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Chapter I Brief Introduction...
defines the acceptance angle (Δθ) as full angle at half maximum (FAHM),
where θ is 0 0 , is phase-matching direction.
Figure: 1.13 Plot of conversion efficiency Vs acceptance angle.
Moreover, there are certain other parameters to be considered along with the
crystal acceptance, which are the spectral acceptance (Δλ) of crystal and the
spectral bandwidth of the laser as well as crystal temperature acceptance
(ΔT) and the temperature change of environment.
Beam Walk-Off
Due to the birefringence in NLO crystals, the extraordinary wave (ne)
will experience Poynting vector walk-off as shown in figure (1.14), which is the
angular deviation between ordinary ray and extra ordinary ray as they
propagate through the crystal. If the beam size of input beam is separated at
walk-off angle (ρ) in the crystal, it will cause low conversion efficiency.
Therefore, for a focused beam or intra-cavity SHG experiment the walk-off is
a main limitation to high conversion efficiency.
Figure: 1.14 The angular deviation between ordinary ray and extra ordinary ray.
Two other methods of phase matching avoids beam walk off by forcing
all frequencies to propagate at a 90 0 angle with respect to the optical axis of
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