Research On Few-mode Fiber Laser

In the context of the widespread application of technologies such as cloud computing,big data,artificial intelligence and Internet of Things and the exponential growth of worldwide data traffic,as well as the government’s policies on accelerating the progress of infrastructure constructions such as 5G and data center,the establishment of optical fiber communication systems at higher data rates is extremely urgent.Since traditional single-mode optical fiber communication systems faced the problem of bandwidth depletion,mode-division multiplexing optical fiber communication systems based on few-mode fibers have attracted great attention.Few-mode fibers are used as transmission media in mode-division multiplexing optical fiber communication systems,where multiple modes are transmitted simultaneously to multiply the transmission capacity of optical fiber communication systems.How to generate lasing with different modes is an important research topic in mode-division multiplexing optical fiber communications,which is of great significance for establishing optical fiber communication systems at higher data rates.This dissertation carried out research into few-mode fiber lasers.Based on the in-depth analysis of the characteristic of mode transmission in few-mode fibers and the principle of mode coupling in few-mode fiber gratings,the method of mode generation and the characteristic of mode output for few-mode fiber lasers have been intensively explored.The main content and innovation of this dissertation are summarized in three aspects as follows.1.Few-mode fiber lasers with switchable output of multiple modes have been studied.Firstly,a mode interferometer-based few-mode fiber laser with switchable output of multiple modes has been proposed and the feasibility of this scheme has been experimentally verified.In this few-mode fiber laser,a few-mode long-period fiber grating is used to obtain different modes simultaneously,and a mode interferometer is formed in the core of an optical fiber.A few-mode fiber laser with switchable output of LP₀₁,LP_11a,LP_11b,LP_21a,LP_21b and LP₀₂ modes can be realized by combining a mode interferometer with a mode-selective photonic lantern.Secondly,a fiber grating-based few-mode fiber laser with switchable output of multiple modes has been proposed and the feasibility of this scheme has been experimentally verified.In this few-mode fiber laser,LP₀₁ and LP₁₁ modes can be simultaneously obtained by a few-mode long-period fiber grating,or LP₀₁,LP₁₁ and LP₂₁ modes can be simultaneously obtained by cascading different few-mode long-period fiber gratings.Under the condition that LP₀₁ and LP₁₁ modes can be reflected by different few-mode fiber Bragg gratings at the same wavelength,the switchable output of lasing with LP₀₁ mode and LP₁₁ mode at the same wavelength can be achieved by switching different few-mode fiber Bragg gratings.Under the condition that a single-mode fiber Bragg grating functions as a wavelength selector under the fundamental mode,the output of lasing with LP₀₁ and LP₁₁ mixed modes,or LP₀₁,LP₁₁ and LP₂₁ mixed modes can be realized by switching different few-mode long-period fiber gratings.Multiple modes can be simultaneously excited in such a fiber laser,and lasing with one,two or three mixed modes at the same wavelength can be output on demand.Finally,a few-mode oscillation-based few-mode fiber laser with switchable output of multiple modes has been proposed and the feasibility of this scheme has been experimentally verified.In this few-mode fiber laser,different modes are excited by a mode-selective photonic lantern.Feedback can be achieved by the principle of stimulated Brillouin scattering.The few-mode fiber functions as the Brillouin gain medium.Each mode oscillates directly in a resonator composed of all few-mode fibers,and the lasing wavelength in each mode can be continuously tuned.The switchable output of lasing with LP₀₁,LP_11a,LP_11b,LP_21a,LP_21b and LP₀₂ modes can be realized by switching the port of the mode-selective photonic lantern.2.A few-mode fiber laser with simultaneous output of multiple modes has been proposed and the feasibility of this scheme has been experimentally verified.In this few-mode fiber laser,a mode-selective photonic lantern is used to simultaneously excite different modes.A single-mode fiber Bragg grating functions as a wavelength selector under the fundamental mode,and different few-mode fiber Bragg gratings are used to reflect different modes at the same wavelength.The simultaneous lasing of LP₀₁,LP₁₁,LP₂₁ and LP₀₂ modes at the same wavelength can be achieved for this fiber laser with the collaboration of multiple sub-linear-cavities.Furthermore,the wavelength stability of lasing with different modes is explored at different temperatures.Compared with LP₀₁ mode and LP₀₂ mode,LP₁₁ mode and LP₂₁ mode are closer to the cladding of the few-mode fiber during transmission,which makes LP₁₁ mode and LP₂₁ mode more sensitive to changes in temperature.Therefore,within the same temperature variation,the wavelength shifts with temperatures in LP₁₁ mode and LP₂₁ mode are larger than those in LP₀₁ mode and LP₀₂ mode.The simultaneous lasing of different modes at the same wavelength can be achieved by this fiber laser,which can be used in mode-division multiplexing optical fiber communication systems.3.A multi-wavelength few-mode fiber laser with simultaneous output of multiple modes has been proposed and the feasibility of this scheme has been experimentally verified.In this few-mode fiber laser,LP₀₁,LP₁₁ and LP₂₁ modes can be obtained by few-mode long-period fiber gratings at different periods.LP₀₁,LP₁₁ and LP₂₁ modes can be reflected by few-mode fiber Bragg gratings at different periods.As a result,the simultaneous output of lasing with LP₀₁,LP₁₁ and LP₂₁ modes at the same wavelength can be achieved.A few-mode fiber Bragg grating functions as a discrete filter.By adjusting a polarization controller,the switchable output among single-,dual-and triple-wavelength lasing can be realized,and three wavelengths correspond to LP₀₁mode,LP₀₁ and LP₁₁ mixed modes,and LP₁₁ mode.The simultaneous lasing of LP₀₁,LP₁₁ and LP₂₁ modes at the same wavelength and the simultaneous lasing of LP₀₁mode,LP₀₁ and LP₁₁ mixed modes and LP₁₁ mode at different wavelengths can be realized for this fiber laser with the collaboration of multiple sub-ring-cavities.Therefore,lasing with different modes at the same wavelength,as well as lasing with different modes at different wavelengths can be output simultaneously for this fiber laser,which can be used in mode-division multiplexing-wavelength-division multiplexing optical fiber communication systems.Based on the in-depth analysis of the characteristic of mode transmission in few mode fibers and the principle of mode coupling in few mode fiber gratings,several few mode fiber lasers with original configurations have been proposed in this dissertation.The method of mode generation and the characteristic of mode output for few-mode fiber lasers have been explored,which lays a solid foundation for mode-division multiplexing optical fiber communication systems.