Coherent Backscattering from Multiple Scattering Systems - KOPS ...

3 Setups
3.1 Laser System
The key piece of the light scattering laboratory is the picosecond pulse laser system (fig. 3.1)
which serves as light source for all light scattering setups.
The system consists of a Rhodamin6G dye laser (699 from Coherent) which is pumped by the
514.5 nm-line of an Ar + -laser (Innova400 from Coherent) [49], a laser type which operates in
continuous wave (cw) mode. The lasing medium in the dye laser is a jet of Rhodamin6G, so
that the wavelength can be tuned roughly between 570 nm and 620 nm.
To be able to alternatively operate the system in pulsed mode, the Ar + -laser was modified
with a mode locker (from APE), which provides 100 ps-pulses at a repetition rate of 76 MHz.
The cavity lengths of pump and dye laser are matched to synchronize and overlap the pulses
coming in from the pump laser with those already circulating in the cavity of the dye laser. The
inversion in the dye jet is completely broken down by the front flank of the circulating pulse,
so that the pulse is shorted to approximately 12-15 ps. After about 30 cycles in the cavity of
the dye laser the now significantly amplified pulse can be released by a cavity dumper (from
APE).
3.2 Wide Angle Setup
The setup used for the study of the wide-ranged angular distribution of the backscattered
light consists of 256 photosensitive diodes attached to a semicircular arc with a diameter of
1.2 m (fig. 3.2). In its center the sample is located, facing the incoming cw laser beam which is
focussed through a tiny hole between the topmost diodes [22, 23, 47]. A simple motor rotates
Figure 3.1: Laser system. The Rhodamin6G dye laser and the Ar + -pumplaser were
modified to run also in pulsed mode. The acousto-optical modulator (AOM) in the
Ar + -laser serves as mode locker, the one in the dye laser as cavity dumper.