###### Abstract  
Optical communication systems based on silicon (Si) multimode interference (MMI) wavelength-division multiplexing (WDM) technology can suffer from back reflection. This undesirable characteristic causes losses and is a key problem that can lead to performance limitations. To overcome this limitation, we proposed a new study on how to divide two wavelengths by understanding the light coupling mechanism of the silicon nitride (SiN) MMI coupler over the C-band window and showed four different options to design a two-channel demultiplexer. The best option for a two-channel SiN MMI coupler with low back reflection losses operating in the C-band spectrum was selected. Based on simulation results, the proposed device can transmit two channels with a spacing of 20 nm between wavelengths in the C-band. Moreover, the device has a low power loss range of 0.895-0.936 dB, large bandwidth of 16.96-18.77 nm, and good crosstalk of 23.5-25.86 dB. Usually, a unique design such as angled MMI is required when using Si MMI technology to reduce the back reflection losses. Due to the use of SiN, which has a low refractive index, we obtained a 40.4 dB back-reflection loss without using this angled MMI design. Therefore, this MMI demultiplexer based on SiN can be used in optical communication systems based on the WDM technique to obtain a high data transfer rate in conjunction with low back-reflection losses. SiN; buried waveguide; back reflection; MMI; BPM; PIC; WDM; FDTD +
Footnote †: journal: Applied Physics Letters  
_Applied_  
_Acticle_  
A Two-Channel Silicon Nitride  
its compactness, flexibility in the design of multiport devices, and acceptable fabrication tolerances [18; 19; 20], MMI couplers have evolved into a variety of signal-splitting devices for applications in optical communication. The MMI device is a type of waveguide that is designed to support a large number of modes. Access waveguides are positioned at the input and output of that multimode waveguide in order to project light into and retrieve light from it. The self-imaging phenomenon, which is the basis for MMI coupler operation, duplicates the electric field profile that enters the device into numerous images at periodic intervals along the propagation direction of the waveguide [21]. Recent developments in quantum technology and photonic bandgaps may provide a reliable way to transmit entanglement over long distances and potential application with quantum technology [22] and in Lorentzian environments such as in the recent paper [23]. Moreover, phase modulation of coherent states plays a role in quantum communication channels [24] and in the use of probabilistic noiseless linear amplifiers both at the encoding stage [25], where the information is coded on phase shifts, and at the decoding stage [26]. One of the key challenges that may degrade the effectiveness of the transmitter system is back reflection, specifically, the reflection back into the laser source in the opposite direction. Due to the self-imaging phenomenon and the mismatch in the refractive indices of Si and SiO2, reflections may occur in Si-based MMI couplers [27]. According to studies, polycarbonate polymer optical fiber can be used as a multiplexer or demultiplexer for RGB signals with insertion losses (ILs) ranging from 0.6 dB to 1.2 dB [28; 29]. A four-channel demultiplexer in the green light spectrum was also implemented using a structure of a multi-slot waveguide based on gallium nitride (GaN) [30]. Moreover, by using a GaN MMI, researchers have been able to divide four [31] and eight [32] channels in the visible and C-band spectrums, respectively. The back-reflection effect, which is crucial for the functioning of the transmitter, was not taken into account in these studies. The MMI coupler can generally experience different reflections, which can be divided into different types. Internal resonance modes are the first type of reflection, where there are many self-images at once.