Vortex retarders are used generate laser beam with certain polarization features. These include radial and azimuthal laser beam resulting in a vortex laser beam, i.e. a dark region in the center of the laser beam as seen on the right side of these photos. The number m=1 and m=2 are the socalled topological number.
One can imagine these topological numbers with phase shifts for a particular wavelength of light entering the vortex beam retarder.
That is the reason why each vortex retarder is designed for a particular wavelength.
The photos on the left and right show the vortex laser beam with a homogeneous donut shaped laser beam profile on the left side of these photos. The left side of these fotos is a result of a polarization filter placed after the vortex beam. How these patterns forme is illustrated below.
Experimental setup for the polarization analysis of the vortex beams
Radial laser beam from vortex retarder m = 1
If the retardation is 2π per wavelenght of the incident light (m=1), either azimuthal or radial light is created depending on how the linear polarized light falls onto the vortex retardar. This is visualised in the photos below. In this case, the polarisation field of the incident light is perpendicular to he floor.
Photos: Silvia Müllner; Illustration: Torlabs (in section Comparison)
Azimuthal laser beam from vortex retarder m = 1
Photos: Silvia Müllner; Illustration: Torlabs (in section Comparison)
Vortex retarder with m = 2
If the retardation is two times 2π per wavelenght of the incident light (m=2), a mixture of radial and azimuthal light is generated as seen in the photos below
Photos: Silvia Müllner; Illustration: Torlabs (in section Comparison)
λ/4 plate
Home
The λ/4 plate is used to create eliptical or circular polarized light from linear polarized light.
Circular polarized light is generated when the direction of polarization field of the incoming light is rotated
by 45° from the fast axis. If the polarization field of the incident light is tilted clockwise from the fast axis, right circular
polarisation is achived and left circular polarisation if this tilt is counterclockwise as seen in the picture right below.
The slow axis means that the light in this direction passes through the λ/4 plate a quater wavelength slower as the one in the fast axis. When the polarisation field of the incident light is aligned with the fast or slow axis the outcome is also linear polarized light. Linear polarised light entering anywhere in between the fast and slow axis (with exception of the 45°) results in elliptical porarisation.
Source of upper two illustrations: Newport; lower four illustrations: Wikipedia
Home
Twisted light can be created by various techniques. One that we use is the vortex retarder in combination with circularly polarized light. With linear polarized light either a radial or azimuthal laser profile is created. Circular polarized light transforms into twisted light after exiting the vortex beam retarder. Twisted light is similar to circular polarized light with the differenct that the polarization field at a particular point of space changes with time and it carries an orbital angular momentum.
The interference pattern of linear polarized light and twisted light with the topological number m=1 and m=2 right. The pattern with one emerging line is from the vortex retarder with m=1 and with two emerging lines from the m=2 vortex retarder
The less overlay between the interfering laser light patters, the narrower the lines, hence more visible lines within the projected laser profile.
In the case where the overlay of the interferening laser paths is identical a swirl-like pattern is created as seen right the emerging line photos. One swirl going towards the center means m=1 and with two squirls, m=2, as seen in the photos on the right. If the handedness of the incident circular polarized light is changed, the swirl-direction will also change direction, so we either have a clockwise or anticlockwise pattern.
Last update: 4th of February, 2021
The slow axis means that the light in this direction passes through the λ/4 plate a quater wavelength slower as the one in the fast axis. When the polarisation field of the incident light is aligned with the fast or slow axis the outcome is also linear polarized light. Linear polarised light entering anywhere in between the fast and slow axis (with exception of the 45°) results in elliptical porarisation.
Source of upper two illustrations: Newport; lower four illustrations: Wikipedia
Twisted light
Home
Twisted light can be created by various techniques. One that we use is the vortex retarder in combination with circularly polarized light. With linear polarized light either a radial or azimuthal laser profile is created. Circular polarized light transforms into twisted light after exiting the vortex beam retarder. Twisted light is similar to circular polarized light with the differenct that the polarization field at a particular point of space changes with time and it carries an orbital angular momentum.
Experimental setup for the polarization analysis of the vortex beams
Interference pattern of Twisted light in Mach-Zehnder interferrometer
The interference pattern of linear polarized light and twisted light with the topological number m=1 and m=2 right. The pattern with one emerging line is from the vortex retarder with m=1 and with two emerging lines from the m=2 vortex retarder
The less overlay between the interfering laser light patters, the narrower the lines, hence more visible lines within the projected laser profile.
In the case where the overlay of the interferening laser paths is identical a swirl-like pattern is created as seen right the emerging line photos. One swirl going towards the center means m=1 and with two squirls, m=2, as seen in the photos on the right. If the handedness of the incident circular polarized light is changed, the swirl-direction will also change direction, so we either have a clockwise or anticlockwise pattern.
Theory of interference pattern from Twisted light in an Mach-Zehnder interferrometer
Last update: 4th of February, 2021