Compact, Scalable and Flexible Multi-Mode-Converting Model Employing Subwavelength Gratings

Orthogonal eigenmodes in multimode waveguides are an indispensable physical dimension for enhancing the capacity of optical communications, and efficient mode converters are therefore of vital functional significance. However, the conventional single-pair mode conversion in a single device is having difficulty meeting the ever-increasing demand for optical capacity. By contrast, the multi-pair mode converter can improve the multiplexing capacity and be further applied in spatial-mode-oriented passive signal switching and permutation cipher. Here, a scalable multi-mode converting model, combing the multimode interference and beam shaping by exploiting subwavelength metamaterials, is firstly reported to enable simultaneous multi-spatial-mode manipulation and break its polarization-dependency limit. Based on this model, five proof-of-concept converters are designed, realizing polarization-independent, and multi-mode conversions, or a combination of both within compact lengths (< 17.436 μm). For the first time, TE₀/TM₀-to-TE₂/TM₂, TE₀/TE₁/TE₂-to-TE₂/TE₃/ TE₄, TE₀/TE₁/TM₀/TM₁-to-TE₂/TE₃/TM₂/TM₃, and TE₀/TE₁-to-TE₁/TE₂ converters are reported and experimentally demonstrated, covering a double/triple/quadruple-mode manipulation in a single device. From measurements, these converters exhibit low insertion losses (0.72 to 1.65 dB) and intermodal crosstalks (−30.26 to −12.19 dB) at 1550 nm, and have a 56-nm/30-nm bandwidth of loss < 1.4 dB/1.9 dB and crosstalk < −13 dB/−10.5 dB, for the best/worst case. The proposed multi-mode converting model can manipulate multiple input modes in parallel and beak the polarization-dependency limit for multi-spatial-mode conversion, which holds great potential for spatial-mode-oriented nanophotonic systems.