Fig. 1. Sensing system and WFBG cryogenic properties. (a) Schematic of WFBG-based sensing system; (b) reflection spectra, (c) reflectivity variations, and (d) wavelength shifts of acrylate coated WFBG and bare WFBG at different temperatures

Fig. 2. Influencing factors of optical fiber loss at cryogenic temperature. (a) Schematic of series-connected WFBGs with fusion joints; images of optical fibers with bending diameter of (b) 10 cm at room temperature and with bending diameters of (d) 10 cm, (f) 6.7 cm, and (h) 5 cm at LN₂ temperature, corresponding to the reflection spectra of (c), (e), (g), and (i)

Fig. 3. Encapsulation and calibration of WFBG sensor. (a) Image of a customized strain sensor; (b) substrate encapsulation schematic of strain sensor; (c) tube encapsulation schematic of temperature sensor; (d) quasi-static tensile testing system at cryogenic temperature

Fig. 4. WFBG sensor calibration. (a) Wavelength varying with applied strain in LN₂ after second-order polynomial fit of experimental data; (b) wavelength varying with temperature after third-order polynomial fit of experimental data (inset: results near LN₂ temperature)

Fig. 5. Schematic of test platform for cryogenic static test. (a) Position of oxygen tank in launch vehicle; (b) stress state of oxygen tank during operation; (c) ground test platform for simulating stress state of oxygen tank

Fig. 6. Loading process of axial force, F₁, F₂, and internal pressure during pressurization (inset: pressurization schematic).

Fig. 7. Strain measurement in cryogenic static test. (a) Hoop strain at bottom of oxygen tank measured by WFBG1 strain sensor (inset: spectrum of three WFBGs); (b) error of hoop WFBG1 sensor; (c) longitudinal strain at welded line on side of oxygen tank measured by "WFBG2" strain sensor (Inset: spectrum of three WFBGs); (d) error of longitudinal "WFBG2" sensor

Fig. 8. Temperature variation measured by "WFBG3" temperature sensor (upper inset: reflection spectrum of three WFBGs; lower inset: temperature fluctuation during pressurization)