Operating Principle

Light Transmission

The technology of transmitting light in a long, narrow dielectric by means of total reflection has been known for quite some time. John Tyndall demonstrated in 1854 that one can enclose light in a thin stream of water and guide its transmission along the stream. Soon afterwards, glass tubes and later filaments made of silica glass were used to demonstrate this effect even more impressively. After the laser was invented in 1960, the scientific community recognized the advantages that could be gained by transmitting information by light instead of by an electrical current. In the course of the past 40 years, optical fiber technology has undergone such tremendous development that we now are familiar with three areas of application: direct transmission of images and illumination, fiber optics for communication technology and novel types of sensor. The following details exactly how light is transmitted by optical glass fibers and the particularities of polarization-maintaining fibers from a theoretical viewpoint.

How Light Transmission Works

Transmission of light in a glass fiber uses the principle of total (internal) reflection (see Fig. 1). The fiber itself consists of a core and a cladding, where nCR > nCL applies to its refractive index. Usually, both the core and the cladding are made of silica glass (SiO2), and the core may be doped with germanium in order to raise the core's refractive index. It is also possible to dope the cladding with a different material to lower its refractive index. In practice, optical fibers are fabricated with a protective coating made of a synthetic material, called a jacket, to protect the cladding from damage. This coating does not have any optical effect. That is why it is not considered in the following descriptions. The schematic diagram in Fig. 1 shows the input end of a fiber (without a jacket), with a meridional beam that propagates inside the core. On account of the total reflection, a maximum value of θmax of the angle θi is produced. Below this value, the beam traveling inside the core hits the wall of the core at the critical angle θc and is totally reflected. Beams that hit the end surface of the fiber at an angle greater than θmax reach the interior wall at a larger angle than θc. Therefore, they are only partially reflected by the surface and thus absorbed within the cladding. Hence, θmax defines the half angle of the entrance cone of the fiber within which all beams guided by total reflection lie. If the entrance side of the fiber is located in air (NA ≈ 1), the numeric aperture NA of the fiber is yielded by the critical angle of total reflection.

Fig. 2 outlines the three most commonly used optical fiber structures with their respective core and cladding. In the fiber shown in Fig. 2a, the core is relatively thick; the refractive indices of the core and cladding are constant over the cross-section of the fiber. This is the so-called step index fiber, which has a homogeneous core with a diameter of 50 to 150 µm or larger and a cladding with an outer diameter of approx. 100 to 250 µm. Because of its relatively thick core, the fiber is rugged , making it easy for light to enter. Depending on the angle of incidence, hundreds of different beam paths or modes exist (of which three are shown in Fig. 2), over which light can propagate all along the core. In this case , we call this a multi-mode fiber (MM fiber) in which every mode corresponds to a different travel time. Beams with a greater angle of incidence travel further than do those that propagate along the axis. As the reflexion at the boundary layer between the core and the cladding is considerable, beams of the former type - a greater angle of incidence - take more time than do those of the latter type to travel from one end of the fiber to the other. This effect is called modal dispersion. A result of the different travel times is a broadened rectangular pulse at the end of the fiber, which was initially sharply defined.

Differences in the travel time can be substantially reduced by gradually decreasing the refractive index of the core towards the cladding (see Fig. 2b). In this case, beams do not propagate in a zigzag manner through the fiber, but rather spirally around the center axis. As the refractive index is greater near the axis than at the edges, beams that have shorter distances to cover are slowed along the axis. Beams that propagate spirally near the cladding move faster on longer paths. On account of their refractive index gradients, these fibers are called gradient index fibers, or GRIN fibers for short.

The last and best solution for minimizing modal dispersion is to choose the smallest possible core diameter that provides just enough space for a single mode in which beams propagate parallel to the central axis (see Fig. 2c). For this reason, these fibers are called single-mode fibers, or SM fibers for short. Their typical core diameters ranging from 2 to 9 µm enable modal dispersion to be eliminated for the most part.

If fibers for light transmission are used, the lowest possible attenuation is usually required to keep the amount of light lost along the travel path as low as possible. For fibers used in data transmission in the field of telecommunications, this amount is approx. 0.1 dB/km.