Research On Bending Characteristics And Parameter Measurement Of Multi-Core Fiber

Multi-core fiber based on space division multiplexing technology can greatly increase communication capacity and improve space utilization,so it is expected to become a key technology to break through the bottleneck of existing communication capacity.With the advancement of commercial application,multi-core fiber is developing towards the performance of low loss,low crosstalk,and large mode area.However,these properties are easily affected by bending.Improper bending will lead to its rapid deterioration,which will affect the normal use of optical transmission systems and optical devices based on multi-core fiber,and even damage the whole system.Therefore,the study of bending characteristics and parameter measurement of multi-core fiber is of great significance for its development and application.In this paper,the bending characteristics and parameter measurement of multi-core fiber are studied theoretically and experimentally.The main contents and innovations are as follows:1.The influence of introducing a high-index ring into the core of a single-core large mode area fiber on its bending characteristics is studied,and a large mode area fiber with double-layer high-index rings in the core is proposed.The simulation results show that adding a high-index ring in the core can not only increase the difference between the loss of high-order mode and the fundamental mode（FM）to improve the single-mode operation under bending but also make power distribution in the core flatter and thus increase the mode area.Based on this conclusion,a large mode area segmented cladding fiber with inner and outer high-index rings in the core is further proposed.The inner ring helps to enhance the single-mode performance of the fiber and makes the top of the electric field distribution of the fundamental mode flatter,and the outer ring can make the waist of the electric field distribution of the fundamental mode wider.The analysis shows that when the bending radius is 15 cm,the fiber can realize a single-mode operation in any bending direction in the wavelength range of 1.06～1.58μm,and the mode area is greater than 835μm².2.The high-order mode suppression ability of double trench fiber with a parabolic-profile core is studied,and multi-core fiber with a parabolic-profile core and a large mode area is proposed.The fiber can increase the core diameter at a small cut-off wavelength,achieve a large mode area,and has the advantages of low bending loss and low crosstalk.The study of its bending characteristics shows that the mode field area of the fiber is little affected by the bending radius and has good bending resistance.When the bending radius is less than 1 m,the core radius is 7μm,the relative refractive index difference between core and cladding is 0.3%,and the core pitch is 45μm,the fiber can achieve single-mode operation in C+L band with crosstalk of lower than-40 d B/100 km and a mode area of over 100μm².3.The effect of index-trench on the cut-off wavelength and bending characteristics of the step-core trench-assisted（Trench Assisted,TA）19-core fiber was studied.The variation of the cut-off wavelength with the inner diameter and thickness of the index trench is analyzed.In order to avoid the extension of the cut-off wavelength to the C-band,the influence of the limited gully parameters on the fiber loss,crosstalk,and mode field area is studied.The bending characteristics of the fiber with the cut-off wavelength within1530 nm are further discussed.The results show that the cut-off wavelength is not only affected by the index trench around the core,but also by the index trench of adjacent cores.The cut-off wavelength is positively related to the inner diameter and thickness of the index trench and extends with the decrease of core pitch and the increase of the refractive index difference between the core and cladding.When the core pitch is 35μm,the effect of the cut-off wavelength on the coupling coefficient and the mode area becomes prominent.The effect is further enhanced when the refractive index difference between the core and claddingΔ₁ is 0.35%.WhenΔ₁ is 0.39%,since the trench parameter is limited by the cut-off wavelength,the TA 19-core fiber can only achieve low crosstalk below-30d B/100 km at the wavelength of 1625 nm under the bending radius of smaller than 160mm.4.Based on the research of large mode area fiber with a high-index ring in the core,TA-19-core fiber with a high-index-ring assisted core is proposed.Compared with the step-core,the high-index ring assisted core has less influence on the cut-off wavelength,which can reduce the crosstalk with shorter extending of cut-off wavelength,and improve the small range of refractive index difference between the core and cladding available for the traditional step-core 19-core fiber to suppress crosstalk and avoid the cut-off wavelength extending to C band.When the bending radius is between 15 and 500 mm,the fiber with a 200μm diameter can achieve crosstalk of lower than-30 d B/100 km and a mode area of over 70μm² when the cut-off wavelength is less than 1530 nm.5.Using asynchronous programming,the operating software of the multi-core fiber parameter measurement instrument is developed,and the bending loss and refractive index of multi-core fiber are measured experimentally.Using based on object-oriented programming technology and the WPF framework,the operating software of multi-core fiber parameter measurement instrument is compiled,which realizes the system integration of various hardware functions,data acquisition of multi hardware cooperation,data processing of calling refractive index reconstruction algorithm,and chart display and export of measurement results.The software runs smoothly,is concise and easy to use.The refractive index of a 7-core fiber is measured,and the loss of a 4-core and 7-core fiber at different bending radii and wavelengths is measured by using the measurement system based on a spatial optical coupling device.The effects of bending radius and wavelength on the output spot of these two fibers are analyzed.The results show that the total bending loss of the two fibers increases exponentially with the decrease of bending radius,which is more than 200 d B/m when the bending radius is below 7 mm.The brightness of the output spots of the two fibers generally increase with the increase of bending radius,and the variation law with the wavelength is not obvious.