All‐Glass GRIN 3D Waveguide for Miniature Polarization Optics

Abstract Precise and compact polarization control is vital in fields ranging from classical optics to high‐dimensional physics. Fiber‐based polarization optics offer low‐loss and high flexibility but are limited in miniaturization and functional diversity, while planar integrated optics often suffer from polarization sensitivity and poor fiber compatibility. A 3D polarization optics platform is presented, based on laser‐written gradient‐index (GRIN) waveguides in bulk glass. This approach enables high‐fidelity polarization engineering with record‐low birefringence (2 × 10 −8 )—over an order of magnitude lower than standard fibers—and low coupling losses (as low as 0.02 dB per facet). Besides, this platform supports the high‐fidelity implementation of polarization‐encoded 3D photonic circuits, integrating essential polarization operations such as splitting, conversion, and arbitrary waveplate functions, by customizing polarization‐dependent GRIN waveguides with birefringence accuracy of 10 −8 and arbitrary rotation of the optical axis over a full 0‐π range. Leveraging these capabilities, controlled polarization retardance covering the entire fiber‐optic communication band (1030–1700 nm) is further demonstrated, and realize a unidirectional waveguide array via spatially engineered birefringence and coupling strength, achieving >20 dB crosstalk suppression without external fields. This platform bridges the gap between fiber and chip‐scale photonics, offering a versatile route to broadband optical interconnects, polarization‐encoded quantum optics, and nonreciprocal topological photonics.