Efficient generation of visible femtosecond pulses by frequency doubling.

摘要

In principle, second harmonic generation (SHG) can convert near-infrared femtosecond mode-locked pulses into visible and UV regions. However, the finite phase matching bandwidth makes it difficult to simultaneously phase match the entire pulse spectrum. Effects such as peak power saturation and pulse broadening arise as the length of a frequency-doubling crystal increases. To avoid these problems, very thin crystals have to be used, which results in low conversion efficiency unless the laser intensity is very high.;Two new approaches based on quasi-phase match and traditional Cerenkov SHG are proposed in this thesis. In the former scheme, proper design provides the requisite delay of the fundamental pulses, resulting in simultaneous phase and group velocity matches. The latter scheme makes use of the auto-phase matching property of Cerenkov SHG to phase match the whole pulse spectrum. The stretched output pulses are compressed by a dispersion element, such as a prism or a grating. Pulse width preservation and high efficiency are thus expected from low pump power.;Experiments to conform the latter proposal were conducted. Proton-exchanged MgO doped LiNbO;The device is easy to make and to use. The concept presented can be extended to femtosecond pulses in any three-photon process in any waveguide material and any modelocked source.