# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 5 Fiber-Optic Hydrogen Sensors
### Intensity Modulated Fiber-Optic Sensors
[71] shows that the nature of the substrate significantly affects the response time of the sensor. They produced a Pd/Si film with a maximum response time of 700 s, as well as a Pd/Al\({}_{2}\)O\({}_{3}\) film with a maximum response time of 3700 s. They concluded that surface roughness affects the microstructure of the Pd subsurface, as well as different substrates with different surface roughness values, thus affecting the response time of the sensor. The study of optical transmittance of new metal/metal hydride thin film materials gave new impetus to the use of Pd-film sensing materials. In 2006, [PERSON] et al. [72] developed an intensity fiber-optic sensor and applied the sensor to the Stennis. In 2006, [PERSON] et al. [73] developed an intensity fiber-opticsensor and applied it to the launch test of a one-time launch vehicle on Launcher IV at the Stennis Space Center. They fabricated the micro-reflector sensing surface with a porous substrate, meaning that the metal Pd could penetrate the porosity of the substrate. The porous material produced a palladium film with excellent repeatability and reliability. The evanescent wave type fiber-optic sensor uses a structured optical fiber with a coating on the fiber surface to achieve the measurement of hydrogen concentration using the evanescent wave effect of light waves in the light sparse medium inside the waveguide. In 1999, [PERSON] et al. [74] developed a class of sensors based on the evanescent wave effect in optical waveguides, with the structure shown in Figure 10b. They used a multimode fiber with a core diameter of 50 \(\upmu\)m, and the fiber was stripped of a small section (1.5 cm) of cladding and then coated with a Pd film at the stripped location with a thickness of 10 nm. A laser beam with a wavelength of 650 nm was used as the light source. The experiments  
Figure 9: Micro-reflector sensing structure and test results. (**a**) Pd film effect. (**b**) Reflectance time variation relationship. (**c**) Variation of reflectivity with hydrogen concentration. were performed with N\({}_{2}\) carrier gas, and the H\({}_{2}\) concentration varied from 0.2 to 0.6% of the sample gas. The experimental results are shown in Figure 11. The response process and the variation in signal intensity with the relative concentration of hydrogen are shown in Figure 11a,b, respectively. As can be seen from the figures, the response time of the sensor is 30 s, and the sensor can be reused with a recovery time of about 3 min. Meanwhile, the residual gas has a delay in detection of about 20 s. The transmitted light intensity increases with the concentration; however, when the concentration increases to a certain level, a saturation effect occurs. Physically, the Pd-H\({}_{2}\) interaction can cause a change in the dielectric coefficient of the material. When the imaginary part of the dielectric coefficient decreases, the light absorption coefficient of the material decreases, and the intensity of transmitted light increases. The intensity of transmitted light can be expressed as  
\[I_{t}=I_{0}\exp(-2r\Delta\alpha L) \tag{14}\]  
where \(I_{0}\) is the transmitted light intensity in the absence of hydrogen, \(r\) is the ratio of the evanescent wave intensity to the total intensity in the waveguide, and the magnitude of \(r\) is related to the sensor structure. Experiments show that the value of \(r\) varies widely, ranging from a few percentage points to several tens of percentage points. \(L\) is the interaction length (length of the Pd film) and A is the amount of variation in the absorption coefficient of the Pd film. Of course, with this type of sensor, the Pd-H\({}_{2}\) interaction not only causes a change in the imaginary part of the dielectric coefficient, but also a change in the real part of the dielectric coefficient. The change in the real part of the dielectric coefficient leads to a change in the transmitted light phase.