dispersion-flattened fibers (original) (raw)

Acronym: DFF = dispersion-flattened fiber

Definition: fibers with a non-standard zero dispersion wavelength

Category: article belongs to category fiber optics and waveguides fiber optics and waveguides

Related: Dispersion Engineering for Telecom Fiberschromatic dispersiondispersion-engineered fiberstelecom fibers

DOI: 10.61835/sne Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Dispersion-flattened Fibers?

Dispersion-flattened fibers are a class of dispersion-engineered telecom fibers. They are designed so that their chromatic dispersion remains nearly constant — and often small — across a wide wavelength range, typically covering much of the low-loss transmission window of fused silica (around 1300–1625 nm).

An earlier dispersion-engineered fiber type is dispersion-shifted fibers, which have their zero dispersion wavelength shifted into the 1.5-μm telecom wavelength region. Such fibers have a substantial dispersion slope (specified with units of ps/(nm2 km), related to third-order dispersion), which can be problematic particularly in wavelength division multiplexing, where different signals experience substantially different GVD values. Even when only a single signal wavelength is used, it is inconvenient that the dispersion slope strongly restricts the usable wavelength range.

For such reasons, dispersion-flattened fibers with a much reduced dispersion slope, i.e., with relatively constant group velocity dispersion over some wavelength range, have been developed. In other words, they exhibit low higher-order dispersion. They can, for example, exhibit near zero dispersion or some moderate relatively constant amount of GVD in the telecom C band. Such fibers, defined in ITU standard G.656 [1], are important not only for transmission fibers, but also e.g. for pulse compression, for signal processing using four-wave mixing and wavelength channel translation, and for the generation of broadband frequency combs.

Refractive Index Profile

Dispersion-flattened fibers achieve their performance through careful tailoring of their refractive index profile, which influences both material dispersion and waveguide dispersion.

A typical DFF design features a W-shaped refractive-index profile, consisting of:

mode field

Figure 1: Example of a W profile fiber design and the calculated mode amplitude profile. For details, see a case study for concrete examples of dispersion engineering of fibers.

This structure has several design parameters allowing fine control of the wavelength dependence of mode confinement: the relative core radius, the refractive index contrast, and the ring width.

Alternative designs may employ:

The design process involves complex trade-offs among:

Achieving these balances requires precise control during fiber fabrication — especially in fiber preform fabrication, where deposition, doping concentration and geometry are crucial. Techniques such as modified chemical vapor deposition (MCVD), plasma chemical vapor deposition (PCVD) and outside vapor deposition (OVD) are often employed with stringent process control to maintain the designed profile.

See a case study for concrete examples of dispersion engineering of fibers:

Case study: Dispersion Engineering for Telecom Fibers

Case Studies

Dispersion Engineering for Telecom Fibers

We explore different ways of optimizing refractive index profile for specific chromatic dispersion properties of telecom fibers, resulting in dispersion-shifted or dispersion-flattened fibers. This also involves automatic optimizations.

Frequently Asked Questions

What is a dispersion-flattened fiber?

A dispersion-flattened fiber is a type of dispersion-engineered fiber with a greatly reduced dispersion slope. This means its group velocity dispersion remains relatively constant over a specific wavelength range.

What is the advantage of dispersion-flattened over dispersion-shifted fibers?

While dispersion-shifted fibers shift the zero-dispersion wavelength, they still have a substantial dispersion slope. Dispersion-flattened fibers also reduce this slope, which is crucial for applications like wavelength division multiplexing where all channels should experience similar dispersion.

How is a flat dispersion profile achieved in a fiber?

Dispersion-flattened fibers have complex refractive index profiles, such as a W-shaped profile. This structure modifies the fiber's waveguide dispersion to counteract the wavelength dependence of the material's group velocity dispersion.

For which applications are dispersion-flattened fibers used?

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