What are Dispersion Modified Fibers?

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- GoPhotonics

Sep 20, 2023

Dispersion modified fibers are specialized fiber optic cables that have been designed and engineered to reduce or control the effects of chromatic dispersion. These fibers are optimized to ensure that light pulses maintain their integrity and arrive at their destination with minimal distortion and delay, enhancing the efficiency of data transmission in optical communication systems.

Dispersion is the phenomenon where different wavelengths or colors of light travel at different speeds through an optical fiber. This leads to a spreading out or broadening of the light pulses as they travel, ultimately affecting the quality and speed of data transmission. Fibers are often categorized based on their dispersion characteristics, as dispersion plays an important role in determining both the bandwidth available for information transmission and the fiber's suitability for communication purposes. 

The two primary types of dispersion in optical fibers are: 

Chromatic dispersion occurs due to different wavelengths of light having varying refractive indices within the fiber. As a result, light with different colors experiences different propagation velocities, causing them to spread out over distance. Modal dispersion arises when light takes multiple paths (modes) through the fiber. This happens in multimode fibers, where light rays travel through different angles and arrive at the receiver at different times.

Engineering Chromatic Dispersion with Modified Fibers

Dispersion modified fibers are specifically engineered to address chromatic dispersion. These fibers offer control and management of chromatic dispersion to ensure that light pulses arrive at their destination with minimal distortion and delay. 

Dispersion modified fibers come in various types, with two notable categories:

  • Dispersion-shifted fibers
  • Dispersion-flattened fibers

The figure below shows that silica single-mode fiber exhibits zero chromatic dispersion at approximately 1300 nm.

Dispersion-Shifted Fibers

A fiber that shifts its dispersion curve so that the zero point aligns with the central range of useful communication channels, roughly spanning 1500-1600 nm, is known as a zero dispersion-shifted fiber. The shifting of a fiber's dispersion curve is achieved through specific manufacturing techniques and design choices. It involves controlling the refractive index profile of the fiber and its core material composition. By altering these parameters, manufacturers can position the dispersion zero point at a desired wavelength range within the fiber. 

Similarly, a nonzero dispersion-shifted fiber is a type of optical fiber where the dispersion zero point is deliberately positioned outside the central range of useful communication channels (e.g., beyond 1600 nm or before 1500 nm). A reduced slope fiber refers to an optical fiber designed to minimize the dispersion slope (Dch vs. wavelength slope) (commonly denoted as 'S') within the central communication wavelength range, typically spanning 1500-1600 nm. The dispersion slope measures how the dispersion changes with variations in wavelength. This design is particularly advantageous for applications like wavelength division multiplexing (WDM).

The term nonzero dispersion fiber generally refers to a fiber with nonzero dispersion within its operational wavelength range. The 1460-1625 nm region is especially significant in telecommunications due to its low fiber losses. It includes three key communication bands labeled as S (1460-1525 nm), C (1525-1565 nm), and L (1565-1625 nm). 

There are also negative dispersion-shifted fibers and positive dispersion-shifted fibers. Negative dispersion-shifted fibers shift the zero dispersion wavelength to longer wavelengths, typically in the 1.5 μm range. Positive dispersion-shifted fibers shift the zero dispersion wavelength to shorter wavelengths, typically in the 1.3 μm range.

The fundamental concept of controlling chromatic dispersion (Dch), represented by the behavior of Dch vs. λ, is quite straightforward and involves the adjustment of the waveguide dispersion coefficient, Dw. Dw is a parameter that characterizes the inherent dispersion properties of the optical waveguide or fiber. It quantifies how the velocity of light varies with wavelength within the waveguide, which ultimately affects chromatic dispersion. Chromatic dispersion is a phenomenon in optical fibers where different wavelengths or colors of light travel at different speeds through the fiber. This leads to a spreading out or broadening of the light pulses as they travel, which can affect the quality and speed of data transmission. λ represents the wavelength of light, typically measured in nanometers (nm).

Dispersion-Flattened Fibers and Waveguide Geometry

Modifying the waveguide dispersion characteristics (Dw vs. λ) can be achieved by altering the waveguide's geometry. Waveguide dispersion arises from the dependence of the group velocity, Vg, on wavelength, λ. As the wavelength increases, the optical field penetrates deeper into the cladding, resulting in a change in the distribution of light energy between the core and cladding, consequently influencing Vg. Therefore, by altering the waveguide's geometry, specifically by modifying the refractive index profile, Dw can be effectively controlled, ultimately achieving the desired chromatic dispersion behavior. This desired behavior often helps achieving zero dispersion at a specific wavelength or attaining the desired dispersion slope across a range of wavelengths.

The dispersion-flattened fiber is an optical fiber designed with a refractive index profile that minimizes and maintains a consistent level of chromatic dispersion across a specific wavelength range. This dispersion is 'flattened' between the wavelengths λ1 and λ2, as illustrated in the figure below.

The refractive index profile of such a fiber resembles a 'W,' wherein the cladding consists of a thin layer with a reduced refractive index. This type of fiber is referred to as 'doubly clad.' The simple step-index fiber is 'singly clad.' For greater control over waveguide dispersion, multiple clad fibers can be employed, although they are more challenging to manufacture. However, they can achieve excellent chromatic dispersion characteristics, typically around 1-3 ps nm-1 km-1 within the 1.3 - 1.6 µm wavelength range. This low dispersion across a wavelength range facilitates wavelength multiplexing, allowing the utilization of various wavelengths as independent communication channels.

An important characteristic of a fiber is not only the magnitude of the dispersion but also its slope with respect to the wavelength. The slope of a fiber is given by,

And it is measured in ps nm-2 km-2. Fiber specifications typically include not just the typical dispersion values at different important wavelengths but also the dispersion slope.

Applications of Dispersion Modified Fibers

Dispersion modified fibers enable high-speed data transmission over long distances, crucial for applications like internet backbones and intercontinental data links. By controlling dispersion, dispersion modified fibers facilitate WDM systems, where multiple data streams are sent over different wavelengths in a single fiber. Dispersion management is essential for transmitting data across vast distances without signal degradation. These fibers find use in optical sensing applications, where precise control of optical properties is crucial.