Optical Fiber Communication

In this lecture, we are going to learn about Optical fiber communication, a Block diagram of optical fiber communication systems, types, and modes of optical fiber, and the advantages and applications of optical fiber communication.

So let’s start with the basic knowledge of what is communication and then we will learn about the optical fiber communication system in detail.

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What is Communication?

The term communication may be defined as the transfer of information from one point (source) to another point (destination). For the information to be transferred over a distance, a communication system is usually required.

Within a communication system, the information is transferred made possible by modulating or superimposing the information onto an electromagnetic wave which acts as a carrier for the information signal. This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation.

Electromagnetic waves operating at radio frequencies, as well as microwave frequencies, are being used for communication. However, it was also discovered that communication may also be made possible using an electromagnetic wave that is selected from an optical range of frequencies.

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Optical Fiber Communication System

A communication system that uses light as the carrier of information from a source to a destination through a guided fiber cable (glass or plastic) is called an optical fiber communication system.

The information-carrying capacity of a communication system is directly proportional to its bandwidth, i.e the wider the bandwidth, the greater its information-carrying capacity. Light frequencies used in fiber optical systems are between 10⁴ and 4 x 10¹⁴ Hz ( 10000 to 400,000 GHz)as a result they have higher information-carrying capacity.

In addition to the capability of carrying a tremendous amount of information, fibers have an extremely low loss of about 0.2dB/km i.e., 0.5% of power loss over a distance of 1 km. Because of high information-carrying capacity and low attenuation, nowadays fibers are finding a wide application in telecommunication, local area networks, sensors, computer networks, etc.

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Construction Of Optical Fiber Cable

There are many different cable designs available today. Depending on the configuration, the cable may include a core, a cladding, a protective tube, a polyurethane compound, and one or more protective jackets.

The fiber cable consists of a core at the center and cladding outside a core. The core is generally a cylindrical dielectric glass with a refractive index of n1, and the cladding is the second’s sheath or cover made of glass with a lower refractive index of n2 than the core refractive index.

The cladding in turn is covered by a buffer jacket, This buffer jacket provides protection for the fiber from external mechanical influences that could cause fiber breakage or excessive optical attenuation.

Surrounding the puffer jacket there is a layer of strength members called Kevlar which increases the tensile strength of the cable.

When light propagates through the fiber, the light is transmitted within the core. The cladding keeps the light waves within the core because the refractive index of the cladding material is less than that of the core.

the type of cable construction used depends on the performance requirement of the system and both the economic and environmental constraints.

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Principle of Optical Fiber Communication

The transmission of light in an optical fiber involves the phenomena of total internal reflections at the interface between the core and cladding. Let us consider it in detail.

When light enters one end of a glass fiber under the right conditions, most of the light will propagate or move down the length of the fiber and exit from the far end.

A small part of the light will escape through the sidewalls of the fiber, and some will also be lost due to internal absorptions. But, a portion of the light will be contained and guided to the far end. Such fiber is called a “light pipe” or “light guide”.

Total internal reflection within the fiber wall can occur only if two conditions are satisfied.

The refractive index n1 of the core must always be greater than the refractive index n2 of the cladding.

The light must approach the wall with an angle of the incident \phi that is greater than the critical angle \phi_{c} given by,

\sin \phi_{c}=\frac{n_2}{n_1}

When \phi = \phi_{c}, then by snell’s law,

n_1\sin \phi_{c}= n_2\sin90^{\circ}

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Block diagram of Optical Fiber Communication system

Fiber optic communication link is the transmission of information by the propagation of the optical signal through optical fibers over a required distance. This involves deriving an optical signal from an electrical signal at the transmission end and conversion of the optical signal back to an electrical signal at the receiving end.

The important components of fiber optic communication links are:

1. Transmitter
2. Fiber optic cable
3. Receiver

Transmitter

The transmitter consists of an analog or digital interface, a voltage-to-current converter, a light source, and a source-to-fiber connector or coupler.

Fibre Optic Cable

The fiber optic cable is either an ultra-pure glass or plastic cable.

Receiver

The receiver includes a fiber-to-detector coupler, a light detector, a current-to-voltage converter followed by an amplifier, and an analog or digital interface.

Working of Optical fiber communication system

The message’s origin may take several physical forms. A suitable transducer can be used to convert a non-electrical message into an electrical signal.

For example, microphones are used for converting sound waves into currents, and video cameras are used for converting images into currents. In some cases, such as data transfer between computers or parts of a computer the message is already in electrical form.

In any case, the information must be in the electrical form before transmission for either electronic or optic communication. In a fiber optical transmitter, the light source can be modulated by a digital or an analog signal. For analog modulation, the input interface matches impedances and limits the input signal amplitude.

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Types of Optical Fiber

The optical fibers can be classified into different types based on:

1. Materials of which the fibers are made
2. The mode of propagation and
3. The refractive index (Index profile)

Types Of Optical Fibers Based On Material

There are three major types of optical fibers based on the material type of make. In all three types, the core, as well as the cladding, can be made of either glass or plastic. The three types are:

1. Plastic core with plastic cladding.
2. Glass core with plastic cladding (Also called PCS fiber, plastic-clad-silica).
3. Glass core with Glass cladding (Also called SCS fiber, silica-elad-silica)

1. Plastic Core with Plastic Cladding

• This type of fiber is commonly used because in this both the core and the cladding being made up of the same material simplifies the production process.

• The advantages of these fibers are, that they are more flexible, easy to install can better withstand stress, are less expensive, and weigh approximately 60% less than glass fibers. The major disadvantage is that the attenuation characteristic of these plastic fibers is high. But, even then they are frequently used for short-distance computer applications with information capabilities of about 6 Mbps over a distance of 50-200 meters.

Example:

1. A polystyrene core and a methyl methacrylate cladding.
2. A polymethyl methacrylate core and a cladding of its copolymer.

2. Glass Core with Plastic Cladding (PCS)

• fibers with glass cores exhibit low attenuation characteristics. The PCS fibers are less affected by radiation and hence more suitable for military applications.

3. Glass Core with Glass Cladding (SCS)

The attenuation characteristics of this SCS fiber are slightly better than PCS fibers. Then fibers have the best propagation characteristics and can also be easily terminated. SCS Fibers are the least rugged and they are more susceptible to an increase in attenuation in the radiation area

Example:

1. SiO, core, P.O.-SiO, cladding.
2. P,0,-SiO, core, Si O, cladding.

Types Of Optical Fibers Based On Mode of Propagation

In fiber optics terminology, the word mode simply means path. That is, it defines the number of paths being taken by the light to propagate down the cable. There are only two modes of propagation.

1. Singlemode fiber (also known as mono mode or Fundamental fiber)
2. Multimode fiber.

1. Single-mode Fibre

• If there is only one path for the light to propagate down the cable, it is called a single-mode fiber, In order for the light to take a single path the diameter of the core must be very small (about 7 to 10 µm).

• Singlemode fibers are capable of wide bandwidths (up to 40 GHz) and are ideally suited for long-haul communication. Low cost and high capacity circuits for the transmission of telephone and cable television etc., are the additional features of this fiber.

2. Multimode Fiber

• If the light takes more than one path to propagate down the cable it is called a multimode fiber, Fibers with cores of about 20 to 00 um diameter, support many waveguide modes (i.e. light can take many paths). These fibers are ideally suited for high bandwidth (a few GHz) medium-haul applications. The upper limit of the model is determined by the core diameter and numerical aperture.

Also Read: Difference between Single-mode and Multimode Fiber

Types Of Optical Fibers Based on Refractive Index (Index Profile)

In any optical fiber, the whole material of the cladding has a uniform refractive index value But, the refractive index of the core material may either remain constant or vary in a particular way.

The curve which represents the variation of refractive index (along the vertical axis) with respect to the radial distance (along the horizontal axis) from the axis of the fiber is called the refractive index profile. There are two basic types of index profiles:

1. Step-index fiber
2. Graded-index-fiber.

1. Step-Index Fiber

• A step-index fiber has a central core with a uniform refractive index. The core is surrounded by an outside cladding with a uniform refractive index less than that of the central core. Figure 6.5(a) shows the index profile for a step-index fiber. From the Figure, it is seen that there is an abrupt change in the refractive index at the core/cladding interface and hence the refractive index profile takes the shape of a step. Hence, the name step-index fiber.

2. Graded Index Fibre (GRIN Fibre)

• In graded-index fiber, the refractive index of the core is non-uniform, it is highest (maximum) at the center and decreases gradually with distance toward the outer edge.

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Also Read: Difference between Step Index Fiber and Graded Index Fiber

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Advantages of optical fiber communication

The primary advantages of optical fiber communication compared to metallic cable (electrical) communication are as under:

1. Extremely Wide (Large) Bandwidth

• The bandwidth available with single glass fiber is more than 100 GHz. With such a large bandwidth, it is possible to transmit thousands of voice conversations or dozens of video signals over the same fiber simultaneously. Whereas, only a very small number(40-50) of independent signals alone can be sent through metallic cables.

2. Immunity to Electrostatic Interference

• As optical fibers are made of either glass or plastic (nonconductors or electricity) external electrical noise and lightning do not affect the energy in a fiber cable. The result is a noise-free transmission. however, this is not true for metallic cables made of metals, as they are good conductors of electricity.

3. Elimination of Cross Talk

• Fiber systems are immune to cross-talk between cables caused by magnetic induction. Whereas, in metallic cables, cross-talk results from the electromagnetic coupling between two adjacent wires.

4. Lighter Weight and Smaller Size

• Fibers are very small in size. This size reduction makes fibers the ideal transmission medium for ships, aircraft, and high-rise buildings where bulky copper cables occupy too much space. Reduction in size results in a reduction of weight also.

5. Lower Cost

• The material used in fibers is silica glass or silicon dioxide which is one of the most abundant materials on earth, resulting in lower cost. Optical fiber costs are continuing to decline. The costs of many systems are declining with the use of fiber and that trend is accelerating.

6. Security

• Fiber cables are more secure than metallic cables. Due to its immunity to electromagnetic coupling and radiation, optical fiber can be used in the most secure environments. Although it can be intercepted or tapped, it is very difficult to do so because at the receiving user’s end, an alarm would be sounded.

7. Greater Safety

• In many wired systems, (metallic cables) the potential hazard of short circuits require precautionary designs. Whereas, the dielectric nature of optical fibers eliminates the spark hazard.

8. Corrosion

• Fiber cables are more resistant to environmental extremes. They operate over large temperature variations than their metallic counterparts and are less affected by corrosive liquids and gases.

9. Longer Life Span and Ease of Maintenance

• A longer life span of 20 to 30 years is predicted for fiber optic cables as compared to 12 to 15 years for conventional cables.

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Also Read: Acceptance Angle and Numerical Aperture

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Application of optical fiber communication

The application of optical fibers in various fields is given below:

1. Optical Fibres used in the Medical industry

• Because of its extremely thin and flexible nature, it is used in various instruments to view internal body parts by inserting them into hollow spaces in the body. It is used as a laser during surgeries, endoscopy, microscopy, and biomedical research.

2. Optical Fibres Used in Communication

• In the communication system, telecommunication has major uses of optical fiber cables for transmitting and receiving purposes. It is used in various networking fields and even increases the speed and accuracy of the transmission data. Compared to copper wires, fiber optics cables are lighter, more flexible, and carry more data.

3. Optical Fibres used for Defense Purposes

• Fiber optics are used for data transmission in high-level data security fields of military and aerospace applications. These are used in wirings in aircraft, hydrophones for SONARs, and seismic applications.

4. Optical Fibres are used in Industries

• These fibers are used for imaging in hard-to-reach places such as they are used for safety measures and lighting purposes in automobiles both in the interior and exterior. They transmit information at lightning speed and are used in airbags and traction control. They are also used for research and testing purposes in industries.

5. Optical Fibres used for Broadcasting

• These cables are used to transmit high-definition television signals which have greater bandwidth and speed. Optical Fibre is cheaper compared to the same quantity of copper wires. Broadcasting companies use optical fibers for wiring HDTV, CATV, video-on-demand, and many other applications.

6. Uses of Optical Fibre for Lightening and Decorations

• By now, we have a fair idea of what is optical fibre and it also gives an attractive, economical, and easy way to illuminate the area and that is why it is widely used in decorations and Christmas trees.

7. Optical Fibres used in Mechanical Inspections

• On-site inspection engineers use optical fibers to detect damages and faults which are hard-to-reach places. Even plumbers use optical fibers for the inspection of pipes.

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Also Read: Noise In Communication System

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FAQs on optical fiber communication

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