| The rapid development of information technology requires the continuous expansion of the capacity of optical fiber transmission systems.The system capacity of traditional singlemode fiber is almost close to the physical limit of its transmission,which will be difficult to meet the needs of future communication network bandwidth.The multi-core few-mode fiber based on the principle of space division multiplexing is regarded as one of the most promising methods to solve the communication capacity crisis.However,while the multicore few-mode fiber greatly improves the transmission capacity,it also brings the problems of inter-core crosstalk and inter-mode crosstalk that cannot be ignored.Therefore,how to suppress crosstalk is an urgent scientific and technical problem to be solved for practical application of multi-core few-mode fibers.In this paper,from the perspective of suppressing the crosstalk between cores and modes of multi-core fibers,two low-crosstalk multi-core and few-mode fiber structures are designed,and the transmission characteristics are numerically analyzed by the finite element method.The experimental preparation and performance test of the seven-core five-mode fiber with high and low refractive index double-ring (HL-DR)structure are carried out.The specific work is as follows:Firstly,taking seven-core fiber as an example,the calculation formula of inter-core crosstalk based on power coupling theory and the characterization method of inter-mode crosstalk are given,and the effective schemes for suppressing inter-core crosstalk and intermode crosstalk are analyzed.The theoretical basis for numerical simulation of basic transmission characteristics such as effective mode field area,nonlinear coefficient,dispersion and bending loss of optical fibers is introduced.The relative core multiplicity factor,an important parameter describing the space utilization efficiency of multi-core fibers,is given,which provides a theoretical basis for the design and performance analysis of multi-core few-mode fibers.Secondly,a seven-core five-mode fiber assisted by the superposition-assisted structure(SAS),composed of trenches and air holes,is designed.The design idea is to use the superposition of trenches and air holes to more effectively suppress inter-core crosstalk.By optimizing its parameters,the SAS fiber can support five LP modes in the C+L band,and all the effective refractive index difference values between the modes are greater than 1 × 10-3.Moreover,under the cladding diameter of 125 um,the inter-core crosstalk is lower than -50 dB/100km,and the relative core multiplexing factor is as high as 97.55,which can realize the low crosstalk and high-density transmission of the fiber at the same time.However,the setting of the air hole structure increases the difficulty of fiber preparation.Thirdly,a seven-core five-mode fiber with HL-DR structure is designed.The design idea is to suppress the inter-core crosstalk and inter-mode crosstalk at the same time through the action of double rings,and reduce the difficulty of fiber preparation.The research shows that the designed HL-DR fiber not only has the characteristics of inter-core crosstalk lower than-30 dB/100km,but also has a strong ability to suppress inter-mode crosstalk (the effective refractive index difference between modes is greater than 2 × 10-3).Moreover,the characteristic of its all-solid structure is also conducive to the preparation of optical fibers.Finally,the designed HL-DR seven-core fiber is actually prepared.The fiber performance characterization platform is built independently,and the characteristics of the prepared fiber,such as transmission mode,transmission loss,and inter-core crosstalk,are experimentally tested.Comparing the observed mode field diagram with the simulated mode field diagram,it is verified that the fiber can transmit five LP modes clearly.In addition,the transmission loss and the inter-core crosstalk of the prepared seven-core fiber at 1550 nm are lower than 0.4 dB/km and-30 dB/100km,respectively.The results of this paper can provide theoretical and experimental basis for the preparation and application of multi-core fibers. |
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