Difference Between Modal and Chromatic Dispersion

The key difference between modal and chromatic dispersion is that modal dispersion can occur with a monochromatic light source, whereas chromatic dispersion cannot occur with a monochromatic light source.

Modal and chromatic dispersion are important terms in describing the optical properties of optical fibers. Modal dispersion is a type of distortion mechanism in which the distortion occurs in multimode fibers and other waveguides. Chromatic dispersion is the phenomenon by which different spectral components of a pulse travel at different velocities.

CONTENTS

1. Overview and Key Difference
2. What is Modal Dispersion
3. What is Chromatic Dispersion
4. Side by Side Comparison – Modal vs Chromatic Dispersion in Tabular Form
5. Summary

What is Modal Dispersion?

Modal dispersion is a type of distortion mechanism in which the distortion occurs in multimode fibers and other waveguides. Here, the signal spreads with time because the propagation velocity of the optical signal is not the same for all modes. There are some other names for modal dispersion such as multimode distortion, multimode dispersion, modal distortion, intermodal distortion, etc.

During the process of modal dispersion in ray optics analogy, the rays of light enter the fiber with different angles to the fiber axis (the difference occurs up to the acceptance angle of the fiber). The light rays that enter the fiber with shallow angles cam travel a more direct path. Also, these rays arrive faster than the rays that enter at a steeper angle.

Moreover, modal dispersion can limit the bandwidth of multimode fibers. Using a fiber core having a graded refractive index profile can considerably reduce the modal dispersion, but it cannot completely eliminate it. For example, multimode graded-index fiber containing a bandwidth exceeding 3.5 GHz.km at 850 nm is manufactured for use.

PMD or polarization mode dispersion is a special case of modal dispersion which is a fiber dispersion that is usually associated with single-mode fibers. Typically, PMD occurs when there are two modes that normally travel with the same speed due to fiber core geometric and stress symmetry that travel at different speeds due to random imperfections that can break the symmetry.

What is Chromatic Dispersion?

Chromatic dispersion is the phenomenon by which different spectral components of a pulse travel at different velocities. A chromatic dispersion occurs mainly due to two reasons. As the first reason, the refractive index of silica (silica is the material we use to make most optical fibers), is dependent on the frequency of light. We can call it the material dispersion component in chromatic dispersion.

The second reason is the waveguide dispersion. The light energy of a mode is propagated partly in the core and partly in the cladding. Moreover, the effective index of a mode lies between the refractive indices of the cladding and the core. The actual value of the effective index depends on the proportion of power that is contained in the cladding and core. If most of the power is in the core, the effective index becomes closer to the core refractive index. If most of the power is in the cladding, then the effective index is closer to the cladding refractive index. The power distribution of a mode between the core and the cladding of a fiber is a function of the wavelength of light that passes through the fiber. E.g. longer the wavelength, more power in the cladding. Therefore, even in the absence of material dispersion, the effective index of the mode changes when the wavelength changes, which is named as the waveguide dispersion.

What is the Difference Between Modal and Chromatic Dispersion?

Modal and chromatic dispersion are important in describing the optical properties of optical fibers. Modal dispersion is a type of distortion mechanism in which the distortion occurs in multimode fibers and other waveguides. Chromatic dispersion is the phenomenon by which different spectral components of a pulse travel at different velocities. The key difference between modal and chromatic dispersion is that modal dispersion can occur with a monochromatic light source whereas chromatic dispersion cannot occur with a monochromatic light source.

Summary – Modal vs Chromatic Dispersion

Modal and chromatic dispersion are important in describing the optical properties of optical fibers. The key difference between modal and chromatic dispersion is that modal dispersion can occur with a monochromatic light source whereas chromatic dispersion cannot occur with a monochromatic light source.

Reference:

1. “What Is Chromatic Dispersion ? (Material Dispersion and Waveguide Dispersion).” Fosco Connect, Available here.

Image Courtesy:

1. “Fiber Optical Cable” By chaitawat – (CC0) via Commons Wikimedia
2. “Prism rainbow schema” (CC BY-SA 3.0) via Commons Wikimedia