# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 5 Fiber-Optic Hydrogen Sensors
### Interference-Modulated Fiber-Optic Sensors  
In 1984, [PERSON] et al. [62] developed the first interferometric fiber-optic sensor for hydrogen leak detection. The sensor optical path structure is shown in Figure 7, being similar to the Mach-Zehnder fiber interferometer. The optical path consists of two single-mode fibers, both of which are stripped of the same length (3 cm) of cladding and coated with different material films at the stripped location. One of the fibers was sputtered with a 10 nm thick Ti film, followed by a 10 \(\upmu\)m thick Pd film. The Ti coating is used to increase the adhesion between the palladium and the fiber; the other fiber is sputtered with a Pt film at the corresponding location to match the response of the two fibers to temperature. The fiber with the Pd coating is used as the sensing optical path, and the fiber without the Pd coating is used as the reference optical path. To prevent the effect of mechanical vibration, the two fiber sensing sites are closely pasted on a quartz plate. The light source is a 0.5 mW He-Ne laser, and the laser beam is split and focused into the optical fiber by the lens. When the Pd film on the fiber surface encounters H\({}_{2}\), the Pd film expands, causing the fiber to stretch, thus changing the optical range of the transmitted light. The change in the optical range causes the movement of the interferometer interference fringe. The output of the interferometer is equipped with a photoelectric detector for detecting the number of interferometric fringe movements. The number of moving stripes is related to the H\({}_{2}\) concentration that is measured. For the experiments, different carrier gases, such as argon, nitrogen, and dry air, were used. The results showed that though the response times were different for different carriers, they were all in the order of minutes. The measurement range was 10-4% (1 ppm)-3%. In 1988, researchers improved the manufacturing process of Pd film to improve measurement accuracy [63]. Figure 8 shows the variation in streak motion versus hydrogen concentration. In the figure, the right vertical dashed line is the boundary between phases and the transition phase, the straight solid line represents the theoretical value described via Sieverts' law, the curved solid line represents the theoretical value described via Langmuir's equation, and the curved dashed line is the experimentally fitted value. As can be seen from the figure, when the H\({}_{2}\) concentration is greater than 0.1%, the experimental value conforms to [PERSON] law, and the stripe movement in this interval is linear with the hydrogen concentration. In other words, the lattice constant increase in this interval is proportional to the hydrogen content. At lower hydrogen concentrations, the experimental results deviate more clearly from [PERSON] law, indicating that the low concentration case cannot be fully explained based on the bulk phase solubility. As the metal Pd surface is more capable of capturing hydrogen molecules at lower concentrations, more molecules are adsorbed on the Pd surface; thus, it agrees with the Langmuir equation. The sensitivity of the improved sensor reached 6-10% (20 ppm). The hydrogenation of Pd leads to material strain, and the strain changes the optical range of the transmitted light, causing the stripe to move. In 2015, [PERSON] et al. [64] proposed an fiber-optic F-P hydrogen sensor based on Pd-Y film by measuring the streak contrast variation at different hydrogen volume fractions. The experimental results showed that the sensor has high sensitivity and the performance of the Pd-Y film is better than that of the Pd film. A 0.5 dB reduction in streak contrast was detected when the hydrogen volume fraction varied from 0 to 5.5%, and the sensor's temperature response was also measured. [PERSON] et al. [65] proposed the Sagnac interferometer fiber-optic hydrogen sensor and conducted an experimental study of the sensor performance.