- Dispersion-Compensating Fiber
- 1. Introduction
- Chromatic Dispersion - EXFO animated glossary of Fiber Optics
- Dispersion Compensation Schemes – A System Perspective
- Dispersion compensating fiber pdf creator
- Dispersion Compensating Fibres for Fibre Optic Telecommunication Systems
- Dispersion compensation by using DCM
- chapter and author info
Chromatic dispersion CD of optical pulses in an optical fibre influences their width. It refers to changes in propagation of particular frequency components contained in optical pulses causing extension of optical pulses. The fibre reach is then significantly limited, unless signal regenerators are used.
Because the pulse spread degrades optical systems, it is necessary to prevent its origination or to eliminate its results. CD ought to be suppressed or one shall prevent its origination by specialty optical fibres, including both conventional and microstructured optical fibres MOF. To keep the pulse length nearly constant, it is possible to use Dispersion Compensating Fibre DCF that shall be installed at the regenerators that provide amplification, renewal of timing and pulse duration.
DCFs have negative dispersion parameter; CD accumulating between regenerators is then suppressed. Consequently, bit error rate could be improved and the fibre reach could be extended [ 1 ].
We systematize design approaches for specialty dispersion-tailored optical fibres in order to offer a guideline to flexibly design optical fibres used in telecommunications whose optimized CD is a key property. Last but not least, we present selected fibre designs prepared in last few years, including CD plots and information about structural parameters.
The knowledge about how to design fibres could be useful to design optical fibres, such as for example a submicron flat CD compensating fibre. Additives using these materials are promising for extending the application towards infrared region, where fluoride glasses are usually transparent. DCFs could regenerate signals that are spread as a consequence of CD. As a result, potential bit rate could be increased. Another approach is to use already mentioned DCFs at the signal regenerators.
Sample compensating fibre was published in Refs.
Very low CD parameter is observed whose properties could benefit in compensation of CD accumulating in some optical network. Resultant CD is a balance between waveguide and material component. In order to obtain flat CD, the evolution of waveguide dispersion has to be exactly opposite to the one of material dispersion, which means that both ought to be optimized at each channel step by step.
A very important assumption is that the balance between both dispersion components could be a very useful tool in designing fibres for CD tailoring by optimizing material dispersion through the material properties and waveguide dispersion as a result of optimization of the shape of the waveguide [ 3 ] [ 6 ].
Such narrowband fibres can have very low value of CD that is possible without using additives e. In Ref. Other works concentrate rather on the optimization of the structural parameters of MOFs to obtain flat CD diagram.
Microstructured fibres offer much greater flexibility because geometry could be optimized not only through the core and cladding diameter, but also by changing the air filling fraction, and the lattice pitch including its arrangements.
Index guiding MOFs the one without the central inclusion, having solid core offer high flexibility in CD compensating fibres design.
For example microstructured based DCFs could have dual cores [ 9 ] or they could be doped [ 10 ]. Negative value of CD parameter in DCFs with dual core structure could also be combined with the idea of making some rings with smaller holes [ 7 ].
The performance of DCFs with the concentric cores could be increased by using germanium dioxide in the core [ 12 ]. The relation between the amount of additives and resultant CD is shown in Ref. Some other substrates could be used to lower the effective index n eff , such as for example fluorides [ 14 ].
On the contrary, germanium dioxide raises n eff. Fibres with flat CD, i. They must have CD exactly opposite to dispersion in each channel. In addition, the hole radius in the 1 st ring should be larger to enhance dispersion.
CD slope property is studied in Ref. Another slope compensating DCF is shown in Refs. The 1 st ZDW is the one utilized at visible wavelengths, whereas the longer ZDW is used at telecom wavelengths [ 18 ].
The high index difference between the air-filled microstructure and pure or doped silica core enables tight mode confinement resulting in a low effective area and, thereby, a non-conventional behaviour of CD. Modifying the periodic cladding i. Then, the strong wavelength dependence in the characteristics of the fibre will be used to determine either huge CD with large slope or nearly-zero flat CD.
The bandwidth, at which MOFs are designed to have zero CD, could be divided into three categories: working in the region between 0. The 3 rd region, starting at 1. Both core size and the radius of holes exhibit significant influence on the location of both ZDWs.
As it has already been mentioned above, MOFs are suitable for designing the compensating fibres because they allow huge index contrast and offer many parameters to be optimized core size, hole radius, lattice pitch, amount of rings of holes in order to optimize waveguide dispersion [ 2 ].
Currently, flat CD fibres could for example be referred to as near zero CD transmission medium. An interesting fibre with CD parameter close to zero with CD fluctuations less than 0. Results were obtained by careful optimization of holes in particular rings. Instead, one could consider using octagonal [ 25 ] and decagonal [ 26 ] lattice, or more sophisticated lattice arrangements [ 2 ].
The value of CD and the width of the working range is a compromise. Practically, predicted CD diagram is a wide parabola.
Considered properties are related to the diameter of the core and the radius of the holes in particular rings. Both were found to have different impact on the origination of each ZDW.
Chromatic Dispersion - EXFO animated glossary of Fiber Optics
Changing the air filling fraction and lattice pitch is not the only idea that could be used to optimize CD. For example elliptical holes instead of circular holes exhibit potential low and very stable CD properties, i. Finally, there are a few examples that are worth to be noticed: a DCF in Ref.
Understanding of the mechanisms governing CD tailoring is necessary not only permits for the fibre design, CD suppression and avoidance, but also to predict the potential manufacturing tolerances.
There are a few techniques used to obtain the expected CD and we describe them in more details in the following sections. Because the waveguide dispersion is related to it and because the length of propagated light waves is of the range that is comparable to the core size, the inserted region will be responsible for very low waveguide dispersion component. There is another technique that could be used to optimize dispersion, it assumes to inject some liquids into the microstructured holes.
Dispersion Compensation Schemes – A System Perspective
This means that such a fibre could for example be used for some experiments in laboratory conditions e. Germanium dioxide is very often used to raise the effective index. But this solution is not perfect in terms of effective area of the mode which is rather small in the doped fibres, as well as from the perspective of increased attenuation caused by material absorption, which is high for germanium dioxide [ 1 ].
Another technique to be described and systematized is the one using air holes located close to the fibre centre. The 1 st mode could be confined within the core by the index contrast, and the effective diameter of a mode is then usually small enough to control CD.
Dispersion compensating fiber pdf creator
Stronger confinement could be obtained in a MOF using other type of a lattice, like for example octagonal, decagonal or spiral. An alternative way, how to control the effective index, is to have core defects. This technique uses weak interactions between the core-guided mode and a mode localized in an intentionally introduced defect of the crystal. Finally, the idea of concentric cores could be considered.
What feature in dual core technique is responsible for low CD?
Dispersion Compensating Fibres for Fibre Optic Telecommunication Systems
It is large value of n eff produced by significant asymmetry between the two cores where light is coupled. Light is propagated in both cores concurrently. Low value of CD parameter is possible in concentric cores. They could be obtained by removing [ 7 ],[ 10 ] or reducing [ 7 ],[ 31 ] the air fraction in some rings of holes [ 1 ].
There are many aspects that could influence resultant CD; it could be coupling modes and the phase matching wavelength, at which CD control is possible, could be obtained in many ways, for example by fibre bending. We found these conclusions as interesting, because many works assume that the most important consequence of bending a fibre is increased attenuation. It could be shown that large negative value of CD parameter could be the accompanying process.
They could be enhanced by very careful optimizing the material composition. For example fluorides could be used as additives, but they could also be used as a background material instead of silicon dioxide, which could be evaluated as significant progress in the field [ 32 ]. Last but not least, these materials are transparent in infrared region as well as in the telecommunication bands which makes them interesting for some applications in sensors.
The properties of fluorides allow using them in MOFs [ 33 ]. They would lower the effective index of the cladding or the index at the second, larger concentric core.
Dispersion compensation by using DCM
It is usually combined with doping the main core by using germania. This idea could be considered as a significant progress in fibre optic technology. Then a fibre with a W-type profile or refractive index could be created. For example, the core doped by using germanium dioxide would exhibit the n eff of about 1. Similar solution in general modified W-type index profile fibre could be employed in systems with WDM [ 32 ],[ 34 ].
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All this combined with the application of new materials that should be processed in different way compared to siliceous materials could result in both attenuation and CD properties far from the expectations [ 32 ]. At the telecommunication band both ZBLAN and ZBGA showed similar attenuation properties like the one of silicon dioxide, but their CD properties significantly differ [ 35 ], in addition fluorides have different mechanical properties and they have to be fused at different temperature than temperature for silica.
The theoretical attenuation could even be of 0. The n eff in telecom band is 1. To describe propagation of light in a given optical glass, we could consider the constant of propagation of a wave that is related to the phase variation [ 32 ]:.
Because MOFs are dielectric it could be assumed that:. Both real and imaginary part of propagation constant in 3 could be simplified to a form:. It refers to CD and is less than the speed of electromagnetic wave in vacuum [ 32 ]:. In simulations it is more suitable to use material dispersion equation described by using Sellmeier approximation. CD equation suitable for Sellmeier approximations is following [ 39 ]:. The distribution of light could be calculated by discretizing electric 13 and magnetic 14 field, it is described in Refs.
FDFD is used in many commercial simulators of mode distribution and n eff calculators. In most of them, it is possible to use user friendly graphical interface, but when a very accurate result is demanded, many iterations ought to be done by creating a loop using an appropriate scripting language.