For a linear cavity of optical length L, resonances repeat every c divided by 2L, so a 1.5 m cavity has an FSR of 100 MHz and a 1 mm air-spaced etalon about 150 GHz. It sets the spacing of longitudinal modes, the repetition rate of a mode-locked laser and the unambiguous range of a scanning interferometer. Two lines one FSR apart look identical on a Fabry-Perot analyzer.
FSR on the analyzer is 1.5 GHz, so that second peak could be a real mode or the next order of the same one.
Also heard as
- FSR
- mode spacing
Related terms
Longitudinal mode
One of the discrete frequencies at which a standing wave fits a whole number of half-wavelengths into the resonator.
Finesse
The sharpness of an optical cavity's resonances: free spectral range divided by the resonance linewidth.
Etalon
A pair of parallel reflecting surfaces that transmits a comb of sharp frequency peaks, used to select or measure wavelengths.
Rep rate
Clipped form of repetition rate: how many pulses per second a pulsed laser emits.