LAUSR

The integration of 2D materials with optical fibers enables multifunctional fiber devices, such as polarizers, modulators, and sensors. Recent advances in direct vapor deposition growth further enhance light‐2D material interactions to centimeter‐scale lengths, overcoming the micrometer‐scale limitations of transferred 2D materials. However, conventional methods for growing 2D materials in fibers typically produce isotropic material architectures due to uniform precursor deposition, limiting applications that require birefringence, such as a phase retarder. Here, a selective vapor deposition method is proposed to realize the non‐circular symmetric growth of 2D molybdenum disulfide (MoS2) into photonic crystal fibers (PCFs), achieving anisotropy‐engineered phase retardation. The high refractive index of MoS2 efficiently breaks the degeneracy of polarization modes in PCF and enables phase retardation with a manageable beat length of ≈7.7 cm. The MoS2‐PCF phase retarder reliably works in varying conditions, including outdoor exposure, large deformation, and high temperature/humidity. Its phase retardation exhibits an extremely small fluctuation of ≈3.3° between 25 and 200 °C, which is two orders of magnitude lower than that of commercial polarization‐maintaining fibers (≈2.0°/°C). The work provides a new solution for the fiber device preparation and pave the way for robust polarization manipulation in all‐fiber systems.