Research On The Mode Converter In Optical Fiber Mode Division Multiplexing Communication System

The mode multiplexing technology uses few-mode?multi-mode?fiber to carry different optical signals in different transmission modes to form independent transmission channels,thereby increasing the system capacity several times to meet the needs of higher fiber communication capacity.Therefore,the mode converter that is used to realize the fiber fundamental mode(LP₀₁ mode)to high-order mode conversion has become a focus of mode multiplexing technology research.In this thesis,a mode converter is first proposed,which is realized by multi-stage tapered core structure.The mode converter is used to realize LP₀₁?LP₀₂ and LP₀₁?LP₀₃ mode conversion.The mode converter is formed by connecting a single mode fiber,a tapered multimode core,and a few-mode fiber.The tapered fiber core is used to convert LP₀₁ mode to LP₀₂ mode,and the few-mode fiber is used to cut the modes higher than LP₀₂ mode.The simulation results show that the extinction ratio?a quantity used for measure of mode contribution?is up to?45 dB and the insertion loss is 11.8 dB at the working wavelength of 1550 nm.In the same time,the proposed LP₀₂ mode converter provides 20 dB extinction ratio in a very wide optical bandwidth of 200 nm from 1465 to 1665 nm,and low insertion loss?less than 2.4 dB,i.e.?66%conversion efficiency?.The structure can be used for any bands with small structural changes:It is found that by optimized the core radius of the few-mode fiber,mode converter for each band of O-,E-,S-and U-band etc.is obtained.The mode converter insertion loss is further reduced by inserting a tapered core with a gradually decreasing radius between the tapered multimode core and the few-mode fiber.The simulation results show that the extinction ratio of?20 dB is obtained at the working wavelength of 1550 nm,and the insertion loss is much smaller?-0.45 dB?.Moreover,the extinction ratio of 11 dB is obtained in the optical bandwidth of 415 nm with the working wavelength from 1260 to 1675 nm,with low insertion loss?less than 1.56 dB,i.e.?70%conversion efficiency?.A mode converter that implements LPoi to LP₀₃ mode conversion by connecting a tapered multimode core and a few-mode fiber by using two different cascaded reverse tapered fiber cores is studied.The simulation results show that the conversion efficiency is?80.9%at the working wavelength of 1550 nm,and the conversion efficiency is?80%at the wavelength from 1550 to 1590 nn,with the extinction ratio of?10dB.A mode converter based on a double-core tapered core that efficiently implements mode conversion of the fundamental mode(LP₀₁)to a higher-order mode of LP11,LP2i,LP3,,LP12,and LP22 at a working wavelength of 1550 nm is then proposed.Conversion efficiencies of?99%,?96%,?92%,?93%,and?92%are obtained.At the same time,the above mode converter has a wide bandwidth,which is 205 nm,125 nm,60 nm,65 nm?and 60 nm corresponding to the above.In addition,the structure is also used to implement the LP₀₁-LP₄₁,LP₀₁-LP₃₂,LP₀₁-LP₄₂,and LP₀₁-LP₂₃ mode converters,achieving?81%conversion efficiency at 1550 nm.