib-Azar, B. Sutapun, R. Petrick, and A. Kazemi, “Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions,” Sens. Actuators B Chem. 56(1-2), 158–163 (1999). [CrossRef]

4.

C. Perrotton, R. J. Westerwaal, N. Javahiraly, M. Slaman, H. Schreuders, B. Dam, and P. Meyrueis, “A reliable, sensitive and fast optical fiber hydrogen sensor based on surface plasmon resonance,” Opt. Express 21(1), 382–390 (2013). [CrossRef] [PubMed]

5.

P. Bhatia and B. D. Gupta, “Surface plasmon resonance based fiber optic hydrogen sensor utilizing wavelength interrogation,” Proc. SPIE 8351, 83511V(2012). [CrossRef]

6.

C. L. Tien, H. W. Chen, W. F. Liu, S. S. Jyu, S. W. Lin, and Y. S. Lin, “Hydrogen sensor based on side-polished fiber Bragg gratings coated with thin palladium film,” Thin Solid Films 516(16), 5360–5363 (2008). [CrossRef]

7.

A. Trouillet, E. Marin, and C. Veillas, “Fibre gratings for hydrogen sensing,” Meas. Sci. Technol. 17(5), 1124–1128 (2006). [CrossRef]

8.

A. Hosoki, M. Nishiyama, H. Igawa, A. Seki, Y. Choi, and K. Watanabe, “A surface plasmon resonance hydrogen sensor using Au / Ta2O5 / Pd multi-layers on hetero-core optical fiber structures,” Sens. Actuators B Chem. 185, 53–58 (2013). [CrossRef]

9.

J. Homola, “Optical fiber sensor based on surface plasmon excitation,” Sens. Actuators B Chem. 29(1-3), 401–405 (1995). [CrossRef]

10.

M. Mitsushio, S. Higashi, and M. Higo, “Construction and evaluation of a gold-deposited optical fiber sensor system for measurements of refractive indices of alcohols,” Sens. Actuators A Phys. 111(2-3), 252–259 (2004). [CrossRef]

11.

K. Watanabe, K. Tajima, and Y. Kubota, “Macrobending Characteristics of a Hetero-Core Splice Fiber Optic Sensor for Displacement and Liquid Detection,” IEICE Trans. Electron. 83(3), 309–314 (2000).

12.

M. Iga, A. Seki, and K. Watanabe, “Hetero-core structured fiber optic surface plasmon resonance sensor with silver film,” Sens. Actuators B Chem. 101(3), 368–372 (2004). [CrossRef]

13.

M. Iga, A. Seki, and K. Watanabe, “Gold thickness dependence of SPR-based hetero-core structured optical fiber sensor,” Sens. Actuators B Chem. 106(1), 363–368 (2005). [CrossRef]

14.

K. Takagi and K. Watanabe, “Near Infrared Characterization of Hetero-Core Optical Fiber SPR Sensors Coated with Ta2O5 Film and Their Applications,” Sensors (Basel) 12(12), 2208–2218 (2012). [CrossRef] [PubMed]

15.

D. Monzón-Hernández, D. Luna-Moreno, and D. Martinez-Escobar, “Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers,” Sens. Actuators B Chem. 136(2), 562–566 (2009). [CrossRef]

16.

Y. Tack Lee, J. M. Lee, Y. J. Kim, J. H. Joe, and W. Lee, “Hydrogen gas sensing properties of PdO thin films with nano-sized cracks,” Nanotechnology 21, 165503 (2010).

17.

Z. Zhao and M. A. Carpenter, “Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films,” J. Appl. Phys. 97(12), 124301 (2005). [CrossRef]

18.

F. A. Lewis, “The Palladium Hydrogen System, (Academic Press, London and New York, 1967).

19.

X. Bévenot, A. Trouillet, C. Veillas, H. Gagnaire, and M. Clement, “Hydrogen leak detection using an optical fibre sensor for aerospace applications,” Sens. Actuators B Chem. 67(1-2), 57–67 (2000). [CrossRef]

OCIS Codes
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(310.6845) Thin films : Thin film devices and applications
(240.6380) Optics at surfaces : Spectroscopy, modulation

ToC Category:
Sensors

History
Original Manuscript: April 25, 2014
Revised Manuscript: June 3, 2014
Manuscript Accepted: June 20, 2014
Published: July 24, 2014

Citation
Ai Hosoki, Michiko Nishiyama, Hirotaka Igawa, Atsushi Seki, and Kazuhiro Watanabe, "A hydrogen curing effect on surface plasmon resonance fiber optic hydrogen sensors using an annealed Au/Ta2O5/Pd multi-layers film," Opt. Express 22, 18556-18563 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-15-18556


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References

  1. J. Villatoro, D. Luna-Moreno, and D. Monzon-Hernandez, “Optical fiber hydrogen sensor for concentrations below the lower explosive limit,” Sens. Actuators B Chem.110(1), 23–27 (2005). [CrossRef]
  2. J. Villatoro and D. Monzón-Hernández, “Fast detection of hydrogen with nano fiber tapers coated with ultra thin palladium layers,” Opt. Express13(13), 5087–5092 (2005). [CrossRef] [PubMed]
  3. M. Tabib-Azar, B. Sutapun, R. Petrick, and A. Kazemi, “Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions,” Sens. Actuators B Chem.56(1-2), 158–163 (1999). [CrossRef]
  4. C. Perrotton, R. J. Westerwaal, N. Javahiraly, M. Slaman, H. Schreuders, B. Dam, and P. Meyrueis, “A reliable, sensitive and fast optical fiber hydrogen sensor based on surface plasmon resonance,” Opt. Express21(1), 382–390 (2013). [CrossRef] [PubMed]
  5. P. Bhatia and B. D. Gupta, “Surface plasmon resonance based fiber optic hydrogen sensor utilizing wavelength interrogation,” Proc. SPIE8351, 83511V(2012). [CrossRef]
  6. C. L. Tien, H. W. Chen, W. F. Liu, S. S. Jyu, S. W. Lin, and Y. S. Lin, “Hydrogen sensor based on side-polished fiber Bragg gratings coated with thin palladium film,” Thin Solid Films516(16), 5360–5363 (2008). [CrossRef]
  7. A. Trouillet, E. Marin, and C. Veillas, “Fibre gratings for hydrogen sensing,” Meas. Sci. Technol.17(5), 1124–1128 (2006). [CrossRef]
  8. A. Hosoki, M. Nishiyama, H. Igawa, A. Seki, Y. Choi, and K. Watanabe, “A surface plasmon resonance hydrogen sensor using Au / Ta2O5 / Pd multi-layers on hetero-core optical fiber structures,” Sens. Actuators B Chem.185, 53–58 (2013). [CrossRef]
  9. J. Homola, “Optical fiber sensor based on surface plasmon excitation,” Sens. Actuators B Chem.29(1-3), 401–405 (1995). [CrossRef]
  10. M. Mitsushio, S. Higashi, and M. Higo, “Construction and evaluation of a gold-deposited optical fiber sensor system for measurements of refractive indices of alcohols,” Sens. Actuators A Phys.111(2-3), 252–259 (2004). [CrossRef]
  11. K. Watanabe, K. Tajima, and Y. Kubota, “Macrobending Characteristics of a Hetero-Core Splice Fiber Optic Sensor for Displacement and Liquid Detection,” IEICE Trans. Electron.83(3), 309–314 (2000).
  12. M. Iga, A. Seki, and K. Watanabe, “Hetero-core structured fiber optic surface plasmon resonance sensor with silver film,” Sens. Actuators B Chem.101(3), 368–372 (2004). [CrossRef]
  13. M. Iga, A. Seki, and K. Watanabe, “Gold thickness dependence of SPR-based hetero-core structured optical fiber sensor,” Sens. Actuators B Chem.106(1), 363–368 (2005). [CrossRef]
  14. K. Takagi and K. Watanabe, “Near Infrared Characterization of Hetero-Core Optical Fiber SPR Sensors Coated with Ta2O5 Film and Their Applications,” Sensors (Basel)12(12), 2208–2218 (2012). [CrossRef] [PubMed]
  15. D. Monzón-Hernández, D. Luna-Moreno, and D. Martinez-Escobar, “Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers,” Sens. Actuators B Chem.136(2), 562–566 (2009). [CrossRef]
  16. Y. Tack Lee, J. M. Lee, Y. J. Kim, J. H. Joe, and W. Lee, “Hydrogen gas sensing properties of PdO thin films with nano-sized cracks,” Nanotechnology21, 165503 (2010).
  17. Z. Zhao and M. A. Carpenter, “Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films,” J. Appl. Phys.97(12), 124301 (2005). [CrossRef]
  18. F. A. Lewis, “The Palladium Hydrogen System, (Academic Press, London and New York, 1967).
  19. X. Bévenot, A. Trouillet, C. Veillas, H. Gagnaire, and M. Clement, “Hydrogen leak detection using an optical fibre sensor for aerospace applications,” Sens. Actuators B Chem.67(1-2), 57–67 (2000). [CrossRef]

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