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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 100atm, 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 ∼300 K to 4 K and back. Additional samples were subjected to either one or three temperature cycles and held at 4 K 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.