For an optical fiber, the square of the product of the diameter of the near-field pattern and the sine of the Radiation angle of the far-field pattern. The diameter of the near-field Radiation Pattern is defined here as the full width at half maximum and the Radiation Angle at half maximum intensity.

Note: Effective Mode volume is proportional to the breadth of the relative distribution of power amongst the Modes in a Multimode fiber. It is not truly a spatial volume but rather an “optical volume” equal to the product of area and solid angle.