Mode volume

The mode volume M is approximately given by ${\displaystyle V^{2} \over 2}$ and ${\displaystyle {V^{2} \over 2}\left({g \over g+2}\right)}$, respectively for step-index and power-law index profile fibers, where g is the profile parameter, and V is the normalized frequency, which must be greater than 5 for this approximation to be valid.