| Benefited from the compact design,short pulse duration,and high peak power,the ultrashort fiber lasers are always the research hotspots all over the world,which are attractive for a diverse range of applications,including biomedicine,material micromachining,optical communications,and nonlinear optics.In recent years,various low-dimensional nanomaterials which have saturable absorption effect and broadband response have been discovered with the rapid progress of research on low-dimensional nanomaterials.On one hand,fiber lasers based on low-dimensional nanomaterials saturable absorbers(SA)are one of the research hotspots in the field of the research of ultrashort pulse fiber lasers.Realizing the pulse compression and generating different types of solitons have become important research goals and directions.On the other hand,ultrashort pulse fiber laser based on nonlinear polarization rotation(NPR)mode-locking mechanism is also a hot spot in this field.Researchers are committed to developing fiber laser with ultrashort pulse duration and wide-spectrum output,which are very suitable for optical frequency combs,multiphoton microscopy imaging,and attosecond science.This thesis mainly uses the carboxyl-functionalized graphene oxide(GO-COOH)saturable absorber and NPR mode-locking mechanism to study ultrashort pulse erbium-doped fiber lasers and thulium-doped fiber lasers,and explores the generation and evolution of modelocked pulse.The specific research contents is as follows:First,a ring cavity based on GO-COOH was constructed to study erbium-doped fiber laser with different types of output solitons.The net dispersion in the cavity was adjusted to make the laser work in different dispersion regions.The conventional soliton,dispersion-managed soliton and dissipative soliton were successfully obtained.The spectrum bandwidth is 6 nm,11.38 nm,9.7 nm,and the pulse duration are 460 fs,320 fs,3.26 ps,respectively.The laser system is simple in structure,easy to start,stable and reliable,etc.It can provide an ideal experimental platform for the study of different types of solitons in the field of nonlinear optics.Second,the study of single and dual-wavelength erbium-doped femtosecond lasers based on GO-COOH has been carried out with usage of the filtering effect induced by the fiber birefringence in the optical resonator.The switchable single and dual-wavelength femtosecond soliton was obtained in single-mode Er-doped mode-locked fiber laser when the net dispersion was negative.At the low pump power,fiber laser works in a stable single pulse mode-locking state,with a center wavelength of 1560.1 nm,3 d B bandwidth of 5.1nm,and a pulse duration of 548.1 fs.With the increase of pump power and further adjustment of PC,the laser can be operated in a dual-wavelength mode-locked state.At 450 m W pump power,the center wavelengths of the dual-wavelength mode-locked pulses are located at1555.2 nm and 1531.9 nm with a spacing of approximately of 23.3 nm.The 3 d B bandwidth of the pulse spectrum is 6.2 nm and 1.46 nm,respectively.The laser system is simple to adjust and easy to construct,it can provide a feasible solution for generation of dualwavelength femtosecond pulse in erbium-doped fiber laser.Third,an erbium-doped mode-locked fiber oscillator operating in the C+L band was constructed,which can output a self-starting,high signal-to-noise mode-locked pulse.By controlling the structure of the resonant cavity and optimizing the gain-shift effect,the stable C+L band of conventional soliton was achieved.When the gain fiber length is varied from0.6 m to 1.9 m,the center wavelength of the output pulse with a wavelength range of 32.42 nm from 1560.88～1593.3 nm.The spectral bandwidth increases with the gain fiber,and the widest bandwidth is 6.53 nm,corresponding to pulse width of 479.6 fs.Fourth,a ring cavity based on NPR was constructed,a self-similar soliton with a spectral bandwidth of 59.7 nm and a pulse duration of 85.9 fs after compression outside the cavity was successfully realized through suitable control of intracavity dispersion and polarization,which was ejected into a bidirectionally pumped erbium-doped fiber amplifier(EDFA)as a seed source.The EDFA adopts a first-stage amplification,and only a section of SMF is spliced at the output as a compressor.The shortest pulse duration is 59 fs under the maximum pump power,the spectral coverage is from 1400 nm to 2000 nm,and the spectral bandwidth of-20 d B is 553 nm.Then,the dispersion and polarization state were further optimized in the same oscillator,we experimentally demonstrated the generation of the switchable dualstate solitons in a same cavity.The spectral bandwidths are 17.9 nm and 63.1 nm,and the pulse widths after compression are 219.5 fs and 117 fs,respectively.The self-similar soliton was adopted as seed light and coupled into the EDFA with only backward pumping,and the octave supercontinuum spectrum covering the spectrum from 928 nm to 2400 nm was obtained by using 3.1 m SMF at the maximum pump power.Our experimental results provided a possibility to achieve the SC generation in the SMF-EDFA laser system.The laser system is easy to start,stable and reliable,etc.It can provide an ideal wide spectrum femtosecond light source for multiphoton imaging system.Fifth,two sets of thulium-doped fiber lasers based on the NPR mode-locking mechanism were constructed.In the first experiment,a pulsed laser with a center wavelength of was1810.3 nm successfully generated,the spectral bandwidth was 3.6 nm,and the pulse width was 1.14 ps,which proved that the customized fiber device can suppress the longwavelength emission of thulium-doped fiber,making it possible to achieve 1.7 μm tunable ultrashort pulses.In the second experiment,a tunable picosecond ultrashort pulse output with a wavelength tuning range of 1788 nm-1827 nm was realized using the filtering effect by the diffraction grating in the ring cavity.The laser system is simple in structure and easy to start,it can provide an ideal tunable ultrashort pulse light source for the fields of multiphoton imaging and nonlinear optics. |
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