The optical fibre is a flexible, transparent fibre made of glass (silica) or plastic, slightly thicker than a human hair. Optical fibers are used most often as a means to transmit light and is of wide usage in telecommunications.
Optical fibres are used as optical communication channels because of its high BW, data rates of Gbps, and capacity of transmission. Thousands of channels can be multiplexed together over one strand of fibre. Moreover, optical fibres have very low attenuation, about 0.2 dB/km, and relative low cost. All those characteristics result of great interest for communications over large distances.
Fig. 13 shows the structure of an optical fibre. The thin glass centre of the fiber where the light travels is called the core. The outer optical material surrounding the core that reflects the light back into the core is called the cladding. A buffer coating or jacket protects the optical surface.
An optical fibre has a central region, core, of higher refractive index n1 than the surrounding region, cladding, which has a refractive index n2. If the light hits the interface at any angle larger than the critical angle, φ1c defined in section 2.3, it will not pass through to the second medium and it will be reflected back into the core due to the TIR process.
A fibre optic has a core of pure Si with refractive indexes:
n = 5.57 for wavelengths of 0.4 µm and n = 3.78 for wavelengths of 0.7 µm.
Calculate the times needed for lights of both wavelengths to travel along 2 km of that fibre optic.
The velocity of Light of different wavelengths in the core will be different due to the different values of the refractive index at the given wavelengths. This value is given by: .
First we must calculate the velocities at each one of the wavelengths:
Then we can calculate the time needed to travel 2 km as follows:
Multimode fibres are fibres than may carry more than one mode at a specific light wavelength. Some fibres have very small diameter core and they can carry only one mode, single mode fibres, which travels as a straight line at the centre of the core. To get a propagating wave along a guide it is necessary to have a constructive interference. All the rays interfere with each other. Only certain angles are allowed. Each allowed angle represents a mode of propagation.
The maximum acceptance angle of the fibre defines a cone that fix the light entering into the fibre that will propagate through the fibre in different propagation modes. The half-angle of this cone is the acceptance angle, Φmax, determined only by the indices of refraction. The NA ( numerical aperture) of the fiber is defined by the following equation:
where n is the refractive index of the medium light is traveling before entering into the optical fibre.
The number of propagation modes, M, depends on the optical fibre parameters as follows:
, being V the V-number or normalized frequency, defined by the following equation:
, where 2a is the core thickness.
When V < 2.495 there is only one mode of propagation in the fibre, the fundamental mode (single mode fibre). For values of V > 2.495 the fibre is multimode.
Main transmission losses in the fibre are related to absorption and scattering mechanisms. Rayleigh scattering due microscopic irregularities in the Fiber is an intrinsic source of losses. Absorption is due to the presence of impurities in the fiber material. In optical fibres fabricated from silica (SiO2) there are three main peaks of attenuation due to absorption caused by OH- ions at wavelengths of 1050 nm, 1250 nm and 1380 nm.
A second source of losses or attenuation is the fibre bending. Some radiation is lost in the region where the fiber is bent. The amount of losses depends on the curvature of the bending. If the curvature radius of the bending is similar to the diameter of the fibre including the cladding, D, we are in presence of microbendings, while bending with a curvature radius larger than D is called a macrobendings. Typically, macrobending occurs when the fiber is bent during the installation of a fiber optic link such as turning the fiber around a corner, while microbending effects are due to manufacturing flaws that can result in variations in the fiber geometry over small distances.
Types of optical fibres |
Characteristics |
Plastic |
Losses around 102 dB/km Very flexible, inexpensive, lightweight |
Other glass fibres |
Materials : Chalcogenide, fluoroaluminate Used in long wavelengths communications |
Fused Silica (SiO2) |
Can be extremely pure and then doped to obtain the desired concentration of carriers. Low loss and dispersion at λ = 1.55 μm |
We can couple two fibres if they are of compatible types. The fibres must be accurately aligned with each other, matching of NA, and the ends of the fiber must be brought together in close proximity.
Advantges |
Drawbacks |
Not affected by electromagnetic interference |
High initial cost in installation |
Lower attenuation than coaxial cable or twisted pair. Lower-power transmitters can be used. |
Point to point communications system |
No protections for grounding and voltage problems are needed |
Jointing of fibre and splicing are not easy. Adding additional nodes is difficult. |
High signal security because there is no radiated energy any antenna or detector cannot detects it. |
More fragile than coaxial cable. |
Wide bandwidth |
More expensive to repair and maintain. |
Photonic crystals are artificial multi- dimensional periodic structures with a period of the order of optical wavelengths. These materials are structured to have a periodic modulation of the refractive index.
It is possible to fabricate optical fibres by using photonic crystals. In these fibres, both the core and cladding use the same material, usually silica. One of the fibre regions, the core or the cladding, have air holes, while the other region is totally solid. The presence of air holes in one region, e.g. cladding, results in an effective refractive index that is lower than the solid core region.
On the other hand, it is possible to supress spontaneous emission by using photonic crystals.