Numerical Demonstration of 800 Gbps WDM Silicon Photonic Transmitter with Sub-Decibel Surface-Normal Optical Interfaces  
[PERSON]  
1 School of Electronic and Information Engineering, Tiangong University, Tianjin 300387, China; [EMAIL_ADDRESS] (M.L. ); [EMAIL_ADDRESS] (K.Z. ); [EMAIL_ADDRESS] (T.L. ); [EMAIL_ADDRESS] (H.J. ); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (Q.W. ); [EMAIL_ADDRESS] (J.X. ); [EMAIL_ADDRESS] (B.Y. ); [EMAIL_ADDRESS] (B.Y. ); [EMAIL_ADDRESS] (H.L. ); [EMAIL_ADDRESS] (H.J. ); [EMAIL_ADDRESS] (H.J. ); [EMAIL_ADDRESS] (B.Y. ); [EMAIL_ADDRESS] (H.J. ); 213107  
Silicon photonics is considered an economical candidate for optical interconnects by taking advantage of the existing mature CMOS technology to offer unprecedented bandwidth scalability using spatial division multiplexing [5], wavelength-division multiplexing [6], polarization-division multiplexing [7], and advanced modulation formats. Normally, spatial division multiplexing is quite suitable for the short reach optical interconnect applications, i.e., 500 m, while wavelength-division multiplexing is commonly used for targeted range of a few kilometers. Especially, the coarse wavelength-division multiplexing (CWDM) is utilized in today's silicon photonic products such as the 100G CWDM4 QSFP28 optical transceiver and the recently released 400 G optical transceiver using 4-level pulse amplitude modulation (PAM4) by Intel [8]. Recently, there are also many other demonstrations of silicon optical transceivers which are further extending the data transmission speed to more than 400 Gb/s [9; 10; 11]. Towards 800 G silicon photonic optical modules, there are many viable solutions such as PSM 4 \(\times\) 200 G, PSM 8 \(\times\) 100 G, and WDM 8 \(\times\) 100 G [12; 13; 14]. There are also a few demonstrations of Terabit/s optical I/O chiplets integrated with multiple wavelength lasers and ASICs [15; 16], which show the great potential for large capacity optical communication using the WDM techniques. As an important building block of the silicon photonic transceivers, silicon optical transmitters achieved great success during the past few years [17; 18; 19]. Currently, silicon optical transmitters are more densely integrated and most of the performance metrics, i.e., data capacity and power efficiency, are comparable to its III-V alternatives [20]. However, the rapid commercialization of silicon photonic products is still hindered by two main drawbacks. One is the relatively large optical loss of the silicon photonic circuits, which is mainly contributed by the fiber-chip coupling loss and the on-chip insertion loss of the photonic devices. As the on-chip optical amplification function is not currently available, the power budget of the silicon photonic circuits is of paramount importance and directly related to the quality of optical communications. Thus, high efficiency grating couplers and low-loss photonic devices are urgently needed. Another main roadblock would be the viable and cost-effective on-chip laser source for silicon photonic circuits. Although heterogeneous integrated III-V lasers on silicon platform have been successfully demonstrated, but issues around the CMOS process compatibility, yield, and reliability remain to be resolved [21; 22; 23]. In recent years, hybrid integration of VCSELs is emerging as a promising solution as a hybrid laser source via grating coupling. Unfortunately, the tilting coupling scheme of conventional grating couplers leads to a large obstacle for the coplanar integration of VCSEL and silicon photonic chips. Although many attempts are proposed to solve this problem, such as using a high-contrast grating as the bottom reflector [24], adding a SUB prism above the grating coupler [25], and controlling the reflow of the solder ball deposition [26], these techniques suffer from low coupling efficiency, complex fabrication process and high cost. To decrease packaging complexity, high-efficiency, perfectly vertical grating couplers would be the key to promoting the integrated hybrid silicon-based VCSELs.