Research On Saturable Absorbers Based On New Nanoparticles For Ultrafast Lasers

With the merits of short pulse duration and high pulse peak power,femtosecond lasers are widely used in the fields of surgery,high precision measurement,sensing,femtosecond laser micromachining and fiber communications.It’s an effective method to obtain ultrashort pulses by inserting a saturable absorber（SA）in the laser cavity.The successful application of nanomaterials such as carbon nanotubes（CNTs）and graphene in the field of mode-locked lasers has greatly promoted the development of ultrafast lasers.However,CNTs is wavelength-selective when applied in mode locking.The modulation depth of single-layer graphene is very low and multi-layer graphene will introduce higher insertion loss.For this reason,people are committed to exploring other nanomaterials with excellent optoelectronic properties and continue to promote the development of ultrafast lasers.Metal oxide particles are an important class of nanomaterials,which have the characteristics of rich valence,diverse morphology,and excellent photoelectric properties.However,due to the relatively large band gaps of most metal oxides,they are generally used in Q-switched lasers,and there are few studies in the ultrafast field.With narrow band gap,broadband absorption property,MnO₂ nanoparticles can be applied to 1.55μm ore Ven mid-infrared ultra-fast lasers to achieve ultrashort pulse output.In addition,the commonly used methods for preparing saturable absorbers have certain problems.For example,the damage threshold of the SAs based on composite film method is relatively low;SAs based on the tapered fiber deposition method are susceptible to environmental pollutants,and SAs based on the filled microstructure fiber method requires complicated fusion splicing parameter.In this thesis,the preparation of saturable absorbers based on novel nanoparticles,the measurement of nonlinear absorption characteristics and their ultrafast laser applications are theoretically analyzed and experimentally demonstrated.SAs is fabricated by depositing fiber tapers and filling hollow fibers,and various soliton states are obtained.The major research works and achievements are outlined as follows:（1）A passively mode-locked fiber laser experiment was performed with a saturable absorbers based on MnO₂ nanoparticles:MnO₂ nanoparticles are characterized by different characterization methods,which prove that MnO₂ nanoparticles have saturable absorption characteristics in the communication band.The modulation depth of the saturable absorber prepared by deposit nanoparticles on fiber taper is 2.74%and the unsaturable loss is 71.5%.After excluding the NPR mode locking caused by the polarization-dependent loss introduced by the tapered fiber,mode locking based on the fabricated SA with center wavelength of 1530nm,a 3d B bandwidth of 2.4nm,and pulse width of 1.63 is obtained.By adjusting the polarization controller,23rd harmonic bound state mode-locked with a repetition frequency of 119MHz can be obtained.The pulse interval is 3.28 ps,and the single pulse duration is 975 fs.By adjusting the parameters in the cavity,dual-wavelength/triple-wavelength mode locking can be obtained.Mode-locked laser experiments confirm that MnO₂ nanoparticles have great potential to be used in ultrashort fiber lasers.（2）Saturable absorbers made by depositing nanomaterials on the waist of fiber tapers suffer from pollution in the air and have poor long term stability.We fill MnO₂nanoparticles into a hollow fiber to fabricate a saturable absorber with excellent performance and stability.By adjusting the net dispersion in the laser cavity,conventional soliton,dissipative soliton and noise-like pulse are obtained,respectively.When the net dispersion of the laser cavity is set as-0.35ps²,conventional soliton with 3d B bandwidth of 2.2nm,pulse width 1.14ps and repetition rate of 5.09MHz is achieved.By using a section of fiber with large normal dispersion as the dispersion compensation fiber,the net dispersion in the cavity is set as+0.379ps².A bandpass filter plays the role of spectral filtering and dissipative soliton is obtained by carefully adjusting the polarization controller and pump power.When the net dispersion in the cavity is set as+0.287ps²,noise-like pulse is obtained with femtosecond peak located on the top of a broad picosecond pedestal.（3）Broadband wavelength tunable switchable multi-wavelength Q-switched fiber laser:In order to obtain a tunable pulsed light source,we propose a wavelength tunable switchable multi-wavelength Q-switched fiber laser based on a homemade Sagnac-Lyot filter.Compared with the traditional Sagnac filter,its wavelength tunable performance is better.The saturable absorber and the Sagnac-Lyot filter are inserted in the laser cavity to achieve a single-wavelength Q-switched fiber laser with tuning range over 13nm.Dual-/triple-wavelength switchable Q-switched fiber laser with wavelength range over38nm（1523～1561nm）can also be obtained.（4）A wavelength-switchable passively mode-locked fiber laser based on Sagnac filter is obtained:A noise-like pulse mode-locked fiber laser with switchable center wavelength of 1538nm and 1552nm are obtained based on a Sagnac filter,respectively.The spectra are smooth and nearly triangular,and the autocorrelation curve are characterized by a narrow peak located on the top of a broad pedestal.Stable bound state soliton with center wavelength of 1527.8nm,the pulse interval of 3.04ps,and pulse duration of 1.16ps can be obtained by adjusting the polarization controller.Dip-type sideband soliton is obtained in the experiment with center wavelength of 1528.7nm,3d B bandwidth of 6.2nm,and pulse duration of 533fs.Numerical simulations are conducted to verify that it is a stable state of mode locking.