Mode volume

In fiber optics, mode volume is the number of bound modes that an optical fiber is capable of supporting.

The mode volume M is approximately given by $4V^2 \over \pi^2$ and ${V^2 \over 2} \left({g \over g+2}\right)$ , 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.