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Radial-Azimuthal Polarization
Converter
The
Radial Polarization converter
from
Arcopix (former polaroptic)
 is
a worldwide unique device that converts a
conventional linearly polarized beam into a beam that has a
CONTINUOUS
radial or azimuthal polarization distribution and
stable in time. As illustrated in the figure below the
orientation of the polarization vector vary spatially but locally the
polarization sate is considered as linear.
Thanks to special alignment of the liquid crystal molecules, the
polarization converter rotates locally the orientation of the linearly
polarized beam. Depending of the orientation of the device in respect to the
entrance polarization we may obtain either azimuthally or a radially
polarization distribution as described in the figure above.
The polarization rotator can be delivered with adequate polarizers and
housing (figure below). The specially designed housing permits to align our
polarization converter precisely with the rest of your optical system. Also
an additional Arcoptix electrical tunable phase
shifter can be inserted in this housing.
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Housing |
Product Overiew |
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DOWNLOAD
detailed description of the polarization converter |
For further information contact ARCoptix:
info@arcoptix.com
Radial polarization converter Specs
| wavelength range |
350-1700 nm |
| active area |
10 mm diameter |
| transmission |
better than 75% (in the VIS) |
| retarder material |
Nematic Liquid-Crystal |
| Substrates material |
Glass |
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Local extinction ratio
(input Intensity/ouput intensity) when placed between
crossed polarizers |
~100 @ 633nm |
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Output intensity
homogenity |
< 1/100 RMS
variation |
| temperature range |
15°-35° |
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Save operating limit |
500 W/cm2 CW
300 mJ/cm2 10 ns, visible
200 mJ/cm2 10 ns, 1064 nm |
| Total size of the housing |
6 cm x 4 cm x 1.5 cm |
Driver (optional)
The Polarization
converter can be driven with one or two standard labor function generators
but it can also be driven the the USB ARCoptix LC Driver.
The Arcoptix LC (Liquid Crystal) driver is a USB computer
controlled electrical power supply optimized for driving the polarization
converter. The variable phase retarder (phase step compensation) and the TN
cell (switch between azimuthal and radial polarization) inside the
polarization conveter can be driven with the two outputs of the LC Driver.
 The
LC driver has two independent outputs (Lemo connectors). They are controlled
via a simple windows compatible software. The output has a variable square
amplitude with polarity inversion and a frequency of 1.6 KHz. This
guarantees a homogenous variation of
the LC layer inside the cell.
An external trigger input can be provided on demand.
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DOWNLOAD
detailed description of the polarization converter |
Principle
The radial polarization converter is a
nematic liquid crystal cell composed of one uniform and one circularly
rubbed alignment layer. The local alignment of the LC in the
polarization converter is that of a twisted cell, with a twist angle
given by the local alignment layers These twist angles are always
smaller than pi/2. A thin disinclination line appears in the LC cell
(line in the figure below) but is unnoticeable for most types of
experiments. As shown in figure above, when linearly polarized light is
shining through a polarization converter and the polarization direction
is parallel or perpendicular to the uniform alignment layer, azimuthally
or radially polarized light emerges on the other side. So by simple
rotation of the entrance polarization the polaroptic polarization
converter can switch from radial to azimuthal polarisation distribution.
A more detailed description can be found in “Stalder et.
al., Optics
Letters, volume 21, page 1948, published in 1996”. |
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Front view LC molecule twist inside
the theta-cell |
Applications
Dougnut focal point
(or reduiced size focal spot)
For some applications such as confocal microscopy for example one is
interested to produce a doughnut shaped focal point at the front focal
plane of a high NA objective. Rigorous electromagnetic calculations
shows that doughnut shaped focal points can be obtained by focusing
beams having a radial polarization distribution. This may lead to
interesting applications in the field of fluorescence microscopy.
Polarization axis finder (PAF)
When a polarization converter is used in combination with a polarizer, a
device results that can be used as polarization axis finder (PAF).
Watching the PAF a dark segment appears when the entrance polarization
is linear. The orientation of the dark segment gives the direction of
the polarization.
Inspection of birefringent materials
When placing a brefrigent material between two PAFS (two polarizers with
two polarization converter), one can analyze the birefringent properties
of the sample in one glimpse (characteristic interference colors and
main axis). Neither the sample nor the polarizers have to be rotated.
Optical trapping
A Doghnut shaped focal point created by focusing a radial polarized beam
may increase the traping force. Also it may enable trapping particles
with lower refractive index than its surrounding fluid.
Laser cutting
The polarization direction of a laser beam when cutting materials is an
important parameter. The cutting speed using p-polarized light is more
then twice as fastcompared to using s-polarized light.Most cutting
machines are therefore releasing circular polarized light which results
in an average cutting speed and in cutting direction independnce.
Radially polarized light may eventually increase cutting speed compared
to circular polarized light...In principle the polarization converter
can withstand high intensities (500W/cm^2).
Inspection of the polarization of the sky
The blue sky light due to scattering of sun light in theatmosphere is
partially polarized and therfore be visualized with a PAF. Combined with
a compass a sun dial could be built which indicates the local time.
References
We have alread sold our radial polarization converter all over the
world. Some groups have already obtained interesting results, here are
some references of articles where the LC radial polarization converter
has been used:
1) M. Stalder and M. Schadt, "Linearly polarized light with axial
symmetry generated by liquid-crystal polarization
converters," Opt. Lett. 21, p. 1948- (1996)
2) S.F Periera and al. “Frequency spectra and waveguiding of a family of
daisy modes in vertical-cavity
surface-emitting lasers”, opt. comm. 179, p.485, (2000).
3) E. Descrovi and al., “Collection of transverse and longitudinal
fields by means of apertureless nanoprobes with
different metalic coating characteristics”, appl. phys. lett. 85 (22),
p. 5340, (2004).
Articles may be obtained upon request.
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