# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 5 Fiber-Optic Hydrogen Sensors
### Intensity Modulated Fiber-Optic Sensors  
In 1991, [PERSON] et al. [66] developed an intensity-modulated fiber-optic sensor. This sensor uses a miniature reflector as the sensing surface, and its structure principle is shown in Figure 9. One port of the multimode fiber is coated with a Pd film with a thickness of 10 nm and a diameter of 125 \(\upmu\)m. The Pd film is used as a sensing surface, on one hand, and a light-reflecting surface, on the other hand, as shown in Figure 9a. The other part of the fiber is connected to a Y-shaped coupler, which is coupled to the other two fibers. One fiber is used as the input light, which is connected to the light source, and the other fiber is used as the output signal light, which is connected to the photoelectric converter. When the sensor is placed in an H\({}_{2}\) environment, the reflectivity R (or transmittance T) of the Pd film is changed due to the Pd-H\({}_{2}\) interaction, and the amount of change in reflectivity is related to the hydrogen concentration. Therefore, the amount of change in output light intensity is detected to obtain information on hydrogen concentration. Figures 9b and 10c show the variations in reflectance with time and hydrogen concentration, respectively, which were obtained experimentally. As can be seen from the figures, the reflectivity decreases with increasing concentration, and the response time varies for different concentrations. Of particular importance is the fact that the relative amount of change in reflectance in correlation with concentration is in good agreement with the P-C-T plot of Pd. This finding indicates that the relative value of the change in reflectance can be used to test the components of palladium hydride [67]. This type of sensor has the simplest structure and stable performance [68]. From the previous experimental results, it is clear that the sensing effect of microscopic mirrors depends on the extent to which the Pd-H\({}_{2}\) interaction affects the reflectance or transmittance of the mirrors. [PERSON] et al. [69] studied the optical reflectance of Pd films with thicknesses between 1 nm and 30 nm. They performed single- and multiple-repetitive adsorption/desorption for Pd films of different thicknesses to characterize the phase change process for Pd films from \(\alpha\)-phase to \(\beta\)-phase for a different number of cycles. The results demonstrate that the variation in reflectance is related to various factors, such as film thickness, film structure, and substrate properties. [PERSON] [70] reported the effect of film surface microstructure on reflectance (or transmittance). The study selected Pd films with thicknesses ranging from 10 to 70 nm and observed that when the Pd film adsorbs H\({}_{2}\), molecules, microbubbles, and microcracks appear on the film surface due to the interaction between the two factors, with these two defects directly affecting the reflectivity of the film surface. They also selected thick Pd films with film thicknesses over 100 nm and found microbubbles and micro cracks on the film surface. Comparing such micro-defects in thin and thick films, the results show that micro-bubbles and micro-cracks appear in thin films after adsorption, and the defects on the film surface disappear after desorption, i.e., the defects on the film surface are reversible, while the defects in thick films are permanent mechanical damage and are irreversible. To eliminate such micro-defects, a 1-2 nm thick Ni film is plated on the fiber end face, while a Pd film is plated on top of the Ni film. The experimental results show that the reflectivity of Ni/Pd film decreases after Ni coating, and the height and width of the slowly changing reflectivity interval are different. The adhesion of Pd film to the cladding affects the microstructure of the film surface, which affects the sensing performance. The adhesion of Pd on the quartz substrate is smaller than that on the Ni substrate, and increasing the adhesion can eliminate microbubbles and micro cracks on the film surface. The study by [PERSON] et al.