# Effective mode volume

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See also: [Mode volume](/source/Mode_volume)

For an [optical fiber](/source/Optical_fiber), the **effective mode volume** is the square of the product of the diameter of the [near-field](/source/Near_and_far_field) pattern and the sine of the [radiation angle](/source/Radiation_angle) of the [far-field](/source/Near_and_far_field) pattern. The diameter of the near-field [radiation pattern](/source/Radiation_pattern) is defined here as the [full width at half maximum](/source/Full_width_at_half_maximum) and the radiation angle at half maximum [radiant intensity](/source/Radiant_intensity). Effective [mode](/source/Transverse_mode) volume is [proportional](/source/Proportionality_(mathematics)) to the breadth of the relative distribution of [power](/source/Power_(physics)) 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. The power divided by the effective mode volume is proportional to the [radiance](/source/Radiance) of the light emitted by the fiber.

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