The cross section of optical fibers consists of a circular core region surrounded by a cladding region. The core region has a higher refractive index than the cladding region. Typically, the core refractive index is about 1% higher than the cladding refractive index. Light propagating in the core is guided inside the core by means of total internal reflection. The condition of total internal reflection can be inferred from Snell’s law. A light ray is totally internally reflected whenever it is incident at the core-cladding boundary. In a ray-optics picture, light rays propagating inside the core follow a zigzag path.

There are three types of optical fibers used in communication systems. These types are the (i) step-index multimode fiber, (ii) graded-index multimode fiber, and (iii) single-mode fiber. The three types of fibers are shown in Fig. 22.1 along with the refractive index profiles.

Step-index multimode fibers have a relatively large core diameter. Typical core diameters are 50, 62.5, and 100 pm for silica fibers used in communication systems. Plastic optical fibers have larger core diameters, typically 1 mm. An important advantage of multimode fibers is easy coupling of the light source to the fiber. Usually, a ± 5 pm accuracy of alignment is sufficient for multimode fibers with a core diameter of 50 |im. The main disadvantage of multimode fibers is the occurrence of modal dispersion.

Because the core diameter in multi-mode fibers is much larger than the operating wavelength, several optical modes can propagate in the waveguide. These optical modes have different propagation constants so that different modes arrive at the end of the fiber at different times, even if they were launched at the same time. This leads to a broadening of the optical pulse and limits the maximum bit rate that can be transmitted over a fiber of a given length.

Modal dispersion is reduced by graded-index multimode fibers. Graded-index multimode fibers have a parabolically graded core index leading to a reduction in modal dispersion.

Single-mode fibers have such a small core diameter that only a single optical mode can propagate in the fiber. Typical single-mode core diameters are 5-10 |im. The main advantage of single-mode fibers is the lack of modal dispersion. The main disadvantage of single-mode fibers is difficult coupling due to the small core diameter. A small core diameter requires light sources

with high brightness such as lasers. However, LEDs, in particular edge-emitting LEDs and superluminescent LEDs are also occasionally used with single-mode fibers. Coupling of light into single-mode fibers requires precise alignment with tolerances of a few micrometers.

If the optical power to be transmitted over an optical fiber is of prime interest, the core diameter should be as large as possible and the core-cladding index difference should also be as large as possible. Specialty fibers with core diameters of > 1 mm are available. Such fibers are not suitable for communication applications due to the large modal dispersion.