Polarizing prism
Polarizing prisms are optical prisms that are used to change or select the polarization state of light . As polarizing functional elements , they belong to the polarizers and are used to detect or investigate linearly polarized light.
functionality
The way polarization prisms work is based on the birefringent properties of crystals, which are generally weakly absorbent . These mainly include optically uniaxial materials such as calcite (very strongly birefringent), quartz or tourmaline , but also synthetic crystals made of ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP). If a ray of light falls on the cut surface of a birefringent crystal, the ray splits into two partial rays. Both partial beams are polarized perpendicular to each other and are called ordinary and extraordinary beams depending on their optical behavior . The splitting occurs due to different propagation speeds and thus different refractive indices depending on the polarization and the direction of propagation. The beam path is thus primarily determined by the position of the interfaces to the optical axis of the birefringent material used.
In the case of a simple prism with parallel entry and exit surfaces (e.g. a rhombohedron of calcite), two parallel offset beams of different linear polarization would result after complete passage, which could be selected for example by a slit . Since the splitting of the two beam components is relatively small (in the range smaller than 10 °), one would need very thick prisms for a complete separation of the two components in the case of beam bundles with larger cross-sections. This is particularly important because the crystals must have optical properties that are as homogeneous as possible and therefore only single crystals with a comparatively small diameter are available. Since these are mostly natural materials, they are relatively expensive. Larger crystals can also cause problems with space during installation.
In order to achieve a greater separation of the two partial beams, two methods are essentially used in common polarization prisms:
- In one, a stronger lateral separation of the partial beams is created by a combination of two birefringent prisms in order to be able to hide one of the two beams,
- in the other one of the beams is guided out of the original beam path by total reflection at an intermediate layer
The Rochon prism, introduced by Alexis-Marie de Rochon in 1801, belongs to the first group of polarizing prisms . It consists of two calcite partial prisms at the same angle (Icelandic Spat), in which the optical axis of one is perpendicular to the entry surface and the other is parallel to the exit surface. The two prisms were joined on their sloping sides. Due to the perpendicular incidence on the optical axis of the first partial prism, the light beam is not split either with regard to its polarization or its wavelength (see dispersion (physics) ). The extraordinary beam is only deflected in the second partial prism. The degree of deflection is also dependent on the wavelength of the light. Modifications of this principle are the Wollaston prism (1820) and the Sénarmont prism (1857), in which the optical axes of the two partial prisms are oriented differently.
With the other group of polarizing prisms, a corresponding prism made of a birefringent material is cut into two parts in a certain orientation and then joined together again with an adhesive such as Canada balsam . The intersection and the directions of propagation of the rays can be coordinated in such a way that due to the different angles of incidence on the intersection and the different refractive indices of the two rays, one ray is totally reflected at the intersection and the other is transmitted into the second prism part. William Nicol published in 1828 for the first time a corresponding prism consisting of an assembled with a thin layer of Canada balsam Calcitrhomboeder, the Nicol prism . With this prism, both partial prisms are oriented in the same way and the adhesive Canada balsam has a refractive index ( n ≈ 1.54) in the range between the refractive index for the ordinary and extraordinary ray in calcite.
There are other variants of polarization prisms, which are mostly based on a structure that is slightly different from the Nicolian prism and thus achieve different optical properties, such as which beam part continues in the original beam path. For example, they use air as a layer between the partial prisms, have different sections with respect to the optical axis or use partial prisms whose optical axes are rotated against each other. The two partial beams are often not separated by total reflection, but by a corresponding beam deflection in the crystal. In addition to the Nicolian prism, the Glan-Thompson prism (1880), the Ahrens prism or its modification, the Nomarski prism, should be mentioned. The latter is used, among other things, in polarization microscopes and enables examination methods such as differential interference contrast .
The use of a thin intermediate layer of air has the advantage over the use of an adhesive layer that the function of the prism is not influenced by the destruction of the adhesive layer at higher laser power. In addition, the optical behavior of the adhesive limits the usable spectral range, for example they are usually not usable in the UV range. For power applications, prisms with an air gap (instead of the adhesive layer) are therefore used, for example the Glan-Taylor prism .
literature
- Heinz Haferkorn: Optics: Physical-technical basics and applications . Wiley-VCH, 2003, ISBN 978-3-527-40372-1 , pp. 431-435 .