Experimental demonstration of a flexible and high-performance mode-order converter using subwavelength grating metamaterials.

Mode-order converters, transforming a given mode into the desired mode, have an important implication for the multimode division multiplexing technology. Considerable mode-order conversion schemes have been reported on the silicon-on-insulator platform. However, most of them can only convert the fundamental mode to one or two specific higher-order modes with low scalability and flexibility, and the mode conversion between higher-order modes cannot be achieved unless a total redesign or a cascade is carried out. Here, a universal and scalable mode-order converting scheme is proposed by using subwavelength grating metamaterials (SWGMs) sandwiched by tapered-down input and tapered-up output tapers. In this scheme, the SWGMs region can convert, TEp mode guided from a tapered-down taper, into a TE-like-mode-field (TLMF) and vice versa. Thereupon, a TEp-to-TEq mode conversion can be realized by a two-step process of TEp-to-TLMF and then TLMF-to-TEq, where input tapers, output tapers, and SWGMs are carefully engineered. As examples, the TE-to-TE, TE-to-TE, TE-to-TE, TE-to-TE, and TE-to-TE converters, with ultracompact lengths of 3.436-7.71 µm, are reported and experimentally demonstrated. Measurements exhibit low insertion losses of < 1.8 dB and reasonable crosstalks of < -15 dB over 100-nm, 38-nm, 25-nm, 45-nm, and 24-nm working bandwidths. The proposed mode-order converting scheme shows great universality/scalability for on-chip flexible mode-order conversions, which holds great promise for optical multimode based technologies.