Mode Field Diameter (MFD)

The typical core diameter of communication single mode fibers is from 8~10um for operating wavelength 1.31um to 1.5um. Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics as we did in the explanation of step-index multimode fiber. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation.

So even though the fiber cladding confines the light within the fiber core, some light does penetrate into the cladding, despite the fact that it nominally undergoes total internal reflection. This occurs both in single mode and multimode fibers, but this phenomenon is more significant in single mode fibers.

For a Gaussian power distribution (lasers used in communications are Gaussian power distribution) in a single mode optical fiber, the mode field diameter (MFD) is defined as the point at which the electric and magnetic field strengths are reduced to 1/e of their maximum values, i.e., the diameter at which power is reduced to 1/e2 (0.135) of the peak power (because the power is proportional to the square of the field strength). For single mode fibers, the peak power is at the center of the core.

Mode field diameter is slightly larger than the core diameter, as shown in the following illustration.

September 6, 2011

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