The cutoff wavelength for any mode is defined as the maximum wavelength at which
that mode will propagate. The cutoff wavelength λof LP11 is an important
specification for a single-mode fiber. The operation wavelength must be greater than
the cutoff wavelength of LP11 to operate the fiber in a single mode regime. λc 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.