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 $$V^2 \over 2$$ 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.