Effective Mode Volume

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 as the full width at half maximum and the radiation angle at half maximum intensity. Effective mode volume is proportional to the breadth of the relative distribution of power amongst the modes in a multimode fiber. It is not a spatial volume but rather an optical volume equal to the product of area and solid angle.