Novel Mode-Division Multiplexer/Demultiplexer with Ultra-Large Bandwidth and Ultra-Low Insertion Loss Based on Five-Core Photonic Crystal Fiber
Objective Multicore photonic crystal fibers(PCFs)have attracted considerable attention.The cladding and core of PCFs typically contain air holes of different sizes and pitches that run the entire length of the fiber,allowing light waves to be confined to the core area.By adjusting the structure,size,and arrangement of these air holes,PCFs can achieve unique light-guiding characteristics that traditional fibers cannot,such as a large mode field area,high nonlinearity,endless single mode,high birefringence,and tailored dispersion.Compared with other types of MUX/DMUXs,PCF-based MUX/DMUXs generally offer a large number of mode multiplexes,a wide operating bandwidth,and low insertion loss.They can be easily integrated into existing optical-fiber communication systems and utilized in various engineering applications.Additionally,with the development of the mobile Internet across different vertical application fields,there is a need to further enhance the transmission rate of optical-fiber communication systems in backbone networks.With the emergence of MDM systems,the communication capacity of a single fiber has greatly improved.As a key component of the MDM system,the PCF-based MUX/DMUX has an inherent advantage in compatibility with existing single-mode optical-fiber systems.However,to date,PCF-based MUX/DMUX systems with satisfactory performance have been rarely reported.Therefore,developing a PCF-based MUX/DMUX with ultra-low insertion loss,ultra-wide bandwidth,and multiple multiplexing modes for ultra-large-capacity,ultra-high-speed,and ultra-long-distance MDM systems is highly desirable.Methods First,we design a five-core PCF-based structure MUX/DMUX.We then conduct numerical analysis on the main kernel of the proposed five-core PCF-type MUX/DMUX using the eigenmode finite difference(FDE)method,verifying that the main kernel can simultaneously transmit five modes without crosstalk.Next,using the control variable method,we study the fundamental mode characteristics in the side core by optimizing the radius of the side core pores,the distance between the side core pores,and the pores at the connection between the side core and the main core to obtain the optimal structure.Finally,the key performance parameters of the five-core PCF mode division multiplexer are analyzed using the eigenmode expansion(EME)method,including mode coupling efficiency,insertion loss,crosstalk,and process tolerance.Results and Discussions On the basis of previous research,our paper proposes an MUX/DMUX based on five-core PCF using silica as the substrate.We use FDE and EME methods to simulate and analyze its structure and performance.By adjusting the size and arrangement of the cladding air holes of the side core,we match the effective refractive index of the basic modes in the side core with the effective refractive index of the corresponding higher-order modes in the main core,achieving effective mode coupling.The results indicate that when the main core parameters are r0=0.5 μm and A1=5 μm,and the side core structural parameters are r1=0.65 μm,d1=0.37 μm,Δ2=6 μm,r2=0.95 μm,d2=0.48 μm,r3=1.68 μm,d3=0.45 μm,r4=1.96 μm and d4=0.53 μm,the device achieves optimal performance.At the center wavelength of 1.55 μm,the device has a mode coupling efficiency of 96.7％[Fig.8(a)],an ultra-wide bandwidth of 620 nm[Fig.8(b)],a low insertion loss of 0.15 dB[Fig.9(a)],and a maximum crosstalk of-11.34 dB[Fig.9(b)].We also investigate the effect of structural parameter deviations on device performance.When the device size changes by±0.5％,the device bandwidth is reduced to 490 nm and 470 nm,respectively,which is still within a reasonable range.This provides an important reference for future multi-core PCF type MUX/DMUX with ultra-high bandwidth and ultra-low loss.Conclusions In this study,we propose a novel mode-division MUX/DMUX with an ultra-large bandwidth and an ultra-low IL based on a five-core PCF.The device consists of a main core(supporting fundamental mode and higher-order mode transmission)and four side cores(supporting only fundamental mode transmission).By optimizing the geometrical structures based on the FDE method,we couple the LP01 modes from the four side cores at the input port to the main core,converting them to the LP11,LP21,LP31,and LP12 modes.Multiplexing of the LP01,LP11,LP21,LP31,and LP12 modes is realized in the main core.Conversely,if the output port of the device is used as the input port,mode division demultiplexing of the five modes from the main core can be realized.The proposed MUX/DMUX improves the number of mode-division multiplexing and mode coupling efficiency,greatly reduces the insertion loss,and shortens the device length.It can operate efficiently over a bandwidth of 620 nm(1.33-1.95 μm,covering the E-,S-,C-,L-,and U-bands)with an insertion loss of 0.15 dB and a crosstalk of-11.34 dB over a length of 1.84mm.Compared to previous schemes,the proposed MUX/DMUX offers the advantages of ultra-large bandwidth,ultra-low insertion loss,and short device length,with extensive application potential in future large-capacity MDM systems.
mode division multiplexer/demultiplexermulti-core photonic crystal fibermode coupling efficiencyinsertion loss
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