This study investigates the degradation of multimodal interferometric signal quality in sapphire fiber-based Fabry-Perot temperature sensors under extreme operational conditions. Through rigorous analysis, we elucidate the mechanistic influence of optical fiber parameters and source characteristics on interferometric signal fidelity. Our findings reveal that increased fiber length, core diameter, and numerical aperture significantly exacerbate modal dispersion and phase noise, thereby diminishing fringe visibility. Concurrently, source spectral broadening directly impairs temporal coherence, further compromising signal integrity. To overcome the fundamental challenge of multimodal coherence control, we pioneer a novel spatial coherence measurement system based on shear interferometry of sapphire fiber output. By implementing non-uniform beam path separation and phase modulation compensation, we achieve precise quantification of complex coherence modulus—a critical advancement in interferometric metrology. This work establishes a theoretical framework and provides innovative methodologies for enhancing high-temperature sensing precision, with direct applications in next-generation marine propulsion systems for the renewable energy sector.