The significance of coherent flow structures for the turbulent mixing ...

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4.2 Four-pulse-laser System
focused particles everywhere within the image plane. Therefore, each camera lens must be
connected to a specially designed one-axis tilt-adapter (Scheimpflug-adapter). The axis of rotation
should coincide with the centreline of the CCD sensor to ensure that all particle images
along this axis remain in focus under rotation. This simplifies the installation of the system and
the focusing process since the image location of the centreline in object space and the opening
angle between corresponding camera pairs remain constant under Scheimpflug adjustment.
For magnification and field of view adjustments (necessary for maximising the amount of
stereo information) all Scheimpflug-adapters should be mounted on a two-axis linear translation
stage which allows high precision translations by thumb screws. In order to simplify
the adjustment procedure without restricting the flexibility of the system the left and right
recording systems can be connected to different base-plates with individual rotation stage.
4.2 Four-pulse-laser System
For the illumination of the tracer particles the beams of four independent laser-oscillators
need to be combined in such a way that the linearly polarised light-sheets can be positioned
independently with respect to each other. This can be easily and precisely done by the fourpulse
system shown in figure 4.2. The lasers consist of a Neodymium-Yttrium-Aluminium-
Granat rod embedded in an unstable resonator with a variable reflectivity output mirror (the
transmission decreases from the centre to the edge in order to eliminate the intensity maxima
of higher orders introduced by the unstable resonator) and a white light flash-lamp for the
excitation of the crystal-atoms [60]. The monochromatic unpolarised radiation spontaneously
emitted by the laser material (1) immediately after the excitation of the atoms will be linearly
polarised when it emerges from the dielectric Glan-Laser polariser (6) and circularly polarised
behind the retardation plate (5) which consists of a plane-parallel slice cut out of a uniaxial
crystal such that the cut-plane is parallel to the crystalline optic axis. The thickness of the
plate is such that the phase difference (retardation of the slow ray in comparison to the fast
nm).
ø
ray at emergence) is åŽø
for normal incident and a given wavelength (here
The direction of propagation of the circularly polarised beam is reversed by reflection in a
mirror (2) and thus the sense of the circular polarisation as well (due to the ¤ ÷aóaü
phase shift
induced by the mirror) before the second transformation into linearly polarised light. Thus, the
linearly polarised wave of the reflected beam, as it finally emerges from its second path through
the retarder (5), is orthogonal to the incident linearly polarised wave and can be rejected by
means of the dielectric polariser (6) constructed from an air-spaced right-angle prism pair
with parallel optical axis. In this configuration the polariser quarter-wave plate combination
acts as an isolator (or closed switch) when the angle between the plane of linear polarisation
and the crystalline optical axis equals exactly ¢ ø ü either (for other angles the transformation
is from linear to elliptical polarisation due to the different amplitudes of the ordinary and
extraordinary beam behind the retardation plate). For the stimulated emission of radiation the
electro-optical block (4) can be made equivalent to a quarter-wave plate by appropriate choice
of block length, bias voltage and suitable orientation with respect to the quarter-wave plate
such that the beam can pass the Glan-Laser polariser when the population inversion reaches
its maximum. Using this optical arrangement the laser power can be increased by reducing
the pulse duration and the output energy of the laser is adjustable by changing the time delay
between the flash-lamp and the Pockels cell bias voltage.
In figure 4.2 the linearly orthogonally polarised infra-red beams released by the resonator
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