What is a Twyman-Green Interferometer?

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- GoPhotonics

Feb 6, 2023

Twyman-Green Interferometers are interferometers used to test optical components. It was invented by Frank Twyman and Arthur Green in 1916. It is used to measure the surface shape and transmitted wavefront quality of optical components, assemblies, systems, and optical-grade mechanical surfaces. A point-like light source such as lasers or LEDs is used in the Twyman-green interferometer. The Twyman-Green interferometer has a mirror and beam splitter arrangement like a Michelson interferometer. The only difference is the way the interferometers are illuminated. The Twyman-Green interferometer uses a monochromatic point source that is located in the principal focus of a well-corrected lens, in contrast to the Michelson interferometer which uses an extended light source.

Figure 1: Basic layout of Twyman-Green Interferometer

The Twyman-Green interferometer consists of a light source (S), Mirror 1 (M1), Mirror 2 (M2), a beam splitter (BS), and a screen. 

The light source is used to provide the input light for the interferometer. The beam splitter is used to split the incoming light from the source into two separate beams. One of the beams reaches M1­ reflects the light from the source and directs it toward the beam splitter. The other beam from the beam splitter reaches M2 and gets reflected back to the BS. The interference pattern is created when the two beams recombine at the beam splitter and interfere with each other. The screen is used to display the interference pattern produced by the interferometer. The distance between M and M2 is adjustable, allowing for the measurement of phase shifts in the interference pattern produced by the interferometer. By analyzing the interference pattern, it is possible to obtain information about the properties of the object being tested, such as its surface topography or the presence of stress and strain in a material sample.

The interference is precisely equivalent to thin-film interference at normal incidence if the mirrors M1­ and M2 are parallel to one another and the beam-splitter BS forms a 45° angle with the normal of each mirror. When d = m/2, where d is the path length difference between the two arms adjusted by translating M1, we obtain totally constructive interference. When d = (m + 1/2)/2, we get completely destructive interference. Since the angle of incidence is constant, if we rotate M2, we will observe fringes on the screen that are of equal thickness. This situation is similar to interference seen when collimated light and a thin screen of different thicknesses are used. One of the mirrors is purposefully tilted to create fringes for the testing of optical components. When the component is placed in the interferometer, the quality of the component can then be determined based on how the fringe pattern changes. For measuring the focal length and determining aberrations, lens testing is very important.

Advantages

  • Versatile
  • Contactless testing
  • Chance of scratching the surface under test is less.

Disadvantages

  • High-quality optical components are required.

Applications

The Twyman-Green interferometer is often used to test the quality of optical systems such as lenses, mirrors, and telescopes, as well as to quantify the wavefront aberrations present in these systems. They can be used to test the properties of materials, such as the stress and strain present in a sample, by measuring the phase shift in the interference pattern produced by the interferometer.

The Twyman-Green interferometer can be used to analyze the surface topography of an object, including the shape and roughness of the surface, by measuring the phase shift in the interference pattern produced by the interferometer. They are used to perform high-resolution microscopy, as well as to measure the size and shape of small particles and other structures. They are also used in the design and optimization of optical systems, as well as to test the performance of these systems during the manufacturing process.