Visible polarization analyser

Abstract

The invention relates to a polarization analyser allowing direct observation and measurement of rotation of polarization, especially optical Faraday rotation. With this invention, a polarization analyser producing a resultant image having a pattern directly representing a rotation of polarization due to a sample is provided. This pattern may even be used for the determination of rotation angle of polarization by visual inspection only. An angular sweep of the beam is not needed. A method for producing a pattern directly representing a rotation of polarization and employing said pattern for the measurement of rotation of polarization is also provided.

Description

[0001] VISIBLE POLARIZATION ANALYSER

[0002] Technical Field

[0003] The invention relates to a polarization analyser allowing direct observation and measurement of rotation of polarization, especially optical Faraday rotation.

[0004] Prior Art

[0005] A conventional polarization analyser for the observation of Faraday rotation essentially comprises, in order along a predetermined direction, a laser source, a first linear polarizer, a magnet in relation to which a magneto-optic sample medium such as a magneto-optic crystal rod is to be positioned under the influence of the magnetic field and aligned such that its axis lies along the magnetic field, and a second linear polarizer for the detection of the rotation angle of the polarization after passing through the sample, said rotation angle being related to the sample’s material properties. The resultant image obtained from the second linear polarizer however is not directly representative of said rotation angle and the rotation angle must be determined by aligning the second linear polarizer to find the extremum of the measured intensity of light, further requiring the incorporation of a detector.

[0006] The document numbered WO2022119660A1 discloses a polarimeter employing a beam reflected from a sample. The use of an incident optical vortex beam has been explained in this document. However the detectors used in this setup are either polarization state detectors or 2D sensors following a rotatable polarizer and the measurement of a change in polarization through the measurement of image orientation has not been suggested.

[0007] In Liu et al., 2017 (Liu, Z., Liu, Y., Ke, Y., Liu, Y., Shu, W., Luo, H., & Wen, S. (2017). Generation of arbitrary vector vortex beams on hybrid-order Poincare sphere. Photonics Research, 5(1), 15-21.) and Liu et al., 2020 (Liu, J., Chen, X., He, Y., Lu, L., Ye, H., Chai, G., Chen, S., & Fan, D. (2020). Generation of arbitrary cylindrical vector vortex beams with cross-polarized modulation. Results in Physics, 19, 103455.), generation of vector vortex beams has been presented. The figures regarding the verification of generated beams (namely Fig. 7 of Liu et al., 2017 and Fig. 4a, Figs. 6b,e,h of Liu et al., 2020) show resultant images with intensity distributions split into two regions for polarization order of 1. Objects of the Invention

[0008] The object of the invention is to provide a polarization analyser producing a resultant image having a pattern directly representing a rotation of polarization due to a sample. This pattern may even be used for the determination of rotation angle of polarization by visual inspection only. An angular sweep of the beam is not needed.

[0009] A further object of the invention is to provide a polarization analysing method, producing a pattern directly representing a rotation of polarization and employing said pattern for the measurement of rotation of polarization.

[0010] Detailed Description of the Invention

[0011] The method implemented according to the objects of the invention described with the use of the attached figures.

[0012] Figure 1 is a polarization analyser according to the prior art.

[0013] Figure 2 is a polarization analyser according to the invention.

[0014] Figure 3 (a), (b), (c) are schematic representations of the polarization of the beam at first, second and third points respectively according to the prior art. The directions of the arrows represent the direction of polarization while the lengths represent the intensity.

[0015] Figure 4 (a), (b), (c), (d) are schematic representations of the polarization of the beam at fourth, first, second and third points respectively according to the invention. The directions of the arrows represent the direction of polarization while the lengths represent the intensity. Figure 5 is the intensity distribution of the beam at the fifth point, corresponding to the output of the laser.

[0016] Figure 6 is the resultant intensity distribution of the beam according to the prior art.

[0017] Figure 7 (a), (b), (c), (d), (e) are resultant intensity distributions of the beam according to the invention for samples having Verdet constants of 0, 0,25, 0,5, 0,75 and 1 respectively.

[0018] The features in the figures have been numbered and corresponding numbers are defined below.

[0019] 1. Laser

[0020] 2. Second linear polarizer

[0021] 3. Optical vortex retarder 4. Magnet

[0022] 5. Sample

[0023] 6. Linear polarizer

[0024] 7. Observation component

[0025] A. First point

[0026] B. Second point

[0027] C. Third point

[0028] D. Fourth point

[0029] E. Fifth point

[0030] The polarization analyser for observation and measurement of rotation of polarization according to the invention essentially comprises a light source emitting a beam of light, a radial polarizer for transforming the beam to a beam having a radial polarization, a polarization rotator, a linear polarizer (6) for filtering the beam with respect to a specific polarization direction.

[0031] The radial polarizer is preferably an optical vortex retarder (3). The optical vortex retarder (3) can be a half wave plate displaying birefringence or a metamaterial. The optical vortex retarder (3) may follow a second linear polarizer (2) for transforming the beam to a beam having a linear polarization or a light source emitting a linearly polarized beam can be used.

[0032] The light source can be coherent source, i.e., a laser (1).

[0033] The polarization rotator is preferably a Faraday rotator having a magnet (4) producing a magnetic field having a component aligned with the beam, and a magneto-optic sample (5) positioned along the beam and under the influence of the magnetic field. Thus, the polarization analyser according to a preferred embodiment of the invention constitutes a polarimeter. However, polarization rotator can have other configurations producing a rotation of the polarization, allowing the invention to be employed for measuring the rotation of polarization of other media or for testing of equipment which constitutes a polarization rotator. An observation component (7) for observing the resultant intensity distribution is also provided. The observation component (7) can a screen, an eyepiece or an array sensor (photo-detector).

[0034] As can be seen in Figures 4a-d, a linearly polarized beam at a fourth point (D) is converted to a radially polarized beam at a first point (A) after passing through an optical vortex retarder (3). This radially polarized beam is then rotated separately at every point of the beam when passing through the sample, resulting in a polarization distribution dependent on the angular position inside the beam at a second point (B). Accordingly, when filtered by the linear polarizer (6), high intensity regions representing the rotation of polarization is obtained at a third point (C) corresponding to the observation component (7). The difference between the angular orientation of the resultant pattern that is defined by the two split regions and the polarizing angle of the linear polarizer (6) is equal to the rotation of polarization due to the polarization rotator, i.e., the rotation of polarization due to the total length of the sample (5). Therefore this difference can be determined for any linear polarizer (6) angle.

[0035] For the same laser (1) output at a fifth point (E) as seen in Figure 5, the prior art would output a pattern having an intensity distribution having the same shape as the input but lower intensity as seen in Figure 6. The rotation of polarization would have to be determined by finding the angular position corresponding to extremum of the intensity by an angular sweep. On the other hand, the rotation of polarization can directly be determined from the resultant intensity distributions for samples (5) of different Verdet constants obtained with different pattern orientations seen in Figures 7a-e. While these examples have been obtained for radially polarized vortex beams, radially polarized full beams can also be employed similarly, with a bar indicating the orientation of the resultant pattern instead of two split regions.

[0036] The measurement of the angular orientation of the resultant pattern can easily be performed by visual inspection however automated measurement can also be employed.

[0037] A polarization analysing method for observation and measurement of rotation of polarization is also introduced with the invention. The method essentially comprises the steps, providing a radially polarized beam of light, passing the beam through a polarization rotator, passing the beam whose polarization has been rotated through a linear polarizer (6) with a known polarization angle, measuring the angular orientation of the intensity distribution of the resultant beam, finding the rotation of polarization by calculating the difference between the polarization angle of the linear polarizer (6) and the measured angular orientation of the intensity distribution.

[0038] When a vortex beam is employed, the angular orientation is measured by determining the centres of the two split high intensity regions, providing a virtual line joining said centres, measuring the angular orientation of said line.

[0039] If the rotation of polarization is due to a sample (5) of known length along the beam, the rotation per unit length can be determined by dividing the calculated angle to said length of the sample (5).

[0040] In an exemplary embodiment of the invention, a radially-polarised Laguerre-Gaussian (LG) beam is used. The result of this is that the doughnut shape of the LG beam is split into two lobes separated by a dark intensity gap. The orientation of the intensity gap is perpendicular to the polarisation direction of the linear polarizer (6). The LG beam emerging from the optical vortex retarder (3) is sent through a terbium gallium garnet sample (5) which is subject to an axial magnetic field. The role of the material sample (5) is to rotate the polarisation direction. The linear polariser (6) after the sample (5) is used to produce the splitting and observe the change in polarisation with the naked eye. In an experiment it was to shown that the Faraday rotation can be directly observed by the naked eye. For the experiment, a laser beam was first passed through the second linear polariser (2). It is then passed through the optical vortex retarder (3) WPV10L-633 which converts the linearly polarised beam into the radially polarised (doughnut mode) LG beam with topological charge = 1. The radially polarised beam then passes through the linear polariser (6) which results in splitting of the ring-shaped doughnut intensity distribution into two intensity lobes where the intensity gap is perpendicular to the polarisation direction for a radially polarised vortex beam. This setup includes a magnetic field which is applied to the polarisation sensitive material. In contrast to the conventional Faraday rotation experiments in which the polarisation rotation is deduced from the recorded change of the intensity of the light, the invention which employs a radially polarised doughnut beam, lead to the direct and easy observation of the visible rotation. The experiment was also repeated with a flint glass sample (5) yielding similar results as the angle of rotation represented by the split between two intensity lobes. While the invention has been described above for the case of a radially polarized beam, any beam having a vector polarization, such as an azimuthally polarized beam can be employed in the same manner.

[0041] In order to obtain such a polarization, the polarization analyser according to the invention comprises azimuthal polarizer for transforming the beam to a beam having an azimuthal polarization, instead of the radial polarizer. Accordingly, the method according to the invention comprises the first step of providing an azimuthally polarized beam of light instead of providing a radially polarized beam of light.

[0042] Such an azimuthally polarized beam is then rotated separately at every point of the beam when passing through the sample and filtered by the linear polarizer (6), high intensity regions representing the rotation of polarization is obtained at a third point (C) corresponding to the observation component (7). For the case of the azimuthal polarization, the orientation of the intensity gap is parallel to the polarization direction of the linear polarizer (6).

Claims

CLAIMS1. A polarization analyser for observation and measurement of rotation of polarization comprising a light source emitting a beam of light, a polarization rotator, a linear polarizer (6) for filtering the beam with respect to a specific polarization direction and an observation component (7) for observing the resultant intensity distribution characterized by further comprising a radial polarizer for transforming the beam to a beam having a radial polarization positioned before the polarization rotator.

2. A polarization analyser according to claim 1, characterized by comprising an azimuthal polarizer for transforming the beam to a beam having an azimuthal polarization, instead of the radial polarizer.

3. A polarization analyser according to claim 1, characterized by the radial polarizer being an optical vortex retarder (3).

4. A polarization analyser according to claim 1, characterized by comprising a second linear polarizer (2) between the light source and the radial polarizer.

5. A polarization analyser according to claim 1, characterized by the light source being a laser (1).

6. A polarization analyser according to claim 1 , characterized by the polarization rotator having a magnet (4) producing a magnetic field having a component aligned with the beam, and a magneto-optic sample (5) positioned along the beam and under the influence of the magnetic field.

7. A polarization analyser according to claim 1, characterized by the observation component being one of a screen, an eyepiece or an array sensor.

8. A polarization analysing method for observation and measurement of rotation of polarization characterized by the steps providing a radially polarized beam of light, passing the beam through a polarization rotator,passing the beam whose polarization has been rotated through a linear polarizer (6) with a known polarization angle, measuring the angular orientation of the intensity distribution of the resultant beam, finding the rotation of polarization by calculating the difference between the polarization angle of the linear polarizer (6) and the measured angular orientation of the intensity distribution.

9. A polarization analysing method according to claim 8 employing a vortex beam, characterize by the steps determining the centres of the two split high intensity regions, providing a virtual line joining said centres, measuring the angular orientation of said line for measuring the angular orientation.

10. A polarization analysing method according to claim 8, characterized by the step of providing an azimuthally polarized beam of light instead of providing a radially polarized beam of light.

11. A polarization analysing method according to claim 8 wherein the rotation of polarization is due to a sample (5) of known length along the beam, characterized by dividing the calculated angle to said length of the sample (5) for determining the rotation per unit length.

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