Glan–Thompson prism

The Glan-Thompson prism ( by Paul, Silas Glan Thompson Phillips ) is based on double- refraction and total reflection polarizer, the non-polarized light is linearly polarized ( s-polarization, i.e., the Polarisationssebene is perpendicular to the incidence plane ). The principle was introduced in 1880 by Paul Glan and improved in 1881 by Silvanus Phillips Thompson.

Construction

The Glan-Thompson prism is similar to the earlier ( 1828) developed Nicol prism of a birefringent crystal ( calcite is a typical material ). The crystal is cut into two right-angled prisms, that its optical axis is parallel to an end face. The two prisms are joined together with a transparent adhesive to a cuboid.

The angle of intersection of the two prism halves is chosen so that the ordinary ray is totally reflected at the interface while the extraordinary ray almost without reflection is allowed through. For this, the adhesive must have a refractive index between the ordinary and the extraordinary ray is for the birefringent material. In the case of calcite with at a wavelength of between ( extraordinary ray ) and ( ordinary ray ), for example, the previously often used Canada balsam. Nowadays materials are used which have better transmission characteristics in the near infrared, such as crystalline glucose or glycerol.

Operation

Upon entry into a material of the incident light beam is refracted according to the Snell's law of refraction. Because of the anisotropic refractive index birefringent materials, the incident beam is ( polarized perpendicular to the optical axis of the crystal) into an ordinary and an extraordinary split ( parallel to the optical axis polarized ) beam. Since the cut surfaces of the crystal are parallel to the optical axis of the Glan- Thompson prism, both beams are equal broken at normal incidence - by the vertical incidence angle of incidence equals the angle of refraction, ie 0 °. The two beam components thus move on a common path in the crystal, although because of the different refractive indices at different speeds.

Meeting now both beams on the cut surface, then the two beams due to an appropriately selected materials reflected differently. For the ordinary ray, the adhesive as compared to an optically thinner medium calcite dar. It is, therefore, totally reflected at the interface, so that it impinges on an outer surface of the Glan-Thompson prism and there be absorbed by an optionally -placed absorber. For the extraordinary ray calcite has a lower refractive index. It therefore passes through the sectional area in accordance with the Fresnel equations almost undisturbed (T ≈ 99.9%). This is repeated when it passes into the second half prism. It is important to make sure that the angle of intersection (and thus the angle of incidence) is not too large, since the beam is otherwise totally reflected at this interface. When passing through the adhesive layer, the beam experiences a displacement of the beam, but this is minimal because of the low thickness of the adhesive. Upon exiting the crystal, therefore, is present only the linearly polarized extraordinary ray whose polarization corresponding to the alignment of the optical axis.

The exiting beam is largely, but not completely polarized. The degree of polarization in addition to the manufacturing tolerances and the materials used, which are not ideal dielectrics in general, also of the thickness of the adhesive layer -dependent. As a ray of light upon total reflection due to continuity conditions of the Maxwell equations in the following optically thinner material penetrates ( in the form of an evanescent wave ), it can at very low film thicknesses of the adhesive for so-called optical tunneling effect ( prevented total reflection ) can occur. Here, depending on the film thickness reaches a part of the actually total reflected radiation in the second half prism. The emergent beam is therefore never one hundred per cent linearly polarized. Since the influence of this effect, however, decreases exponentially with the layer thickness can be determined by a sufficiently thick layer ( > 10 microns) are almost completely reduced.

Demarcation from other prism types

This principle is substantially similar to that of the Nicol prism, a benefit is that by the aligned sections at the Glan- Thompson prism is no offset between the incoming and emergent beam occurs.

A Glan - Thompson prism, the glue used instead of an air gap is referred to as a Glan prism Faucault. It is mainly used for high -performance applications for the Glan- Thompson prism is less suitable because the adhesive heat here and thus can be destroyed. More priming from Glan- Thompson - type are the Ahrens prism are arranged in the two Glan-Thompson prisms side by side, and the Big prism, essentially a Ahrens prism with air gap.

The similarly constructed Lippich or Glan -Taylor prism, also can be due to the extraordinary ray is generated, however, because of the 90 ° rotated in the plane of incidence of an optical axis by 90 ° different from polarized beam (p- polarized).