EC2402 - OPTICAL COMMUNICATION AND NETWORKING

GENERAL COMMUNICATION SYSTEM

OPTICAL COMMUNICATION SYSTEM

GOOD COMMUNICATION NEEDS

(1)Bandwidh (BW) [Bandwidth of a system is more or less proportional to the frequency of operation, use of higher frequency facilitates larger BW]

(2)Good signal to noise ratio (SNR) i.e. low loss.

UNIT I INTRODUCTION SYLLABUS

Introduction Ray theory transmission Total internal reflection Acceptance angle Numerical aperture Skew rays Phase and group velocity cylindrical fibers SM fibers

INTRODUCTIONOptical Communication is the most modern mode of wired communication.Optical communication is also the youngest mode of communication. However its capabilities supersede all other modes of communication.Before optical communication, the most of the communication was in radio and microwave domain which has lower frequency range than the optical.

ELECTRO MAGNETIC SPECTRUM

TRANSMISSION MEDIA ALTERNATIVE TO THE OPTICAL COMMUNICATIONThere are various wired and wireless media used for long and short distance communication. Their broad characteristics are summarized in the following.

The first two media have a very limited bandwidth.

Microwave links and Satellite communication has comparable bandwidths as in principle their mode of operation is same but the spatial reach of satellite is far greater. Before Fiber optic communication became viable, satellite communication was the only choice for long distance communication.

Medium / Link CarrierInformation CapacityCopper Cable(short distance)1 MHz1 MbpsCoaxial Cable (Repeater every 4.5 km)100 MHz140 Mbps (BSNL)UHF Link2 GHz8 Mbps (BSNL), 2 Mbps (Rly.)MW Link(Repeater every 40 km)7 GHz140 Mbps (BSNL), 34 Mbps (Rly.)OFC 1550 nm2.5 Gbps(STM-16 Rly.)10 Gbps (STM-64)1.28 Tbps (128 Ch. DWDM)20 Tbps (Possible)

Frequency Vs Attenuation In Various Types of Cable More information carrying capacity fibbers can handle much higher data rates than copper. More information can be sent in a second

ADVANTAGES OF OPTICAL FIBRE

ThinnerLess ExpensiveHigher Carrying CapacityLess Signal Degradation& Digital SignalsLight SignalsNon-FlammableLight Weight

ADVANTAGES OF FIBER OPTICS Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiberImmunity to Noise - Immune to electromagnetic interference (EMI).Safety - Doesnt transmit electrical signals, making it safe in environments like a gas pipeline.High Security - Impossible to tap into.

ADVANTAGES OF FIBER OPTICS Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation)Reliability - More resilient than copper in extreme environmental conditions.Size - Lighter and more compact than copper.Flexibility - Unlike impure, brittle glass, fiber is physically very flexible.

FIBER OPTIC ADVANTAGES*greater capacity (bandwidth up to 2 Gbps, or more)smaller size and lighter weightlower attenuationimmunity to environmental interferencehighly secure due to tap difficulty and lack of signal radiation

DISADVANTAGES OF FIBER OPTICS Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers)

Splicing fiber optic cable is also more difficult.

FIBER OPTIC DISADVANTAGESexpensive over short distancerequires highly skilled installersadding additional nodes is difficult*

AREAS OF APPLICATIONTelecommunicationsLocal Area NetworksCable TVCCTVOptical Fiber Sensors

ATTENUATION HISTORY

FIRST WINDOW

Initially in early 1970s due to technology limitation, the optical fiber had a low loss window around 800nm. Also the semiconductor optical sources were made of GaAs which emitted light at 800nm. Due to compatibility of the medium properties and the sources, the optical communication started in 800nm band so called the First window.

SECOND WINDOW As the glass purification technology improved, the true silica loss profile emerged in 1980s. The loss profile shows two low loss windows, one around 1300nm and other around1550nm. In 1980s the optical communication shifted to 1300nm band , so called the Second Window' . Support the highest data rate due to lowest dispersion.

THIRD WINDOW

In 1990s the communication was shifted to 1550nm window, so called Third Window' due to invention of the Erbium Doped Fiber Amplifier (EDFA). The EDFA can amplify light only in a narrow band around 1550nm. Also this window has intrinsically lowest loss of about 0.2 dB/Km . This band has higher dispersion, meaning lower bandwidth. However, this problem has been solved by use of so called dispersion shifted fibers'.

BASICS OF LIGHTICharacteristics of light areIntensity (Power per unit solid angle)Wavelength (Color)Spectral width ( purity of color)Polarization - Linear - Circular - Elliptical

The first three parameters are scalar characteristics of light whereas the last parameter, polarization, describes the vector nature of light.

PHYSICS OF LIGHTPhotons (light particles)light represented by tiny bundles of energy (or quanta), following straight line paths along the rays.

PHYSICS OF LIGHTPLANCKS LAWEp =hf

Where,

Ep energy of the photon (joules)h = Plancks constant = 6.625 x 10 -34 J-s f frequency o f light (photon) emitted (hertz)

INDEX OF REFRACTION

SNELLS LAW

When light travels from one medium to other, it gets refracted. The relation between the angle of incidence and the angle of refraction is given by the Snell's law.

SNELLS LAW

EXAMPLE:Let medium 1 be glass ( n1 = 1.5 ) and medium 2 by ethyl alcohol (n2 = 1.36 ). For an angle of incidence of 30, determine the angle of refraction.

Answer: 33.47

SNELLS LAW

OPTICAL FIBEROptical fiber consists of a core, cladding and a protective outer coating, which guides light along the core by total internal reflection.

OPTICAL FIBER CONSTRUCTIONCore thin glass center of the fiber where light travels.

Cladding outer optical material surrounding the core

Buffer Coating plastic coating that protects the fiber.

OPTICAL FIBERThe core, and the lower- refractive index cladding, are typically made of high-quality silica glass, though they can both be made of plastic as well.

FIBER OPTIC LAYERSconsists of three concentric sections*

FIBER OPTIC CABLE*

FIBER-OPTIC CABLEContains one or several glass fibers at its core

Surrounding the fibers is a layer of glass called cladding

3 TYPES OF OPTICAL FIBERSPlastic core and claddingGlass core with plastic cladding ( called PCS fiber- Plastic Clad Silica )Glass core and glass cladding ( called SCS - Silica Clad Silica ) 3 TYPES OF OPTICAL FIBER

PROPAGATION OF LIGHT IN AN OPTICAL FIBER

1. Optical fiber is basically a solid glass rod. The diameter of rod is so small that it looks like a fiber.2.Optical fiber is a dielectric waveguide. The light travels like an electromagnetic wave inside the waveguide. The dielectric waveguide is different from a metallic waveguide which is used at microwave and millimeter wave frequencies.

3. In a metallic waveguide, there is a complete shielding of electromagnetic radiation but in an optical fiber the electromagnetic radiation is not just confined inside the fiber but also extends outside the fiber.4. The light gets guided inside the structure, through the basic phenomenon of total internal reflection .

WORKING PRINCIPLE

Total Internal ReflectionWhen a ray of light travels from a denser to a rarer medium such that the angle of incidence is greater than the critical angle, the ray reflects back into the same medium this phenomena is called total internal reflection.In the optical fiber the rays undergo repeated total number of reflections until it emerges out of the other end of the fiber, even if the fiber is bent.r is bent.f the fiber is bent.r is bent.

TOTAL INTERNAL REFLECTION IN FIBER

CRITICAL ANGLE, CThe minimum angle of incidence at which a light ray ay strike the interface of two media and result in an angle of refraction of 90 or greater.

ACCEPTANCE ANGLE /CONE HALF-ANGLEThe maximum angle in which external light rays may strike the air/glass interface and still propagate down the fiber.

ACCEPTANCE ANGLE /CONE HALF-ANGLE

ACCEPTANCE ANGLE /CONE HALF-ANGLE

in (max) = sin-1

Where,in (max) acceptance angle (degrees)n1 refractive index of glass fiber core (1.5)n2 refractive index of quartz fiber cladding ( 1.46 )

NUMERICAL APERTURE (NA)Used to describe the light-gathering or light-collecting ability of an optical fiber. In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light

NUMERICAL APERTURE (NA)The numerical aperture in respect to a point P depends on the half-angle of the maximum cone of light that can enter or exit the lens.

CLASSIFICATION OF OPTICAL FIBER

Optical fiber is classified into two categories based on :-1) The number of modes, and2) The refractive index

On the basis of number of modes 1.Single mode fiber (SMF)

2.Multi-mode fiber (MMF)

SINGLE-MODE FIBERSIn single mode fiber only one mode can propagate through the fiber. This type of fiber has small core diameter(5um) and high cladding diameter(70um) and the difference between the refractive index of core and cladding is very small. There is no dispersion i.e. no degradation of signal during travelling through the fiber.The light is passed through the single mode fiber through laser diode.

MULTI-MODE FIBERMulti mode fiber allows a large number of modes for the light ray travelling through it.The core diameter is (40um) and that of cladding is(70um)

MULTI-MODE FIBERThe relative refractive index difference is also larger than single mode fiber.There is signal degradation due to multimode dispersion.They are not suitable for long distance communication due to large dispersion and attenuation of the signal.

REFRACTIONRefraction is the changing direction of light when it goes into a material of different density.

On the basis of Refractive index

There are two types of optical fiber:-(i) Step-index optical fiber(ii) Graded-index optical fiber

STEP INDEX FIBERThe refractive index of core is constantThe refractive index of cladding is also constantThe light rays propagate through it in the form of meridiognal rays which cross the fiber axis during every reflection at the core cladding boundary.

GRADED INDEX FIBER

In this type of fiber core has a non uniform refractive index that gradually decrease from the centre towards the core cladding interface.The cladding has a uniform refractive index.The light rays propagate through it in the form of skew rays or helical rays. They do not cross the fiber axis at any time.

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Optical Fiber Communication System

FIBER-OPTIC COMMUNICATIONThe process of communicating using fiber-optics involves the following basic steps: Creating the optical signal using a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, and receiving the optical signal and converting it into an electrical signal.

Information source- dataElectrical transmitter- converts data to electrical signalOptical source- converts electrical signal to optical signal. It may be a semiconductor laser or an LED.

Optical cable: It serves as transmission medium.Receiver : Optical detector - optical to electrical conversion of data and hence responsible for demodulation of the optical carrier. It may be a photodiodes, phototransistor, and photoconductors.

Electrical receiver: It is used for electrical interfacing at the receiver end of the optical link and to perform the signal processing electrically.Destination: It is the final point at which we receive the information in the form of electrical signal.

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