The cutoff wavelength for any mode is defined as the maximum wavelength at which

that mode will propagate. The cutoff wavelength λ*c *of *LP _{11}* is an important

specification for a single-mode fiber. The operation wavelength must be greater than

the cutoff wavelength of

*LP*to operate the fiber in a single mode regime. λ

_{11 }*c*can be

determined analytically for some specified fiber profiles. For a general fiber profile, a

highly accurate numerical mode solver should be involved to calculate cutoff

wavelength. OptiFiber currently implements two different approaches for finding the

cutoff and respectively two different values can be calculated for each mode:

## A “Theoretical” cutoff value

This is the wavelength, above which the given mode cannot propagate, even in short

and unperturbed samples of this fiber. This value is calculated using the general

numerical mode-solvers of OptiFiber. It is defined as the wavelength, above which the

Eigenvalue problem formulated for the current fiber design and for the given mode

does not have real solutions.

## An “Estimated ITU-T” cutoff value

These values are obtained by emulating the actual experimental cutoff

measurements, as described in the ITU-T / TIA / EIA recommendations (see [ITUT[

23]] in the Technical references). It employs a macrobending loss formula [J. Sakai

and T. Kimura, 1978 [26]] applicable to any mode in arbitrary-index profile optical

fibers. The ITU-T/EIA-455-80A recommendation is formulated only for the cutoff of

LP11.