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

4 Multiplane Stereo Particle Image Velocimetry
pair are combined using a dielectric polariser at the Brewster angle (8a) which reflects the
p-polarised light partially at every dielectric interface within the multi-layer coating while
transmitting the s-polarised light with almost no reflection. A retardation plate behind the
Brewster window (5) transforms both linearly polarised beams into circularly polarised light
before they enter a properly cut and temperature stabilised highly doped KD*P (Kaliumdihydrogenphosphat)
crystal (10) for polarisation selection and generation of the second harmonic
(532 nm) from the fundamental wavelength (1064 nm), see [70] for details. To separate the
two wavelengths, a high energy harmonic separator (9) is used consisting of a specially coated
substrate which reflects the harmonic max at 532 nm) and transmits the fundamental (! max
ì
at 1064 nm) wave. As the linearly polarised light emerging from the frequency doublers possesses
the same state of polarisation, a åaú
retardation plate (11) has to be inserted before the
superposition of the four beams by means of another dielectric polariser (8c) can take place.
As the orientation of the frequency doubler (10) affects the efficiency of the second harmonic
generation and may change the angle between the incoming and outgoing beam, problems
such as different output energy may occur when using a system where two beams pass the
same frequency doubler crystal.
4
1
2 5 6 3
11
7
8a
12
5 10
12
7b
12
9
8b
11
8c
FIGURE 4.2: Four-pulse four frequency doppler 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.
An alternative system which has been tested as well is shown in figure 4.3. The recombination
takes place behind the frequency doubler in the visible wavelength range so that each
beam energy can be optimised independently by appropriate orientation of the frequency doubler
crystal. The superposition can be adjusted again using the dielectric polariser 8c. As
the light from each laser pair is linearly orthogonally polarised the Q-switches of the outer
oscillator pair in figure 4.3 are connected with a Pockels cell (4) in the four beam combination
optics. When these oscillators deliver the light, the corresponding Pockels cell is switched
simultaneously and turns the state of polarisation of the incident beam by an angle of û óü . The
advantage of the laser system shown in figure 4.3 lies in its ability to maximise the output energy
for each laser independently. The price is two extra frequency doublers and Pockels cells
and slightly more complicated combination optics. In addition as the beams take the same
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