Three‐Dimensional Ultra‐Compact Mode Converter Enabled by Femtosecond Laser Differential Exposure

Multimode photonic devices hold significant value and promising potential in communications, sensing, and computing. However, efficient on‐chip excitation of higher‐order modes remains a critical challenge. Here, we propose a 3D mode converter based on a femtosecond laser differential exposure (FLDE) strategy, where localized high‐refractive‐index regions are directly inscribed within glass. This enables the introduction of a π ‐phase delay without occupying additional space, achieving ultracompact and path‐independent conversion from the LP 01 mode to LP 11 /LP 21 modes. With a record‐small footprint of 2.52 × 10 −4 mm 3 for femtosecond direct laser‐written on‐chip converters, the device simultaneously achieves excellent mode purity (94.18%) and broad‐band polarization insensitivity (780–1000 nm). This work provides a robust and scalable platform for achieving high integration density and functional flexibility in future photonic integrated circuits.