The impact of fiber optic transmission in multiservices networks in rwanda

2.2 OPTICAL FIBER CABLE

To guide signals carrying information from a transmitter to a receiver, the technique of telecommunications uses primarily two means such as:

> Material support between the transmitter and the receiver; > Transmission by radio waves.

Among the transmission material medium, best adapted for significant traffics to high flow is the optical fiber; this is why, this chapter is devoted to the meticulous treatment of this transmission resource.

2.2.1 Definition

Fiber optics are long, thin strands of very pure glass about the diameter of a human hair, They are arranged in bundles called optical cables and used to transmit light signals over long distances that light contains information which are being transmitted, the basic structure of an optical fiber consists of three parts; the core, the cladding, and the coating or buffer. The core is a cylindrical rod of dielectric material, Dielectric material conducts no electricity, Light propagates mainly along the core of the fiber, The core is generally made of glass, the core is described as having a radius and an index of refraction. The core is surrounded by a layer of material called the cladding, even though light will propagate along the fiber core without the layer of cladding material, the cladding does perform some necessary functions.[6]

The following picture shows the basic structure of the optic fiber

Figure 3 : Basic structure of an optical fiber

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The cladding layer is made of a dielectric material with an index of refraction; the index of refraction of the cladding material is less than that of the core material. The cladding is generally made of glass or plastic, the cladding performs the following functions it Reduces loss of light from the core into the surrounding air, reduces scattering loss at the surface of the core ,Protects the fiber from absorbing surface contaminants

For extra protection, the cladding is enclosed in an additional layer called the coating or buffer. The coating or buffer is a layer of material used to protect an optical fiber from physical damage, the material used for a buffer is a type of plastic. [6]

2.2.2 Principle of optical transmission.

The fiber carries a signal encoded beam of light by virtue of total internal reflection (TIR). TIR occurs at the interface of a transparent medium with another medium, if the transparent medium has a higher index of refraction than the surrounding medium, TIR occurs. This makes the optical fiber a waveguide for frequencies in the range 10¹⁴ to 10¹⁵ Hz, covering the visible and part of the Infrared spectrum

Figure 4 : Principle of optical transmission

The figure above illustrate the principle of total internal reflection as used in fiber optical communications, Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.

Figure 5: Total internal reflection

So the critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees.

Let's consider two different media creatively named medium i (incident medium) and medium r (refractive medium). The critical angle is the èi that gives a è_r value of 90-degrees. If this information is substituted into Snell's Law equation, a generic equation for predicting the critical angle can be derived. The derivation is shown below.[7]

ni
· sine(èi) = n_r
· sine (è_r) (2.2)

when è_r=90⁰

ni
· sine(ècrit) = n_r
· sine (90 degrees) (2.3)

ni
· sine(ècrit) = n_r

sine(ècrit) = nr/ni

ècrit = sine^-1 (n_r /ni) = arcsin (n_r/ni) (2.4)

Two stages transducers thus should be added (the equipment intended to convert the signals), one at the beginning, to ensure conversion electricity/light; the other on arrival for opposite conversion. In the first case, it is about a laser diode or LED diode ; in the second a photo diode.

By convention, an impulse of light indicates a bit to 1 and the absence of light to 0, But as any ray whose incidence reaches the critical angle undergoes an internal reflection, many rays are propagated under various angles in optical fiber. it is said that each one has a different mode a fiber presenting this property is thus called fiber multimode.[8]

However, if the diameter of fiber is tiny in proportions such as only one luminous ray can there be propagated, then the fiber acts like a guide of waves and the light can be propagated only in straight line without reflection is called a monomode fiber, it is more expensive than fiber multimode but is largely used at longer distances because it transmit data to 50 Gbit/s out of 100 km without amplification.