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In order to obtain high conversion efficiency, the
phase vectors of input beams and generated beams have to be matched:
Δk = k3 - k2 - k1 = 2πn3/λ3 -
2πn2/λ2 - 2πn1/λ1 = 0 (for sum frequency generation)
Where:Δk is phase mismatching, ki is
phase vector at λi and ni is refractive index at
λi. In low power case, the relationship between conversion
efficiency and phase mismatching is:
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Where:Δk is phase mismatching,
ki is phase vector at λi and ni is
refractive index at λi. In low power case, the relationship
between conversion efficiency and phase mismatching is:
η∝[(sinΔkL)/ΔkL]2
It is clear that the conversion efficiency will
drop dramatically if Δk increases. |
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Critical Phase Matching |
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 (θ) isn't equal to 90o or
0o, we call it critical phase-matching (CPM). Otherwise,
90o non-critical phase-matching (NCPM) is for
θ=90o and 0o NCPM is for
θ=0o. |
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
type I phase-matching |
type II phase-matching |
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