Mechanism Investigation Of Pulsed Fiber Laser Based On Saturable Absorption Properties Of Carbon Nanomaterials

Ultrashort pulse lasers can be used in material processing,optical transient imaging,high-speed optical communication,biomedical research,security detection and and various other scientific domains due to their high peak power,high energy density and narrow pulse width.Among them,passive mode-locked fiber lasers are one of the hot directions to research ultrashort pulses.Passive mode-locked fiber lasers have emerged as a prevalent technology for the generation of ultrashort pulses in diverse scientific fields due to many advantages such as high photoelectric conversion efficiency,simple structure and easy maintenance.Passive mode-locking techniques typically involve the use of a saturable absorber,with carbon nanomaterials such as single-walled carbon nanotubes(SWCNT)and graphene being widely used to fabricate saturable absorber devices due to their superior nonlinear absorption properties.Compared with the use of semiconductor saturable absorber mirrors(SESAM)as mode-locked devices,they have advantages in various aspects such as saturation absorption capacity,damage threshold,integration method with the fiber and cost,which provides a new idea and direction for the preparation of high-performance,high-stability ultrashort pulse fiber lasers.This thesis presents a comprehensive investigation,both theoretical and experimental,based on the utilization of carbon nanomaterials in mode-locked fiber lasers.Two carbon nanomaterials,SWCNT and graphene,are used to make saturable absorption devices,which are applied to fiber lasers to realize mode-locked pulse outputs with different characteristics.The main contents of this thesis include:1.A graphene-based mode-locked fiber laser utilizing grating frequency selection was constructed.Fiber Bragg gratings and chirped fiber Bragg gratings were employed as output cavity mirrors in graphene erbium-doped fiber lasers to achieve traditional soliton modelocked pulses.The resulting spectral widths were measured to be 0.22 nm and 0.14 nm,respectively.The influence of different output cavity mirrors on the properties of soliton pulses was examined.Stable mode-locked pulse output was achieved by altering the resonant cavity structure,reducing the cavity length,and employing the coupler as the output cavity mirror.The mode-locked pulse output exhibited a center wavelength of 1561.5 nm,a spectral width of 0.51 nm,and a pulse width of 7.79 ps.Furthermore,the influence of pump power on the characteristics of the pulse output was examined.2.A thulium-doped fiber laser based on single-walled carbon nanotubes(SWCNT)was constructed to generate stable single-wavelength and dual-wavelength mode-locked pulse outputs.The laser emits a single-wavelength pulse at 1940 nm with a pulse duration of 3.7 ps and a spectral bandwidth of 3.3 nm.Additionally,a dual-wavelength mode-locked pulse output with two central wavelengths at 1880 nm and 1895 nm is achieved.The corresponding repetition rates are approximately 17.055 MHz and 17.056 MHz,respectively.The paper introduces the method of single-wavelength and dual-wavelength pulse switching and analyzes the realization principle of dual-wavelength pulse.3.In this study,different methods were employed to tune the wavelengths of output pulses from graphene-based erbium-doped fiber lasers and SWCNT-based thulium-doped fiber lasers.For the erbium-doped fiber laser,a six-layer graphene saturable absorber was used,and the access mode of the graphene was changed while adjusting the length of the air cavity.This resulted in a wavelength tuning range of 4.5 nm.In the case of the thulium-doped fiber laser,the polarization controller in the cavity was adjusted to achieve wavelength tuning.The tuning range of the single-wavelength pulse was 40 nm,while the tuning range of the dual-wavelength pulse reached 22 nm.Additionally,the working principle of wavelength tuning was studied.4.Simulation studies were conducted for both a graphene-based erbium-doped fiber laser and a SWCNT-based thulium-doped fiber laser.A MATLAB program was developed to solve the nonlinear Schr?dinger equation,which describes the pulse transmission in the laser cavity.The program was employed to investigate the effects of dispersion and self-phase modulation on pulse transmission.Additionally,the program was used to carry out simulation research on the experimental lasers,which allowed for better verification of the experimental results.Parameters such as the modulation depth of graphene in the erbium-doped fiber laser and the coupling ratio of the coupler in the thulium-doped fiber laser were analyzed through simulation to determine their impact on pulse output.