Tunable Mode-filter Characteristics Of Liquid-filling Microstructured Optical Fiber

The rapid development of the Internet and technologies,such as 5G networks,IoT and car networking,has made the capacity of optical fibers must be continuously improved.In recent years,the transmission capacity of single-mode fibers has approached the Shannon-limit with nonlinear effects.In order to further enhance the optical fiber transmission capacity,researchers proposed the mode division multiplexing?MDM?technology to break through the transmission conditions of single-mode fiber.The MDM technology uses the few-mode fiber?FMF?as the transmission main part.It used different modes in FMF to independently transmit different information.In addition to the application of mode division multiplexing,FMF can also achieve dispersion compensation,ultrashort pulse transmission and nonlinear applications by selectively exciting high-order modes.Therefore,the control and selection of modes has become a key technology.Wavelength filter is an important device for wavelength selective in wavelength division multiplexing?WMM?.The mode-filter is also an important mode control device in the applications with FMF.At present,there are some mode-filter devices that can provide separation and multiplexing of modes.However,they still have some disadvantages such as low loss of mode-filter and inability to tunable filter mode in time.This paper proposed a petal-shaped micro-structure fiber to achieve higher efficiency in mode-filter.This structure utilizes the principle of the cladding super-mode group to couple the core-mode to the cladding.Because of the high-refractive-index outer cladding layer,the total internal reflection conduction mechanism is broken.The cladding modes get a high confinement loss which is the reason for the core-mode filter.The petal-shaped inner cladding structure can reduce the distance between the rods and the high refractive index outer cladding layer so that it can get a higher loss for the cladding-mode.The results show that the petal-shaped cladding structure can increase the loss of the filtration mode by an order of magnitude compared to the circular cladding structure.This fiber can effectively filter the LP₀₁ mode without affecting the transmission of the LP₁₁ mode,and the fiber length is only 46mm.This paper not only analyzed the reason why the petal-shaped cladding structure enhances the filter mode,but also proposed a new petal-shaped core structure,which can separate the LP₂₁ and LP₀₂2 mode in similar refractive indices.Based on the petal-shaped inner cladding structure,this paper proposed a solution for the tunable mode-filter fiber with filling the cladding rods with liquid.Therefore,the effective refractive index interval of the super-mode group formed by the cladding can be adjusted by the ambient temperature.Thus,the purpose of tunable mode-filter is achieved.At the same time,the super mode group formed by the LP₁₁ mode transmitted in the liquid rods is used as the coupling mode,which effectively increases the working bandwidth and temperature tuning range.The numerical simulation results show that the mode-filter fiber with a length of only 71.4mm can selectively filter the four modes in the few-mode fiber,and the loss of the specific mode to be filtered can reach 20dB or more,while the other modes lose less than 1dB.The fiber proposed in this paper can be used as a filter in a few-mode fiber transmission system to reduce mode crosstalk of mode converters,multiplexers/demultiplexers,and optical switches and optical routing.