Research Of 2 ?M Graphene Mode-locked Thulium-doped Fiber Laser Based On The Tapered Microfiber

Owing to their wide applications in laser medicine,human-eye safety radar,environmental monitoring,and non-metal processing,2?m mode-locked fiber sources have attracted too much attention as a novel laser source.As one of the saturable absorber materials for obtaining mode-locked laser pulse,graphene has shown excellent performance in the fields of optics and electricity.At present,the construction methods of mode-locked device with graphene can be generally classified into transmission type and reflection typethere.However,due to their low modulation depth and difficulty in achieving all-fiber operation,the above two methods can not achieve large-scale application.Based on the above problems,a novel structure of mode-locked devices with the excellent performance of graphene is designed by studying the transmission principle of light in micro-nanofibers,and used to study the characteristics of mode-locking in the thulium-doped fiber laser.First of all,based on the status of the domestic and international research of 2?m passively mode-locked fiber lasers in the last decade,the merit and demerit of mode-locked technologies,mode-locked structures and gain fiber types are summarized.Next,the overall scheme of 2?m graphene mode-locked fiber lasers is established by using a ring resonator,the pump source with 793 nm central wavelength,and the thulium-doped fiber.To improve the quality of output laser,the cavity is also added to the isolator to maintain the unidirectional transmission of light,the standard single-mode fiber increases the peak power of the laser and the large numerical aperture fiber to compensate for the negative dispersion in the cavity.Secondly,the optimization of pump mode and gain fiber length is studied under the overall scheme.This study shows that thulium-doped fiber laser has three main types:³H₆-³H₅,³H₆-³F₄ and ³H₆-³H₄.Through the analysis of the excitation and transition process,the pump methods of the ³H₆-³H₅ and ³H₆-³F₄ are not superior to the ³H₆-³H₄ in terms of the quality of output laser and difficulty of inverting particle formation.Therefore,based on the pump method of ³H₆-³H₄,the rate-equation model of thulium-doped fiber laser is builded,and the distribution of the pumping optical power and the signal optical power along the thulium-doped fiber under different doping concentrations are studied.The results show that the length of thulium-doped fiber has the optimal value under a certain doping concentration,and the maximum output power can be obtained by the laser under this length.Meanwhile,the theoretical basis has been provided for the construction of subsequent mode-locked thulium-doped fiber laser.Then,considering that modulation depth of the mode-locked device has a greater influence in the mode-locked thulium-doped fiber lasers.A novel all-fiber mode-locked device is designed using single-layer graphene and tapered microfiber.And the key device of the mode-locking device consists of graphene and tapered microofibers.The process of design is mainly divided into three parts:?1?the preparation and the Raman characterization of the single-layer graphene,?2?the simulation and preparation of the tapered microfiber,?3?the combination and the testing of the composite waveguide.The results show that the graphene is a single-layer structure using the intensity ratios of the G and 2D of graphene Raman spectra.By comparing domestic and international literature,it verified that this mode-locking device can achieve stable mode-locking.Finally,using previously designed thulium-doped fiber lasers and mode-locked device,a 2?m mode-locked thulium-doped fiber laser system is built with the help of fiber fusion splicer.In the net anomalous dispersion regime,the characteristic of the mode-locked pulses,the central wavelength,full width at half maximum,repetition frequency,pulse width and signal-to-noise ratio are 1992.17 nm,2.02 nm,563 ps and6.92 MHz and 40 dB,are obtained by application of the mode-locked device in the thulium-doped fiber laser.Continue to increase pump power,2⁶ multi-soliton mode-locked pulses are also observed.Near zero dispersion regime,the dispersion management solitons of center wavelength,full-height half-width,repetition frequency,pulse width and signal-to-noise ratio of 1986.27 nm,10.73 nm,10.06 MHz,812 ps and55 dB are obtained.Through dispersion management technology,the mode-locked device can not only achieve stable mode-locking in a 2?m thulium-doped fiber laser,but also has a better output characteristic than when it operates in the net negative dispersion regime.