| Owing to the characteristics of compact structure,narrow pulse width,high peak power,and wide spectral range,2?m mode-locked fiber lasers working in the “eye-safe” band haveimportant application prospects in materials processing,gas detection,biomedical,free-space optical communications,etc.However,because 2?m lasers were developed late compared with 1?m and 1.55?m band,there are few researches on the nonlinear dynamics of mode-locked pulses at2?m band.Besides,optical parameters of fibers at 2?m band such as group velocity dispersion,nonlinearity,gain bandwidth of rare-earth ion doped fibers and so on,are significantly different from those of fibers at 1?m and 1.55?m band.Therefore,it is of great academic significance and application value to realize different type mode-locked pulses in 2?m fiber lasers and compress the pulses.Based on passively mode-locked fiber lasers using nonlinear polarization rotation technology,this paper studies the generation of traditional solitons,stretched pulses and dissipative solitons at 2?m band.Three types of pulses are compressed by means of shortening the cavity length,thedispersion compensation of the ultra-high aperture fibersat extra-cavity or compressing by the space gratingsat extra-cavity.The specific research contentsare as following:1.The effect of cavity length on the narrowing characteristics of traditional soliton pulses is studied,and the complex nonlinear dynamic characteristics of soliton molecules are experimentally analyzed.Since symmetrically distributed Kelly sidebands oftraditional solitons are related to the dispersion,reducing the net cavity dispersion by shortening the cavity length can suppress the generation of Kelly sidebands andreduce the number of Kelly sidebands,which broadensthe spectrum andthus achieves the purpose of compressing the pulse width.Using this method,the cavity length is shortened from 6.92 m to 1.96 m,andthe corresponding pulse width is reduced from 700 fs to 440 fs.On the other hand,it isexperimentally found that the interaction between dispersive waves associated with Kelly sidebands and solitons resulted in the formation of soliton molecules with a fixed phase-difference.2.The extra-cavity compression of the stretched pulse with Martinez spatial grating pairor different types of ultrahigh numerical aperture fibers is studied.Firstly,the intra-cavity dispersion is compensated to near zero by ultra-high numerical aperture fiber,which completely suppresses the Kelly sideband generation and achieves the stretched pulses.Secondly,three kinds of ultra-high numerical aperture fibers,UHNA4,UHNA7 and SM2000 D,are used for pulse compression and the compression effects are compared.The pulse width of 1.19 ps is compressed to 390 fs by using UHNA4 fiber.Finally,the direct output of thestretched pulse from the resonator is pre-stretching through a long single-mode fiber,and then the pulse is compressed to 287 fs through the Martinez compressor.Simultaneously,the nonlinear dynamic characteristics of the bound state of stretched pulse are studied,and the formation of bound state caused by thedirect overlap of pulse tailsis analyzed.3.The generation of dissipative solitons in thenormal-dispersion regime is studied,The spectrum of the dissipative soliton has a steep edge and its top is relatively flat.The 3-d B bandwidth of the spectrum is up to 52 nm.The dissipative soliton is extra-cavity compressed by using single-mode fibers and the pulse width decreases from 12.1ps to 364 fs. |
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