# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 5 Fiber-Optic Hydrogen Sensors
### Intensity Modulated Fiber-Optic Sensors
Several similar [PERSON] fiber-optic sensors are also given in Figure 10, the first two of which have the simplest structure, while the last two are slightly more complex. Figure 10c uses multi- and single-mode fiber coupling, as shown in Figure 10d with wedge fiber. The sensing principle can also be understood in terms of numerical aperture variation for sensors of this type of structure. Due to the Pd-H\({}_{2}\) interaction, the numerical aperture of the fiber is changed, which, in turn, leads to a change in the intensity of the transmitted light. Figure 10: Schematic diagram of [PERSON] et al.’s fiber-optic sensor. **(a)** Traditional Structure. **(b)** Traditional Structure. **(c)** Multi-film and single-film fiber coupling. **(d)** Wedge Fiber. The wedge fiber sensor shown in Figure 10d was studied in more depth by [PERSON] et al. [75]. Their studies included multi- and single-mode wedge fibers [76,77], as well as nanoparticle wedge fiber. They fabricated single-mode wedge fiber sensors with a film thickness of 12 nm and an interaction length of 1.5 cm. The variation in optical transmittance versus H\({}_{2}\) concentration was measured for wedge sensors with different tapers using Ar carrier gas, with a sample gas varying from 1.8 to 10% concentration. They found that this sensor does not change the polarization characteristics of the transmitted light. They also investigated the single-mode wedge fiber dispersion effect and found that the transmitted light dispersion effect was significant for light wavelengths in the 900-1600 nm range. The experimental results of [PERSON] et al. also showed that although the sensitivity of the multi-mode wedge fiber sensor was lower than that of the single-mode fiber, the stability and ease of use of the multi-mode fiber sensor were much better. They also fabricated multiple multimode wedge fiber sensors with a film thickness of 15 nm, an interaction length of 1 cm, and a wedge fiber girdle diameter between 30 \(\upmu\)m and 70 \(\upmu\)m. The light source was a LED lamp with a power of 20 \(\upmu\)w and a wavelength of 850 nm, and the light transmission rate of the wedge sensors with different tapers was obtained as a function of concentration by measuring the Ar carrier gas with a sample gas in the concentration variation range of 0.3-3.5%. It was found that the sensor transmission intensity increased with the hydrogenation of Pd; however, the light signal saturated after the concentration exceeded a certain value. The response time was 30 s and the recovery time was 90 s at a concentration of 2%; after repeated use of the sensor, the response time and the response time of the sensor was 1.5-2.5 ms. The response time was 1.5 ms. The  
recovery time were 40 s and 100 s, respectively, and the response time and recovery time increased after multiple uses compared with the first use. The researchers also fabricated a nanoparticle wedge fiber with a film thickness of 4 nm, an interaction length of 2 mm, and a waist spot diameter of 1300 nm. The optical transmittance was measured as a function of concentration using a semiconductor laser with a wavelength of 1550 nm and an Ar carrier gas with a concentration range of 0.8 to 5.2% of the sample gas. In contrast to several other types of sensors, the light intensity of the nanoparticle sensor decreases with increasing hydrogen concentration. The advantage of wedge-shaped fiber-optic sensors is the short waist interaction length and sensitive sensing; however, at the same time, due to the special structure, it is easy to cause waste breakage [78; 79]. In 2007, [PERSON] et al. [80] developed a structured fiber optic-sensor based on the light wave abruptness effect, which has high stability and sensitivity. They used small segments of single-mode fiber in the length range of 3-8 mm and coated the fiber surface with a Pd film and a Pd alloy film with a film thickness of 10 nm. This thickly coated single-mode fiber segment was then sandwiched between two multi-mode fibers (as shown in Figure 10b).