What is a Fizeau Interferometer?

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- GoPhotonics

Jul 13, 2023

The Fizeau Interferometer is a two beam interferometer that employs a reference beam and a test beam to measure the surface flatness. It was invented by Armand Fizeau, a French physicist, in the 19th century. This laser interferometer enables the determination of refractive indices in transparent materials. The Fizeau interferometer offers a simpler alternative to the Twyman-Green interferometer. It allows interference between two closely spaced components, and it is necessary to carefully control the relative distance between them. There is a similarity between the Fizeau interferometer and the Fabry-Pérot interferometer.

Construction of Fizeau Interferometer

The Fizeau interferometer consists of a light source, a beam splitter, and a partially silvered mirror. It also has two reflecting surfaces, typically parallel glass plates that are positioned facing each other. The rear surface of the first reflector and the front surface of the second reflector act as partially reflecting mirrors. These reflecting surfaces allow some of the incident light to pass through while reflecting a portion of it.

Working of Fizeau Interferometer

In the figure above, a monochromatic light beam, such as from a laser, is directed through a pinhole using a converging lens. This configuration causes the light rays to converge and become parallel, as the converging lens acts as a collimator. The incoming light beam is then directed towards the beam splitter. The beam splitter divides the light into two paths: the reference beam and the test beam. One portion of the light reflects off the beam splitter towards the reference surface, while the other portion transmits through the beam splitter towards the surface under test. The test beam interacts with the test surface, undergoing reflection or transmission based on the surface's properties.

The reference beam then travels towards the high-quality reference surface, which is an integral part of the instrument and remains fixed in its position. Part of the incident light is partially transmitted towards the test surface, and another portion is reflected by the rear surface of the reference surface.

When the two reflected beams recombine at the beam splitter, they superimpose and interfere with each other. The interference occurs because the two beams have travelled different optical path lengths.

The interference pattern is observed by directing the combined beam onto a screen or an imaging device. The pattern consists of alternating bright and dark fringes. The bright fringes occur when the two beams interfere constructively, while the dark fringes occur when they interfere destructively. The interference pattern contains valuable information about the surface under test.

The Fizeau interferometer relies on a difference in optical path length between the reference beam and the test beam. This path length difference is essential for the formation of interference fringes. In contrast, interferometers like the Michelson and Mach-Zehnder interferometers often introduce a controlled path length difference using movable mirrors or beam displacers.

The reference surface is adjusted until the image of the pinhole disappears from the eyepiece. This adjustment ensures that the reference surface is perpendicular to the parallel light beams.

By manipulating the spacing between the two reflecting surfaces, the thickness of the air gap between them can be adjusted. This change in the air gap thickness alters the path length difference of the two reflected beams, leading to a variation in the interference fringes observed. The property of the Fizeau interferometer allows for measurements of surface flatness, surface quality, and even refractive indices of transparent materials.

Applications of Fizeau Interferometer

The Fizeau interferometer has found applications in various fields, including optics, metrology, and interferometric microscopy.

  • Surface metrology: It is used to measure the flatness, surface irregularities, and quality of optical components such as lenses, mirrors, and prisms.
  • Thin film analysis: By measuring the interference patterns produced by thin films, the thickness and refractive index of these films can be determined. It is crucial in industries involved in manufacturing optical coatings and semiconductors.
  • Refractive index determination: Fizeau interferometers enable the measurement of the refractive index of transparent materials by comparing interference patterns with and without the sample.
  • Optical testing: The interferometer is used to test and validate the performance of optical systems, including telescopes, microscopes, and cameras.
  • Precision measurements: Fizeau interferometers contribute to high-precision measurements in fields such as nanotechnology, microelectronics, and semiconductor manufacturing. They are used to measure small displacements, distances, and thicknesses with high accuracy.