Research On Novel Ultrashort Pulsed Fiber Lasers

Ultrashort pulsed fiber lasers have widespread applications in optical fiber communications,lidar,consumer electronics,precision micromachining,biomedicine,aerospace and automobile manufacturing because the resultant output pulses feature extremely narrow pulse width,ultra-high peak power,high repetition rate and good stability.In recent years,with the sustained innovation and development of new materials,new technologies and new pulse-shaping mechanisms,ultrashort pulsed fiber lasers based on passive mode locking have become one of the frontier research subjects in the world.Our country has laid a good foundation and obtained fruitful research results in this high-tech field,which is expected to play an important role in promoting traditional and emerging industries.Therefore,it is of great scientific and practical importance to study novel ultrashort pulsed fiber lasers.In this thesis,theoretical and experimental investigations on ultrashort pulsed fiber lasers are carried out from the aspects of new materials for mode locking,new pulse-forming mechanisms and novel pulse dynamics.The main research contents of this dissertation are as follows:1.Generation and evolution of dissipative soliton resonance（DSR）are numerically investigated in a passively mode-locked fiber laser.The new mechanism to achieve DSR pulses by utilizing real saturable absorbers（SAs）is revealed.It is found that the RSA effect induced by the SA material itself plays a dominant role in generating DSR pulses.The influence of key SA parameters on the features of DSR pulses in the cavity is discussed.The existence of period-doubling bifurcations of DSR pulses in a fiber laser mode-locked by nonlinear amplifying loop mirror（NALM）is verified theoretically.The effects of cavity parameters on the DSR-pulse properties under the dynamic bifurcations are analyzed.It is shown that the increase of the saturation power of NALM is beneficial for the formation of period-doubled DSR pulses.A passively mode-locked fiber laser with nonlinear optical loop mirror is numerically investigated.Depending on the strength of cavity birefringence,various types of vector DSR pulses,including the polarization-locked DSR,the polarization rotation-locked DSR,and the group velocity-locked DSR states are obtained in the laser cavity.Their polarization-evolution dynamics are systematically discussed.2.Compact L-band（1565-1625 nm）fiber lasers mode-locked by graphene/WS₂（Gr/WS₂）heterostructure SAs are presented.The strong interlayer coupling between Gr and WS₂ in the as-prepared Gr/WS₂ heterostructure film is confirmed by Raman and photoluminescence spectra.In comparison with single-layer WS₂,the Gr/WS₂heterostructure exhibits superior saturable absorption properties in terms of larger modulation depth and lower saturation intensity.By utilizing the Gr/WS₂heterostructure as a mode locker,erbium-doped fiber lasers（EDFLs）operating in different dispersion regimes are designed and constructed.Conventional solitons centered at 1601.9 nm with 660-fs pulse duration as well as dissipative solitons featuring the central wavelength of 1593.5 nm and the pulse duration of 55.6 ps are achieved in the cavity.In addition,by incorporating the Gr/WS₂-based SA into a C-band（1530-1565 nm）mode-locked fiber laser,conventional-soliton pulses with a pulse width of 420 fs can be realized at 1563.3 nm.The proposed femtosecond pulsed fiber laser is simulated numerically and the simulation results are consistent with those of the experiment.3.A variety of soliton pulsating behaviors are experimentally invesitgated in a single-wall carbon nanotube mode-locked fiber laser via the dispersive Fourier transform（DFT）technique.In the case of single-periodic pulsation,the pulsating period varies from dozens to hundreds of roundtrips while the pulsating behaviors of the two orthogonal polarization components can be either synchronous or asynchronous.We also observe double-periodic pulsation in the cavity for the first time,to the best of our knowledge.It is shown that two modulation periods are involved in this case.By adjusting the pump power and the intra-cavity polarization controller（PC）,various pulsating period-doubled solitons（PDSs）,including single-and double-periodic PDS pulsations are found in the laser.The polarization-resolved transient spectra of these pulsating PDSs are measured by virtue of the DFT method.It can be seen that the polarization states of pulsating PDSs are modulated with a combination of two or three periods,which correspond to those of the pulse-energy modulation.4.A normal-dispersion EDFL mode-locked by nonlinear polarization rotation technique is designed and constructed.By tuning the pump power and intra-cavity PC,stationary dissipative solitons and pulsating dissipative solitons are generated in the laser.It is found that each transient spectrum of the former is almost indistinguishable while the latter manifests obviously spectral breathing behaviors.In addition,a unique dissipative-soliton pulse with a narrow peak emerging on the top of its spectrum is observed in the cavity.With the help of the DFT technique,it is confirmed that this spectral component stems from the mode-locked pulse itself rather than the continuous wave.The buildup and annihilation of the stationary dissipative solitons and pulsating dissipative solitons are studied.The evolution characteristics of their transient spectra are discussed in detail.It is shown that the starting processes of these two mode-locked pulses include unstable mode-locking,quasi-Q-switched mode-locking,quasi-stable and stable mode-locking stages.The width of the single-shot spectrum increases with the pulse energy,which is accompanied with the developemt of fringes at the edges of the spectrum.When the pump power is switched off,the intensity and width of the transient spectrum drop sharply,and the output pulse gradually disappears after several cavity roundtrips.