Free spectral range

The free spectral range shortly FSB (English: free spectral range, FSR ) is characterized a term used in optics and laser technology, optical devices, such as resonators or Fabry -Perot interferometer. It specifies the frequency separation of different modes supported by the device or interference orders.

Definition and calculation

A resonant cavity for the free spectral range is given by the frequency spacing of the axial ( Gaussian ) resonator. In a resonator length of a standing wave can form if and only if is an integer multiple of the half wavelength fits into the resonator:

Expressing this in frequencies, one gets the free spectral range, wherein the speed of light. If the resonator is filled with a dispersive medium, it must be replaced by the group velocity in the medium.

More generally one defines the free spectral range as the inverse orbital period (english: inverse round- trip time ) of an optical pulse.

In a Fabry -Perot interferometer called the free spectral range ( at a given wavelength ), the wavelength difference at which overlap the fringe -order at the wavelength and -order at the wavelength. By tilting the angle α can be changed with the transmission wavelength of the Fabry -Perot interferometer. Then for the resulting spectrometer:

N: refractive index of the medium in the resonator.


  • Free Spectral Range in the Encyclopedia of Laser Physics and Technology (English )
  • E. Hecht optics. Oldenbourg Verlag, 4th edition 2005, ISBN 3-486-27359-0
  • Laser Physics