Ultra-compact dual-polarization silicon mode-order converter.

The mode-order converter is a building block in a multimode optical transmission and switching system. It can be used for switching signals carried on different mode channels. However, such devices constructed by conventional structures can commonly accomplish one mode-order conversion process. It is because the phase-matching condition, which is utilized by a majority of designs, can usually be fulfilled between only one mode pair for specific device geometry. In this Letter, we propose a dual-polarization mode-order converter referring to the concept of a silicon planar metasurface. It can realize mode-order conversions on transverse electrical and transverse magnetic polarizations in parallel. In order to verify our concept, we design and experimentally demonstrate a prototype that can realize conversions from TE0 to TE1 and from TM0 to TM1 simultaneously. The footprint is 4  μm×1.6  μm. The measured insertion losses for both polarizations are smaller than 2.3 dB, and the crosstalk is lower than -11.5  dB within the wavelength range of 1525-1565 nm. We envision that the device can be a building block in polarization and mode multiplexed optical switching systems and endow the systems with simpler structure and a more compact footprint.