Technical and Application Notes

A Reference Guide to Optical Fibers and Light Guides

Types of Optical Fibers

There are two parameters used to distinguish fiber types, mode and index.

The term mode relates to the use of optical fibers as dielectric wave-guides. Optical fibers operate under the principle of total Internal reflection. As optical radiation passes through the fiber it is constantly reflected back through the center core of the fiber. The resulting energy fields in the fiber can be described as discrete sets of electromagnetic waves. These discrete fields are the modes of the fiber. Modes that propagate axially down the fiber are called guided modes. Modes that carry energy out of the core to dissipate are called radiation modes.

The number of modes allowed in a given fiber are determined by a relationship between the wavelength of the light passing through the fiber, the core diameter of the fiber, and the material of the fiber. This relationship is known as the Normalized Frequency Parameter, or V number.

The mathematical description of the V number is:

V = 2π NA a/λ

where:
NA = numerical aperture (see below)
a = fiber core radius (microns)
λ = wavelength (microns)

For any fiber diameter, some wavelengths will propagate only in a single mode. This single mode condition will occur anytime the V number works out to less than 2.405. For the purposes of this discussion, we will say that there are 2 mode conditions for optical fibers, single mode, and multi-mode. The exact number of modes in a multi-mode fiber is usually irrelevant.

A single-mode fiber has a V number that is less than 2.405 for most optical wavelengths. It will propagate light only in a single guided mode.

A multi-mode fiber has a V number that is greater than 2.405 for most optical wavelengths and therefore will propagate light in many paths through the fiber.

The term index refers to the refractive index of the core material. As illustrated in Figure 1, a step index fiber refracts the light sharply at the point where the cladding meets the core material. A graded index fiber refracts the light more gradually increasing the refraction as the ray moves further away from the center core of the fiber.

Mode and index are used to classify optical fibers into three distinct groups. These are shown in Figure 1. Currently, there are no commercial single-mode/graded index fibers. A brief description of the advantages and disadvantages of each type follows.

Multi-Mode/Step Index

These fibers have the greatest range of core sizes (50 to 1500 microns), and are available in the most efficient core to cladding ratios. As a result, they can accept light from a broader range of angles. The broader the acceptance angle, however, the longer the light path for a given ray. The existence of many different paths through the fiber causes "smearing" of signal pulses, making this type of fiber unsuitable for telecommunications. Because of their large core diameters, they are the best choice for illumination, collection, and use in bundles as light guides.

Multi-Mode/Graded Index

These fibers have the next largest range of core size (50-100 micron). The graded index core has a tendency to bend rays from wider incoming angles through a sharper curve. This results in less pulse smearing than with step index fibers, so they are often used in short range communication. They are usually not bundled due to difficulties in obtaining them in appropriate protective buffers.

Single-Mode/Step Index

These fibers have the smallest range of core sizes (5-10 micron). This small size makes them difficult to handle, so they are given thicker cladding. They only operate in a single guided mode, with very low attenuation, and with very little pulse broadening at a predetermined wavelength (usually in the Near IR). This makes them ideal for long distance communications since they require fewer repeating stations. They have inherently small acceptance angles, so they are not generally used in applications requiring the collection of light.

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