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              首頁 > 光學元件 > 偏振光學 > 徑向偏振轉換器
              Radial-Azimuthal Polarization Converter
              PDF 文檔: Download徑向偏振轉換器details Document

              Arcopix 偏振轉換器能將一束具有線性極化的光轉換成具有放射性極化或圓環狀極化的光束。

              Radial-Azimuthal Polarization Converter
              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 varies 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.


              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
              Local extinction ratio (input Intensity/ouput intensity)   when placed between crossed polarizers  ~100 @ 633nm
              Output intensity homogenity  < 1/100 RMS variation
              temperature range  15°-35°

              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


              PrincipleThe 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”.
                Front view LC molecule twist inside the theta-cell

              ApplicationsDougnut focal point (or reduced 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 Doughnut 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 independence. 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 the
              atmosphere is partially polarized and therefore be visualized with a PAF. Combined with a compass a sun dial could be built which indicates the local time.



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