High-power, Linearly-polarized, Picosecond Pulse Ytterbium-doped All-fiber Laser And Its Frenquency-doubling Characteristics

High average power fiber laser sources operating in the picosecond regime havea wide range of applications including materials processing, precise manufacturing,because of the good beam quality, convenient heat dissipation and compactness. But,the requirement of high power linearly-polarized fiber laser sources in nonlinearoptics frequency conversion field, such as second harmonic generation, makes it agreat application prospect and practical research value. Furthermore, pulsed greenlaser are more widely used in industrial processing because the absorption of somematerials in visible wavelength region is more apparent than in the infrared region. Inthis paper, a high power linearly-polarized picosecond pulse Ytterbium-dopedall-fiber laser and green laser achieved by second harmonic generation aresystematically reported. The main subjects are as following:(1) A picosecond pulse Ytterbium-doped fiber laser mode-locked bysemiconductor saturable absorber mirror (SESAM) is reported. Considering theadvantages of SESAM and passively-mode-locking technique, a ring cavityconfiguration added by a filter with bandwidth of2.68nm to define the centerwavelength is adopted. Stable mode-locked pulse trains occur at an incident pumppower of200mW. The laser generates30ps pulses at a repetition rate of28MHz. Itscenter wavelength and the spectrum bandwidth are measured to be1064.02nm and2.59nm respectively. The steep edge in the optical spectrum is a clear indication of thedissipative solitons (DSs) in this all-normal-dispersion mode-locked fiber laser. DSgeneration is a result of the mutual interactions among normal cavity dispersion, thefiber nonlinear Kerr effect, laser gain and loss, and the effective cavity gainbandwidth filtering. The maximum output average power is42mW when the incidentpump power reaches250mW, which is the highest pump power without pulsebreaking.20mW linearly polarized laser output is achieved after the broadbandpolarization maintaining isolator. Such low-power fiber laser with compact structureand superior performance is suitable as seed source for high power fiber amplifier.(2) A linearly-polarized Ytterbium-doped fiber amplifier based on MOPAstructure is experimentally studied. The fiber amplifier consists of two stages seed bya mode-locked Ytterbium-doped fiber laser with SESAM.10W average power isachieved when the available incident pump power reaches23W with the optical efficiency of46%, corresponding to the single pulse energy of357nJ and a peakpower of11.9kW, respectively. The pulse width and the polarization extinction ratioare measured to be30ps and17dB, respectively. Even at the highest output power, noundesirable spectral peaks on the right side of the center wavelength induced bystimulated Raman scattering (SRS) are observed, this is because that SRS has athreshold power. Only the peak power reaches a certain value, the energy transfer canhappen from pump source to the Stokes light. The obvious spectral broadening occurswith the spectrum bandwidth of6.18nm. It can be explained by the frequency chirpinduced by self-phase modulation, which is related to nonlinear phase shift dependingon the intensity.(3) Second harmonic generation process pumped by a picosecond pulseYtterbium-doped fiber laser is performed from theoretical analysis and experiment.Firstly, proceeding from Maxwell equations, the theory of second harmonicgeneration about ultra-short pulse laser source is derived. It is suggested that not onlythe phase-matching but also the group-velocity matching should be met. Secondly,nonlinear crystals and phase-matching technique are described, with emphasis onBBO and the angle phase-matching technique as well as LBO and noncriticalphase-matching technique. Thirdly, a commonly used nonlinear software SNLO isintroduced and the simulation of frequency doubling is carried out. Lastly, asingle-pass extracavity frequency doubling device in linear cavity configuration isdesigned by using BBO and LBO. The green laser output at532nm with Watt level isrealized at the highest fundamental power in both conditions. The bandwidth offundamental light, the crystal length and the degree of polarization are three keyfactors which affect the efficiency mostly. In the future work, high-power, high degreeof polarization and narrow-linewidth fundamental laser source is desired. With suchexcellent pump sources and the appropriate nonlinear crystal, higher frequencydoubling efficiency can be achieved.