With exponentially growing data traffic in data centers and high-performance computing systems due to the rise of 5G, IoT, AI and other applications, silicon photonics (SiPh) based wavelength-division multiplexing (WDM) optical interconnects have appeared as a promising technology to achieve large communication capacity, which achieves parallel signal transmission by independently encoding data on multiple wavelength channels. In the past decade, silicon photonic WDM transmitters based on Mach-Zehnder modulators, micro-ring modulators and electro-absorption modulators have been extensively demonstrated for large-capacity transmission. However, silicon WDM techniques always impose stricter requirements on multi-wavelength laser sources and wavelength precise controls, increasing the system complexity, power consumption and cost.

To further increase the link bandwidth, other promising dimensions of data transfer, like separate guided modes, can also be utilized for multiplexing. Specifically, multiple parallel signal transmission can be carried with a set of orthogonal spatial modes in multimode optical waveguides, and each mode channel can support a full WDM link. Compared with WDM, mode division multiplexing (MDM) only needs one single-wavelength light source, which has obvious advantages in cost, power consumption and reliability, and the wavelength multiplexers mostly use cascaded MZIs or array waveguide grating structures, featuring large size (usually in the order of 100 microns) and high insertion loss, while the tapered-asymmetric directional coupler (TADC) based mode multiplexers have the advantages of compact size, low loss and large scalability. Therefore, significant research attention has been made towards hybrid MDM-WDM silicon photonic integrated transmitters to fully utilize the wavelength and mode degrees of freedom for greatly extending the data transmission capacity.

Recently, the research team from Shanghai Jiao Tong University led by professor Du jiangbing demonstrate a four-channel silicon photonic WDM-MDM transmitter chiplet in Chinese Optics Letters, Volume 22, Issue 12, 2024 (X.Wang, et al., High-density MDM-WDM silicon photonic transmitter chiplet based on MRMs and dual-mode GC for 4×56-Gbps 3D co-packaged optical interconnects).

Four-channel optical signals, divided into two groups, are launched into the transmitter via corresponding grating coupler. Each group containing two wavelength channels is modulated by two add-drop MRMs sharing the same drop waveguide. With integrated TiN heaters, the resonant wavelength of each MRM can be thermally tuned to place each channel's operating wavelength near the resonance peak, thus effectively realizing simultaneous modulation and two-wavelength multiplexing. The optical signals of the two groups are further combined by a TADC-based mode multiplexer, which converts one group of the modulated optical signals into the TE₁ mode while the other two channels remain in the TE₀ mode. Ultimately, the optical modulated signals of the four channels (represented as TE₀-λ₁, TE₀-λ₂, TE₁-λ₁, and TE₁-λ₂, respectively) are coupled into either LP₀₁ or LP₁₁ mode in the output few-mode fiber (FMF) through a dual-mode grating coupler, which is of vital importance to realize low coupling loss over the operating wavelength range. The dual-mode grating coupler is etched on a 16 μm-wide multimode waveguide. As the effective refractive indices of TE₀ and TE₁ mode in this multimode waveguide are nearly consistent, the two modes can simultaneously achieve approximately equal diffraction efficiency in the same grating structure. Additionally, a fully etched side-distributed Bragg reflector (DBR) structure is placed after the grating to reflect the light in forward propagation, further strengthening the upward diffraction light field. Simulations of the grating and DBR structure are carried out using the Lumerical-FDTD method, and both TE₀-LP₀₁ and TE₁-LP₁₁ coupling exhibit a peak coupling efficiency above 25% and a 1-dB bandwidth beyond 14 nm, effectively supporting the MDM optical fiber interface.

High data rate up to 56-Gbps DMT signaling is experimentally demonstrated for each channel, featuring applications like 200G Quad Small Form-factor Pluggable (QSFP) transceivers and indicating significant potential for high-density 3D co-packaged optical interconnects.