Diploma thesis

Material: PPLN
Process : z → z + z
775 nm → 1550 nm + 1550 nm
Λ: 0.35 µm
m: 1
Pump FWHM: 0.21 nm
Dimensions: 4 µm x 4 µm x 5 mm
Table 3.5: Crystal parameters for a counterpropagating setup
Figure 3.44: Pump envelope, phasematching function and joint spectral
amplitude of the counterpropagating process
This is because the phasematching contours are orientated along the signal axis.
Additionally, most of these higher order spatial modes in the suggested waveguide
are unlikely to propagate, and therefore may not be observed at all.
In copropagating PDC the spectral distributions of signal and idler photons are
comparable, as opposed to counterpropagating-PDC where the spectral distribution
of the idler photon is more than one magnitude narrower than that of the signal
photon (see Figures 3.51 and 3.52). Again this is a result of the narrow phasematching
function orientated along the ωs-axis. This narrow spectrum may prove useful.
On one hand, the detector of the heralding photons may be optimized to match the
exact wavelength and hence operate on a high performance level. On the other hand,
this very narrow spectrum is optimal for the propagation through optical fibers, and
may be used to distribute quantum states over large distances.
The phasematching contour along the signal axis may also be used to create
a tuneable pure heralded single photon source. The heralding detector can stay
unchanged while the signal spectrum is changed to an arbitrary wavelength by a
simple tuning of the pump laser central frequency (Figure 3.53).
The feasibility of this photon source depends on the grating periods that can be
manufactured. To date the lowest grating period produced is Λ = 0.8µm in KTP
[25]. Hence, further technology progress is required to produce the needed Λ =
0.35µm. But owing to the importance of nonlinear materials substantial progress
during the next years is expected. One major drawback of counterpropagating PDC
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