In order to obtain high conversion efficiency, the phase vectors of input beams and generated beams have to be matched: Dk = k3 - k2 - k1 = 2pn3/l3 - 2pn2/l2 - 2pn1/l1 = 0 (for sum frequency generation)
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Where: Dk is phase mismatching, ki is phase vector at li and ni is refractive index at li.
FIn low power case, the relationship between conversion efficiency and phase mismatching is:h¡Ø(sin(DkL)/DkL)2
It is clear that the conversion efficiency will drop dramatically if Dk increases.
Conversion Efficiency Vs Dk
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The phase-matching can be obtained by angle tilting, temperature tuning or other methods. The angle tilting is mostly used to obtain phase-matching as shown in the left figure. If the angle between optical axis and beam propagation (q) isn't equal to 90o or 0o, we call it critical phase-matching (CPM). Otherwise, 90o non-critical phase-matching (NCPM) is for q = 90o and 0o NCPM is for q = 0o.
Critical Phase Matching
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Two types of phase-matching are classified in consideration of polarization of lasers. If the polarizations of two input beams (for sum frequency) are parallel to each other, it is called type I phase-matching. If the polarizations are perpendicular to each other, it is called type II phase-matching
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Type I Phase Matching |
Type II Phase Matching |
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