Coherent Backscattering from Multiple Scattering Systems - KOPS ...

3 Setups
Figure 3.2: Wide angle setup. 256 photodiodes arranged in a semicircle around the
sample capture the the backscattered radiation over a range of nearly 180 ◦ .
the sample around an axis parallel to the incoming laser beam, providing a sample average
which eliminates the speckle pattern.
3.2.1 Optical setup
The intensity of the coherent backscattering cone varies rapidly around the backscattering direction,
while the variations at larger scattering angles are comparatively slow. Accordingly,
the angular resolution of the setup has to be rather high in the center, while at larger angles
the diodes can be set further apart. To meet these requirements, for angles θ < 9.75 ◦ eight
photodiode arrays with 16 photodiodes each (S5668 from Hamamatsu) are used, yielding an
angular resolution of 0.15 ◦ . For angles θ > 9.75 ◦ , single photodiodes (S4011 from Hamamatsu)
provide angular resolutions of 0.7 ◦ for 9.75 ◦ < θ < 19.55 ◦ , ∼ 1 ◦ for 19.55 ◦ < θ < 60 ◦
and ∼ 3 ◦ for 60 ◦ < θ < 85 ◦ . Still, it is not possible ro resolve the very tip of the cone at θ ≈ 0,
as this is the position where the laser beam passes through the diode arc. For this task the
small angle setup (see sec. 3.3) has to be used.
The desired angular resolution also implies some prerequisites for the optical setup: The
laser source has to be imaged completely on a single photodiode, without any light of a
certain scattering angle missing its diode or even illuminating a neighboring one. On the
other hand, the incoming laser beam is supposed to have a considerable width at the sample
surface to avoid finite size effects [32]. Therefore the incoming laser beam is first widened
and parallelized by a telescope, and then focussed on the point between the photodiodes,
diverging again on its way to the sample. Light scattered with a certain scattering angle is
then focussed again when it reaches the photodiodes.
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