基于级联非对称Y分支的紧凑宽带高效的LP01-LP11a模式转换器

Compact,Broadband,and Efficient LP_(01)-LP_(11a)Mode Converter Based on Cascaded Asymmetric Y-Junction

提出了一种基于级联非对称Y分支的紧凑、宽带、高效的LP_(01)-LP_(11a)模式转换器。制作的聚合物波导模式转换器具有1.5 mm×14.0μm的紧凑尺寸,对于C+L波段的x偏振和y偏振光,其模式转换效率大于98%,串扰小于-17.5 dB,插入损耗低于5.8 dB。所提出的模式转换器可以应用在宽带模分复用传输系统中。

Objective The current information capacity of communication systems based on single-mode fibers(SMFs)is approaching its physical limits.To solve this problem,spatial-division multiplexing based on mode-division multiplexing(MDM)has been intensively investigated.Due to its orthogonal characteristics,MDM can help realize more multiplexed channels,and thus the capacity of existing optical fiber communications can be enhanced.Mode converters are critical devices in optical-fiber communication systems and are essential for improving the performance of future MDM systems applicable in long-distance and high-capacity optical-fiber communication.Mode-conversion efficiency is a major index of mode converters.Mode converters based on asymmetric Y junctions on polymer platforms offer the advantages of low cost,high fabrication tolerance,and wide bandwidth.Thus,the design and fabrication of mode converters with compact structures and high mode-conversion efficiencies based on asymmetric Y junctions on polymer platforms are essential to meet the increasing demands in data traffic.Methods The proposed mode converter consists of two identical inversely connected asymmetric Y junctions.The stem of the Y junction is a straight two-mode waveguide designed to support only the Ei11 and Ei21 modes(i=x or y,indicating the polarization direction),which correspond to the LP01 and LP11a modes(polarized in the i direction)of the optical fiber,respectively.This two-mode core gradually branches into two single-mode cosine S bends with different widths,forming an asymmetric Y junction.Based on the mode evolution principle of the asymmetric Y junction,the parameters of the proposed mode converter based on a cascaded asymmetric Y junction(Fig.1)are optimized in this study.These parameters include the widths of the core(w1),arms A and D(w2),and arms B and C(w3),as well as the length of the arm(L),width of the Y-junction end(w4),and distance between the two Yjunction ends(w5).A three-dimensional finite-difference beam-propagation method(3DFD-BPM)is used to simulate the modeconversion characteristics of the proposed mode converter.Under these device parameters,the proposed mode converter is fabricated with in-house microfabrication facilities.In addition,an experiment is conducted to characterize the mode-conversion performance of the proposed mode converter.Results and Discussions In the proposed mode converter,the waveguide core height is fixed at 4μm,and w1,w2,and w3 are set to 9.0,6.3,and 2.7μm,respectively.The refractive-index difference between the core and cladding is sufficient to achieve mode conversion.The mode-conversion efficiency between the LP01 and LP11a modes is optimal when the length of the arm,width of the Yjunction end,and distance between the two Y-junction ends are 1.5μm,3.1μm,and 6.8μm,respectively.The simulation results show that the mode-conversion efficiencies for the x polarized LP01-LP11a and LP11a-LP01 are 99.3%and 99.2%,respectively(Fig.4).A experiment is conducted to characterize the mode-conversion performance,and the near-field spots detected by the infrared camera indicate that the device can implement mode conversion(Fig.6).Over a wavelength of 1530‒1600 nm,the insertion losses are between~4.8 dB and~5.8 dB and between~3.5 dB and~5.1 dB for the x and y polarizations,respectively(Fig.7).To investigate further the mode-conversion and crosstalk characteristics at the asymmetric Y junction of the device,the device is cleaved at the middle position to obtain an asymmetric Y junction.The results show that,under the premise of neglecting the radiation losses of the asymmetric Y junction and propagation losses of the waveguide,the mode-conversion efficiencies are greater than~98%and~98.1%for the x and y polarizations over the C+L band,respectively,and the mode crosstalk is less than-17.5 dB(Fig.9).Conclusions We propose and demonstrate a mode converter constructed using two identical asymmetric Y junctions connected inversely.Our proof-of-concept mode converter,designed for the conversion of the LP01 and LP11a modes and fabricated using an optical polymer material,has a miniature footprint of approximately 1.5 mm×14.0μm.The results show that over the C+L band and for both polarizations,the mode-conversion efficiencies are greater than~98%,the crosstalk is less than~-17.5 dB,and the insertion loss is less than~5.8 dB.Our proposed mode converter with polymeric materials is easy to fabricate and inexpensive.In particular,the same structure can be implemented with other high refractive index contrast material platforms such as lithium niobite on insulators,silicon nitride,and silicon on insulators to realize more advanced integrated photonic circuits.