Spatial Characterization Of Two-Photon States - GAP-Optique

4. OAM transfer in noncollinear configurations
Laser and
spatial filter HWP lenses
crystal
lenses
filter
collection
system
camera
pinhole
Figure 4.10: By rotating the crystal and the pump beam polarization, it was possible
to measure the coincidences between pairs of photons at different positions over
the emission cone. The photons were collected using multimode fibers and 300 µm
pinholes to increase the resolution. The values of the experimental parameters are
listed in table 4.3.
given by the angle ρ0 = 4.9 ◦ , while the generated photons did not exhibit
spatial walk-off.
Table 4.3: The parameters of the experiment described in section 4.3.
Parameter Value
Crystal liio3
L 5 mm
ρ0 4.9◦ Laser cw diode
wp 136 µm
λp 405 nm
∆λp 0.4 nm
λ0 s
ϕs
810 nm
4◦ We measured the relative position of the pump beam in the xy plane at
the input and output faces of the nonlinear crystal using a ccd camera. At
the output of the crystal, the beam displacement due to the Poynting vector
walk-off allowed us to determine the direction of the crystal’s optical axis, and
to fix its direction parallel to the pump polarization, as shown in figure 4.11.
Right after the crystal, each of the generated photons passed through a
2 − f system with focal length f=50 cm. Cut-off spectral filters removed the
remaining pump beam radiation. After the filters, the photons were coupled
into multimode fibers. In order to increase the spatial resolution, we used small
pinholes with a diameter of 300 µm.
We kept the idler pinhole fixed and measured the coincidence rate while
mapping the signal photon’s transversal shape by scanning the detector with
a motorized xy translation stage, as in the right part of figure 4.3. Finally,
we rotated the crystal and the pump beam polarization to measure the spatial
correlations at different azimuthal positions on the down-conversion cone. After
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