| Fiber lasers are widely used in various fields of scientific research and production due to their excellent output,small size and easy integration.In high-power fiber lasers,since the gain fiber is sensitive to temperature,it is particularly important to establish a heat distribution model in the gain fiber in order to ensure stable operation of the laser.In all-fiber mode-locked lasers,it is a long-standing goal to find a saturable absorber with a simpler structure,lower cost,and more stable output.Therefore,this paper focuses on the thermal distribution model of fiber lasers and the research of new saturable absorbers based on nonlinear multimode interference effects.Aiming at the temperature distribution and output power in the gain light,an accurate heat distribution model and output model are established,and the experimental verification is carried out.At the same time,the mechanism of red light conversion into blue light in the thulium fiber is explored.In this paper,a single-mode double-clad erbium-doped fiber laser is used as the prototype,and the rate equation including the cross-relaxation effect,the output model and the heat distribution model of the laser at steady state are established.The water-cooled fiber laser with MOPA(power amplifier of the main control oscillator)was built.The accuracy of the output model and the thermal distribution model was verified by comparing the quantum efficiency and heat distribution of the laser in the experiment and simulation.It is also experimentally and theoretically explained that the maximum temperature of the double-ended pumped thulium fiber laser is not at the fusion point.The ultimate output capability of single-mode double-clad erbium-doped fiber(Nufern,SM-TDF-10P/130-M)was tested to be 103.51 W.Aiming at the phenomenon that erbium-doped fiber is converted into 457 nm blue light by erbium-doped fiber in thulium fiber laser,the mechanism of this phenomenon is explained from the perspective of thulium ion energy level.Due to the needs of the application,a 60 W air-cooled erbium-doped fiber laser with stable operation was designed and built.The accuracy of the output model and the heat distribution model was proved again from simulation to experiment.Aiming at the study of the structure of saturated absorbers of nonlinear multimode interference structures,this paper proposes a novel hybrid multimode fiber structure(gradient refractive index multimode fiber-step index multimode)which can be used as a saturable absorber.Fiber-gradient index mode fiber).Through theoretical analysis,four factors affecting the modelling of multimode fiber structure locks and how to design nonlinear multimode interference structures are clarified.The erbium-doped fiber laser was used to verify the mode-locking effect of the structure.The mode-locked pulse output with a pulse width of 354 fs,a center wavelength of 1559.71 nm,a spectral width of 10.73 nm and a frequency of 10.83 MHz was obtained.And realize the switching of the normal mode-locked pulse output and the soliton-mode-locked pulse output.Experiments compare the output characteristics of different structures of multimode fiber structures,the differences in spectral and pulse stability,and further demonstrate the excellent characteristics of the proposed multimode fiber structure.This paper provides a reference for establishing the output model and thermal distribution model of fiber lasers,and improves the design of lasers based on the current status of experience.It also provides a new structure for the saturable absorber based on nonlinear multimode interference and provides a method for the design of nonlinear multimode interference structures. |
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