Research On Modified Nonlinear Amplifying Loop Mirror And Thulium-doped Fiber Femtosecond Laser

Fiber lasers are of compact size and they can output laser pulses with excellent spectrum quality from flexible cavities.As a result,fiber laser become to be a paramount tool for mechanical machining,biological imaging,information communication,national defence and other fields.Nowadays,mode-locking methods of fiber lasers are always in the category of passive mode-locking,such as nonlinear polarization evolution,real saturable absorber,nonlinear loop mirror and so forth.However,these typical mode-locking methods are limited by some of their own shortcomings and thus cannot be widely used.Consequently,there is an increasing need for a mode-locking method which has an outstanding output characteristic and a high mode-locking stability.Apart from traditional Er-doped fiber lasers and Yb-doped fiber lasers,the 2 micrometer range of Thulium-doped active fiber laser has emerged as a promising branch.2 micrometer range has attracted researchers' attentions for it is in the eye-safe spectrum and it consists of the absorption peak of water.As has been stated in the previous paragraphs,this thesis chose modified nonlinear loop mirror and thulium-doped fiber laser as research objects.The main subjects are summarized in the following outline: 1)Discuss the basic mechanism and physical process of ultrafast laser pulses1)Discuss the basic mechanism and physical process of ultrafast laser pulses forming.Introduce the meaning of nonreciprocal elements and state peculiarities of laser cavity with nonreciprocal elements.2)Realized mode-locking in typical “figure eight ” and "?-like" cavity,adding merely nonreciprocal,without real saturable absorbers.And the repetition rate of modified nonlinear amplifying is much higher then typical value.Provide a possibility of establishing an all polarization-maintaining fiber laser with simple optical elements.The laser works in soliton regime and generates 308 fs pulses with an average power of 23.6 m W at a repetition of 22 MHz.A non polarization-maintaining Er-fiber laser based on a modified“s-like”cavity working at stretched pulse regime generates pulses with an average power of 36 m W at a repetition of 80 MHz.The same laser working at normal dispersion domain generates pulse train with an average power of 14 m W at a repetition of 53.6 MHz.3)Establish mode-locked thulium-doped fiber lasers with carbon nanotubes and a semi-conductor saturable absorber mirror.Analyze thulium-doped fibers with different doping concentrations and core diameters.Realize mode-locking in a non polarization-maintaining ring cavity with carbon nanotubes.Obtain laser output of pulses centered at 1906 nm with a spectrum width of 2nm.Stable Q-switching are obtained from a SESAM linear cavity thulium-doped all polarization-maintaining fiber laser.An all polarization-maintaining ring cavity based on a circulator SESAM thulium-doped fiber laser generates pulses centered at 1928 nm with 1nm spectrum width at a repetition rate of 28 MHz.