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

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4.2 Four-pulse-laser System
path behind the first dielectric polariser (8a,b), but need different Pockels cell voltage for exit,
the cavities of each laser pair cannot be fired simultaneously. This is usually not a restriction,
as only orthogonally polarised light needs to be delivered simultaneously for multiple plane
stereo recording.
4
1
2 5 6 3
10 11
7
8a
9
12
12
4
8c
8b
7b 12
FIGURE 4.3: Four-pulse laser system. 1 Pump cavity, 2 Full reflective mirror, 3 Partially transmitting
mirror, 4 Pockels cell, 5 "#%$ retardation plate, 6 Glan-Laser polariser, 7 Mirror, 8 Dielectric polariser,
9 Dichroic mirror, 10 Frequency doubler crystal with phase angle adjustment, 11 "#& retardation plate,
12 Beam dump.
4.2.1 Performance of spatial light-sheet separation
The appropriate method of adjusting the displacement between the orthogonally polarised
light-sheets depends on the desired distance. Small separations between the orthogonally
polarised light-sheet pairs (up to a few millimetres) can be generated by a simple rotation of
mirror 8c in the re-combination optics around the axis perpendicular to the laser-beam plane.
This is possible as the divergence between the s- and p-polarised beams is negligible when
the separation is small with respect to the distance from the measurement position to mirror
8c. A translation of mirror 8c on the other hand cannot be applied in order to separate the
orthogonal polarised beams as the focal lens in the light-sheet optics will superimpose parallel
rays at the focal point. The relative separation between the beams can be easily controlled by
means of a target located at the measurement position which is slightly tilted with respect to
the light-sheets in order to increase the resolution.
For a wider range of light-sheet spacings (up to a few cm) and independent positioning of
both beam-pairs, it is useful to remove mirror 7b along with the beam dump (12) such that
two spatially separated laser beams with orthogonally polarised radiation emerge. Using two
separate light-sheet-optics (one for each polarisation) each with a ü mirror behind the last
ø
lens, all positions are possible by moving the mirrors. Once calibrated, the actual position of
each pair of light-sheets is given by a micrometer scale. For the conservation of the polarisation
the mirror needs a dielectric broad-band coating optimised for 532 nm and ü angle ø
of incidence to provide a 99 % reflectance in both s- and p-planes also when the angle is not
exactly ó J/cmî for 15 ns pulse duration) is
ø
53
ü . In addition, a high damage threshold ('