Lithium niobate on insulator (LNOI) has emerged as a promising integrated photonic platform for on-chip optical interconnection and optical communications, which is due to its outstanding characteristics, such as excellent electro-optic properties and strong optical nonlinearity. Diverse important active photonic circuit components have been proposed, including electro-optic modulators and wavelength converters. In particular, the electro-optic bandwidth of the LNOI modulator has reached up to 100 GHz and beyond, thus improving the data capacity of optical links significantly. However, with the continuous data capacity growth, the modulation rate of a single modulator will be challenging to meet the application demands. Therefore, developing a series of advanced optical multiplexers is necessary to further improve the data capacity. Recently, some advanced optical (de)multiplexing technologies have been intensively studied and reported in the LNOI platform, such as wavelength division multiplexing, polarization division multiplexing, and mode division multiplexing. Multidimensional hybrid multiplexing technologies are more important for further improving the data capacity of optical links. Among them, the dual-polarization mode (de)multiplexer (MMUX) has become a promising device as it can achieve multichannel multiplexing by only employing a single-wavelength laser, which thus lessens the cost and power consumption of optical links. However, challenges remain in realizing dual-polarization MMUX in the LNOI platform compared with the silicon-on-insulator (SOI) platform. Specifically, mode hybridization will occur in special width waveguides due to the vertical asymmetry and material anisotropy of the LNOI platform, which will introduce a lot of mode crosstalk in the bus waveguide. Therefore, we propose and demonstrate a dual-polarization MMUX with six data channels in the LNOI platform, with a subwavelength-grating taper adopted to engineer the dispersion of different modes to suppress the mode hybridization.

We employ the finite element analysis method and 3D finite-difference time-domain (3D-FDTD) method. First, we calculate the effective refractive indices of different modes in the straight waveguide as a function of the width of the waveguide (Fig. 2). The results show that the mode hybridization occurs at some specific waveguide widths when the light propagates along with the Y direction, which will introduce inter-mode crosstalk when the waveguide width changes gradually. Then, we adopt the SWG taper waveguide to suppress the mode hybridization. To design the SWG taper waveguide, we input a 1550 nm light with TM₀ and TE₀ mode to the left of the taper waveguide, and simulate the insertion loss and crosstalk of TM₀ and TE₀ mode channel as a function of the length of the SWG taper and ordinary taper waveguides respectively by the 3D-FDTD method (Fig. 5). Meanwhile, we select the 3D-FDTD simulation method to optimize the width of different taper waveguides and the coupling length of different mode coupling regions (Fig. 7).

As proof of the concept, we fabricate a pair of dual-polarization MMUX and employ 12 TE-polarization grating couplers to interface the waveguides to optical fibers (Fig. 8). Then, an amplified spontaneous emission (ASE) and optical spectrum analyzer (OSA) are utilized to characterize the static performance of the fabricated device. The measured transmission spectra of different optical links are normalized by the PSRs with the same parameters, which are fabricated close to the device. The insertion losses of TE₀, TM₀, TE₁, TM₁, TE₂, and TM₂ mode channels are below 1.7 dB, 1.4 dB, 1.7 dB, 2.8 dB, 2.5 dB, and 3.1 dB respectively, while the crosstalk of TE₀, TM₀, TE₁, TM₁, TE₂, and TM₂ mode channels are below -10.5 dB, -17.3 dB, -10.9 dB, -11.6 dB, -10.1 dB, and -12.0 dB respectively, within the wavelength range of 1525-1565 nm (Fig. 9). Finally, the dynamic data transmission experiment with a total capacity of 1.52 Tbit/s net data rate has also been conducted (Fig. 10). To the best of our knowledge, this is the first experimental demonstration of dual-polarization MMUX with six mode channels in the LNOI platform. The proposed device has a huge potential in applications of high-speed and large-capacity optical interconnection. Thus, it has the potential to be followed by many researchers and will be applied to a large number of relevant investigations.

In summary, we propose, design, and demonstrate a dual-polarization MMUX with six mode channels based on the LNOI hybrid platform. The SWG taper waveguide is adopted to suppress the mode hybridization along the Y direction of the LNOI platform. The measured insertion loss and crosstalk of the fabricated device are lower than 3.1 dB and -10.1 dB respectively, within the wavelength range of 1525-1565 nm. Additionally, a high-speed data transmission experiment with a net data rate of 1.52 Tbit/s is demonstrated. Meanwhile, the fabricated device can be combined well with wavelength division multiplexing technology to realize a huge data transmission capacity. The results show that the fabricated dual-polarization MMUX has sound static and high-speed data transmission performance, which can be expected to be employed as the key to future high-speed and large-capacity optical interconnection.