Mechanism And Experimental Study On The Generation Of Orbital Angular Momentum Based On Optical Fiber Mode Interference

With the explosive development of broadband consumptive services such as Internet of everything,big data,cloud computing and virtual reality,the existing optical fiber communication system is facing great pressure and challenges in terms of transmission capacity.At present,the communication system has almost developed the optical wave dimensions such as amplitude,frequency/wavelength,phase,polarization state and time,and its transmission capacity has gradually approached the limit of non-linear Shannon.Therefore,it is urgent to find a new multiplexing method to improve communication capacity.Orbital Angular Momentum?OAM?mode has attracted wide attention due to its ability to expand optical communication transmission capacity in the spatial dimension.And the generation of stable OAM mode is one of the hot research topics.Compared with the OAM mode generated in free space,the OAM mode generated directly in the optical fiber has the advantages of low insertion loss,high system integration and good stability.In this thesis,a new method for generating OAM mode based on few-mode fiber mode interference is proposed.The main contents are as follows:?1?A method for generating orbital angular momentum based on the principle of few-mode fiber mode interference is proposed.The mechanism of generating OAM mode in fiber is analyzed,and the principle of mode interference generated by introducing micro-waveguides in few-mode fiber is described.At the same time,the effects of parameters such as the length of micro-waveguide,waveguide spacing and core-offset on the mode conversion efficiency,interference spectral contrast and free spectral range are studied by means of finite element method and beam transmission method,which provided theoretical guidance for subsequent micro-waveguides writing and spectral analysis of devices?2?Mode interference devices are fabricated by introducing micro-waveguides into a two-mode fiber using the femtosecond laser micromachining technology.In the experiment,the femtosecond laser micromachining system is designed,modified and optimized from the aspects of fiber clamping,lens focusing,fiber alignment,imaging system and power correction.Then,the effects of processing energy,the length of waveguide,the writing speed and the core-offset on the transmission loss of the device are studied in the single-mode fiber,which forms a relatively complete waveguide writing process.Finally,all-fiber mode interferometers are fabricated by writing two micro-waveguides at a certain distance in the two mode fiber.The waveguide has a length of 450?m,an eccentricity distance of 3.8?m,and the interferometers have an insertion loss of 1.3dB,a resonance peak depth of 22dB.In addition,the relationship between waveguide spacing and free spectral range is studied by fabricating mode interference devices with different waveguide spacing.?3?The intensity distribution and spiral phase of the OAM mode generated by the above mode interference device are detected by the spatial interferometry method.The hollow energy distribution field and the"fork"or spiral pattern formed by interference with the planar Gaussian beam or the spherical Gaussian beam indicated that the fiber mode interference can successfully transform the left-or right-handed circular polarization HE₁₁ mode into orbital angular momentum mode with a topological charge of±1 at the resonant wavelength.