Study On The Cylindrical Vector Beams Mode-Locked Fiber Laser And Fewer Mode Amplifier Based On Raman Effect

In fiber laser systems,Raman fiber serves as a gain medium with an exceptionally broad Raman gain spectrum.Its notable advantages include wavelength versatility and high output power,rendering it widely applicable in various fields such as long-haul fiber communication,optical sensing,and biomedical applications.Moreover,Raman fiber amplifiers can achieve optical amplification without requiring specialized gain media,the amplified wavelength closely correlates with the pump light.With the ability to achieve amplification across the entire spectrum under suitable pump conditions.Consequently,this device finds extensive applications in the field of wavelength-division multiplexed fiber communication systems.Furthermore,since the generation of cylindrical vector beams(CVB)has been a highly researched area,the all-fiber approach for achieving CVB offers advantages such as high efficiency and excellent flexibility.In this thesis,stable output of CVB was achieved by combining a passive mode-locked Raman fiber laser with a mode-selective coupler.Additionally,mode-multiplexing technology is a breakthrough in the case of insufficient transmission capacity in single mode fiber(SMF)optic communication systems.Due to the ability of RFA to amplify optical signals at different wavelengths,this thesis combines RFA with wavelength division multiplexing mode selection coupler(WDM-MSC)to obtain amplified output of signal light.The main research focuses on the following aspects:1.Investigation of a passive mode-locked fiber laser based on the Raman effect for generating CVB.The threshold conditions of the passive mode-locked Raman fiber laser were studied and calculated.By suppressing the impact of stimulated Brillouin scattering inside the cavity under a certain total loss,a backward-pumped Raman fiber laser was designed and constructed.Experimental results demonstrated stable mode-locking pulses with a center wavelength of 1562 nm,an output power of 3.55 m W,a repetition rate of 210.76 k Hz,and a signal-to-noise ratio of 74.5 d B,achieved at a pump power of 402 m W.A mode-selective coupler was fabricated and connected at the output end to generate stable CVB with a purity of over 90%.2.Research on a few-mode Raman amplifier based on wavelength-division multiplexing mode-selective coupler(WDM-MSC).The operation principle of the WDM-mode-selective coupler was investigated based on the coupled-mode theory.Finite element method and beam propagation method simulations were performed to obtain the radius ratio between the few-mode fiber and single-mode fiber.Subsequently,based on the simulation results,device fabrication was carried out using the fusion tapering technique.The fabricated device was integrated into the cavity structure of the few-mode Raman amplifier to reduce intensity noise and nonlinear effects through backward pumping.Experimental results showed that the input seed light with a power of 4.2 m W was amplified within the cavity,obtaining an output power of 140.3 m W.The total optical gain of 15.24 d B and a slope efficiency of approximately 16.2% is achieved.Additionally,it was observed that the mode purity of the output light continuously increased with the increasing pump power.