Spatial Characterization Of Two-Photon States - GAP-Optique

Coincidences
800
0
-4
4.2. Effect of the pump beam waist on the OAM transfer
x
y
4mm
Singles
6x10 5
~
-1
y x
Singles
5x10 4
~
1mm
Figure 4.4: The ellipticity decreases as the pump beam waist increases from wp = 30
µm in the left to 500 µm in the right image. As the pump beam waist affects
the efficiency of the process, the coincidences on the left were collected over 600
seconds and the ones in the right over 20 seconds. Solid lines are the best fit to the
experimental data shown as points. The values of the experimental parameters are
listed in table 4.2.
Coincidences
region width
2.5m
0.5
0
0
x dimension
y dimension
L nc=1.1mm
Pump width 330 500m Figure 4.5: The pump beam waist controls the width of the profile in the x direction,
while it does not affect the width in the y direction. The width in both directions
becomes similar when the noncollinear length is of the same order of magnitude as
the crystal length. The vertical line shows the point where, according to the fits, both
dimensions are equal. In this point the noncollinear length is 1.1 mm. The values of
the experimental parameters are listed in table 4.2.
transverse cuts to Gaussian functions, the waist in the x direction is wx 120
µm and the waist in the y direction is wy 180 µm. The waist in the x
direction is much smaller than before and of the same order as the waist in y
which implies a decrease of the ellipticity.
Figure 4.4 illustrates the changes in the width in the x and y directions as a
function of pump beam waist. In the range where the waist varies from 32−500
µm, the width in y remains almost constant while the width in x decreases by
80%. After a certain threshold the width in x becomes stable at a value close
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