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