Free spectral range
The free spectral range , FSB for short (English: free spectral range , FSR), symbol , is a term from optics and laser technology that characterizes optical devices such as resonators or Fabry-Pérot interferometers . It indicates the frequency spacing of various modes or interference orders supported by the device .
Definition and calculation
For a cavity resonator , the free spectral range is given by the frequency spacing of the axial (Gaussian) resonator modes.
A standing wave can form in a resonator of length if and only if an integral multiple of half the wavelength fits into the resonator:
- With
If you express this in frequencies , you get the free spectral range:
at 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 defines the free spectral range than the inverse rotation period (English: inverse round-trip time ) of an optical pulse.
In a Fabry-Perot interferometer , the free spectral range (at a given wavelength) denotes the wavelength difference at which the -th order interference fringes overlap at the wavelength and -th order at the wavelength .
The transmission wavelength of the Fabry-Pérot interferometer can be changed by tilting by the angle α . The following then applies to the spectrometer obtained in this way :
- .
with the refractive index n of the medium in the resonator.
swell
- Free Spectral Range in the Encyclopedia of Laser Physics and Technology (engl.)
- E. Hecht optics. Oldenbourg Verlag, 4th edition 2005, ISBN 3-486-27359-0