intermodal dispersion (original) (raw)
Author: the photonics expert (RP)
Definition: the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode
Alternative term: modal dispersion
Category: 
fiber optics and waveguides
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- acceptance angle
- bend losses
- cut-off wavelength
- differential mode delay
- effective mode area
- effective refractive index
- group velocity dispersion
- intermodal dispersion
- modal bandwidth
- mode radius
- polarization beat length
- propagation constant
- propagation losses
- V-number
- waveguide dispersion
- zero dispersion wavelength
- (more topics)
Related: Telecom Fiber With Parabolic Index Profiledifferential mode delaydispersionchromatic dispersionfibersmultimode fibershigher-order modes
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What is Intermodal Dispersion?
Intermodal dispersion (also called modal dispersion) is the phenomenon that the group velocity of light propagating in a multimode fiber (or other waveguide) depends not only on the optical frequency (→ chromatic dispersion) but also on the propagation mode involved.
Figure 1 shows a numerical simulation, where a 200-fs ultrashort pulse is launched into a 50 cm long multimode fiber such that multiple modes are excited. After the fiber, the corresponding contributions appear at different times due to different group velocities of the modes. The fundamental mode comes first, as it is the fastest.
Figure 1: Output power versus time for a 200-fs input pulse injected into a 50 cm long multimode fiber.
The numerical simulation has been done with the RP Fiber Power software.
Figure 2: Time-dependent output beam profile for the same situation as in Figure 1.
The strength of intermodal dispersion can be quantified as the differential mode delay (DMD). It depends strongly on the refractive index profile of the fiber in and around the fiber core. For example, for a step-index profile the higher-order modes have lower group velocities, and this can lead to differential group delays of the order of 10 ps/m = 10 ns/km. It is then hardly possible to realize data rates of multiple Gbit/s in an fiber-optic link with a kilometer length.
In systems for optical fiber communications based on multimode fibers, intermodal dispersion can severely limit the achievable data transmission rate (bit rate). In order to avoid strong signal distortion, it is usually necessary to keep the pulses long enough to maintain a reasonable temporal overlap of components from different modes, and this unavoidably sets a limit on the data rate.
The natural way of eliminating intermodal dispersion is to use fiber links based on single-mode fibers: if there is only one propagation mode available (disregarding possible polarization mode dispersion and cladding modes), there can be no difference between propagation times. However, intermodal dispersion can also be minimized by using multimode fibers with a parabolic refractive index profile, which greatly reduces this effect.
Frequently Asked Questions
What is intermodal dispersion?
Intermodal dispersion, also called modal dispersion, is a phenomenon in multimode waveguides where the group velocity of light depends on the propagation mode. This causes different modal components of a light pulse to travel at different speeds and arrive at different times.
How does intermodal dispersion affect fiber-optic communications?
In optical fiber communications using multimode fibers, intermodal dispersion causes signal pulses to spread out in time. This distortion severely limits the achievable data transmission rate, especially in links of a kilometer length or more.
How can intermodal dispersion be reduced or eliminated?
Intermodal dispersion is eliminated by using single-mode fibers, which support only one propagation mode. In multimode fibers, it can be minimized by using a fiber with a parabolic refractive index profile, which helps to equalize the propagation times of the different modes.
Bibliography
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| [8] | Y. Painchaud, M. A. Duguay and F. Ouellette, “Interferometric time measurements of intermodal dispersion in optical fibers by using a CCD photodetector array”, Opt. Lett. 17 (20), 1423 (1992); doi:10.1364/OL.17.001423 |
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| [10] | J. Liang et al., “Differential mode group delay measurement for few-mode fibers based on cepstrum analysis”, Opt. Lett. 50 (15), 4766 (2025); doi:10.1364/OL.562030 |
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