new york city college of technologywebsupport1.citytech.cuny.edu/faculty/mseip/files/ee/...d....

D. Mynbaev TCET 4102, Module 1, Spring 2008 1

NEW YORK CITY COLLEGE of TECHNOLOGYTHE CITY UNIVERSITY OF NEW YORK

DEPARTMENT OF ELECTRICAL AND TELECOMMUNICATIONS

ENGINEERING TECHNOLOGY

Course : TCET 4102 Fiber-optic communicationsModule 1: Introduction and physics of light

Prepared by: Professor Djafar K. MynbaevSpring 2008


TCET 4102 course modules:

Table of contents

1. Introduction and physics of light.

2. Optical fiber: Attenuation.

3. Multimode optical fiber: modes and attenuation.

4. Multimode optical fiber: modal dispersion.

5. Multimode and singlemode fibers: attenuation and bandwidth. Reading the data sheets of optical fibers.

6. Fiber-optic cables. Splicing, connectors and adapters.

7. Light sources: LED.

8. Light sources: LD. Transmitters.

9. Photodetectors: PD. Receivers.

10. Optical networks: physical and intelligent levels. Components: splitters and switches, repeaters and optical amplifiers.

11. WDM network: definition and main characteristics. DWDM and CWDM. ITU-T grid.

12. Transmission in WDM networks. Power budget.

13. Access networks. PON architecture, operation and components.

14. Topic by student’s request or General architecture of optical networks, control plane and data plane, standards. Review for final examination.

15. Final examination.

Key words

• Telecommunications

• Optical fiber

• Transmission capacity

• Carrier signal

• Frequency

• Laser diode (LD)

• Photodiode (PD)

• Optical network

• Electromagnetic waves

• Wavelength

• Speed of light

• Light beam (ray)

• Total internal reflection

• Photon

• Light absorption

• Light radiation


• Note: You are advised that black and white figures and text are the quotations from the textbook [1], whereas color figures and text are new materials aimed to deliver the updated information.

• Textbook: Djafar K. Mynbaev and Lowell L. Scheiner, Fiber-Optic Communications Technology, Prentice Hall, 2001, ISBN 0-13-962069-9.

Notes:

The figure numbers in these modules are the same as in the textbook. New figures are not numbered.

Always see examples in the textbook.

TCET 4102 course modules


Module 1: Introduction to optical communications and physics of light

Introduction:

Block diagram of a fiber-optic communications system

Tx RxInformation Information

Electronic domain Electronic domainOptical domain

Optical fiber

Figure 1.1 Block diagram of a fiber-optic communications system.

Note: The figure numbers in these modules are the same as in the textbook. New

figures are not numbered.


• Basic block diagram of a fiber-optic communications system

A fiber-optic communications system is a particular type of telecommunications system. Information to be conveyed enters a transmitter (Tx), where its electronics process it, that is, prepares the information signal for transmission. The transmitter also converts the signal into optical form and the resulting light signal is transmitted over optical fiber. At the receiver end, an optical detector converts the light back into an electrical signal, which is processed by the receiver’s electronics to extract the information and present it in a usable form (audio, video, or data output).

Introduction:



Electronics

Electronics

Light source (Laser diode, LD)

Photodiode (PD)Transmitter (Tx)

Receiver (Rx)

Optical fiber

Detailed block diagram of a fiber-optic communications system.

Info

Info

Introduction:



• The heart of the transmitter is a light source. The major function of a light source is to convert an information signal from its electrical form into light. Today’s fiber optics communications systems use, as a light source, laser diodes (LDs). They are miniature semiconductor devices that effectively convert electrical signals into light. They need power-supply connections and modulation circuitry. All these components are usually fabricated in one integrated package.

• The transmission medium in fiber-optic communications systems is an optical fiber. The optical fiber is the transparent flexible filament that guides light from a transmitter to a receiver. An optical information signal entered at the transmitter end of a fiber optics communications system is delivered to the receiver end by the optical fiber. So, as with any communications link, the optical fiber provides the connection between a transmitter and a receiver and, very much the way copper wire and coaxial cable conduct an electrical signal, optical fiber “conducts” light.

• The main type of optical fiber is made from a type of glass called silica. It is about a human hair in thickness. Bare optical fiber shielded by its protective coating, is encapsulated in several other layers that, all together, make up fiber-optic cable -- the structure that protects very fragile optical fiber from hostile environments and mechanical damage.

• The key component of an optical receiver is its photodetector, which a semiconductor photodiode (PD). The major function of a photodetector is to convert an optical information signal back into an electrical signal (photocurrent). This miniature device is usually fabricated together with its electrical circuitry to form an integrated package that provides power-supply connections and signal amplification.

Introduction:



Why use optical communications?

Why do we need to convert electrical signal to optical and develop

the whole new type of telecommunications system, which has

replaced the traditional copper-based transmission? Here is the

answer:

• Transmission capacity (bandwidth) is proportional to the

frequency of a carrier, fC, that is, C (bit/s) ~ fC (Hz).

• Light has the highest fC (hundreds of THz) among practical signal

carriers Optical communications systems—that use light as a

signal carrier—can achieve the highest transmission capcity.

• Theoretical transmission capacity of optical fiber at a single

wavelength is about 100 Tbit/s; with wavelength-division

multiplexing (WDM) it could reach tens of petabits per second.

Introduction:

Fiber-optic communications link


Source: Atlas of cyberspace.

http://www.cybergeography.org/atlas/alcatel_large.gif


•The trend today is to produce more and more information at a

faster and faster rate.

•Telecommunications, which is responsible for delivering

information from one point to another, plays an increasingly

important role in modern society.

•Optical communications is the linchpin of the

telecommunications industry; thus, the health and

development of telecommunications depends on the health and

development of optical communications.

Introduction:

Optical communications today - general


•Optical networks carry more than 98% of the domestic

telecommunications traffic in the United States; the same percentage

is true for other industrialized countries. Optical cables serve as

pipelines delivering a tremendous volume of information.

Optical networks operate as a means of transport ferrying

information from one point to the other. The scale of operation

ranges from intercontinental to continental to metropolitan to local-

access networks.

Figure in Slide 8 shows fiber-optic submarine networks. In addition to these

networks, all territory of the United States and other developed countries are

covered by extensive optical terrestrial networks.

Today, the global optical network carries most of the world’s traffic. This optical

network is supported by a satellite communications system. Signals from the

global network deliver information to our office desks and to our homes.

Introduction:

Optical communications today - general


metro

access

metro

access

access

accessCPE

metro

access

Three types of networks: traffic aggregation

access

access

Long-distance

Introduction

Long-distance, metro and access networks - Classification


• We subdivide optical networks into three major categories: long-haul (long-distance), metro, andaccess. (See Figure in Slide 11.) No strict boundaries separate these networks. We distinguishamong them by the scale of their operation, the line rate (data rate or bit rate) of their traffic,and their features. No standards define these networks. Other classifications, such as backboneand access networks, are often used.

• Access networks aggregate the traffic from many individual customers and bring this traffic to ametro network. Access networks cover distances in the range of units of miles and carry trafficat line rate from hundreds of megabits per second (Mbit/s) to hundreds of gigabits per second(Gbit/s). Metro networks aggregate traffic from many access networks and deliver this traffic toa long-haul network. Metro networks cover the distances in the range of tens of miles and carrytraffic at the range from hundreds of Gbit/s to hundreds of Tbit/s (terabits per second). Long-haul networks aggregate traffic from many metro networks and carry this traffic over longdistances. These networks cover the distances from hundreds to thousands of miles and carrytraffic at the range from hundreds of Tbit/s to hundreds of Pbit/s (petabits per second).

• Because the development of the long-haul and metro networks has advanced rapidly and theircapacity much exceeds demand, tremendous pressure now falls in the local networks to providecustomers with access to the global telecom infrastructure. Building a broadband access networkenabling fast delivery of high-volume traffic is the current task of network operators.

Introduction

Long-distance, metro and access networks - Classification


Economy dictates how technology will develop Key issues for optical networks:

The main problem is reducing the cost per bit per transmitted

kilometer Solutions:

•Putting more bandwidth on fiber

•Increasing channel bit rate up to 100 Gb/s

•Use of WDM technology and increase the number of channels per fiber

•Use Forward Error Correction (FEC)

•Use new modulation formats based on differential phase-shift keying (DPSK)

•Use new technologies (fiber and other components)

•Sharing equipment among many channels

•Developing un-regenerated transmission systems

•Developing all-optical (without O/E/O conversion) networks

Introduction:

Optical communications today – economy and technology


Physics of light: A brief overview

• Fiber-optic communications technology uses light as a signal carrier. Light is electromagnetic radiation. To understand the basics of the technology, we need to examine various aspects of this type of radiation. Therefore, we will consider this phenomenon from three vantage points: as electromagnetic (EM) waves (the wave view), as a ray, or beam (the geometric optics view), and as a stream of photons (the quantum view). This section briefly reviews these basic concepts of light theory.



Electromagnetic waves

• Our concern in this subsection is light as electromagnetic waves. (See Figure 2.1.)

• Let’s consider light propagation in free space, a vacuum. Here both vectors--electric E and magnetic H--are perpendicular to the direction of propagation, which is the z axis in Fig. 2.1. Only the electric component of the electromagnetic wave is shown in Fig. 2.1, but what follows is also true for the magnetic component of the wave. Observe in particular that the electromagnetic wave is developing with respect to both time and space simultaneously.

• This brings us to two important concepts: wavelength and period. Wavelength is the distance between two identical points (the points having the same phase) of two successive cycles of a wave. Period is the time it takes a wave’s two identical points (the points having the same phase) to pass the same space location.

• The wavelength and the period of the wave are related through wave velocity. Period T is the time it takes a wave to travel a distance equal to one wavelength λ at velocity c. On the other hand, a wavelength λ is the distance traveled by a wave per one period T at velocity v. Therefore, light velocity is equal to wavelength divided by period, c = λ/T. Since the frequency, f (Hz) is equal to 1/T(s), we thus arrive at the very well-known and important formula:

• c = λ f , (2.1)

• where: c = speed of light (m/s), λ = wavelength (m), f = frequency (Hz).



Electromagnetic waves

The speed of light in free space is c = 3 x 108 m/s and the central frequency of visible light is

about 6x1014 Hz; hence, the center of visible wavelengths has the order of value λ = 0.5x10-6

meters or 0.5 micrometers (μm). Fiber-optic communications technology usually measures

wavelength in nanometers (nm); a nanometer is 10-9 m; thus, this wavelength is 500 nm.

Light is electromagnetic waves that occupy a specific range of the electromagnetic spectrum.

Scientifically, the word light means visible and invisible ultraviolet and infrared radiation.

The visible spectrum includes wavelengths between 400 nm and 700 nm, although there are

no hard and fast boundary limits this region. The different wavelengths represent different

colors. For example, green light has a wavelength of about 500 nm and red light has a

wavelength of about 650 nm.

Figure 2.1


• We know from our everyday experience that light can be treated as a beam (ray). The clearest example of this phenomenon is light radiated by a laser, which emerges in pinpoint fashion as a ray, or beam.

• Light rays propagate within different media at different velocities. It seems, too, as though different media resist light propagation with different intensities. The characteristic that describes this property of a medium is called the refractive index, or index of refraction. So, if v is the light velocity within the medium and c is the speed of light in free space, then the refractive index, n, can be determined by the following formula:

• n = c/v (2.2)

• The refractive indexes for some media are given in Table 2.1 .

• Material Air Water Glass Diamond

• Refractive Index 1.003 1.33 1.52-1.89 2.42

• Refractive indexes also indicate how much a light beam will be bent when the beam penetrate from one medium to the other. (See Figure 2.2.)


Beams (rays)



Beams (rays)

Figure 2.4

Snell’s law: Θ1 = Θ3 and n1 sin Θ1 = n2 sinΘ2, (2.3)

where Θ1 is the angle of incidence, Θ3 is the angle of reflection, and Θ2 is the angle of

refraction.


• Total internal reflection

These simple examples bring us to a very important point in our discussion: What happens if we increase the angle of incidence from glass to air? Fig. 2.5 demonstrates a sequence of the positions of the ray of light. The most important position is shown in Fig. 2.5c, where the angle of incidence, Θ1, reaches the critical value of Θ1C -- critical because no light penetrates the second medium (in this example, air.) The incident angle (Θ1C ) at which the angle of refraction (Θ2 ) = 900

is called the critical incident angle. If we continue to increase the angle of incidence so that Θ1 > Θ1C, all light will be reflected back into the incident medium. This situation is shown in Fig. 2.5d. This phenomenon is called total internal reflection because all light is reflected back to the medium of incidence.


Beams (rays)



Beams (rays)

Figure 2.5


Figure 2.7

That’s basically how optical fiber “conducts” light. Total internal reflection is a

necessary condition to make optical fiber work as a communications link.


Beams (rays)



Beams (rays)

Total internal reflection is what keeps light inside an optical fiber, as shown in figure

above. Without this effect, we could not use optical fiber as a light guide over a

distance, as shown in figure below.


Figure 2.8

Figure 2.9

Figure 2.10


• A photon

• What happens if an atom jumps from an upper level to a lower level, that is, from level E3 to level E2? There is an energy gap between these two levels, ΔE = E3 - E2, and this difference will be released in the form of a quantum of energy. This quantum of energy is called a photon. You can think of a photon as a particle--not a mechanical particle like a speck of dust, for instance, but as an elementary particle that carries a quantum of energy, Ep, and that travels with the speed of light, c. A photon’s energy, Ep, is defined as follows:

• Ep = hf, (2.4)

• where h is Planck’s constant (h = 6.626 x 10-34 J s) and f is the photon’s frequency.[1] Formula 2.4 introduces one of the fundamental concepts of modern physics: The energy of a photon, which is

• an elementary particle, depends on its frequency, which we always associate with waves.Remember, too, that the higher the photon’s frequency, the more energy it carries. That’s why x rays can penetrate our body but light cannot. (See Fig. 2.3, “The Electromagnetic Spectrum.”)

•Two questions concerning photons must be considered at this point: First , what is the nature

of a photon? Simply put, it is electromagnetic radiation. Secondly, if a photon’s frequency, f, is about 1014 Hz, what sort of electromagnetic radiation do we mean? Simply put again, it is light. (Again, see Fig. 2.3.) Hence, light is a stream of photons.


A stream of photons


• Radiation

• The next problem to consider is the relationship between a photon’s energy, Ep = hf, and the energy difference ΔE = E3 - E2 of the energy levels E3 and E2. You’ll recall that a photon was created when an atom jumped from E3 to E2 and released energy (E3 - E2 ). Therefore,

• Ep = ΔE = E3 - E2 (2.5)

• But Ep = hf; hence, hf = E3 - E2 and f = (E3 - E2)/h. On the other hand, λ = c/f. Therefore,

• λ = ch/( E3 - E2) (2.6)

• But the product c x h is the constant and the only variable in Formula 2.5 is the energy gap ΔE = E3 - E2. On the other hand, a stream of photons makes light; thus, we arrive at this very important conclusion: The wavelength (the color) of radiated light is determined by the energy levels of the radiating material.

• Atoms want to exist at the lowest possible energy levels; that’s the law of nature. To raise them to higher levels--and that is necessary to do for the atoms to be able to jump down to produce light radiation--we must energize them from an external source. Atoms absorb external energy, jump to the higher energy levels, and then drop to the lower levels, radiating photons--that is, light. The process of making atoms jump to the higher levels by feeding them external energy is called pumping.

• Fig. 2.9 demonstrates these processes. Keep in mind that this illustration is no more than a convenient model and is only a representation of the real pumping and radiation processes.

• Why atoms jump to the higher energy levels when they absorb energy from an external source is the next obvious question. Suppose an atom is at level E2 (Fig. 2.9). That means the atom possesses an energy value of E2. Now it absorbs external energy in the amount of ΔE = E4 - E2. Its new energy becomes equal to E2 + ΔE = E2 + E4 - E2 = E4. In reality, after absorbing external energy, atoms have a new energy value that we can demonstrate by placing them at different energy levels in our energy-level diagram. In other words, an energy-level diagram is a convenient model that helps us understand the pumping and radiation processes by enabling us to visualize them.


A stream of photons


• Absorption

• What happens if an external photon (light) strikes a medium? If its energy, Ep = hf, is equal to the energy gap, ΔE, the photon will be absorbed by an atom and the atom will jump to the appropriate higher level. If Ep is not equal to ΔE, the photon will pass by the material without interaction. Fig. 2.10 demonstrates both absorption and noninteraction processes.

• You know of course that sunlight is not as bright inside a room as it is outside. But why? It’s because the light is partially absorbed by the window and completely absorbed by the wall. But what happens to the energy that the sun’s photons transmit to the window and the wall? This light energy is absorbed by these objects, which become warmer as a result. Analyze your own everyday experience in terms of the absorption and noninteraction processes shown in Fig. 2.10.

• Optical fiber used as a communications link is made from a highly transparent material. That means a large majority of photons injected into the fiber by an LED or LD will travel through it with little if any interaction with the fiber material. But some impurities have energy gaps close to the energy level of the photons. And that poses a problem. The result of the interaction between the photons and the impurities is absorption and the loss of light power. (This issue is discussed in Chapter 3.)

• Another important point to note about absorption and noninteraction processes is conveyed in Fig. 2.10, which shows that we can use photons (light, remember) to pump atoms to upper energy levels.


A stream of photons


• All matter ultimately consists of atoms. Each atom, in turn, consists of a nucleus surrounded by electrons. For the sake of simplicity, you can use a solar (planetary) model of an atom, where electrons rotate on different orbits around the nucleus. The fact that the nucleus is approximately 105

times smaller than the whole atom might help you to visualize the model. Bear in mind that this model, developed by Niels Bohr at the dawn of the quantum physics era (1913), does not describe atomic properties as we understand them today, but the model facilitates a presentation of the basic ideas.

• Bohr’s model assumes that electrons rotate on stationary orbits and therefore possess a stationary value of energy. Bohr’s breakthrough was the assumption that rotating electrons do not radiate; that is, they do not change their energy value during rotation, as classic electromagnetic theory suggests. Any change in energy occurs only discretely, such as when electrons jump from one orbit to another. This implies that an entire atom possesses discrete values of energy; in other words, an atoms’s energy is quantized.

• We use energy-level diagram shown in Fig. 2.8 to visualize this concept. Possible discrete energy values (that is, what are allowed by the law of quantum physics) are always separated by energy gaps. The lowest energy level is called the ground state. An atom can be at any of these levels, or states, and it can change its energy only by jumping from one level to another; in other words, it can only change its energy level discretely. It should be emphasized that there can be no smooth transition between these states. An atom can have as its energy value, say, E2 or E3, but nothing between E2 and E3.


A stream of photons


• See reading assignment and homework problems in the course’s outline.

• After study this module you must be able to:– Sketch detailed block diagram of a fiber-optic communications system,

explain the function of its components and explain why we need to use light as a signal carrier.

– Explain why telecommunications plays more and more important role in the development of modern society.

– Explain the role of optical communications in telecommunications industry today and describe access, metro and long-haul optical networks.

– Explain the nature of light as electromagnetic waves and show the relationship among speed, wavelength and frequency of light.

– Explain nature of light as a beam (ray) and show the phenomenon of total internal reflection as related to optical fiber.

– Explain nature of light as a stream of photons and show radiation and absorption processes.

Module 1: Assignments

new york city college of technologywebsupport1.citytech.cuny.edu/faculty/mseip/files/ee/...d....

Documents