Optical Fibre Communication System

Optical fibers are mainly used in communication systems due to  rapidly increasing demands for telephone communications throughout the world, multiconductor copper cables have become not only very expensive but also an inefficient way to meet these information requirements.

The frequency limitations in the copper system make a conducting medium for high speed communications necessary. The optical fibre, with its low weight and high frequency characteristics (approximately 40 GHz) and its imperviousness to interference from electromagnetic radiation, has become the choice for all heavy demand long-line telephone communication systems.

An optical fibre communication system is similar in basic concept to any type of general communication system. We may know that the communication in fibers is based on the basic principle of total internal reflection. The function of which is to convey the signal from the information source over the transmission medium to the destination. The communication system, therefore, consists of a transmitter or modulator linked to the information source, the transmission medium and a receiver or demodulator at the destination point. It must be kept in mind that in any trannsmission medium (wires, co-axial cables, free space etc.), the signal is attenuated and is subjected to the degradations due to the contamination by random signals and noise as well as distortions imposed by mechanisms within the medium itself. Therefore, in any communication system there is a maximum permitted distance between the transmitter and the receiver beyond which the system effectively ceases to give intelligible communication.

This is one of the major applications of fiber communication systems.

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This loss is generally expressed in decibel (dB) and is defined as the ratio of injected optical power P_i into the fibre to the received optical power P₀ from the fibre, i.e., attenuation  = 10/L Log (P_i) / (P₀)  dB / km where L is the length of the fibre in km.

Let us discuss them one by one:

Material Absorption

This loss mechanism is basically related to the material composition and the fabrication process of the fiber, which results in the dissipation of some of the transmitted optical power as heat in the waveguide. The absorption of the light signal may be due to inherent property (crystal structure) of the glass or due to impurities present within the glass material.

Linear Scattering

This is due to the transfer of some or all of the optical power contained within one propagating mode to a different mode. In this type of loss, no change of  frequency occurs on scattering. The linear scattering caused by material property (i.e., density fluctuations), produces an attenuation proportional to 1 / ג⁴ known as Rayleigh scattering. On the other hand, linear scattering, caused by imperfections in the fiber geometry (e.g. irregularities in core-cladding interface, core-cladding refractive index difference along the length of the fibre, diameter fluctuations, bubbles etc.), is known as Mie scattering.

Fiber Bending

As the name suggests, this loss is due to the bending in the fiber. This bending can be microbend or macrobend. optical fibres suffer radiation losses at bends on their paths. In this loss, the part of the propagating mode which is outside of the bend is required to travel faster than that on the inside so that a wavefront perpendicular to the direction of propagation is maintained. Hence, part of the mode in the cladding needs to travel faster than the velocity of light which is impossible. As a result, the energy associated with the part of the mode is lost through radiation. The large bending losses tend to occur in the multimode fibres at a critical radius of curvature R_c which is given by

R_c = 3n₁²ג / 4π(n₁² – n²₂) ^3/2

These are the various reasons for losses in optical fibers.

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1. Inter-modal dispersion: The term “Inter-modal” consists of two terms “inter” and “modal”. “inter” means “within different” and “modal” term comes from mode (mode means path followed by the light). Therefore, intermodal dispersion means the dispersion between the different modes of the fiber. Therefore this dispersion can not occur in mono-mode fibers. Thus it can only occur in multi-mode fibers.

Reason: This dispersion arises due to the different time taken by different modes of the fiber. This dispersion depends on the angle at which light ray strikes the core-clad interface. As there are more than one path (mode) through which the rays are propagating therefore each group of rays has its different characteristics group velocity. As the velocity will be different thus the rays will not reach the output at the same time. Therefore, the intermodal dispersion occurs.

2. Intra-modal dispersion: The term “Intra-modal” consists of two terms “intra” and “modal”. “intra” means “within same” and “modal” term comes from mode (mode means path followed by the light). Therefore, intramodal dispersion means the dispersion between the same modes of the fiber. Therefore this dispersion can occur both in mono-mode fibers as well as in multi-mode fibers.

This dispersion is of two types:

a) Material dispersion

b) Waveguide dispersion

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The devices which are used to distribute light signal from one to many and from many to one optical fiber are called couplers. A few of the types of the couplers are:

1. Biconically Tapered  Directional Couplers: In this type, the multimode fibers are made bare by removing the sheath and then these fibers are placed side by side and twisted together. Then the twisted portion of the fiber is heated gently and stretched. If the signal is passing through the first fiber then it will now start passing through the second fiber. Thus the coupler now becomes one input having two outputs.

2. Beam Splitting Couplers:

In this type, light incident from one fiber on a beam splitter inclined at an angle of 45 degree. The beam splitter as the name suggests will split the light ray into two parts, one is reflected part and another is transmitted part. The reflected and transmitted light will incident on two fibers placed at reflected and transmitted sides respectively. Thus this type of coupler will have one input and two outputs.

It can be understood by simple method also. You all know that what a prism does. Suppose a fiber has white light and this light will incident on prism and prism will split the white light into seven parts called VIBGYOR. In front of each part placed a fiber, so the prism will have one input and seven outputs. If we placed the prism oppositely, the prism will convert the VIBGYOR into white light. Thus it will behave as seven inputs and one output.

By same method, beam splitting couplers act.

Note: Try to make the diagram yourself

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Last time I have discussed the permanent technique of joining the fibers that is called splicing. Today I will discuss another technique called connectors:

Connectors: Connectors are the temporary joints between two or more fibers. These can connected or disconnected as and when requires. A few different types of connectors are:

1. Ferrule connectors:

(Meaning of ferrule: A metal ring or cap)

In this method, the fiber ends are inserted into left and right end of the ferrule. Then these ferrules are slid into a tapered sleeve and a butt joint is formed between fiber ends. The joint can be fixed at its position by locking the ferrule arrangement. When in need, the fibers can be again separated by unlocking the ferrule.

2. Expanded beam connectors: In this method, light ray is guided with help of lenses. Light is incident from first fiber on a collimating lens and this lens will make the light parallel and incident on a converging lens and this lens will make the light to converge on a second fiber. In this way light is incident from one fiber to another fiber. When required, the lenses are separated.

Note: Next time I will discuss another method to transmit signal from fiber, called couplers.

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Splicers: Splicers are the permanent joints between two or more fibers.

Splicing: Splicing is the technique to join the fibers.

These are used to extend the length of the fiber or repair the damaged fibers. The two common types of splicing are:

1. Fusion splicing: have you seen the welding done to join the iron? Fusion splicing is just the same. In this case, the fibers are made bare by removing the jacket/sheath. The fiber ends are then placed on adjustable vernier screws. The fiber ends are then aligned using these screws to high degree of accuracy. The fiber ends are then brought closer and using a micro electric arc lamp. The ends are then melt and then fused or joined by this method. The condition of the fibers is that they should have the same refractive index of the core.

2. Mechanical splicing: In this technique, splicing of fiber is done by mechanically.

a) V – groove splicing: In this method, the bared fibers are placed in a V- shape structure called V – groove. The two fibers are slide into the V- groove until they touch each other. The joint is made permanent by a sticky substance called epoxy resin. The epoxy resin should have same refractive index as the core of the two fibers.

b) Precision sleeve splicing: In this type, the bared fibers are placed into the glass sleeve whose inner diameter is slightly loose than diameter of the bared fibers. The fiber ends are then joined permanently by putting a sticky substance called epoxy resin through a hole in the middle of the fiber. The epoxy resin should have same refractive index as the core of the two fibers.

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Answer: A soliton is a pulse or wave that travels along an optical fiber without changing the shape. It is experimentally found that due to fiber non-linearity, the refractive index of the fiber starts depending on the intensity of the light in addition to the wavelength of light. Therefore, the intensity of light itself can influence the velocity of the pulse in the fiber. Thus in solitons, the decrease in velocity due to the decrease in wavelength can be compensated by increasing the intensity of the low wavelength components in comparison to the high wavelength components of the wave. Therefore all the components of the wave travel with equal velocity in an optical fiber and pulse dispersion does not take place.

Question 2: Name the optical sources used for optical fibers.

Answer: Laser diodes and light emitting diodes are the most common sources. These devices require very less power for operation.

Question 3: What are optical detectors?

Answer: Optical detectors are the devices or instruments used at the output terminal of an optical fiber. These devices directly convert optical radiation into electrical signals and respond quickly to the changes in the optic power level.

Example: Photodiode, photomultiplier tube, PIN photodiode etc.

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V = Пd(Numerical Aperture)/λ

Where d is the diameter of the core and λ is the wavelength of light passing through the core of the fiber.

As NA = √n1² – n2² or

NA = Sin θ or

NA = n1√2 Δ

Therefore, V number relation can be changed after putting the above relations in basic relation of V according to the need.

Significance of V number:

If V is less than 2.405 then the fiber is mono mode but if V is greater than 2.405 then fiber is multimode.

V number is also related with the number of modes is the fiber as:

N = V²/ 2 for step index fiber and

Number of modes for graded index fiber is N = V²/ 4.

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Numerical Aperture (NA): NA is the light gathering ability or capacity of an optical fiber. More the NA. the more efficient will be fiber. It is also known as figure of merit.

NA is related to refractive index of core (n1), cladding (n2) and outside medium (n0) as

NA = √n1² – n2²/n0

If the medium is air then n0 =1, then

NA = √n1² – n2²

Acceptance angle (θ): It is the maximum angle made by the light ray with the fiber axis, so that light can propagate through the fiber after total internal reflection.

Relation NA and acceptance angle:

NA = Sin θ

Acceptance cone: It is the cone in which the light incident at acceptance angle or less than the acceptance angle and then the light can propagate through the fiber after total internal reflection.

Fractional Refractive index change (Δ)= Δ = n1 – n2/n1

Relation NA and Δ:

NA = n1√2 Δ

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The light can pass through one path ( that is mono mode or single mode fiber) or more than one path (that is called multi mode fiber). On the basis of this, the fiber is divided into two types:

1. Step index fiber

2. Graded index fiber

Let us discuss the difference between the two:

1. Step index fiber is of two types viz; mono mode fiber and multi mode fiber.

Graded index fiber is of only of one type that is multi mode fiber.

2. The refractive index of the core of the step index fiber is constant through out the core.

The refractive index of the core of the graded index fiber is maximum at the center of the core and then it decreases towards core-cladding interface.

3. Number of modes for step index fiber N =  V²/2, where V is cut off frequency or normalized frequency or V- number

Number of modes for graded index fiber is N = V²/ 4.

4. V number can be less that 2.405 or more that 2.405 for step index fiber

V number is always more than 2.405 for graded index fiber.

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