A review for optical fiber hydrogen sensors based on palladium (Pd) and tungsten oxide (WO 3) thin films is presented, with specific focus on the measurement methods, probe structures, and sensing properties of different sensors. Firstly, the theoretical models behind the optical fiber hydrogen sensors, as well as their practical limitations, are addressed. Secondly, four mainstream measurement methods, including intensity, fiber Bragg grating (FBG), interferometer, surface plasmon resonance (SPR), which have been proposed to sense the physicochemical properties variations of sensitive thin films when exposed to hydrogen, are reviewed. Then, the advantages and disadvantages of all the above measurement methods are also discussed and compared. Finally, the existing problems and future prospects of optical fiber hydrogen sensors are pointed out.
A hydrogen sensor based on high-birefringence fiber loop mirror (HBFLM) with sol-gel Pd/WO 3 coating was demonstrated. The sensing structure was formed by inserting a section of polarization maintaining fiber (PMF) with Pd/WO 3 coating in fiber loop mirror. The Pd/WO 3 coating was prepared by sol-gel method and coated on PMF by dip-coating method, which is simple fabrication, low cost, and has good binding force with fiber. When the hydrogen concentration around Pd/WO 3 coating was changed, it would induce the strain change of polarization maintaining fiber, and then shift the interference spectrum of HBFLM. Therefore, the hydrogen concentration can be measured by monitoring the wavelength shift of the interference spectrum. Experimental results showed that the resonance wavelength had a blue shift with the increase of hydrogen concentration and the total shift of the resonance wavelength was ~2.18 nm within the concentration range of 0-1%. The sensor had simple structure, low cost, high sensitivity, good repeatability, and well stability.
A highly-sensitive and temperature-robust photonic crystal fiber (PCF) modal interferometer coated with Pd/WO film was fabricated and studied, aiming for real-time monitoring of dissolved hydrogen concentration in transformer oil. The sensor probe was fabricated by splicing two segments of a single mode fiber (SMF) with both ends of the PCF. Since the collapse of air holes in the PCF in the interfaces between SMF and PCF, a SMF-PCF-SMF interferometer structure was formed. The Pd/WO film was fabricated by sol-gel method and coated on the surface of the PCF by dip-coating method. When the Pd/WO film is exposed to hydrogen, both the length and cladding refractive index of the PCF would be changed, resulting in the resonant wavelength shift of the interferometer. Experimental results showed that the hydrogen measurement sensitivity of the proposed sensor can reach 0.109 pm/(μl/l) in the transformer oil, with the measurement range of 0-10 000 μl/l and response time less than 33 min. Besides, the proposed sensor was temperature-insensitive without any compensation process, easy to fabricate without any tapering, polishing, or etching process, low cost and quickly response without any oil-gas separation device. All these performances satisfy the actual need of real-time monitoring of dissolved hydrogen concentration in the transformer oil.
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