# Optical Sensing Technologies for Cryo-Tank Composite Structural Element Analysis and Maintenance
## 3 Test Article Description
### Sensors Performance  
Fiber optic sensors are selected as they have well-established advantages. Nevertheless, in this application, the extremely \"on the edge\" operating conditions of a cryo-tank storing liquid hydrogen during flight would expose to the monitoring system to an extremely harsh environment, potentially explosive, characterized by a temperature window ranging from cryogenic to high temperatures.  
For this reason, among the requirements for the selection of specialized sensors, it is mandatory to install devices without affecting the overall gravimetric index, while ensuring safety in a harsh environment and minimizing hydrogen darkening effects, which in turn would produce power losses and signal absence. In [25], the influence of hydrogen gas on fiber Bragg grating (FBG)-based optical fiber sensors has been validated experimentally. The paper demonstrates that the refractive index changes due to the presence of hydrogen in the micro-holes and its penetration into the fiberglass.  
To preliminarily evaluate the sensor's performance and functionality, the first step is to test the Bragg grating in the presence of gaseous hydrogen. The sensors are put inside a pressurized tank under a hydrogen atmosphere at a pressure of 4 bar. After the filling-emptying cycle, the tank is filled with hydrogen, which is kept inside the tank for approximately 2 days. After this time, the hydrogen is removed from the tank using nitrogen, following the same procedure explained before (filling-emptying cycles).  
Absorption may occur at specific wavelengths and is observed as an increase in attenuation. In this case, a pure silica core single-mode fiber with carbon-polyimide coating is specifically designed for hydrogen-rich environments. The silica core prevents photodarkening effects, which are normally associated with germanium-doped fibers [26]. As shown in Figure 6, the central wavelengths exhibiting lower hydrogen absorption are 1530 nm 1550 nm and 1570 nm for the FBGs, while in the case of distributed fiber, the internal laser module emitted wavelengths swept from 1510 to 1570 nm which is in line with the observation.  
The functionality test campaign is then conducted in order to verify the sensing response at specific thermo-mechanical loading conditions, after hydrogen exposure.  
The sensors are tested in a controlled chamber in two steps. First \(-\)70 Celsius (Figure 7a) is targeted. A thermocouple is used as a reference sensor to calibrate the FO output. The fiber optic is left unstrained in the chamber to experience only thermo-optic deformations driven by its CTE (thermal expansion coefficient and refractive index variations). The gauge factor used to convert wavelength to temperature is 10.021 pm/\({}^{\circ}\)C, providing a linear response and consistent temperature values concerning the reference. In the second step, the temperature is cooled down to cryogenic conditions to verify the optical signal stability at \(-\)160 Celsius again (Figure 7b) and to simulate the operating working scenario.  
At this point, to assess possible measurement deviations or uncertainty recorded before and after these exposures, a simple tension solicitation is applied to the sensors, comparing strain signals before and after both hydrogen conditioning and the cryogenic bath. The strain signal comparison is reported for the FBG (Figure 8a) and for the distributed fiber (Figure 8b), showing very good consistency.  
Figure 6: Optical hydrogen losses for very long exposition at different central wavelengths (courtesy of GhT Photonics s.r.l.).  
Figure 7: Fiber optic temperature signal: (**a**) the temperature calibration at \(-\)70 Celsius for FBG sensor; (**b**) the temperature signal stability of FBGs during cryogenic tests.