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Silicon photonics interconnects have emerged as a promising way to exceed the capacity limits imposed by copper interconnects, exploiting different multiplexing technologies like wavelength division multiplexing (WDM) and polarization division multiplexing (PDM). However, the bandwidth demand is still growing and new multiplexing technologies like Mode division multiplexing (MDM) are required to overcome these limitations, enabling the transmission and reception of multiple modes through a single multi-mode waveguide. Several architectures have been proposed to perform mode multiplexing like asymmetrical directional couplers and architectures based on conventional MMIs, which show a narrowband performance. Recently, adiabatic and counter-tapered couplers have been presented showing a broader bandwidth, but both architectures suffer from large footprints. For this reason, a compact mode multiplexer with low insertion losses and low crosstalk over a broad bandwidth is still sought after.
In this work, we present an ultra-broadband two-mode division (de)multiplexer (DE/MUX) that overcomes the restrictions imposed by conventional MMIs by means of sub-wavelength grating waveguides (SWG). SWG structures are composed by a disposition of different alternating materials that are repeated periodically with a pitch smaller than the operation wavelength enabling dispersion engineering. The structure of our broadband mode multiplexer is composed by a sub-wavelength engineered MMI, a 90º phase shifter (PS) and a symmetric Y-junction supporting the first two modes (TE0 and TE1) at the stem. By properly choosing the duty cycle (DC) and the pitch (Λ) of the SWG section of the sub-wavelength engineered MMI, an almost flat beat length can be achieved and, subsequently, a broader operation bandwidth. SWG tapers are included in order to perform an adiabatic transition between the non-periodic waveguides and the periodic structures of the SWG-MMI. On the other hand, the PS consists of two parallel waveguides, where the upper arm comprises two tapers in back-to-back configuration, whereas the lower arm is a straight waveguide. The operation principle of the device working as a MUX is as follows: the TE0 mode injected through the lower (upper) port of the SWG-MMI is equally split with the same amplitude and a phase difference of -90º between the two output ports. The PS generates a +90º phase shift between the upper and lower arms. Therefore, the modes arrives in-phase (out-of-phase) at the Y-junction, producing the output TE0 (TE1) mode.
Silicon on Insulator (SOI) platform was considered for the design of the proposed two-mode DE/MUX with 220-nm-thick and 500-nm-wide waveguides surrounded by SiO2 substrate and cover. Full-3D-simulations of the device working as DEMUX show that insertion loss are less than 0.84 dB (0.61 dB) for the TE0 (TE1) mode in the [1.4 - 1.7] μm wavelength range, and less than 0.49 dB within [1.5 – 1.6] μm. The crosstalk of the two modes is below -20.29 dB in the [1.4 - 1.7] μm wavelength range and decreases down to -27.41 dB within [1.5 – 1.6] μm. In conclusion, we have proposed a SWG based ultra-broadband two-mode DE/MUX with a 300 nm bandwidth and a footprint as small as 36 x 3.7 μm.
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