Download (4Mb) - Etheses - Saurashtra University

Chapter I Brief Introduction...
the crystal. These methods are called temperature tuning method and quasi –
phase matching. In temperature tuning, the pump (laser) frequency
polarization is selected orthogonal to the signal and idler frequency
polarization. The birefringence in some crystals, for example, Lithium niobate
is highly temperature dependent. The crystal is controlled at a certain
temperature to achieve phase matching conditions. In quasi – phase matching
the frequencies involved are not constantly locked in phase with each other,
instead the crystal axis is altered at a regular interval. Hence, these crystals
are called periodically poled. This results in the polarization response of the
crystal to be shifted back in phase with the pump beam by reversing the
Nonlinear susceptibility. This allows net positive energy flow from pump into
the signal idler frequencies. In this case, the crystal itself provides the
additional wave vector K = 2/ to satisfy the phase matching condition. The
quasi – phase matching can be expanded to chipped pulse as it is done in a
dazzler.
Group Velocity Mismatching
For NLO processes of ultrafast lasers such as Ti:Sapphire and Dye
lasers with femto second (fs) pulse width, the main limitation to conversion
efficiency is group velocity mismatching(GVM). The GVM is caused by group
velocity dispersion of NLO crystal.
Damage Threshold
Damage threshold is a property of the crystal to withstand the laser
radiation without damage. The damage threshold is a function of the
wavelength of fundamental and harmonic radiation, pulse duration, beam
16