| Owing to their specific wavelength,2μm mid-infrared fiber lasers have been applied in various areas including the fields of medical treatment,industrial processing,military application and the basic scientific research,etc.,showing great application prospect.Recent years,with the progress of fiber manufacturing techniques and deepening understanding of mode-locking mechanisms,pulse energy of mode-locked1～1.5μm fiber lasers has been greatly improved,and maximum pulse energy of 1μm fiber lasers have surpassed 1μJ.Owing to the restrict of soliton area theorem,pulse energy of 2μm traditional mode-locked fiber lasers is much lower than that of their1～1.5μm counterparts,which places a limit in their practical application.Facing the huge application requirement of high-pulse-energy ultrafast 2μm laser sources,it is very urgent to develop large-pulse-energy mode-locked fiber laser techniques in the 2μm wavelength regime.The research of this dissertation is mainly about how to improve the pulse energy of 2μm mode-locked thulium doped fiber lasers（TDFs）.In the 2μm region,large anomalous dispersion of silica fiber gives rise to the tendency of soliton splitting under high pump power.To deal with that problem,we propose a condensed gain fiber model so as to greatly improve the generated soliton pulse energy through decreasing the anomalous dispersion and nonlinear phase shift.Both numerical simulation and experimental operation produce 2μm dissipative soliton（DS）with pulse energy larger than 10 n J,and harmonic solitons with pulse energy larger than 5 n J have also been achieved.These results show pulse energy is an order of magnitude larger than the traditionl 2μm single-mode solitons,thus confirming the validity of our condensed gain fiber model for improving the pulse energy of 2μm soliton fiber lasers.At the same time,we applied this condensed gain fiber model to a 2-dimensional（2D）material（Mo S2）mode-locked 2μm fiber laser,and also achieved>10 n J DS pulse energy,thus proving that the condensed gain model is not confined by the saturable absorber material.The main research content of this dissertation is as follows:1.A theoretical condensed gain fiber model is constructed for mode-locked 2μm DS fiber laser with total normal-dispersion fiber cavity.Based on this theoretical model,numerical simulation on the evolution of pulse shape and spectrum of laser pulses when traversing through various cavity-elements is carried out.After analyzing the relationship between the pulse amplitude and the nonlinear phase shift,and the pulse output energy versus the gain fiber length,the DS formation mechanism of 2μm mode-locked fiber is clearly disclosed.Theoretical analysis and numerial simulation show that highly-doped short-length gain fiber helps to improve the soliton pulse energy,which increases with reduction of gain fiber length.Numerical simulations show that～12.7 n J pulse energy of 2μm DS can be achieved,when the length of TDF is shortened to～0.12 m.2.Based on semiconductor saturable absormber mirror（SESAM）and dispersion management,we construct a 2μm thulium-doped DSs fiber laser and demonstrated experimentally the output of high pulse energy DSs.263 m W output power and 12.07 n J energy of dissipative soliton is obtained from this experimental configuration at the highest launched pump power of 1.085 W.The pulsing repetition rate is 21.79 MHz and the pulse width is 43 ps.To the best of our knowledge,this is the highest single pulse energy reported to date directly from a passively mode-locked 2μm single-mode TDF laser.Experimental results show good agreement with simulation results,confirming our condensed gain fiber model.3.The multi-pulsing phenomenon and harmonic mode-locking in 2μm DS fiber lasers are also studied.After dispersion management（the total cavity length is 4.71 m）and carefully adjusting the position of the SESAM,stable multiple pulse operation（multiple pulses form a clustered entity）is achieved under the pump power of 693 m W.Further tuning the position of SESAM,stable harmonic mode-locking DS phenomenon can be achieved(harmonics order ranging from the 2nd to the 4th),with the pulse energy and repetition rates being 6.27,4.32,3.29 n J,and 43.4,65.1,86.8 MHz,respectively.The harmonic laser pulse has a pulse width of～30 ps and a center wavelength of～1929nm with a spectral bandwidth of～3.26 nm.Doublet and triplet soliton molecule pulses have also been obtained from the 2nd harmonic mode-locking scheme,and their output power and pulse energy are 72 m W and 95 m W,3.32 n J and 4.38 n J,respectively.4.Given the remarkable optic and electronic properties of the two-dimensional Molybdenum Disulfide（Mo S2）material,we have also implemented research on 2μm mode-locked fiber lasers based on the nonlinear saturable absorption of Mo S2.Through the conventional liquid-phase exfoliation approach,few-layer Mo S2 saturable mirror is successfully fabricated,and a 2μm TDF laser is demonstrated experimentally based the nonlinear saturable absorption of Mo S2.Through dispersion management,the fiber laser operates in the normal dispersion regime,and 2μm Q-switching,fundamental mode-locking and harmonic mode-locking have been realized.At the launched pump power of 743 m W,163 m W mode-locked laser output power is achieved with the repetition rate and pulse energy of being 9.59 MHz and 17 n J,respectively.To the best of our knowledge,this is the highest single pulse energy reported to date from a single-mode TDF laser mode-locked with Mo S2.Stable 4th harmonic mode-locked pulse based on the Mo S2 modulated TDF laser is also realized under the pump power of～783 mw,with repetition rate of 38.36 MHz and pulse width of 1.375 ns.Experimental results prove that 2D Mo S2 is of great potential in the field of 2μm mid-infrared high power photonic devices. |
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