Linear entrance can be transformed in all
kinds of Polarization distributions
Planar wave with circular Polarization
is transformed in a helical wave
Alignment of the liquid crystal within a Spiral-plate with topological charge of 2
The variable spiral plate (VSP) is a passive liquid crystal optical element that is capable to modify a simple laser beam into a vortex beam, radial polarized beam having orital momentum (L=1 or L=2). The VSP is a perfectly transparent optical element without scattering or diffracted ligh losses (apart from the absorption losses of the material). The conversion from homogenous linear polarized beam to, for example, radial, azimuthal (or also lemon, spiral or star distribution) can simply be obtained by placing the VSP in the optical path of your laser beam. The different output Polarization patterns can be obtained by simply changing the bias applied on the Q-plate the VSP.
Orbital momentum or helical wavefront generation-spiral phase plate (SPP)
One interesting application of the VSP is the transformation of a planar wavefront with circular (left or right) Polarization into a beam with an optical vortex (with an undefined phase in the center of the beam). Such a beam carries an orbital momentum (OAM) and has a helical wavefront as shown in the picture below. The retardation of the Q-plate is controlled by an AC bias and can be adjusted to any wanted value between 50-1500nm. As an additional feature, the orbital momentum can switched on and off (within 100ms) simply by changing the bias on the q plate.
Radial and azimuthal Polarization
As the ARCoptix radial Polarization converter product (link), the VSP (with topology q = 0.5) is capable to transform a linear input Polarization into a radial or an azimuthal Polarization (depending of the input Polarization). The same Q-plate can be adapted to any wavelength by simply adjusting an AC voltage (0-5V) that is applied on the VSP.
The variable spiral plate can produce from a simple gaussian beam with a spiral phase. To proove this, we create a well-known Mac-Zehnder interferometer setup coupled to a CDD camera to record the interference pattern at the output of the interferometer. By tilting the two beams of the interferometer, we obtain regular interference fringes (as shown in picture) and by introducing a spiral plate (with half wave retardation), we obtain a phase dislocation (pitchfork hologram). Again, if the VSP retardation is tuned (via an electric bias) to full wave retardation, the dislocation disappear as in pictures below.
VSP switched off-
VSP switched on-
disclocation shows spiral phase
Q=0,5 (OM=1) between crossed polarizers
Q=1 (OM=2) between crossed polarizers
Notice that the spiral plate is similar to our radial Polarization converter product. The principal difference between this product is that with the spiral plate, one can obtain either a spiral or a radial Polarization and with the Polarization converter one obtains always the radial Polarization and the spiral phase at the same time. The differences between the spiral plate and the ARCoptix Polarization converter are summarised in the table below.
By looking at the comparaison in the table, it seems clear that the VSP has many advantages compared to the Polarization converter. Also, the VSP does not have the PI phase step in the middle of the aperture, which makes the device simpler to adjust and beam quality will be better. It is also important to note that Polarization is a proven device that has already been used by many scientists with full satisfaction. The VSP is a new product that does not have reference for the moment. Notice that for optimal quality it is recommended to use a beam size of at least 5mm in diameter. Results obtained with a beam size below 2-3 mm may suffer from inperfection dues fundamental LC manufactring limitatios in the center of the cell.
|Features||Polarization Converter||Variable spiral plate|
|Technology||Alignment of LC nematic with rubbing||Alignment LC nematic with polymers aligned with pol. UV light|
|Topological charge||not relevant||
Q=+/- 0.5 standard
|Minimal Beam Size||2 mm||5 mm|
|Orbital Momentum||OM= +/- 1||OM= +/- 1 on demand +/- 2|
|Broadband wavelength illumination||Yes possible||Max wavelength width 100nm|
|Generation of various singularities||Fix singularity||Singularity can varied with input Polarization and phase retardance|
|Radial or azimuthal Polarization||Yes||Yes|
|Spiral phase||No||Yes (with circular pol.)|
|Pi phase step||Yes need to be compensate with phase compensator||No phase step|
|Electrical driving||Yes USB LC driver recommended||Yes USB LC driver recommended|