ABSTRACT 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.
Duan et al. (Wed,) studied this question.