Quantitative characterization of thermo-optically modulated microbend loss in silica fibers for high-temperature deep-well telemetry.

To mitigate the nonlinear amplification of fiber micro-bend loss in high-temperature deep-well environments (303.15 ~ 483.15 K), this study establishes an equivalent refractive index model integrating thermal, mechanical, and optical fields. The model incorporates thermo-optic effects(TOEs) directly into the micro-bend equivalent refractive index expression, facilitating unified modeling of coupled thermal and bending perturbations. Finite element eigenmode analysis was employed to quantify the synergistic effects of bending radius (0.1 ~ 1.0 mm) and temperature on radiation leakage and mode coupling. Results demonstrate that temperature does not act as an independent loss term. Instead, it reconfigures waveguide confinement by modulating the core-cladding refractive index profile, thereby altering mode-coupling pathways. Multimode fiber (MMF) exhibits oscillatory loss governed by inter-modal phase matching, with attenuation coefficients displaying non-monotonic temperature dependence. Conversely, single-mode fiber (SMF) demonstrates threshold-type loss surge at temperatures T ≥ 440 K. Spectral and phase analyses reveal that MMF undergoes significant phase decorrelation and topological discontinuity under stochastic mode coupling, whereas SMF maintains phase continuity even under high-loss conditions, demonstrating superior coherence stability. Furthermore, wavelength scanning shows that long wavelengths (> 1.6 μm) significantly enhance SMF micro-bend sensitivity, while the stochastic coupling in MMFs is better suited for short-distance, intensity-modulated telemetry. This study establishes a quantitative correlation between temperature, bending, and wavelength, providing essential physical criteria and engineering guidelines for fiber selection and loss compensation in extreme downhole environments.