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Optical Fiber Communications

Objectives To discuss the importance of optical fiber communication To introduce optical fiber communication system To describe the principle of LED To describe the principle of laser To illustrate light propagation in optical fibers To explain total internal reflection To introduce the concept of numerical aperture To introduce the concept of modes in waveguides To study the transmission properties of optical fibers To explain the working principle of photodetector To study optical fiber communication system design

The objective of any communications system is the transfer of information from one point to another. Thisinformation transfer is accomplished most often by superimposing (modulating) the information onto an

electromagnetic wave (carrier). The modulated carrier is then transmitted (propagated) to the destination,where the electromagnetic wave is received and the information recovered (demodulated). Such systems

are often designated by the location of the carrier frequency in the electromagnetic spectrum. In radio

systems, the electromagnetic carrier wave is selected with a frequency from the radio frequency (RF)portion of the spectrum. In an optical fiber communication system, the carrier is selected from the optical

region, which includes the infrared and visible frequencies.

What are the advantages of optical fiber communication system?

1. Wide bandwidthIn any communication system, the amount of information transmitted is directly related to the

bandwidth of the modulated carrier. Thus increasing the carrier frequency increases the availabletransmission bandwidth, and therefore the information capacity of the overall system. The frequencies

in the optical range will have a usable bandwidth approximately 105 times that of a carrier in RF range.

2. Low lossOptical fibers have lower transmission losses than copper cables. In a copper cable, the attenuation

increases with modulation frequency: the higher the frequency, the greater the loss.

Bandwidth is an indication of the rate at which information can be sent.Loss indicates how far the information can be sent.

The combination of high bandwidth and low loss of optical fiber communication system means more

data can be sent over longer distances, thereby decreasing the number of wires and the repeaters

required. This reduction in equipment and components decreases the system cost and complexity.

The typical optical communications system essentially consists of a transmitter with a diode laser, areceiverwith aphoto-diode and an optical fibre between the two serving as the transmitting medium.

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Block diagram of the optical fiber communication system

An optical fiber communication system uses a digital communication scheme.

Optical sources

The fundamental function of optical source in optical fiber communications is to convert electrical energy

in the form of current into optical energy.The main optical sources currently used in optical fiber communications are lasers and light emitting

diodes (LEDs).

Light emitting diode (LED)

AnLED is essentially a semiconductor p-n junction under forward bias.In this condition, electrons cross the pn junction from the n-type material and recombine with holes in the

p-type material. When recombine takes place, the recombining electrons release energy in the form of heatand light. A large exposed surface area on one layer of the semiconductive material permits thephoton to

be emitted as visible light. This process is called electroluminescence. Various impurities are added during

the doping process to establish the wavelength of the emitted light. The wavelength determines the color of

the light and if it is visible or invisible (infrared).

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In the energy diagram it can be seen that, the major carriers from both sides of the p-n junction are injected

to the other side where they become minority. The relatively large local minority carrier population close to

the junction leads to a minority carrier concentration gradient. As a consequence, the excess minority

carriers will diffuse away from the junction recombine with carriers and emit photons.

LEDs are made of gallium arsennide (GaAs), gallium arsenide phosphide (GaAsP), orgallium phosphide

(GaP). Silicon and germanium are not used because they are essentially heat-producing materials and arevery poor at producing light. GaAs LEDs emit infrared (IR) radiation, which is nonvisible, GaAsP

produces either red or yellow visible light, and GaP emits red or green visible light. LEDs that emit blue

light are also available. Red is the most common.

In LED, the dominant photon generation is spontaneous emission in which the electron drops to the lowerenergy level in an entirely random way. The output spectrum of an LED is relatively wide.

Laser

The most frequently used light source in optical communication systems is laser.

Why a laser as the light source?(a) monochromatic: suitable for elimination of white noise

(b) coherent: suitable for synchronous detection;

(c) high power: improves signal to noise ratio;

(d) small divergence: improves efficiency of transmission; and(e) small source size: suitable for use with optical fibres

Laseris an acronym forlight amplification by stimulated emission of radiation.In general, an atomic system is characterized by discrete energy level, and the constituent atoms can exist

in one of the allowed energy levels or states. The atoms can make upward or downward transitions between

any two allowed states by absorbing or releasing, respectively, an amount of energy equal to the difference

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between the two energy levels. Thus, if we consider two levels of an atomic system that participate in aninteraction with optical radiation of photon energy

h = E2 E1where h is Plancks constant, then there could be three types of interactions:

Absorption

An atom in a lower energy state can absorb photons and make an upward transition to the higher energylevel.

Spontaneous emission

An atom in an excited level can make a downward transition spontaneously (i.e., on its own) by emitting a

photon corresponding to the energy difference between the two levels.

Stimulated emission

An atom in an excited level can make a downward transition in the presence of external stimulation by

emitting a photon corresponding to the energy difference between the two levels. The emitted photon is inphase with the incident photon.

ExampleSuppose you use an LED whose energy gap equals 2.5 eV. What is its radiating wavelength?Solution 1 eV = 1.602 10-19 J

Since the energy gap is the photon energy Ep, and Ep = h = hc /, then ashc = 6.62610-34 Js 3108 m/s 2010-26 mJEp = 2.5 eV = 2.5 1.60210

-19 410-19 J

We have = hc /Ep = 2010-26 mJ / 4H10-19 J = 5H10-7 m = 500 nm

What is the condition for light amplification by stimulated emission?

Population inversion

To achieve light amplification, it is necessary that the population of the upper energy level is greater thanthat of the lower energy level, this condition is known as population inversion.

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How to realize population inversion?

Population inversion is achieved by pumping techniques.Pumpingis to excite atoms into the upper energylevel and hence obtain a nonequilibrium distribution by the use of external source.

Optical feedback and laser oscillationIf the population inversion is achieved, light amplification occurs when a photon colliding with an atom in

the excited energy state causes the stimulated emission of a second photon and then both these photons

release two more. When the electromagnetic wave associated with these photons are in phase, amplified

emission is obtained. A positive feedback mechanism has to be provided in the amplifying medium toincrease the net gain and achieve a laser light output.

Totalreflector

Partialreflector

Some light escapes

Some light escapesThe rest is fed back

Eventually, equilibriumis established

Growth of stimulated emission in a resonant laser cavity

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Oscillations occur in the laser cavity over a small range of frequencies where the cavity gain is sufficient toovercome the losses incurred in the amplifying medium. Hence the laser source is not monochromatic. The

spectral emission from the laser source lies within the frequency range dictated by the gain curve.

The type of laser most frequently used in optical communication systems is the semiconductor diode laser,or the fibre laser.

Why semiconductor diode lasers?

(a) They are excited conveniently: by injecting current;

(b) Their operating wavelengths suffer low attenuation in fibres: small loss;(c) Their size is compatible with fibre dimensions: improved coupling;

(d) They work continuously at room temperature;

(e) They do not have moving parts: easier to use, no maintenance;

(f) They are highly efficient;(g) They have a very long life; and

(h) They are relatively cheap.

The laser diode commonly used in optical fiber communication systems is single mode, emits light with a

wavelength of around 1.55 m, within a bandwidth of 0.2 nm. The light emitting area is of the order of

several m2. Pe