Enhanced cryogenic strain sensing of encapsulated distributed optical fiber sensors for Rayleigh-scattering-based measurements

Abstract

Distributed optical fiber sensors (DOFSs) have emerged as key tools for safeguarding the stability and safety of cryogenic structures, attributable to their exceptional sensing performance. However, traditional protective layers used for long-term strain monitoring are unable to meet the demands of cryogenic embedment, primarily due to excessive shear force. Once the embedded DOFSs are damaged or fractured, the resulting strain measurements are rendered invalid. This study addresses the issue by exploring an enhanced strain DOFS encapsulated within minuscule capillary tubing, thereby preserving its mechanical properties in an embedded configuration. A theoretical strain transfer model for a multi-layer packaged DOFS was established to elucidate the effects of mechanical parameters and temperature variations on strain transmission, complemented by extensive mechanical experiments conducted at both room and cryogenic temperatures to inform the design of appropriate packaging processes. In addition, strain measurements from encapsulated DOFSs and polyimide-coated DOFSs were compared under nonuniform thermal stress. Furthermore, the long-term monitoring reliability was verified through multiple thermal shock cycles applied to the encapsulated DOFSs. The experimental results demonstrated that the encapsulated DOFS exhibits high sensitivity to both mechanical and thermal strains. Furthermore, its simplified installation and compact form factor enable deployment within devices for real-time strain monitoring. This advancement not only enhances the endurance capacity of the DOFS but also provides a robust, non-invasive means for precise analysis of internal strain under harsh conditions.