Study On All-normal-dispersion Yb-doped Fiber Laser Based On Long Period Fiber Gratings

Mode-locked lasers have attracted great interests in fundamental science,commercial instruments and other applications. Most of important researches andapplications need the ultroshort pulses laser with narrow linewidth, high power and highpeak power. Recently, all-normal-dispersion (ANDi) fiber lasers have attracted greatinterests because of their simple cavities, higher pulse energies and higher peak energythan soliton or dispersion-managed soliton. In this thesis, all the experimental researchesare concerned of this novel kind of mode-locked fiber lasers to achieve the characteristicsof all-fiber format, multi-functions and practicality. And research the new phenomenonof the ANDi system is also our purpose. The main contents and innovations of thisresearch are as follows.1. Using a cascaded long-period fiber grating (CA-LPFG) as the spectral filter in theytterbium-doped mode locking to achieve femtosecond pulse in the normaldispersive regime was first proposed. The CA-LPFG was fabricated in a single modefiber at wavelength of1064nm using a CO₂laser based system. A stable mode-locking laser output at wavelength of1034nm was observed and measured in thenormal dispersive regime. With the CA-LPFG for suppress and reshape the pulses,an output pulse width408fs and maximum pulse energy of2.03nJ (41.736MHz andmaximum output power of84.7mW) laser pulse is obtained by NPE.2. An all normal dispersion low repetition rate high energy passive mode-lockedytterbium–doped fiber laser with output pulses duration ranging from nanosecondsto picoseconds was reported. The mode-locking mechanism of the laser is based onnonlinear polarization evolution and strong pulses shaping with a cascade long-period fiber grating bandpass filtering in highly chirped pulses. The laser generateshighly stable pulses duration from2.62ns to315ps with a maximum pulse energy of49.5nJ and2.5435MHz repetition rate.3. We report on an all normal dispersion harmonically mode-locked ytterbium-dopedfemtosecond fiber laser for the first time. The modelocking mechanism of the laser isbased on nonlinear polarization evolution with strong pulse shaping. Bandpassfiltering is achieved with a cascaded long-period fiber grating, forming highlychirped pulses. The laser generates stable2.5-ps duration pulses at a repetition rateof125.39MHz that could be dechirped externally to310fs with a maximum pulseenergy of0.36nJ.4. A tunable high-order harmonic and passively mode-locked Yb-doped with all normaldispersion fiber laser is demonstrated. With the mode-locking mechanism ofnonlinear polarization evolution (NPE) effect and a cascade long period fiber grating(C-LPFG) as an all-fiber format spectral filter for the strong pulse shaping, amaximum of14th-order harmonic mode-locked (HML) Yb-doped fiber laser can betunably achieved with the repetition rate of35.497MHz compared with thefundamental repetition rate at2.538MHz, which is the highest harmonic mode-locked laser with all normal dispersion so far. At the14th mode-locked harmonicoutput, the cavity supermodes are suppressed by23dB, and the pulses duration is1.23ns with maximum pulse energy of0.97nJ. It is further shown that the order ofthe HML depends not only on the pump power but the cavity length.5. Dissipative soliton resonance with all-normal-dispersion in Yb-doped fiber laser isreported and demonstrated for the first time. we found experimentally that the squarepulse width increased linearly with the pump strength while the pulse intensity keptalmost constant. In the different cavity length (75m and123m), the maximum pulseduration of2.55ns and13.1ns can be obtained, respectively. It is further shown thatthe pulse duration depends not only on the pump power but the cavity length withthe DSR effect.6. A tunable all-normal-dispersion Yb-doped fiber laser (YDFL) with either passively mode-lockedor continuous-wave (CW) output using a phase-shifted long period fiber grating (PS-LPFG) isproposed and demonstrated. The output wavelength can be tuned continuously and smoothlyover a spectral range of10.5nm and11.4nm, respectively for the mode-locked mode and the CW mode. In addition, the pulse duration can also be tuned continuously from3.6ns¹.2ns. Ata pump power of530mW, the maximum output pulse energy is38.9nJ with a repetition rate of2.499MHz and the maximum output power in continuous-wave mode is124mW, respectively7. A switchable dual-wavelength and passively modelocked ytterbium-doped fiberlaser (YDFL) in the all-normal-dispersion (ANDi) regime is reported anddemonstrated for the first time. Using a phase-shifted long-period fiber grating(PS-LPFG) as an all-fiber format spectral filter in the laser cavity, a stable andswitchable dual-wavelength mode-locked operation is achieved by nonlinearpolarization evolution (NPE). The maximum output energy of the dual-wavelengthpulse is25.76nJ with pulse width of310ps at a repetition rate of2.499MHz.8. A tunable dual-wavelength passively mode-locked ytterbium-doped fiber laser(YDFL) in the all-normal-dispersion (ANDi) regime is first reported anddemonstrated. Using a phase shifted long period fiber grating (PS-LPFG) as anall-fiber format spectral filter in the laser cavity, a self-starting stabilizeddual-wavelength mode-locking operation and a triple-wavelength mode-locking areachieved by nonlinear polarization evolution (NPE). The mode-locked dual-wavelength output can be tuned continuously and smoothly over a spectral range of10nm using the PS-LPFG bandpass filter. Meanwhile, a triple-wavelength mode-locking can be achieved by nonlinear polarization evolution (NPE). The maximumaverage output power of triple-wavelength mode-locked pulses is⁶4mW at thepump power of530mW.