# An Eight-Channel C-Band Demux Based on Multicore Photonic Crystal Fiber
## 1 Introduction  
Dense wavelength division multiplexing (DWDM) is a system [1; 2] that is used to integrate information from different sources over one fiber, while each source carried on its own divided light wavelength at the same time. DWDM has the ability to divide sources up into 80 ports, and allow more information to be multiplexed into a light-stream that is transferred on one fiber. Demux is an essential device in the DWDM system, and its main functionality is to divide signals from one input port into multiple ports. The demux device has several advantages such as a low bit error rate [3], a high data rate, a large bandwidth, low cross-talk [4], and less propagation delay. Therefore, researchers have shown the potential of designing demux-based waveguide techniques such as silicon photonics [5], Y-branch [6], multimode interference (MMI) [7; 8; 9], Mach-Zehnder interferometers [10; 11], MMI in slot waveguide structures [12; 13; 14], etc.  
Photonic crystal fiber (PCF) is a powerful waveguide that is based on a microstructured arrangement of materials of different refractive indexes [15]. The background material is usually pure silica, and the low-index regions are air-holes that are located along the fiber length. Several works have demonstrated the great potential of using PCF structures in comparison to conventional fibers [16; 17; 18]. The main benefit of designing a demux device based on a PCF structure is its ability to integrate different materials that have a high difference in their refractive index values. This is because the light guiding mechanism in PCF is based on the bandgap and modified total internal reflection (MTIR). Another advance is the ability to achieve a lower coupling length, especially in the case of closer coupled ports (cores) [19].  
Several techniques have demonstrated how it is possible to couple light between closer coupled ports (cores) in a PCF structure through methods such as changing the PCF index profile by replacing some air-hole regions with pure silica along the fiber length [19; 20] and using different air-hole sizes in the PCF structure [21; 22; 23; 24].  
The C-band range is set from 1530 nm to 1565 nm in the wavelength, and it is the most efficient and useful range in the optical communication field [14]. The advance of the C-band is the ability to transmit data with a high bitrate over a long distance since the C-band supports DWDM and optical amplifier fiber technologies [25].  
In this work, we demonstrated a 1 \(\times\) 8 wavelength demux in a PCF structure that split eight wavelengths in the C-band range. The operating wavelengths are between 1530-1565 nm with a spacing of 5 nm between two wavelengths. The light coupling between the closer coupled cores was obtained by replacing some air-hole areas with lithium niobate (LiNbO\({}_{3}\)) and silicon nitride (Si\({}_{3}\)N\({}_{4}\)) materials along the fiber length.  
Numerical investigations were carried out on the locations of the LiNbO\({}_{3}\) and Si\({}_{3}\)N\({}_{4}\)s layers and the key geometrical parameters of the multicore PCF structure to obtain high efficiency demultiplexing between the operating wavelengths. The demux PCF structure was analyzed and simulated using a beam propagation method (BPM) and Matlab codes. This device can be useful to increase the data bitrate in the C-band using a DWDM system.  
To the best of our knowledge, this paper is the first to study the controlling of light propagation direction by integrating LiNbO\({}_{3}\) and Si\({}_{3}\)N\({}_{4}\) rods over a multicore PCF structure. This new study was utilized to design a new demultiplexer based on a multicore PCF structure. Thus, the novelty is that the demultiplexer operated within the multicore PCF structure without using additional optical components. This can lead to a new DWDM technique that can be utilized to reduce the costs and size of the optical communication system.