# 16-Channel Wavelength Division Multiplexers Based on Subwavelength Grating
## 1 Introduction  
With the development of technologies such as 5G, big data, the Internet of Things, cloud computing, and artificial intelligence, society's demand for information transmission continues to grow exponentially. With the advantages of high speed, large bandwidth, low latency, and low power consumption, optical interconnection gradually replaces traditional electrical interconnection and becomes an effective way to solve massive data transmission, exchange, and processing, and is gradually applied to backbone network transmission and data centers, high-performance computing and sensing [1; 2; 3; 4]. Silicon-based optoelectronics has become a key technology for optical communication and optical interconnecting due to its advantages of high integration, anti-electromagnetic interference, complementary metal oxide semiconductor (CMOS) process compatibility, low power consumption, and low cost [5; 6; 7; 8; 9; 10]. In recent years, many advances have been made in silicon-based optoelectronic devices. Since silicon (Si) has a band gap of 1.19 eV and is transparent to the wavelengths of the communication band, silicon waveguides can transmit multiple wavelengths of light simultaneously and almost non-destructively. Multiple optical waves are transmitted and calculated on the chip at the same time, which can double the transmission rate and processing rate of the on-chip data. Among them, the application of wavelengths has doubled the transmission rate and processing rate of on-chip optical signals, and wavelength devices such as wavelength division multiplexers (WDMs) and filters have become research hotspots [11; 12].  
To achieve WDM on a chip, researchers designed a variety of structures, including array waveguide grating (AWG) [13; 14], etched diffraction grating (EDG) [15], sub-wavelength grating (SWG) [16; 17], cascaded Mach-Zender interference structures (MZI) [18],micro-ring resonators (MRR) [19; 20], photonic crystal filters [21] sidewall gratings [22], etc. These wavelength devices are essentially based on the phase matching of light. Among these, SWG has the advantages of small size compared with other wave decomposition multiplexing devices, can achieve flat-top filtering, and there is no FSR limitation [23; 24]. In recent years, researchers have conducted studies on SWG waveguide-based WDM, providing new ideas for improving the efficiency of on-chip data transmission [25; 26].  
In this paper, we designed and simulated a 16-channel wavelength WDM based on SWG. Different from the previously reported SWG-based WDM, our design utilizes an SWG perturbation structure to provide an inverse couple condition for each channel, which can lower the transmission loss and enhance fabrication tolerance compared to WDM on SWG waveguide structure. Besides, with specific structure optimization, our design is capable of 16-channel WDM, covering both O-band and C-band. Simulation results show that the 16-channel wavelength demultiplexer has a single-channel loss of less than 0.5 dB and crosstalk of less than 17 dB. When WDM is performed in the O-band, the loss of C-band is low in the bus waveguide, and the modified structure is insensitive to the duty cycle of the SWG, with a process tolerance of more than 20 nm. The single-channel filter has a size of 200 \(\upmu\)m \(\times\) 2 \(\upmu\)m and an overall footprint of 200 \(\upmu\)m \(\times\) 150 \(\upmu\)m, allowing for large bandwidth WDM in a compact structure.