Twyman–Green interferometer

The Twyman- Green interferometer (designated by Frank and Alfred Twyman Green ) is an interferometer, that is an optical measuring device that is used for characterization and testing of optical components, such as lenses or prisms, are used.

Design and operation

The structure of a Twyman-Green interferometer corresponds substantially to that of the Michelson interferometer comprising a light source, a beam splitter, two mirrors and a screen / detector. There are two differences, however, these are firstly the use of a monochromatic point source of light, on the other lenses at the interferometer input and output.

The operation may be summarized as follows. The light from the light source onto the input lens, the light from the point source of a collimated beam ( plane wave ) generated finite dimensions. Then, the collimated light is incident on a beam splitter where it is split into two identical sub-beams, each of which make one of the two mirrors. After reflection on the two mirrors, the light is incident again on the beam splitter and is directed onto the reunited interferometer. The lens at the interferometer provides a final for focusing the light onto a screen or detector. As with all interferometers is carried out at the union of the two partial beams of a superposition of the partial waves and thus interference, depending on the optical path difference of the two partial beams. These differences are then observed as a characteristic stripes or ring patterns and allow conclusions on the surface of the mirror and the sample.

The testing of optical components is performed by the placement of the component in one of the beam paths. It must be ensured that, for example, when testing a prism, the light is incident perpendicular to the second reference mirror. Otherwise, only a portion of light, or no light reaches the beam splitter again. When testing a lens to lens fitting, curved reference mirror must be used instead of a plane reference mirror.

A similar structure is also used in the Fizeau interferometer.

Variants

A slightly modified construction uses instead of a planar beam splitter a polarizing beam splitter cube-shaped and 45 ° linearly polarized light. By this change in construction, the two partial beams are different by the beam splitter, that is, horizontally and vertically linearly polarized and converted at the exit of the cube-shaped beam splitter by a λ/4-Verzögerungsplatte in different rotating circularly polarisertes light. The falling back from the reference mirror and the sample light is then passed once again the λ/4-Verzögerungsplatte and the beam splitter, and enters a different linearly polarized light to the detector. It should be noted that the two component beams due to their different polarization do not interfere with each other and again no light comes to the light source. An interfering signal may be achieved by a further linear polarization.

Applications

Twyman- Green interferometer in the optical test methods are of great importance. It is therefore, inter alia, in the production of optical components such as lenses, prisms and reflecting surfaces, for testing the optical properties, such as homogeneity of flat surfaces are used.