Fizeau-Interferometer

A Fizeau interferometer ( named after its inventor Hippolyte Fizeau ) is a special interferometer, which is used, among other things, assessing the visual quality of surfaces and optics. To a surface defined by means of the interference of light is compared with another surface.

Design and operation

A Fizeau interferometer for evaluating opaque surface is constructed as follows. Monochromatic light (e.g. laser light) is "filtered" through an objective lens and a pinhole diaphragm. The pinhole is in focus ( focal point) of a second lens, the collimating lens. Is a beam splitter between the diaphragm and the second lens.

Kolliminierte the beam hits now on a glass plate whose surface pointing towards the collimating lens of good optical quality. The other surface is of a lower quality, so called λ/20-Oberfläche or better is adequate. It serves as a reference surface, is reflected by the part of the light. The transmitted portion of proceeds to the sample surface. The light reflected from the sample surface portion contains information on the sample caused by the aberration. The wave fronts of the two fractions interfere in the interferometer, and are passed via the beam splitter on a screen or detector. The recorded image will show a sharp image of the sample surface, which is provided with a striped pattern traversed ( the fringe pattern ). In this case, a continuous strip portions is in the same air gap thickness. Adjoining strips, however, indicate a change in the thickness corresponding to half the wavelength of light.

The structure of a Fizeau interferometer is comparable to that of a Fabry -Perot interferometer, which also consists of two partly reflecting surfaces. In a Fizeau interferometer, however, the two surfaces usually are less reflective ( reflectivity around 4-30 % ), such that secondary reflections contribute less to the edge contrast.

The halos are easy to interpret, and the differences of less than λ/20 a wavelength can be measured visually. The classic stripe pattern produced by a Fizeau interferometer, Newton's rings are, therefore, it is sometimes referred to as a Newton's interferometer. These are generated by the comparison of a convex spherical with a flat sample surface.

The beam splitter is in contrast to other interferometers interferometric no function. Furthermore, it is not rotated by 45 ° relative to the collimated beam. This leads to some practical advantages: the beam splitter

Application

Fizeau interferometers are commonly used for measuring the shape of an optical surface (e.g., the parallelism). As a rule, a lens or mirror is compared with a reference piece of the same shape or surface finish. Sometimes, the comparison unit is realized by a diffractive optical element, since these can be made ​​by photolithographic methods, and enable a higher precision in the manufacture. Fizeau interferometer can also be used in fiber optic sensors for measuring pressure, temperature, strain, etc..

In interference microscopes Fizeau interferometers are often used.

With a modified Fizeau interferometer, the influence of the movement of a medium can be (such as water ) as measured at the speed of light. As shown in the adjacent graph is reflected by a tilted beam splitter and split using a lens and a split into two parallel beams. The beams pass through each other part of a pipe in which water moves. After crossing the two beams are reflected by another lens to a mirror that each beam path of the other beam takes back. The two beams are combined at the detector, and form an interference pattern, which depends on the phase difference in their path through the water.