Optical fibers are commonly used for data transmission and sensing in industrial, geophysical, and aerospace markets, where they may be employed in high vacuum and cryogenic environments. The performance and integrity of optical fibers and their coatings is well understood over temperatures of ≈40 to 300 °C and pressures up to 100 atm, but their characteristics at cryogenic temperatures under high vacuum remain relatively unexplored. This study investigates the optical and mechanical reliability of selected fibers operating at cryogenic temperatures. The fiber samples under investigation were prepared with either an acrylate or polyimide coating. Several properties of the fibers were assessed, including optical loss, mechanical strength, and coating integrity. Optical loss was monitored continuously over a single temperature cycle from 300K to 4K and back. Additional samples were subjected to either one or three temperature cycles and held at 4K for extended periods. Mechanical strength of the thermally cycled fibers was determined via a 2-point bend method, and the coating material was characterized using Fourier transform infrared spectroscopy and thermogravimetric analysis.
Geophysical applications of optical fibers for distributed temperature, strain, and acoustic sensing challenges their reliability due to harsh environmental conditions. Which may include high temperature, pressure, presence of hot steam, hydrogen, and other aggressive chemicals. Robustness of silica-based optical fibers is primarily governed by the thermal and environmental stability of their polymer coatings. Among different types of coatings, polyimide materials exhibit favorable properties, such as durability at elevated temperatures, protection against solvents, and long-term mechanical reliability. In this work we investigate optical fibers employed with a novel polyimide coating. Extensive environmental testing was performed, comparing fibers with a standard and the novel coating. Fiber samples were aged in dry air (up to 380 °C), high temperature/pressure water, paraffin oil, crude oil, hydrogen scavenging cable gel and isopropyl alcohol (all up to 300 °C/2000 psi). Mechanical strength of the aged fibers was used as a measure of their performance at harsh conditions. In addition, we studied an adhesion that develops at elevated temperatures between the fibers and a stainlesssteel tube interior. Thermal stability of the polyimides was also evaluated via thermogravimetric analysis. Based on the obtained results, the novel polyimide coating shows a 35 – 38 °C improvement over the standard coating. The findings indicate the superiority of the new coating, which should extend the useful temperature range for this class of optical fibers.
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