Spatial Characterization Of Two-Photon States - GAP-Optique

4. OAM transfer in noncollinear configurations
Laser and
spatial filter lenses
crystal
lenses
filter
collection
system
pinhole
x
idler
pinhole
signal
Figure 4.3: A spatially filtered cw laser is focused into a nonlinear crystal. A coincidence
circuit measures the correlations between the photon counts in a fixed position
for the idler photon with the counts of photons in two orthogonal directions in the
signal transverse plane. The values of the different experimental parameters are listed
in table 4.2.
the measured shape of the coincidence rate, although it does modify the single
counts spatial shape.
Table 4.2: The parameters of the experiment described in section 4.2.
Parameter Value
Crystal liio3
L 5 mm
ρ0
0◦ Laser cw diode
wp 32 − 500 µm
λp 405 nm
∆λp 0.6 µm
λ0 s
∆λ
810 nm
0 s
ϕs
10 nm
17.1◦ The left side of figure 4.4 shows the coincidence measurements in the x and
y directions for a focalized pump beam. As the pump beam waist is wp = 32
µm, the noncollinear length Lnc = 108.8 µm is much smaller than the crystal
length. According with reference [30], in this regime the ellipticity of the signal
photon should be appreciable.
By fitting the result to a Gaussian function, the waist in the x direction
is wx 960 µm, while in the y direction it is wy 150 µm. The waist in x
is about six times larger than the waist in y, and therefore the signal spatial
distribution is elliptical. As a reference, the figure shows the singles counts for
the signal coupler.
The right side of the figure shows the coincidence measurements for a larger
pump beam, wp 500 µm. In this case, the noncollinear length Lnc = 1.7 mm
is of the same order of magnitude as the crystal length. When fitting the
42
y