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