Coherent Backscattering from Multiple Scattering Systems - KOPS ...

3.4 Time Of Flight Setup
Figure 3.8: Time of flight setup. The setup measures a histogram of time delays between
photons arriving at detector 1 (photomultiplier) behind the sample and the signal
of the reference detector 2 (photodiode).
3.4 Time Of Flight Setup
Apart from investigations of the path length distribution in transmission and the search for
Anderson localization [48], the time of flight setup is used to determine diffusion coefficients
and absorption times of the samples. These quantities are obtained by fitting the measured
distribution of the times photons need to travel through the sample with the theoretical prediction
of eqn. 2.12.
To obtain a distribution of photon flight times, the time of flight setup measures a histogram
of time delays between photons arriving at a detector behind the sample and the signal of a
reference detector, as shown schematically in fig. 3.8. The signal from a photon arriving at the
photomultiplier (H5784 from Hamamatsu) starts a time measurement in the single photon
detection card (SPC-140 from Becker & Hickl). The stop signal is provided by a photodiode
which receives a small part of the unscattered pulse and which is followed by a cable delay to
have the signal come in after the signal from the photomultiplier.
It is important to have the samples turbid and thick enough so that at most a single photon
of each pulse actually reaches the detector. This ensures that the probability to trigger a time
measurement is not biased towards photons on short paths.
The measured time delays are sorted into a histogram with 1024 time slots. The width of
this histogram has to be matched to the desired time range, which is of the order of 20-40 ns.
As with the resulting time resolution the shape of the unscattered laser pulse is not a delta
function, the data of a broadened pulse have to be deconvoluted with the systemic answer of
the unscattered pulse.
[a] Deconvoluting the data with the reflection profile would be more straightforward, but is not possible due to
mathematical difficulties.
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