Editorial Feature

An Introduction to Fabry–Pérot Interferometers

An interferometer is a device used to measure various parameters of light by dividing the light into several light beams. The light beams are transmitted across different paths such that the beams interfere with each other when they are combined. The interference occurs in the form of fringes of light and dark bands.

The Fabry-Perot interferometer was developed by Charles Fabry and Alfred Perot in 1899. It consists of two highly reflective parallel mirrors known as etalon. The high reflectivity of the mirrors generates continuous multiple light reflections with intensities which slowly reduce thereby forming sharp, narrow interference fringes. The arrangement of line spectra can thus be analyzed using these fringes.

Working Principle of Fabry–Pérot Interferometers

A typical Fabry-Perot interferometers includes two semi-silvered plane or curved mirrors fixed at a certain distance. One mirror is fixed, while the other is movable. Light from the source is allowed to pass through the mirrors with respect to an optical axis at a specific angle. As a result of multiple reflections from one mirror to the other, the light between the mirrors undergoes successive destructive and constructive interferences.

The constructive interference occurs when the distance between the mirrors is in the order of the wavelength of incoming light. The transmitted light passes through the second mirror only during the event of constructive interference. On the other hand, the destructive interference is created when the mirror separation distance is half the wavelength of incoming light.

The interference pattern forms a group of concentric circles. The resolution of the interferometer is improved with large number of interference rings.

Applications of Fabry–Pérot Interferometers

Fabry–Pérot Interferometers find applications in the following:

  • Optical wavemeters
  • Dichroic filters
  • Laser absorption spectrometry
  • Laser resonators
  • Telecommunication networks
  • Gravitational wave detection


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