Abstract
Although new optical materials continue to open up access to more and more wavelength bands where femtosecond laser pulses can be generated, light frequency conversion techniques are still indispensable in filling the gaps on the ultrafast spectral scale. With high repetition rate, low pulse energy laser sources (oscillators) tight focusing is necessary for a robust wave mixing and the efficiency of broadband nonlinear conversion is limited by diffraction as well as spatial and temporal walk-off. Here we demonstrate a miniature third harmonic generator (tripler) with conversion efficiency exceeding 30%, producing 246 fs UV pulses via cascaded second order processes within a single laser beam focus. Designing this highly efficient and ultra compact frequency converter was made possible by full 3-dimentional modelling of propagation of tightly focused, broadband light fields in nonlinear and birefringent media.
Highlights
Polarization rotation, the two pulses are overlapped in the sum frequency generation (SFG) crystal
As the highest conversion efficiencies in the nonlinear crystals of choice (BBO and LBO) are possible with type I phase matching for second harmonic generation (SHG) and SFG processes, the polarization of the fundamental pulse is rotated by 90° with the quartz half-wave plate
In this configuration the angle tuning of the phase matching for SHG and SFG is performed with two perpendicular axes of rotation
Summary
With this spectral method the fundamental, SH and TH pulse propagation in a stack of crystals is accurately simulated. The third harmonic pulse energy is maximized using a simplex method[21] by varying the beam width and position of the focus with respect to the crystal stack. This procedure is iterated for different thicknesses of SHG and SFG crystals. M2 measurements were based on recording beam profiles in several planes with Basler 102f camera installed on the Aerotech PRO165LM translation stage
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