Fibre gratings for hydrogen sensing

Abstract

Liquid hydrogen has been intensively used in aerospace applications for the past 40 years and is of great interest for future automotive applications. Following major explosive risks due to the use of hydrogen in air, several studies were carried out in order to develop optical fibre sensors for the detection of hydrogen leakage. This paper aims at the presentation of new hydrogen sensors based on the use of fibre Bragg gratings (FBG) and long period gratings (LPG) coated by palladium nanolayers. The sensing principle based on the palladium-hydrogen interaction is presented, as well as experimental results. It is shown that both techniques could be used for hydrogen sensing but with a sensitivity enhanced by a factor up to 500 when using a LPG sensor. FBG sensors appear to be pure strain sensors and LPG sensors are mainly based on the coupling between the cladding modes and evanescent or surface plasmon waves. Preliminary results obtained with an in-fibre Mach-Zehnder interferometer configuration with in-series LPG sensors are also presented. They show potential interest to compensate for the thermal sensitivity of the fibre gratings.

Optical fiber hydrogen sensor based on core diameter mismatch and annealed Pd–Au thin films

“…We observed a drift in the signal of approximately 2% in the devices that were exposed several times to hydrogen. Such a drift is mainly due to the instability of the Pd thin film resulting from the large volume changes that occur during the hydrogenation of the film [16][17][18][19][20][21][22][23][24][25][26][27][28]. The drift may become higher with time or after many times loading and un-loading of hydrogen, which can severely affect the reliability of the sensor.…”

“…In this regard the structure shown in Fig. 1 is better than core-exposed, D-shape, or tapered optical fibers since its fabrication does not require chemical etching, polishing, or tapering the fibers that usually weakens them [18][19][20][21][22][23][24][25][26][27][28]. The losses caused by the insertion of the single-mode fiber between two multimode fibers are typically below 5 dB (measured at 850 nm).…”

“…When the device is exposed to hydrogen the palladium film is converted to a PdH thin film whose optical properties are different than those of a hydrogen-free palladium film. Basically, the exposure of the film to hydrogen causes a decrease of the real and imaginary parts of the refractive index of the film [16][17][18][19][20][21][22][23][24][25][26][27][28]. This in turn causes attenuation changes of the evanescent fields that can be easily monitored with a simple light transmission measuring setup.…”

“…In addition, fiber-based sensors allow punctual, distributed, or remote sensing of hydrogen. Optical fiber hydrogen sensors can be fabricated by exploiting the physical or optical changes, caused by the presence of hydrogen, of a palladium, palladium alloy, or composite thin film (transducer) [16][17][18][19][20][21][22][23][24][25][26][27][28]. Palladium or palladium-based alloys are usually employed as transducers in optical fiber hydrogen sensors due to the high sensitivity and selectivity of palladium towards hydrogen.…”

Nano-structured Pd-long period fiber gratings integrated optical sensor for hydrogen detection

“…Once coated with sensing material on the cladding at the grating region, upon the change of refractive index, the LPFGs can serve as a gas or liquid sensor and show very good sensitivity towards the target materials [16][17][18][19]. In 2006, Trouillet et al [16] reported the use of the Pd-coated LPFGs as a hydrogen sensor. In their work, Pd was deposited on one side of the optical fiber, which was modified into LPFGs.…”