# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 5 Fiber-Optic Hydrogen Sensors
### Wavelength Modulated Fiber-Optic Sensors  
A Bragg grating fiber of a given structure has a certain characteristic wavelength. When a sensor made of Bragg fiber is placed in an H\({}_{2}\) environment, the grating structure changes due to Pd-H\({}_{2}\) interaction, resulting in a shift in the Bragg wavelength, which, in turn, indicates the concentration of hydrogen gas to be measured. In 1999, [PERSON] et al. [83] developed a Bragg grating fiber sensor. They used a single-mode grating fiber with a core of 5-10 \(\upmu\)m, a diameter of 125 \(\upmu\)m, and a Bragg wavelength of 829.7 nm. Before coating, they etched the cladding of the fiber, and the remaining thickness of the cladding was 35 \(\upmu\)m. Figure 12 shows the structure and test results of the grating fiber sensor, with the top panel showing the sensor structure, the middle panel showing the refractive index distribution and the bottom panel showing the transmission and reflection spectral response; Figure 12b also shows the wavelength versus concentration response curve. Due to the Bragg grating written in the optical fiber, the refractive index of the fiber changes periodically, and both transmitted and reflected light are selected for wavelength when the broad-spectrum beam is transmitted. Upon placing the sensor in a hydrogen environment, the fiber stretches, and both the grating constant and refractive index change due to the Pd-H\({}_{2}\) interaction, resulting in a shift in the Bragg wavelength. Experiments were performed with an inert carrier gas, i.e., a sample gas with H\({}_{2}\) concentration variations ranging from 0.3 to 1.8%. Unfortunately, the wavelength drift due to fiber stretching in their experiments was so small (less than 0.1 nm) that, even with a high-resolution spectrometer (resolution 0.14 nm), it could not be resolved. Thus, they instead used simulations for estimation. They also found that when the measured concentration was 1.8%, there was peeling of the Pd film, which affected the reuse of the sensor. For this reason, they proposed the method of plating a solidifying film between the Pd film and the substrate to solve the film-shedding problem. [PERSON] originally designed the grating fiber sensor with only the choice of transmitted light wavelength in mind. Later studies found that hydrogenation caused the grating constant to change. However, the choice of wavelength for the sensor is not just limited to the change in the grating constant of the fiber core. The way the transmitted light is coupled to the photoconductor depends on multiple interfaces, including the core/cladding layer, the cladding layer/Pd, and the Pd/outer gas layer, all of which affect the amount of transmitted light wavelength drift. Using the coupling of multiple interfaces, a larger wavelength drift can be achieved. Based on this principle, in 2006, [PERSON] et al. [84] developed a grating fiber sensor with a large wavelength drift using long-period instead of short-period  
Figure 12: Bragg grating fiber-optic sensor. (**a**) Structure. (**b**) Variation of characteristic wavelength with concentration. grating. This sensor still used a single-mode fiber with a 28 \(\upmu\)m core, and a Pd film with a thickness of 50 nm was coated on the outer surface of the cladding via sputtering, with nanoscale particles on the film surface. They measured the variation in wavelength shift versus hydrogen concentration at various temperatures with an inert carrier gas. The wavelength shift of the new sensor was more than two orders of magnitude greater compared to the Sutapun-type sensor. In 2008, [PERSON] et al. [85] used a Bragg single-mode fiber with a core of 10 \(\upmu\)m and a diameter of 125 \(\upmu\)m. The optical grating period was 0.534 \(\upmu\)m, the length was 20 mm, and the characteristic wavelength was 1545 nm. One side of the fiber was polished, and the polished side was coated with a palladium film with a thickness of 20 nm and an interaction length of 12 mm.