Research On All-Fiber Mode Selective Comb Filter Based On Mach-Zehnder Interferometer

With the rapid development of mobile Internet,cloud computing and the Internet of Things,researchers around the world are urged to find new multiplexing technology to deal with the bottleneck of communication capacity.As a result,the spatial dimension was introduced to optical fiber communication system.The mode division multiplexing technology based on few-mode fiber is considered as one of the most promising technologies to increase the transmission capacity.The mode-wavelength division hybrid multiplexing system that combines wavelength division multiplexing technology and mode multiplexing technology will greatly improve the communication capacity,which opens up a way to further expand the channel capacity of optical communication system.One of the most critical components in the mode-wavelength division hybrid multiplexing system is the multi-wavelength broadband mode conversion device.Based on the above research background,this thesis mainly conducts the following research:1.The coupling theory is introduced in detail.The coupled mode equation is derived based on the perturbation method.The exact solution of the coupled mode equation including mode coupling,self-coupling and butt coupling is given.The calculation method of three kinds of coupling coefficients in the mode selective coupler are given.2.A vector mode selective coupler based on an air core ring few mode fiber is proposed.The vector mode selective coupler can efficiently convert the fundamental mode into the second order vector mode in a wide wavelength range.The coupling characteristics of the vector mode are analyzed in detail,and the influence of wavelength and core distance on the vector mode coupling(mode coupling,butt coupling and self-coupling)in the vector mode selective coupler are studied.Finally,the bandwidth and crosstalk problems of the vector mode selective coupler are analyzed,and the methods of depressing the crosstalk are given.3.Based on the vector mode selective coupler,a mode selective comb filter based on Mach-Zehnder interferometer is proposed.The mode selective comb filter can realize mode conversion and comb filtering at the same time.In the C-band,the proposed filter has good comb filtering effects for both the fundamental mode and the high order modes,and the conversion efficiencies of second order vector modes are higher than66%.Especially,in the wavelength range from 1540 nm to 1560 nm,the conversion efficiencies of the second order vector modes are higher than 96%.The principle and characteristics of the mode selective comb filter are systematically studied.The transmission spectrum of the proposed filter is derived by Jones matrix.The mode conversion characteristics,comb filtering characteristics and free spectrum range of the proposed filter are analyzed in detail.Finally,the bandwidth characteristics and the crosstalk of the proposed filter are analyzed and discussed.