Passive Mode-Locked 2mm Fiber Laser Operating In Fundamental Transverse Mode

2 ?m mode-locked fiber lasers have been widely used in medical surgery,material processing,radar,remote sensing and other fields.And they have important scientific research value in the generation of optical parametric oscillation,Raman self-frequency shift and supercontinuum spectrum,etc.2 ?m mode-locked fiber lasers have an excellent prospect.However,compared with the mature near-infrared mode-locked fiber lasers,the mode-locking technology in 2 ?m band lags behind,and there are many problems,such as the immaturity and low efficiency of the laser modulator,poor pulse stability,limited monopulse energy,strong nonlinear effect and serious thermal effect in the optical fiber.So it has the vital significance to study the 2 ?m mode-locked fiber lasers.This paper carry outs the reaserch for 2 ?m thulium passively mode-locked fiber lasers from two aspects of novel saturable absorber and multimode gain medium with large core,aiming to achieve 2?m mode-locked pulses with high pulse energy,good stability,high beam quality,etc.Firstly,we carried out PbS nanoparticles modulated 2 ?m mode-locked fiber laser research based on thulium single-mode fiber.The saturable absorber was prepared by dripping the PbS nanoparticles solution over a gold mirror.We characterized the surface morphology,Raman spectroscopy,linear absorption and nonlinear absorption characteristics of the saturable absorber,and the results show that PbS nanoparticles have many advantages such as broadband absorption properties,high damage threshold,low saturation intensity,etc.The output of the mode-locked pulses were successfully obtained by inserting the PbS nanoparticles saturable absorber into the ring cavity for nonlinear modulation.The average power,the pulse width and the pulse energy were 115.6 mW,4.24 ps and 7.3 nJ,respectively.The experimental results demonstrate the superior modulation characteristics of Pb S nanoparticles in the mid-infrared band.Secondly,the mode-selective multimode locking experiment was studied based on thulium multimode fiber,and the mode selection principle of fiber Bragg grating(FBG)was analyzed.The spatial mode filtering of semiconductor saturable absorption mirror(SESAM)and the transverse mode-reflection wavelength correlation of FBG were used to select the mode.By controlling the position and angle of SESAM,a switchable continuous wave(CW)mode-locked and Q-switched mode-locked pulse states were achieved,producing the CW mode-locked pulses with an average output power of 0.87 W,a pulse width of 17 ps and a single pulse energy of 52.4 nJ.The beam quality factors in the x and y directions were 1.74 and 1.71,respectively.In addition,the Q-switched mode-locked pulses with an average output power of 1.67 W,a minimum pulse width of 1.55 ?s and a maximum pulse energy of 29.2 ?J were obtained.Preliminary experimental results show that multimode optical fiber has superior output performance compared with single-mode optical fiber,and has great potentials in the fields of high power,high energy lasers and nonlinear optics in the future.Finally,we summarize the work in this paper.We analyze the shortcomings and make the next step of improvement plan.The main work to be done in the future is prospected.