Optical Micro/nanofibers For Dispersion Management And Their Applications In Mode-locked Fiber Laser

Fiber lasers have many advantages,such as relatively low thermal effect,broad gain bandwidth,relatively high gain,easy operation,compact structure,etc.,which has drawn increasing attention in practical applications.For passive mode-locked fiber lasers,the sign and magnitude of the net cavity dispersion mainly determine the pulse dynamics inside the cavity and the output pulse waveform regimes.Therefore,in order to obtain laser pulses with different characteristics,it is necessary to manage the dispersion of the laser cavity.However,due to the large material dispersion and relatively low waveguide dispersion of common fibers,there is a lack of dispersion compensation fiber for lasers operating at 1 ?m.Meanwhile,the dispersion compensation fiber at 2 ?m has low dispersion value and high splicing loss.Other free space dispersion management techniques with prism pairs or grating pairs have the problem of inconvenient alignment,and will break the all-fiber structure of the laser.Optical microfibers are waveguides with diameters close to or smaller than the guided wavelength.Because of the high contrast of the refractive index between the core and cladding material,the evanescent field around the microfibers could be strongly enhanced,which leads to a strong diameter-dependent waveguide dispersion.Thus,by controlling the diameter and length of the microfiber,the desired dispersion value can be obtained.With the properties of strong dispersion,low loss,highly flexibility and fully compatibility with traditional fiber devices,microfibers could be applied in mode-locked fiber lasers as dispersion management or pulse compression components.In the first part of this thesis,a femtosecond mode-locked fiber laser at 1 pm using optical microfiber for dispersion management is proposed.Through theoretical calculation and white light interferometry measurement,we demonstrate that the optical microfiber used in this experiment has an anomalous dispersion around 1 ?m.A microfiber with a diameter of 1 ?m,a length of 25 cm,a dispersion of-120 ps2/km and an ultra-low loss of 0.06 dB is integrated into mode-locked Yb-doped fiber lasers,enabling convenient dispersion management of the laser cavity.Besides,optical microfibers could be adopted to spectrally broaden and dechirp the ultrashort pulses outside the laser cavity.The cavity length of the laser is 1.7 m and the net dispersion is-0.006 ps2.When the pump power is 150 mW,our experimental results show that the average power of the output pulses is 60 mW,the repetition rate is 120 MHz,the central wavelength is 1048 nm,the FWHM of the optical spectrum is 20 nm,and the pulse width is 110 fs.We believe that this demonstration may facilitate all-fiber format high performance ultrashort pulse generation at 1 ?m and may find applications in precision measurements or evanescent-field-based optical sensing.In the second part of this thesis,a microfiber-enabled dissipative soliton fiber laser at 2 ?m is proposed.Through theoretical calculation,we demonstrate that optical microfibers can show very strong normal dispersion(as high as 2700 ps2/km)around 2 ?m,which is a highly desired feature for dispersion management in ultrafast lasers.A microfiber with a diameter of 1 ?m,a length of 10 cm,a dispersion of 2700 ps2/km and a loss of 0.5 dB is integrated into mode-locked Tm-doped fiber lasers to enable dissipative soliton operation.The cavity length of the laser is 4.1 m and the net dispersion is 0.017 ps2.When the pump power is 795 mW,our experimental results show that the average power of the output pulses is 25 mW,the repetition rate is 49 MHz,the central wavelength is 1925 nm and the FWHM of the optical spectrum is 50 nm.The pulse duration is measured to be 195 fs after a dechirping single-mode fiber.Because of its large diameter-dependent dispersion,low loss,easy fabrication process and fully compatibility with traditional fiber devices,optical microfibers can be used as highly efficient and flexible dispersion management components for Tm-doped or Ho-doped fiber lasers.In summary,based on the theory and experiment,optical microfibers have strong diameter-dependent waveguide dispersion,and can be applied to mode-locked fiber lasers at 1?m and 2?m,enabling dispersion-management soliton and dissipative soliton respectively.As efficient and flexible dispersion management components,optical microfibers may find applications in ultrashort pulse generation and other applications based on ultrashort pulse.