0733-8724 (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JLT.2019.2939260, Journal of Lightwave Technology JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 8, AUGUST 2015 1 Ultra-Broadband and Compact Two-Mode Multiplexer Based on Subwavelength-Grating-Slot-Assisted Adiabatic Coupler for the Silicon-on-Insulator Platform Luhua Xu, Yun Wang, Deng Mao, Jinsong Zhang, Zhenping Xing, Eslam El-Fiky, Md Ghulam Saber, Amar Kumar, Yannick D’Mello, Maxime Jacques, and David V. Plant, Fellow, IEEE Abstract—We design and experimentally demonstrate an ultra-broadband and compact two-mode multiplexer based on subwavelength-grating-slot-assisted adiabatic coupler for the silicon-on-insulator platform. The introduction of subwavelength- grating slot effectively increases the refractive index of the gap region between two strip waveguides and enhances the coupling between them, leading to high modal transmission over a short mode-evolution region, particularly at the short-wavelength regime. As a result, our two-mode multiplexer has a compact design footprint, with a mode-evolution region length of only 25 μm and an entire device length of only 55 μm. Simulation shows that the mode-division multiplexing (MDM) link formed by the designed two-mode multiplexers has a record bandwidth of 740 nm spanning from 1260 nm to 2000 nm, where the crosstalk is lower than -18.5 dB and the insertion loss (IL) is lower than 0.32 dB. The MDM link has been fabricated using electron beam lithography and achieves measured crosstalk lower than -18.8 dB and IL lower than 2.6 dB, over a 100 nm bandwidth from 1260 nm to 1360 nm that covers the O-band and a 130 nm bandwidth from 1500 nm to 1630 nm that covers the C-band and the L-band. Index Terms—Mode multiplexer, subwavelength grating, adi- abatic coupler, silicon photonics. I. I NTRODUCTION P HOTONIC integrated circuit based on the silicon-on- insulator (SOI) platform, which features compatibility with the mature complementary metal-oxide-semiconductor (CMOS) fabrication process, is an enabling technology for high-density and cost-effective optical interconnect. To in- crease the transmission capacity of optical communication Manuscript received April 19, 2005; revised August 26, 2015. L. Xu, Y. Wang, D. Mao, J. Zhang, Z. Xing, M. G. Saber, A. Kumar, Y. D’Mello, M. Jacques, and D. V. Plant are with the Photonic Systems Group, Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 0E9, Canada (e-mail: luhua.xu@mail.mcgill.ca; yun.wang5@mcgill.ca; deng.mao@mail.mcgill.ca; jinsong.zhang@mail.mcgill.ca; zhenping.xing@mail.mcgill.ca; md.saber@mail.mcgill.ca; amar.kumar@mail.mcgill.ca; yan- nick.dmello@mail.mcgill.ca; maxime.jacques@mail.mcgill.ca; david.plant@mcgill.ca). E. El-Fiky is with the Photonic Systems Group, Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 0E9, Canada. He is also with the Electrical Engineering Department, Alexandria University, Alexandria, 21544, Egypt (e-mail: eslam.elfiky@mail.mcgill.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2019.XXXXXXX link, multiplexing schemes such as polarization-division mul- tiplexing [1] and wavelength-division multiplexing [2] have been studied extensively. Recently, mode-division multiplex- ing (MDM) has attracted growing attention due to its capa- bility to further enhance the link capacity, by utilizing higher- order modes for each wavelength [3]. The mode multiplexer, which can multiplex independent incoming signals into differ- ent mode channels, is one of the fundamental building blocks for the MDM system. A wide variety of approaches have been proposed to realize mode multiplexers on the SOI platform, such as asymmetric Y-junctions [4]–[9], asymmetric directional couplers [10]– [23], contra-directional couplers [24], multimode interference (MMI) couplers [25]–[29], photonic crystals [30], inverse design [31]–[33], and adiabatic couplers [34]–[41]. Among them, the adiabatic coupler approach has the advantages of broad bandwidth and relaxed fabrication tolerances due to its mode evolution nature, making it a competitive structure for implementing the mode multiplexing function. In addi- tion to mode multiplexers, adiabatic couplers have also been intensively used to realize 3-dB couplers [42]–[54] as well as polarization splitter-rotators [55]–[65]. However, in order to ensure adiabatic evolution of the local modes, the length of the mode-evolution region of an adiabatic coupler has to be sufficiently long, resulting in occupation of a large chip area. Therefore, it is of interest to develop a broadband mode multiplexer based on adiabatic coupler with a compact device footprint. Subwavelength grating (SWG) is a one-dimensional peri- odic structure where the diffraction effect arising from the periodicity is strongly suppressed by using a grating pitch small enough compared to the operating wavelength [66]. SWG could be treated as equivalent to a homogeneous material whose refractive index is related to the duty cycle of the SWG. Therefore, SWG has the flexibility to engineer the refractive index without changing the material composition. SWG has found wide application in devices fabricated on the SOI platform, such as directional couplers [67]–[69], MMI couplers [70]–[75], grating couplers [76]–[79], edge couplers [80], multimode waveguide bends [81], polarization beam splitters [82]–[85], and polarization splitter-rotators [86]–[88]. Recently, noticeable research effort has been focused on the employment of SWG for 3-dB couplers based on adiabatic