In this article we will discuss about:- 1. Fabrication of Optical Fibres 2. Types of Optical Fibres 3. Formation of Light Guide or Optical Guide.
Fabrication of Optical Fibres:
Fibres used for communication are constructed from material systems which must satisfy the following requirements:
i. The dielectric materials used must have low loss (<10 dB/km) in the infrared region of the optical spectrum.
ii. The dielectric material must be capable of being drawn into a fibre.
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iii. The dielectric material in the core must have a greater refractive index than the cladding material.
iv. In order to get a high-bandwidth fibre, the materials used must be capable of producing a graded refractive index profile in the core of the fibre.
The above requirements limit the field of dielectric material systems to glasses and plastics. Two types of glass systems are used in general. This is shown in Fig. 8.3. The first type consists of glasses produced by a deposition process. The vapour phase reaction of oxygen with mixtures of compounds like SiCl4, GeCl4, BCl3 and POCl3 produces an ultrapure glass rod known as preform.
The preform has a doped silica core and a pure silica (SiO2) cladding. The concentration of the dopants GeO2, P2O5 and B2O3 is radially varied to get the desired variation of the refractive index of the core. The preforms are pulled into fibres at high temperatures (typically 2000°C). The resultant fibres are then referred to as ‘high-silica’ fibres. Such fibres are mostly used in optical telecommunication systems.
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The other type of glass system is constructed by pure powdered raw materials that are processed using classical glass- making techniques for producing compound silicate glasses. By means of this technique the two glasses (core and cladding) of different chemical compositions are separately melted at relatively low temperatures (850°C to 1100°C) and fed into two concentric crucibles with a hole at the bottom through which a fibre is pulled. The fibres thus constructed are known as ‘multicomponent glass’ fibres.
Preform Fabrication Techniques:
In these techniques pure silica (SiO2) is used as a base material and a small amount of dopants is added to it for changing its refractive index sufficiently to allow a waveguide to be formed.
In a typical vapour phase reaction, chloride precursors like SiCl4, GeCl4, POCl3 and BCl3 undergo a high-temperature oxidation resulting oxides of silicon and dopant elements as given below:
In the above chemical reactions the dopant materials GeO2 and P2O5 increase the refractive index of silica but B2O3 reduces it as shown in Fig. 8.4. Dopants like phosphorus and boron are used as fining agents. Those are thus used to lower the fusion temperature and the viscosity of the deposited glass and hence making it more homogeneous.
For fabricating preforms two different processes are employed, in general. These are- (i) the inside deposition processes and (ii) the outside deposition processes. The inside and outside refer to the general environment in which the glass materials are deposited.
Deposition in an inside process occurs on the inside surface of a fused silica tube while in the outside process the materials are deposited onto an external target surface directly, called a bait rod. The commonly used inside process is the modified chemical vapour deposition (MCVD) process while the outside processes belong to the- (a) lateral deposition and (b) vapour-phase axial deposition (VAD).
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Types of Optical Fibres:
1. The Step-Index Fibre:
In order to analyse the round optical fibre with a homogeneous core (the step-index fibre) let us assume that the radius b of the fibre cladding is large enough so that the cladding field decays exponentially and approaches zero at the cladding air interface. Thus we may consider the fibre, as a two media boundary-value problem.
The steps to solve the boundary-value problem of the step-index fibre are the following:
1. Mathematically model the step-index fibre.
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2. To partition the wave equations, separation of variables technique is used.
3. Physical requirements to solve the field solutions in the core and cladding.
4. Selection of proper functional form of the wave equation.
5. Use the boundary conditions.
6. The characteristic solution is obtained.
7. Analyse the resulting modes and their cut-off condition.
Now we have to solve the following equations for Ez and Hz in both the core and cladding regions of the fibre:
Equation (8.9) is nothing but Bessel’s equation. Therefore, the field must be finite in the core of the fibre at r = 0 and the field in the cladding of the fibre must have an exponentially decaying behaviour.
The Fields in the Core and Cladding of the Step-Index Fibre:
The field must be finite at the centre of the fibre core, the solution of the equation (8.9) for r>a will be-
Mode Cut-Off Conditions:
A mode is said to be cut-off when its field in the cladding is detached from the guide, i.e., the field does not decay. The rate of decay is determined by the value of the constant γ.
For γ = 0, the field detaches itself from the guide. The frequency at which this occurs is called the cut-off frequency.
The cut-off frequency of a mode can be zero if Kc = 0. Only one mode can exist in the fibre with (ωc = 0. Therefore, it is possible to design and operate a single-mode optical fibre. The single- mode fibre has a very small core diameter.
Single-Mode Optical Fibre:
Let us define cut-off parameter Kca in terms of the parameters of the fibre.
Δ is the fractional refractive index difference between the core and cladding.
For smaller value of Δ
2. The Graded-Index Fibre:
Let us consider a multimode fibre with an in-homogeneous core (Fig. 8.6). The analysis of this fibre is made by following the same approach as used for the step-index fibre.
The propagation constant β of a bound mode must fall within the range-
The solution will lead us to a characteristic equation for the waveguide. For simplification we assume that the refractive index decreases as shown by the dotted curve in Fig. 8.6 and the following simplifying assumptions are made for the multimode fibres:
1. The refractive index profile is circularly symmetric.
2. This multimode fibre has very large core diameter.
3. The mode is considered to be transverse electromagnetic.
4. Index variations are very small over distances of a wavelength. So WKBJ (Wentzel, Kramer, Brillouin and Jeffreys) method of approximation is used.
WKBJ Analysis of the Graded-index Fibre:
Suppose the Z component of the electric field is expressed as-
Transmission Loss Measurements:
We know that various absorption and scattering losses will contribute to the total transmission loss of an optical fibre. The transmission loss is determined by measuring the total power at the two points in the fibre by a length L. The attenuation of a fibre of length L is given by the expression,
Where PFE is the power at the output or far end of the fibre and PNE is the power at the near end of the fibre. When a fibre is excited with its EMD (equilibrium mode distribution), the attenuation constant a is defined as-
Scattering and Absorption Loss Measurements:
In order to measure the scattering loss of a fibre, the light scattered from a short length of fibre is collected and then compared with the light travelling in the fibre core.
The following equation is used to measure the scattering loss:
Where Psc is the scattered power, Ptot is the power transmitted in the core and l is the length of the fibre.
The absorption loss can be determined from the difference between its total and scattering loss.
Non-Destructive Loss Measurements:
When a fibre is packaged with a connector on its end, a non-destructive loss measurement is required.
The insertion loss (IL) is given by-
Characteristics of a fibre are given in terms of its power transfer function H(ω) or in terms of its impulse response h(t) where H(ω) and h(t) are Fourier transform of the form-
Where ω is the radian base-band frequency of the envelope of the modulated optical carrier.
In the time domain, the shape of the impulse response is characterized by its r.m.s. pulse- width (2σ) given by-
A narrow pulse having optical power ƒ(t) is used to excite a fibre and an output voltage g(t) is produced. The output voltage g(t) is a distorted version of the input signal ƒ(t).
The detected output pulses through the full length of the fibre g1(t) and through a short length of the fibre g0(t) are measured.
The corresponding Fourier transform is-
Frequency Domain Measurements:
Here the fibre mode is considered to be independent base-band channels so that Fourier transform of the impulse response yields-
Formation of Light Guide or Optical Guide by Fibres:
Usually, when a transparent solid pipe, made by glass, is covered by another hard tube which is also transparent, an optical guide is formed. The cover of transparent material is called cladding. The inside glass tube is known as core. The refractive index of the transparent material should be less than the refractive index of the inside glass tube. If n1 be the refractive index of the glass tube and n2 that of the transparent material, then n2 < n1.
If the optical ray is incident at an arbitrary angle, then it cannot propagate parallel to the fibre axis. So it will be incident in a manner as shown, at the interface of core and cladding. If this angle of incidence θ0 becomes greater than the critical angle фc then total internal reflection will occur. Due to this total internal reflection, the ray will be situated within the fibre and it will be reflected successively at the interface of core and cladding, and so it will lie within the fibre and will be transmitted along the length of the tube (Fig. 8.11).
Let the optical ray makes an angle θ0 with the axis of the fibre. If it is refracted at the core medium at an angle θ1 and if фc be the critical angle at the interface of the core and cladding then-
Now let us assume that the optical ray is incident from outside at an angle less than θ0. In that case the refracting angle will be less than θ1 and the angle of incidence at the interface of core and cladding will be greater than фc. Consequently, there will be a total reflection of light.
