cut-off wavelength (original) (raw)
Author: the photonics expert
Definition: a wavelength above which a guided mode of a waveguide ceases to exist
Category: 
fiber optics and waveguides
Units: m
Formula symbol: <$\lambda_\textrm{co}$>
DOI: 10.61835/ow7 [Cite the article](encyclopedia%5Fcite.html?article=cut-off wavelength&doi=10.61835/ow7): BibTex plain textHTML Link to this page 
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.
Figure 1: Fraction of the power of various guided modes (where the colors are related to the <$l$> indices of those) which is contained in the fiber core as a function of the wavelength.
The thin vertical lines indicate the calculated cut-off wavelengths of the modes. 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 theoretically no cut-off for the fundamental (LP01) mode, although propagation losses at long wavelengths may still be high, even making the fiber unusable. This is essentially because long-wavelength light would have a mode size well beyond the fiber core, with such mode experiencing light guidance and thus being very sensitive e.g. to microbending. Note, however, that in such situations there is not a well-defined mode cut-off, but rather one sees gradually rising propagation losses at longer wavelengths.
For other (not step-index) fiber designs, in particular for some photonic crystal fibers, there can also be a fundamental mode cut-off.
Fibers with not radially symmetric designs (and strongly bent fibers) can have polarization-dependent cut-off wavelengths.
Just below its cut-off wavelength, the bend losses of a mode can become very high due to the increased mode area. Therefore, even for moderate bending of the fiber one may obtain sharply increasing propagation losses near the cut-off wavelength. Therefore, cut-off wavelengths can not always be precisely determined in experiments.
Simulations on Cut-off Wavelengths
Explore, for example, how mode properties change near a cut-off wavelength, or what influence the cut-off wavelength of a fiber has on the performance of a device. Only with a suitable simulator, you get complete insight and fully optimize performance. The RP Fiber Power software is an ideal tool for such work.
More to Learn
Encyclopedia articles:
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.