When used on high-speed rail or bridges, will long-term severe vibrations loosen the optical fiber?
In engineering environments with long-term severe vibration, dynamic alternating loads, and impacts, such as high-speed railways and large bridges, the stability and durability of fiber optic systems are key considerations in optical engineering design.
I. Principles of Vibration’s Impact on Physical Properties of Optical Fibers and Fiber Optic Systems
Silica (quartz) optical fibers themselves possess extremely high tensile strength. However, without special protective packaging, long-term intense vibration primarily poses the following three engineering risks to optical fibers and their systems:
- Relative Displacement (Loosening) Between Optical Fiber and Protective Sheath
If traditional loose tubes or incompletely filled plastic sheathed fiber optic cables are used, high-frequency or continuous impact vibrations can cause the optical fiber to experience axial sliding, creep, or microbend accumulation within the tube. This not only alters the local strain state of the fiber but also impairs the strain transmission efficiency during sensing measurements. - Microbending Loss and Optical Signal Fluctuations
Under the action of dynamic vibration, the bending radius of the optical fiber undergoes small, high-frequency changes, known as microbending. Microbending causes some optical modes transmitted in the fiber core to leak into the cladding, resulting in dynamic fluctuations in optical signal attenuation or increased loss. - Connector Loosening and Stress Corrosion Fatigue
Vibration can easily lead to micro-displacements or physical loosening of fiber optic connectors (such as ferrule connectors, threaded connections), affecting the precision of interface alignment. Furthermore, at fixed clamping points or where diameter changes occur, long-term alternating stress (e.g., vibration frequencies maintained within the range of f > 10\text{ Hz} to 100\text{ Hz}) can cause the expansion of micro-cracks on the surface of the silica (stress corrosion fatigue), leading to fiber material fracture.
II. Engineering Protection Mechanisms for High-Vibration Environments
To prevent long-term severe vibration from causing fiber optic loosening or damage, engineering practices commonly employ Metal Armored Encapsulation (FIMT - Fiber in Metal Tube) and Polymer Curing Integration techniques:
- Mechanical Isolation:
Utilizing elastic alloy tubes or seamless stainless steel tubes as protective barriers to isolate external mechanical impacts and shear forces. - Physical Locking:
Through the interference fit of metal tubes or tight-fitting processes, the optical fiber is fixed integrally with the protective structure, completely eliminating relative displacement and loosening between the optical fiber and the sheath.
III. Official Related Products and Technical References
OFSCN® has developed armored fiber optic sensing and transmission products with strong anti-vibration characteristics for high-vibration and harsh engineering environments:
1. OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor
This product uses an elastic alloy tube to encapsulate Fiber Bragg Gratings, specifically for strain monitoring in high-vibration environments such as bridges and high-speed railway tracks. The metal tube structure not only withstands high-frequency impacts but also ensures the fiber is fixed within the tube without loosening, achieving high-fidelity strain transmission.
2. OFSCN® 85°C Seamless Steel Tube Fiber Cable
Utilizes a single-layer stainless steel seamless steel tube (default outer diameter 2.0\text{mm} or 3.0\text{mm}) for protection, acting as a high-strength distributed sensing or signal transmission optical cable. The seamless steel tube structure provides extremely high rigidity and resistance to vibration and loosening, effectively preventing fiber optic loosening and mechanical damage in harsh vibration environments.
