Physics

Key unit Competence 

By the end of this unit, I should be able to differentiate optical fiber transmission and other transmitting systems.

My goals 

• Explain the functioning of optical fiber 

• Explain attenuation in optical fiber 

• Identify and explain the components of optical fiber system 

• Solve problem related to attenuation giving answers in decibels 

• Describe telecommunication system 

• Describe functions of amplifiers in optical fiber transmission 

• Distinguish optical fiber and other telecommunication systems

Introductory activity

Investigating the use of optical fiber in RWANDA 

Rwanda plans to connect three million people to the World Wide Web as part of the “Internet for All” project. The project is a World Economic Forum initiative that aims to connect 25 million new Internet users in Kenya, Uganda, South Sudan and Rwanda by 2019.

This goal will partly be achieved by addressing the challenges of affordability, digital skills gap, lack of local content and limited infrastructure, which are hindering growth in the use of Internet across the region (http://www. threastafrican.co.ke, 2017)

                                              Fig.6. 1: The installation and use of optical fiber in Rwanda

Following to the information provided above, answer to the following questions: 

1. Observe the images A, B and C (Fig.6.1) and describe what you can see. 

2. What are the uses of optical fiber in transmission of signals? 

3. How do optical fibers function? In which field? 

4. Discuss other applications of optical fibers.

6.1 PRINCIPLES OF OPERATIONS OF OPTICAL FIBERS

Activity 6.1: Total internal reflection in optical fiber.

                               Fig.6. 2: The total internal reflection in the optical fiber 

Given the illustration above (Fig.6.2), one can see different rays inside the optical fiber. As the angle of incidence in the core increases, as the angle of refraction increases more until it becomes right angle at a certain value of incidence angle called critical angle. 

Discuss: 

  1. What do you understand by the term critical angle? 

  2. What causes the total internal reflection?

  3. Discuss different fields where total internal reflection can be useful.

6.1.1 Definition 

An optical fiber (fiber optics) is a medium for carrying information from one point to another in the form of light. It uses a flexible, transparent fiber made by drawing glass or plastic and has a diameter slightly thicker than that of a human hair. They are arranged in bundles called optical cables and can be used to transmit signals over long distances. 

Fiber optics continues to be used in more and more applications due to its inherent advantages over copper conductors.

An optical fiber is made of 3 concentric layers: 

• Core: This central region of the optical fiber is made of silica or doped silica. It is the light transmitting region of the fiber.  

• Cladding: This is the first layer around the core. It is also made of silica, but not with the same composition as the core. This creates an optical waveguide which confines the light in the core by total internal reflection at the core cladding interface.  

• Coating: The coating is the first non-optical layer around the cladding. The coating typically consists of one or more layers of polymer that protect the silica structure against physical or environmental damage. 

The light is guided down the core of the fiber by the optical cladding which has a lower refractive index. Remember that the refractive index is the ratio of the velocity of light in a vacuum to its velocity in a specified medium. Then light is trapped in the core through total internal reflection. The other outer parts that are the strength member and the outer jacket, serve as protectors.

Connecting two optical fibers is done by fusion splicing or mechanical splicing. It requires special skills and interconnection technology due to the microscopic precision required to align the fiber cores. 

6.1.2 Refractive index of light 

When light falls at the interface (boundary) of two media, it is partially reflected and partially refracted. As it passes from one medium to another it changes its direction

The change in its direction is associated with the change in velocity. The ratio of the speed of light in the vacuum c (or air) and that of light in a certain medium v is called the absolute refractive index n.

                                                                              n =  c/v                                                (6.01)

6.1.3 Total internal reflection 

When light passes from one a medium of higher index of refraction into a medium of lower refractive index the light bends away from the normal as indicated on Fig.6.6. A weak internally reflected ray is also formed and its intensity increases as the incident angle increases. 

6.1.3 Total internal reflection

Increasing the angle of incidence increases the angle of refraction and at a particular incidence, the angle of refraction reaches the 90°. This particular incident angle is called the critical angle θ_c . As the incident angle exceeds the critical angle, the incident beam reflects on the interface between the 2 media and return in the first medium. This effect is called total internal reflection.

For any two media, using Snell’s law the critical angle is calculated using the expression  

                                                    sin θ_c=n₂/n₁                                                            (6.02)

where n₁ and n₂ are respectively the refractive indices of the first and second media.  θ_c increases when approaches  n₁ 

The outer layer of glass, which is also known as the optical cladding, does not carry light but is essential to maintain the critical angle of the inner glass. The underlying main physics concept behind the functioning of an optical fiber is a phenomenon known as total internal reflection. 

Application: 

An optical fiber is basically made of 2 types of glass put together in a concentric arrangement so the middle is hollow. The inner circle of glass also called the Core consists of a glass of higher refractive index than the outside layer as indicated on fig.6.4.

Any light entering the fiber will meet the cladding at an angle greater than the critical angle. If light meets the inner surface of the cladding or the core - cladding interface at greater than or equal to critical angle then total internal reflection (TIR) occurs. So all the energy in the ray of light is reflected back into the core and none escapes into the cladding. The ray then crosses to the other side of the core and, because the fiber is more or less straight, the ray will meet the cladding on the other side at an angle which again causes the total internal reflection. The ray is then reflected back across the core again and again until it reaches the end of the optical fiber. 

Maximum angle of incidence 

The maximum angle of incidence in air for which all the light is total reflected at the core-cladding is given by:
                                                     (6.03)

6.1.4 Checking my progress 

1. Operation of optical fiber is based on: 

    a. Total internal reflection 

    b. Total internal refraction 

    c. Snell’s law 

   d. Einstein’s theory of reality

   e. None of the above 

2. When a beam of light passes through an optical fiber 

   a. Rays are continually reflected at the outside(cladding) of the fiber 

   b. Some of the rays are refracted from the core to the cladding 

   c. The bright beam coming out of the fiber is due to the high refractive index of the core 

   d. The bright beam coming out of the fiber is due to the total internal reflection at the core-cladding interface

   e. All the rays of light entering the fiber are totally reflected even at very small angles of incidence 

3. A laser is used for sending a signal along a mono mode fiber because 

  a. The light produced is faster than from any other source of light  

  b. The laser has a very narrow band of wavelengths

  c. The core has a low refractive index to laser light 

  d. The signal is clearer if the cladding has a high refractive index 

  e. The electrical signal can be transferred quickly using a laser 

4. Given  that  the  refractive  indices  of  air  and  water  are  1  and  1,33,  respectively, find  the  critical angle. 

5. The frequency of a ray of light is 6.0x1014 Hz and the speed of light in air is 3x108 m/s. the refractive index of the glass is 1.5. 

  a. Explain the meaning of refracting index 

  b. A ray of light has an angle of incidence of 30° on a block of quartz and an angle of refraction of 20°. What is the index of refraction of the quartz? 

6. A beam of light passes from water into polyethylene (n = 1.5). If θi = 57.5°, 

    what is the angle of refraction?

7.
 a. What is the critical angle when light is going from a diamond (n= 2.42) to air?

 b. Using the answer to (a), what happens when:

  I. The angle of incidence is less than that angle?

  II. The angle of incidence is more than that angle?

6.2 TYPES OF OPTICAL FIBERS 

Activity 6.2:  Investigating the types of optical fiber. 

Use search internet and discuss different types of optical fiber. Then, differentiate them according to their respective uses. There are three main types of Optical Fibers: Monomode (or single mode), Multimode and special purpose optical fibers.

6.2.1 Monomode fibers 

Those are Fibers that support a single mode and are called single-mode fibers (SMF). Single-mode fibers are used for most communication links longer than 1 000 m.
      

In the monomode fiber, the core is only about 8 μm in diameter, and only the straight through transmission path is possible, i.e. one mode. This type, although difficulty and expensive to make, is being used increasingly. For short distances and low bitrates, multimode fibers are quite satisfactory. 

Following the emergence of single-mode fibers as a viable communication medium in 1983,   they   quickly   became   the   dominant   and   the   most   widely   used   fiber   type   within Telecommunications. Major reasons for this situation are as follows: 

1. They exhibit the greatest transmission bandwidths and the lowest losses of the fiber transmission media. 

2. They   have   a   superior   transmission   quality   over   other   fiber   types   because   of   the absence of modal noise. 

3. They offer a substantial upgrade capability (i.e. future proofing) for future wide- bandwidth services using either faster optical transmitters or receivers or advanced transmission techniques (e.g. coherent technology,). 

4. They are compatible with the developing integrated optics technology. 

5. The above reasons 1 to 4 provide confidence that the installation of single mode fiber will provide a transmission medium which will have adequate performance such that it will not require replacement over its anticipated lifetime of more than 20 years. (John, 2009)

6.2.2 Multimode fibers 

In multimode fiber, light travels through the fiber following different light paths called “modes” as indicated on Fig.6.9. Those are fibers that support many propagation paths. A multi-mode optical fiber has a larger core of about 50 μm, allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors.

The propagation of light through a multimode optical fiber is shown on Fg. 6.9. However, a multi-mode fiber introduces multimode distortion, which often limits the bandwidth and length of the link. Furthermore, because of its higher dopant content, multi-mode fibers are usually expensive and exhibit higher attenuation. 

There are two types of multi-mode optical fibers: multimode step-index and multimode graded index (see Fig.6.10)

• In step-index multimode type, 

the core has the relatively large diameter of 50μm and the refractive index changes suddenly at the cladding. The wide core allows the infrared to travel by several paths or modes. Paths that cross the core more often are longer, and signals in those modes take longer to travel along the fiber. Arrival times at the receiver are therefore different for
radiation of the same pulse, 30ns km-1, being a typical difference. The pulse is said to suffer dispersion, 

it means that it is spread out. 

• In the graded index multimode type, the refractive index of the glass varies continuously from a higher value at the center of the fiber to a low value at the outside, so making the boundary between core and the cladding indistinct. Radiation following a longer path, travel faster on average, since the speed of light is inversely proportional to the refractive index. The arrival times for different modes are the about the same (to within 1ns km-1) and all arrive more or less together at the receiving end. Dispersion is thereby much reduced.

6.2.3 Special-purpose optical fiber 

Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include:  polarization-maintaining fiber  and fiber designed to suppress whispering gallery mode propagation.  

• Polarization-maintaining fiber is a unique type of fiber that is commonly used in fiber optic sensors due to its ability to maintain the polarization of the light inserted into it. 

• Photonic-crystal fiber is made with a regular pattern of index variation. It is often in the form of cylindrical holes that run along the length of the fiber. Such fiber uses diffraction effects in addition to total internal reflection, to confine light to the fiber’s core.

6.2.4 Checking my progress

1. Fiber optics is best known for its application in long-distance telecommunications. 

   a.  True

   b.  False 

2. Choose the basic types of optical fiber: 

   a.  Single-mode

   b.  X-mode

   c.  Microwave-mode

   d.  Graded-index mode      

   e.  Multi-mode 

   f.  A and C      

  g.  B and D 

  h.  A and E 

3. Single-mode fiber has the advantage of greater bandwidth capability.  It has the    disadvantage of: 

  a.  Being harder to bend 

  b.  Smaller mechanical tolerances in connectors and splices 

  c.  Being difficult to couple light into 

  d.  B and C 

  e.  None of the above

4. Describe with the aid of simple ray diagrams: 

  a.  The multimode step index fiber; 

  b.  The single-mode step index fiber. 

  c.  Compare the advantages and disadvantages of these two types of fiber for use as an optical channel. 

6.3 Mechanism of attenuation 

Activity 6.3: Light transmission analysis in optical fiber

                               Fig.6. 11 The images to show the attenuation in optical fiber

Observe the image clearly, and answer to the following questions: 

1. Does all the light from the source getting to the destination? 

2. What do you think is causing the loss in light transmission? 

3. What can be done to minimize that loss in the optical fibers above?

Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) as it travels through the transmission medium. Over a set distance, fiber optic with a lower attenuation will allow more power to reach its receiver than a fiber with higher attenuation.        

Attenuation can be caused by several factors both extrinsic and intrinsic: 

• Intrinsic attenuation is due to something inherent to the fiber such as impurities in the glass during manufacturing. The interaction of such impurities with light results in the scattering of light or its absorption. 

• Extrinsic attenuation can be caused by macro bending and microlending. A bent imposed on an optical fiber produce a strain in that region of the fiber and affects its refractive index and the critical angle of the light ray in that area. Macrobending that is a large-scale bent and microbending which is a smallscale bent and very localized are external causes that result in the reduction of optical power. 

Attenuation coefficients in fiber optics usually are expressed decibels per kilometer (dB/km) through the medium due to the relatively high quality of transparency of modern optical transmission media.  It is observed that the attenuation is a function

6.3.1 Light scattering and absorption 

In the light transmission of signals through optical fibers, attenuation occurs due to light scattering and absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. 

a.  Light scattering 

Scattering losses

The propagation of light through the core of an optical fiber is based on total internal reflection of the light wave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions as it is illustrated on Fig.6.12. This is called diffuse reflection or scattering, and it is typically characterized by wide variety of reflection angles.

Light scattering depends on the wavelength of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident lightwave and the physical dimension (or spatial scale) of the scattering center, which is typically in the form of some specific micro-structural feature. Since visible light has a wavelength of the order of one micrometer (one millionth of a meter) scattering centers will have dimensions on a similar spatial scale. Thus, attenuation results from the incoherent scattering of light at internal surfaces and interfaces.

b. Light absorption 

Material absorption is a loss mechanism related to the material composition and fiber fabrication process. This results in the dissipation of some transmitted optical power as heat in the waveguide. Absorption is classified into two basic categories: Intrinsic and extrinsic absorptions. (John, 2009)

• Intrinsic absorption: is caused by basic fiber material properties. If an optical fiber is absolutely pure, with no imperfections or impurities, ten all absorption will be intrinsic. Intrinsic absorption in the ultraviolet region is caused bands. Intrinsic absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level.

6.3.2 Measures to avoid Attenuation 

The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. 

• Repeaters: Repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than was received, thus counteracting the loss incurred in the previous segment. They mostly used to be installed about once every 20 km.

• Regenerators: Optical fibers link, in common with any line communication system, have a requirement for both jointing and termination of the transmission medium. When a communications link must span at a larger distance than existing fiber-optic technology is capable of, the signal must be regenerated at intermediate points in the link by optical communications repeaters called regenerators.An optical regenerator consists of optical fibers with special coating (doping). 

The doped portion is pumped with a laser. When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new strong light signal with the same characteristics as the incoming weak signal. Basically, the regenerator is a laser amplifier for the incoming signal. 

• Optical Amplifiers: Another approach is to use an optical amplifier which amplifies the optical signal directly without having to convert the signal into the electrical domain. It is made by doping a length of fiber with the rareearth mineral erbium and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.

6.3.3 Checking my progress 

  1. True or False: One of the reasons fiber optics hasn’t been used in more areas has been the improvement in copper cable such as twisted pair.

  2. True or False: With current long-distance fiber optic systems using wavelength-division multiplexing, the use of fiber amplifiers has become almost mandatory.

 3. Fiber optics has extraordinary opportunities for future applications because of its immense bandwidth.

      a. True 

      b. False 

 4. a. What do we mean by attenuation in optical fibers? 

     b. State two ways in which energy is lost in optical fibers. 

     c. If a fiber loses 5% of its signal strength per kilometer, how much of its strength would be left after 20 km?

6.4 OPTICAL TRANSMITTER AND OPTICAL RECEIVER 

Activity 6.4: Investigating the signal sources and signal receiver for optic fibers

With the basic information you know about the functioning process of optical fiber, answer to the following questions. 

1. Where does the light that is transmitted into the optical fiber core medium come from? 

2. What are the type compositions of the light signal propagating into optical fiber?

3. Discuss and explain the function principle of signal generators and signal receivers of light from optical fibers.

The process of communicating using fier-optics involves the following basic steps: 

 1. Creating the optical signal involving the use of a transmitter,  usually from an electrical signal.

 2. Relaying the signal along the fier, ensuring that the signal does not becometoo distorted or weak.

 3. Receiving the optical signal.

 4. Converting it into an electrical signal

6.4.1 Transmitters 

The most commonly used optical transmitters are semiconductor devices such as  light-emitting diodes  (LEDs) and  laser diodes. The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light. 

For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient and reliable, while operating in an optimal wavelength range and directly modulated at high frequencies (see Fig.6.13: Transmitter block). 

In its simplest form, a LED is a forward-biased  p-n junction, emitting light through spontaneous emission, a phenomenon referred to as electroluminescence. The emitted light is incoherent with a relatively wide spectral width of 30–60 nm. LED light transmission is also inefficient, with only about 1% of input power, or about 100 microwatts, eventually converted into launched power which has been coupled into the optical fiber. However, due to their relatively simple design, LEDs are very useful for low cost applications. 

6.4.2 The Optical Receivers 

The main component of an optical receiver is a photodetector (photodiode) which converts the infrared light signals into the corresponding electrical signals by using photoelectric effect before they are processed by the decoder for conversion back into information. The primary photo detectors for telecommunications are made from Indium gallium arsenide (see Fig.6.13). The photodetector is typically a semiconductor-based photodiode. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.

1. Circle the three basic components in a fiber optic communications system. 

    a. Telescope 

    b. Transmitter 

    c. Receiver

    d. Surveillance satellites                                         

    e.  Maser fiber                                      

    f.  Optical fiber                                      

    G. Alternator 

2. Information (data) is transmitted over optical fiber by means of: 

 a. Light  

 b. Radio waves

 c. Cosmic rays           

 d. Acoustic waves  

 e.  None of the above 

3. Connectors and splices add light loss to a system or link. 

 a.  True 

 b.  False 

4. Do fibers have losses?

6.5. USES OF OPTICAL FIBERS 

Activity 6.5: Applications of fiber optics in telecommunication and in medicine 

Use the internet or the library to investigate the applications of optical fiber in medicine and telecommunication systems.

6.5.1. Telecommunications Industry 

Optical fibers offer huge communication capacity. A single fiber can carry the conversations of every man, woman and child on the face of this planet, at the same time, twice over. The latest generations of optical transmission systems are beginning to exploit a significant part of this huge capacity, to satisfy the rapidly growing demand for data communications and the Internet.

The main advantages of using optical fibers in the communications industry are: 

• A much greater amount of information can be carried on an optical fiber compared to a copper cable. 

• In all cables some of the energy is lost as the signal goes along the cable. The signal then needs to be boosted using regenerators. For copper cable systems these are required every 2 to 3km but with optical fiber systems they are only needed every 50km.

• Unlike copper cables, optical fibers do not experience any electrical interference.  Neither will they cause sparks so they can be used in explosive environments such as oil refineries or gas pumping stations.

• For equal capacity, optical fibers are cheaper and thinner than copper cables and that makes them easier to install and maintain.

6.5.2 Medicine Industry 

The advent of practicable optical fibers has seen the development of much medical technology. Optical fibers have paved the way for a whole new field of surgery, called laproscopic surgery (or more commonly, keyhole surgery), which is usually used for operations in the stomach area such as appendectomies. Keyhole surgery usually makes use of two or three bundles of optical fibers. A “bundle” can contain thousands of individual fibers”. The surgeon makes a number of small incisions in the target area and the area can then be filled with air to provide more room.

One bundle of optical fibers can be used to illuminate the chosen area, and another bundle can be used to bring information back to the surgeon. Moreover, this can be coupled with laser surgery, by using an optical fiber to carry the laser beam to the relevant spot, which would then be able to be used to cut the tissue or affect it in some other way.

6.5.3 Checking my progress

The basic unit of digital modulation is:

 a. Zero

 b. One   

 d.     A and B  

 e.     None of the above

6.6 ADVANTAGES AND DISADVANTAGES OF OPTICAL FIBERS

Activity 6.6Advantages and disadvantages of optical fibers 

Use search internet or your library to investigate the advantages and disadvantages of fiber optics. Although there are many benefits to using optical fibers, there are also some disadvantages. Both are discussed below:

6.6.1 Advantages 

• Capacity: Optical fibers carry signals with much less energy loss than copper cable and with a much higher bandwidth. This means that fibers can carry more channels of information over longer distances and with fewer repeaters required.

• Size and weight: Optical fiber cables are much lighter and thinner than copper cables with the same bandwidth. This means that much less space is required in underground cabling ducts. Also they are easier for installation engineers to handle. 

• Security: Optical fibers are much more difficult to tap information from undetected; a great advantage for banks and security installations. They are immune to electromagnetic interference from radio signals, car ignition systems, lightning etc. They can be routed safely through explosive or flammable atmospheres, for example, in the petrochemical industries or munitions sites, without any risk of ignition. 

• Running costs: The main consideration in choosing fiber when installing domestic cable TV networks is the electric bill. Although copper coaxial cable can handle the bandwidth   requirement over the short distances of a housing scheme, a copper system consumes far more electrical power than fiber, simply to carry the signals.

6.6.2 Disadvantages

 • Price: In spite of the fact that the raw material for making optical fibers, sand, is abundant and cheap, optical fibers are still more expensive per metre than copper. Having said this, one fiber can carry many more signals than a single copper cable and the large transmission distances mean that fewer expensive repeaters are required. 

• Special skills: Optical fibers cannot be joined together (spliced) as an easily as copper cable and requires additional training of personnel and expensive precision splicing and measurement equipment.

6.6.3 Checking my progress 

1. List two advantages of using optical fiber. __________________________ 

2. The replacement of copper wiring harnesses with fiber optic cabling will increase the weight of an aircraft.  

       1. True 

       2. False

6.7. END UNIT ASSESSMENT

1. a. An endoscope uses coherent and non−coherent fiber bundle 

I. State the use of the coherent bundle and describe its arrangement of fibers.

II. State the use of the non−coherent bundle and describe its arrangement of fibers.  

 b. Each fiber has a core surrounded by cladding.  Calculate the critical angle at the core−cladding interface. 

Refractive index of core = 1.52

Refractive index of cladding = 1. 

2. a. Fig.6.9 shows a ray of light travelling through an individual fiber consisting of cladding and a core. One part has a refractive index of 1.485 and the other has a refractive index of 1.511.   

                                            Fig.6. 9: Light transmission in optical fiber.

I. State which part of the fiber has the higher refractive index and explain why.

II. (ii)     Calculate the critical angle for this fiber.  

(b) The figure below shows the cross-section through a clad optical fiber which has a core of refractive index 1.50. 

                                                                                                  Fig.6. 10 

Complete the graph below to show how the refractive index changes with the radial distance along the line ABCD in the figure above.

           
                                 Fig.6. 11: Axes for the half life decay curve

1.

    a. What do we mean by attenuation in optical fibers? 

    b. State two ways in which energy is lost along the length of an optical fiber.

    c. If a fiber loses 5% signal strength per km, how much strength would be left after 20 km? 

2. Estimate the length of time it would take a fiber optic system to carry a signal from the UK to the USA under the Atlantic. (Take c = 2 x 108 m/s in the cable. Estimate the length of the cable under the sea.

  a. Estimate the length of time it would take a microwave signal to travel from the UK to the USA a satellite Enk. (Geosynchronous satellites orbit at a height of about 36 000 Ian above the Earth’s surface. 

  b. Which would give less delay in a telephone conversation?