| There is rapid development in the technology of frequency conversion ofultrashort pulses with the appearance of many high quality and efficientnonlinear media. Meanwhile, frequency conversion technology of ultrashortpulses also provides a support for the fabrication of high quality laser sources.The technology of frequency conversion is one of important methods to getvarious kinds of laser sources with different wavelengths, and is widely appliedin industry, medicine.The main contents in this thesis are as follows:Firstly, the current development of the technology of frequency conversionis discussed. Consequently, the basic theory of ultrashort pulse frequencydoubling is introduced, such as coupled wave equation, phase matchingtechnology and walk-off effect. Based on the inhomogeneous vector waveequation, a general integral form of the second harmonic field is derived. Theeffects of temporal and spatial walk-off and loss of fundamental wave areconcerned, the formula of second harmonic wave field out of crystal is derived aswell. In experiment, the properties of frequency doubling of BBO crystal isinvestigated, using Ti-doped sapphire femtosecond laser (Wavelenth tuning rangeof710-900nm, pulse width of80fs) as light sources. Using different thick crystal,we study the relationship between incident angle and frequency doublingconversion, the relationship between phase matching angle and frequencydoubling conversion, and the relationship between frequency doubling power,conversion and fundamental wave power. We find that the field of ultrashortpulse frequency doubling field in the near-field case is approximately Gaussian.In far-field case, the frequency doubling field assumes a distortion: the beamtakes an approximately Gaussian spot in the phase matching condition, while it makes a non-Gaussian peak and accompanying peaks aside in the phasemismatching condition. The spots change with the increase of phase mismatch.The above phenomenon can be explained by a simulating in theory. |
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