Research On Frequency Doubling Of Yb-doped Fiber Laser

Comparing with the traditional solid laser, the fiber laser has thecharacter of high beam quality, high reliability, high efficiency, small size, aircooling etc. The combination of the Yb-doped fiber laser and high efficiencynonlinear crystal can produce high quality green beam. This scheme attractedextensive attentions in recent years.For the reason of nonlinear effects, the peak power of fiber laser may belimited in a low power level. The low peak power of Yb-doped fiber laserwould need a long crystal for high SHG efficiency.When the lenth of crystal islarger than the confocal parameter ( kw0 2), the condition for plane waveapproximation is not fulfilled. So the equation using plane waveapproximation will not work well for focusing Gaussian beam for a longcrystal. In this thesis, the couple equation with plane wave approximation isimproved to describe the case of focusing Gaussian beam under big signal andlarge crystal. The new equations can describe the case of focusing guassianbeam well with the bringing of parameters which can show the decreasing andmismatching seduced by the beam divergence.The analysis shews that the main reasons for the decrease of SHG efficiency forMOPA structure SHG scheme are the shift of wavelength, labefaction of polarizationand amplified spontaneous emission(ASE). Those phenomenons must be overcome,when a high efficiency is wanted. The temperature tuning nonlinear crystal(likeLBO), polarization maintaining fiber, good matching for seed power and pumppower could be chosen to restrain those phenomenons.Another topic of this thesis is on the analysis of frequency doubling for awide spectra Yb-doped fiber laser by KTP and LBO, using the improvedcouble equation. It shows that: 1) For LBO crystal, using none critical phasematch and long fundamental wavelength can bring about an ideal SHG result.2) KTP crystal can be used under noncritical phase match (NCPM) conditionwhen the Yb-doped fiber laser works under 1079nm-1085nm wavelength;under the condition of NCPM, KTP crystal has a big accepted angular andextreme large accepted bandwidth for temperature, but the acceptedbandwidth for spectral is small. So for the case of SHG using KTP crystal afundamental beam with narrow linewidth is needed.