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Definition: a wavelength above which a guided mode of a waveguide ceases to exist
The number of guided modes of a waveguide (for example, an optical fiber) depends on the optical wavelength: the shorter the wavelength, the more modes can be guided.
For long wavelengths, there may be only a single guided mode (→ single-mode fibers) or even none at all, whereas multimode behavior is obtained at shorter wavelengths.
When a particular mode ceases to exist beyond a certain wavelength, that wavelength is called its cut-off wavelength.
For an optical fiber, the cut-off wavelength for the LP11 mode sets a limit to the single-mode regime, as below that wavelength there is at least the LP01 and the LP11 mode.
Just below the cut-off wavelength, the mode properties often vary substantially.
Typically, the mode radius (and thus the effective mode area) increases sharply near the cut-off, and the fraction of power propagating within the waveguide core decreases accordingly.
That effect is shown in Figure 1 for a multimode step-index fiber; similar behavior occurs for fibers with other transverse refractive index profiles.
Fraction of the power which is contained in the fiber core as a function of the wavelength.
The thin vertical lines indicate the calculated cut-off wavelengths.
The diagram has been produced with the software RP Fiber Power.
For LPlm modes of a fiber, only for l = 0 the fraction of the power guided in the core goes to zero when approaching the cut-off.
For modes with higher l, the mode size stays finite there.
In step-index fibers, there is no cut-off for the fundamental (LP10) mode.
For other fiber designs, in particular for some photonic crystal fibers, there can also be a fundamental cut-off.
Fibers with not radially symmetric designs (and strongly bent fibers) can have polarization-dependent cut-off wavelengths.
See also: waveguides, fibers, modes, LP modes
Category: fibers and other waveguides
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