What is free spectral range?

The free spectral range (FSR) is a fundamental property of an optical resonator and refers to the spectral range over which the resonator can support multiple resonant modes. It is defined as the frequency difference between two adjacent longitudinal modes that have equal amplitude. In other words, it is the spacing between consecutive peaks of a resonant cavity in the frequency domain.

The FSR is determined by the physical length of the cavity and the refractive index of the material inside the cavity. A longer cavity or a material with a higher refractive index will result in a smaller FSR. The FSR is important in photonics applications where the resonant modes of a cavity need to be precisely controlled, such as in wavelength division multiplexing (WDM) and optical filters.

In addition to the FSR, there are other important resonator parameters, such as the finesse, Q-factor, and mode volume, that determine the performance of the resonator in different applications.