Diploma thesis

y theory. We detected a very strong type-I process (EzEz → Ez), a very weak
type-I process (EzEz → Ey), and one type-II process (EyEz → Ey). The process
EyEz → Ez is not a type-II downconversion, but the remainant of the strong type-I
PDC. Our future PDC setup producing separable two-photon states will employ the
observed type-II process. Surprisingly at all applied pump wavelengths the type-
II process is always much stronger than the weak type-I process in spite of their
comparable nonlinearities. This may be concering the change in polarisation in the
case of this specific type-I SHG, or may be traced back to the different phasematching
contours.
Figure 4.13: Different measured SHG processes in KTP
The accumulated data stresses the importance of precise control over the pump
polarization in future PDC-experiments. In type-II PDC, any misalignment in the
pump polarisation will introduce two type-I PDC processes that may be mistaken
for type-II downconversion and thus introduce noise.
Phasematching considerations
After identifying the different SHG processes, we restricted ourselves to the type-II
conversion process. To gain insight into its phasematching properties, we investigated
the mapping between the pump spectrum and the SHG spectrum produced
inside the crystal. The spectrum of a SHG pulse generated from a pulsed laser
source is given in Formula 3.7:
� ∞
I3(ω3) =
0
dω1K ′ I 2 pe − (ω 1 −ωp)2
2σ 2 p e − (ω 1 −ω 3 −ωp)2
2σ 2 p sinc 2
� �
∆kL
2
(4.7)
At a SHG far away of the momentum conservation condition, we can treat the
sinc factor as constant. Under these circumstances, we obtain a convolution of two
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