Broadband mode-division (de)multiplexer using nanorod-assisted multimode subwavelength gratings

Abstract

—Subwavelength grating (SWG) allows flexible tuning of the dispersion of waveguides, enabling wider bandwidths for directional coupler-based devices. However, its light field constraint capability for high-order modes is so weak that the mode scattering loss from sidewalls is high, which impedes low-loss transmission and efficient energy coupling from adjacent waveguides. In this paper, we propose nanorod-assisted multimode subwavelength grating (NSWG) waveguides that support stable high-order modes transmission like strip waveguides, and maintain refractive index tunability similar to SWGs. We fully analyze the mode transmission and coupling characteristics of NSWG waveguides for modes TE1, TE2, and TE3. Results show that such waveguides can hold high transmission fidelity and low losses for high-order modes (<0.33 dB/cm, 0.58 dB/cm, and 1.0 dB/cm for TE1, TE2, and TE3). Arbitrary high-order modes among NSWGs can also be excited efficiently by evanescent field coupling. As proofs of concept, we also demonstrate a three-channel mode-division multiplexer with broad bandwidth (136 nm/135 nm/100 nm), low insertion loss (0.29 dB/0.85 dB/0.62 dB), and crosstalk (-21.4 dB/-20.4 dB/-18.0 dB) for TE0/TE1/TE2. And the device has a compact footprint of 5 × 61 μm. At 1.55 μm, all three modes satisfy IL < 0.19 dB and CT < -20.0 dB. The proposed waveguide structures may provide an alternative building block for broadband and compact multimode on-chip directional coupler-based devices.