I am doing research on micro cavity frequency combs and I've been trying to understand things like dispersion, Free Spectral range, resonance, and modes.
I am looking at the definition of free spectral range(FSR), and the definition of it seems to be the frequency spacing of a resonators axial modes.
So when would you use one versus the other? The mathematical definition for FSR is:
While for resonators axial modes is:
Where $n_g$ is the index of refraction of the medium, L is the length of cavity, c is the speed of light, $\lambda$ is the wavelength, and $\nu$ is the frequency.