Optical Communication

Communication by LightCommunication system with light as the carrier and fiber as communication medium

Propagation of light in atmosphere impractical:the great attenuation of the light due to atmospheric effects such as water vapor, oxygen, particles.

Optical fiber is used, glass or plastic, to contain and guide light waves

CapacityMicrowave at 10 GHz with 10% utilization ratio: 1 GHz BWLight at 100 Tera Hz (1014 ) with 10% utilization ratio:100 THz (10,000GHz)

History1880 Alexander Graham Bell

1930 Patents on tubing

1950 Patent for two-layer glass wave-guide

1960 Laser first used as light source

1965 High loss of light discovered

1970s Refining of manufacturing process

1980s OF technology becomes backbone of long distance telephone networks in NA.

Optical FiberFibers of glass

Usually 120 micrometers in diameter

Used to carry signals in the form of light over distances up to 50 km.

No repeaters needed.

Core 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 Fiber: AdvantagesCapacity: much wider bandwidth (10 GHz)

Crosstalk immunity

Immunity to static interference

Safety: Fiber is nonmetalic

Longer lasting (unproven)Security: tapping is difficult

Economics: Fewer repeaters

Non-Flammable

Light Weight

Optical Fiber: Disadvantageshigher initial cost in installation

Interfacing cost

Strength: Lower tensile strength

Remote electric power

More expensive to repair/maintainTools: Specialized and sophisticated

Areas of ApplicationTelecommunications

Local Area Networks

Cable TV

CCTV

Optical Fiber Sensors

Fiber Optic Communication SystemInputSignalCoder orConverterLightSourceSource-to-FiberInterfaceFiber-to-lightInterfaceLightDetectorAmplifier/ShaperDecoderOutputFiber-optic CableTransmitterReceiver

Fiber Optic Communication SystemThe information signal to be transmitted may be voice, video, or computer data.

The first step is to convert the information into a form compatible with the communications medium.

These digital pulses are then used to flash a powerful light source off and on very rapidly.

Fiber Optic Communication SystemLight source: LED or ILD (Injection Laser Diode):amount of light emitted is proportional to the drive current

Source-to-fiber-coupler (similar to a lens):A mechanical interface to couple the light emitted by the source into the optical fiber

Light detector:PIN (p-type-intrinsic-n-type) or APD (avalanche photo diode) both convert light energy into current

Fiber-Optic CableThe fiber, which is called the core, is usually surrounded by a protective cladding. The cladding is also made of glass or plastic but has a lower index of refraction. This ensures that the proper interface is achieved so that the light waves remain within the core. In addition to protecting the fiber core from nicks and scratches, the cladding adds strength. Some fiber optic cables have a glass core with a glass cladding. Others have a plastic core with a plastic cladding. Another common arrangement is a glass core with a plastic cladding. It is called plastic clad silica (PCS) cable.

Fiber-Optic Cable

Characteristics and Behavior of LightLight waves travel in a straight line like microwaves do.

Like electricity, these light rays travel at the speed of light, which is generally considered to be 300,000,000 m/s in free space.

The speed of light depends upon the medium through which the light passes.

When light passes through another material such as glass, its speed is slower.

Total Internal Reflection

Total Internal Reflection in FiberTotal internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than the critical angle with respect to the normal to the surface.

If the refractive index is lower on the other side of the boundary no light can pass through, so effectively all of the light is reflected.

The critical angle is the angle of incidence above which the total internal reflection occurs.

Fiber-Optic CableOptical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.

Fiber-Optic CableGlass has superior optical characteristics over plastic.

Although plastic is less expensive and more flexible, its attenuation of light is greater.

For a given intensity, light will travel a greater distance in glass than in plastic.

For very long distance transmissions, glass is certainly preferred.

For shorter distances, plastic is much more practical.

Type of FibersOptical fibers come in two types:

Single-mode fibers used to transmit one signal per fiber (used in telephone and cable TV). They have small cores(9 microns in diameter) and transmit infra-red light from laser.

Multi-mode fibers used to transmit many signals per fiber (used in computer networks). They have larger cores(62.5 microns in diameter) and transmit infra-red light from LED.

Types Of Optical Fiber

Single-mode step-index FiberAdvantages:Minimum dispersion: all rays take same path, same time to travel down the cable. A pulse can be reproduced at the receiver very accurately.Less attenuation, can run over longer distance without repeaters.Larger bandwidth and higher information rate

Disadvantages:Difficult to couple light in and out of the tiny coreHighly directive light source (laser) is required.Interfacing modules are more expensive

Multimode step-index FibersMultimode step-index Fibers:inexpensive; easy to couple light into Fiberresult in higher signal distortion; lower TX rate

It is also the easiest to make and, therefore, the least expensive.

It is widely used for short to medium distances at relatively low pulse frequencies.

The main advantage of a multimode step-index fiber is the large size. Typical core diameters are in the 50- to 1000 m range. large diameter cores are excellent at gathering light and transmitting it efficiently.

Multimode step-index Fibers

This means that an inexpensive light source such as an LED can be used to produce the light pulses.

The light takes many hundreds or even thousands of paths through the core before exiting.

Disadvantage: Modal Dispersion

Multimode graded-index FiberMultimode graded-index Fiber:intermediate between the other two types of Fibers

Multimode graded index fiber cables have several modes or paths of transmission through the cable, but they are much more orderly and predictable.

Because of the continuously varying index of refraction across the core, the light rays are bent smoothly and converge repeatedly at points along the cable.

The light rays near the edge of the core take a longer path but travel faster since the index of refraction is lower.

All the modes or light paths tend to arrive at one point simultaneously. The result is that there is less modal dispersion.

Multimode graded-index Fiber

It is not eliminated entirely, but the output pulse is not nearly as stretched as in multimode step index cable.

The output pulse is only slightly elongated. As a result, this cable can be used at very high pulse rates and, therefore, a considerable amount of information can be carried on it.

This type of cable is also much wider in diameter with core sizes in the 50 to 100 m range.

Therefore, it is easier to splice and interconnect, and cheaper, less intense light sources may be used.

Fiber types

Acceptance Cone & Numerical ApertureAcceptance angle, qc, is the maximum angle in which external light rays may strike the air/Fiber interface and still propagate down the Fiber with

Bandwidth & Power BudgetThe maximum data rate R (Mbps) for a cable of given distance D (km) with a dispersion d (ms/km) is:R = 1/(5dD)

Power or loss margin, Lm (dB) is:Lm = Pr - Ps = Pt - M - Lsf - (DxLf) - Lc - Lfd - Ps 0

where Pr = received power (dBm), Ps = receiver sensitivity(dBm), Pt = Tx power (dBm), M = contingency loss allowance (dB), Lsf = source-to-Fiber loss (dB), Lf = Fiber loss (dB/km), Lc = total connector/splice losses (dB), Lfd = Fiber-to-detector loss (dB).

Losses In Optical Fiber CablesThe predominant losses in Optical Fibers are:

absorption losses due to impurities in the Fiber material

material or Rayleigh scattering losses due to microscopic irregularities in the Fiber

chromatic or wavelength dispersion because of the use of a non-monochromatic source

radiation losses caused by bends and kinks in the Fiber

modal dispersion or pulse spreading due to rays taking different paths down the Fiber

coupling losses caused by misalignment & imperfect surface finishes

Absorption Losses In Optic Fiber

Cable Attenuation The main specification of a fiber-optic cable is its attenuation.

Attenuation refers to the loss of light energy as the light pulse travels from one end of the cable to the other.

The intensity of the light at the output is lower because of various losses in the cable. The main reason for the loss in light intensity over the length of the cable is due to light absorption, scattering, and dispersion.

Cable AttenuationAbsorption refers to how the light waves are actually "soaked up" in the core material due to the impurity of the glass or plastic.

Scattering refers to the light lost because of light waves entering at the wrong angle and being lost in the cladding due to refraction.

Dispersion refers to the pulse stretching caused by the many different paths through the cable.Although no light is lost as such in dispersion, the output is still lower in amplitude than the input but the length of the light pulse has increased in duration.

DispersionThere are two different types of dispersion in optical fibers.

Intramodal, or chromatic, dispersion occurs in all types of fibers.

Intermodal, or modal dispersion occurs only in multimode fibers.

Each type of dispersion mechanism leads to pulse spreading. As a pulse spreads, energy is overlapped. The spreading of the optical pulse as it travels along the fiber limits the information capacity of the fiber.

Intramodal dispersion Intramodal dispersion occurs because different colors of light travel through different materials and different waveguide structures at different speeds.

There are 2 types:Material dispersion occurs because the spreading of a light pulse is dependent on the wavelengths' interaction with the refractive index of the fiber core. Different wavelengths travel at different speeds in the fiber material. Different wavelengths of a light pulse that enter a fiber at one time exit the fiber at different times. Material dispersion is a function of the source spectral width. The spectral width specifies the range of wavelengths that can propagate in the fiber. Material dispersion is less at longer wavelengths.

Waveguide dispersion occurs because the mode propagation constant is a function of the size of the fiber's core relative to the wavelength of operation. Waveguide dispersion also occurs because light propagates differently in the core than in the cladding.

Intermodal dispersionIntermodal or modal dispersion causes the input light pulse to spread. The input light pulse is made up of a group of modes. As the modes propagate along the fiber, light energy distributed among the modes is delayed by different amounts.

Modal dispersion occurs because each mode travels a different distance over the same time span. The modes of a light pulse that enter the fiber at one time exit the fiber a different times. This condition causes the light pulse to spread. As the length of the fiber increases, modal dispersion increases. It is the dominant source of dispersion in multimode fibers. It does not exist in single mode fibers. Single mode fibers propagate only the fundamental mode. single mode fibers exhibit the lowest amount of total dispersion. Single mode fibers also exhibit the highest possible bandwidth.

Cable AttenuationThe amount of attenuation, of course, varies with the type of cable and its size.

Glass has less attenuation than plastic

Wider cores have less attenuation than narrower cores. The attenuation is directly proportional to the length of the cable.

Doubling the length of a cable doubles the attenuation, and so on.

The attenuation of a fiber optic cable is expressed in decibels per unit of length. decibels per kilometer

The standard decibel formula used isdB = 10 log (Po/Pi) where Po is the power out and Pi is the power in.

Cable AttenuationThe attenuation ratings of fiber optic cables varies over a considerable range.The finest single mode step index cables have an attenuation of only 1 dB/km. very large core plastic fiber cables can have an attenuation of several thousand decibels per kilometer.

A typical cable might have a standard loss of 10 to 20 dB/km. Typically, those fibers with an attenuation of less than 10 dB/km are called low-loss fibers, while those with an attenuation of between l0 and 100 dB/km are medium-loss fibers. High-loss fibers are those with over 100 dB/km ratings.

Cable AttenuationIf a cable has an attenuation of 15 dB/km. then a 5-km length cable has a total attenuation of 5(15) = 75 dB.

If two cables are spliced together and one has an attenuation of 17 dB and the other 24 dB, the total attenuation is simply the sum, or 17 + 24 = 41 dB.

When long fiber optic cables are needed, two or more cables may be spliced together. The ends of the cable are perfectly aligned and then glued together with a special, clear, low-loss epoxy.

Connectors are also used. A variety of connectors provide a convenient way to splice cables and attach them to transmitters, receivers, and repeaters.

Generic Optical Comm. SystemOptical TransmitterComm. ChannelOptical ReceiverOutputElectrical Signal InputModulation CharacteristicsPowerWavelengthLossDispersionNoiseCrosstalksBandwidthResponsivitySensitivityNoiseWavelength

FormatBandwidthProtocol

Considerations:Wavelength: 0.85, 1.3, 1.55, DWDMTransverse mode: SM vs. MMModulation: Direct vs. external vs. integrated modulator

Considerations:Wavelength: 0.85, 1.3, 1.55, DWDMTransverse mode: SM vs. MMDispersionLoss

Optical TransmittersConventional light sources such as incandescent lamps cannot be used in fiber optic systems. The reason for this is that they are simply too slow. An incandescent light source consists of a filament that heats up and emits light.Such a light source cannot be turned off and on fast enough because of the thermal delay in the filament.In order to transmit high speed digital pulses, a very fast light source must be used.

The two most commonly used light sources are LEDsSemiconductor lasers.

Light sourceLight-Emitting Diodes (LED)made from material such as AlGaAs or GaAsPlight is emitted when electrons and holes recombineeither surface emitting or edge emitting

Injection Laser Diodes (ILD)similar in construction as LED except ends are highly polished to reflect photons back & forth

ILD versus LEDAdvantages:more focused radiation pattern; smaller Fibermuch higher radiant power; longer span faster ON, OFF time; higher bit rates possiblemonochromatic light; reduces dispersion

Disadvantages:much more expensivehigher temperature; shorter lifespan

Light Emitting Diode (LED)LED is a PN junction semiconductor device that emits light when forward biased. When a free electron encounters a hole in the semiconductor structure, the two combine, and in the process they give up energy in the form of light.

Most LEDs are GaAs devices optimized for producing red light.

LED is a fast semiconductor device, it can be turned off and on very quickly. Therefore, it is capable of transmitting the narrow light pulses required in a digital fiber-optic system.

Can be designed to emit virtually any color of light desired. The LEDs used for fiber-optic transmission are usually in the red and low infrared ranges.

Typical wavelengths of LED light commonly used are 0.82, 0.94, 1.3, and 1.55 m (all in the near-infrared range just below red light). The light is not visible to the naked eye. have been chosen primarily because most fiber-optic cables have the lowest losses in these wavelength ranges.

Light Emitting Diode (LED)Special LEDs are made just for fiber optic applications.

These units are made of GaAs indium phosphide (GaAsInP) and emit light at 1.3 m. many fiber-optic cables have minimum loss at that frequency.

They come with a fiber-optic "pigtail" already attached for optimum coupling of light. The pigtail usually has a connector that attaches to the main cable.

There are two basic ways that digital data is formatted in fiber-optic systems.Return-to-zero (RZ) Non-return-to-zero (NRZ)

LED TransmitterThe Digital data to be transmitted is converted into a serial pulse train, encoded and then applied to light transmitter.

The light transmitter consists of the LED and its associated driving circuitry.

The binary pulses are applied to a logic gate which, in turn, operates a transistor switch Q1 that turns the LED off and on.

A positive pulse at the NAND gate input causes the NAND output to go to zero. This turns off Q1, so the LED is forward-biased through R2 and turns on. With zero input, the NAND output is 1, so Q1 turns on and shunts current away from the LED.

Injection Laser Diodes (ILD)At some current level, it will emit a brilliant light. The physical structure of the ILD is such that the semiconductor structure is cut squarely at the ends to form internal reflecting surfaces.

One of the surfaces is usually coated with a reflecting material such as gold. The other surface is only partially reflective.

When the diode is properly biased, the light will be emitted and will bounce back and forth internally between the reflecting surfaces.

The distance between the reflecting surfaces has been carefully measured so that it is some multiple of a half wave at the light frequency.

The bouncing back and forth of the light waves causes their intensity to reinforce and build up. The structure is like a cavity resonator for light. The result is an incredibly high brilliance, single-frequency light beam that is emitted from the partially reflecting surface.

Injection Laser Diodes (ILD)Another advantage ILDs have over LEDs is their ability to turn off and on at a faster rate. High-speed laser diodes are capable of gigabit per second digital data rates.

BW: >500MHz for ILD and

Optical ReceiversThe receiver part of the optical communications system is relatively simple.

It consists of a detector that will sense the light pulses and convert them into an electrical signal.

This signal is then amplified and shaped into the original serial digital data.

The most critical component, of course, is the light sensor.The most widely used light sensor is a photodiode. This is a silicon PN junction diode that is sensitive to light.

Light DetectorsPIN Diodesphotons are absorbed in the intrinsic layersufficient energy is added to generate carriers in the depletion layer for current to flow through the device

Avalanche Photodiodes (APD)photogenerated electrons are accelerated by relatively large reverse voltage and collide with other atoms to produce more free electronsavalanche multiplication effect makes APD more sensitive but also more noisy than PIN diodes

Multiplexing in Optical FibreDevelopments in optical components make it possible to use FDM on fiber-optic cable (called wavelength division multiplexing, or WDM), which permits multiple channels of data to operate over the cable's light-wave bandwidth.

WDM uses separate lasers to transmit serial digital data simultaneously on two or more different light wavelengths.

Current systems use light in the 1550-nm range. A typical four-channel system uses laser wavelengths of 1534, 1543, 1550, and 1557.4 nm.

Each laser is switched off and on with the desired data. The laser beams are then optically combined and transmitted over a single fiber cable.

Wavelength-Division MultiplexingWDM sends information through a single optical Fiber using lightsof different wavelengths simultaneously.LaserOptical sourcesl1l2lnln-1l3l1l2lnln-1l3LaserOptical detectorsOpticalamplifierMultiplexerDemultiplexer

WDM SystemWDM increases the carrying capacity of the physical medium (fiber) using a completely different method from TDM.

At the receiving end of the cable, special optical filters are used to separate the light-beams into individual channels.

Each light beam is detected with an optical sensor and then converted into the four individual data streams.

WDM significantly increases the data-handling capacity of fiber-optic cable.

When WDM multiplexer/demultiplexer units are added to existing systems, more data channels and/or higher data speeds can be accommodated.

Systems with 8. 16, or 32 channels are available.

WDM SystemWDM systems are divided in different wavelength patterns, conventional or coarse and dense WDM.

Conventional WDM systems provide up to 16 channels in the 3rd transmission window (C-band) of silica fibers around 1550 nm.

DWDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. Some technologies are capable of 25 GHz spacing (sometimes called ultra dense WDM).

Conventional WDM systems CWDM in contrast to conventional WDM and DWDM uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs. To again provide 16 channels on a single fiber CWDM uses the entire frequency band between second and third transmission window (1310/1550 nm respectively) including both windows (minimum dispersion window and minimum attenuation window) but also the critical area where OH scattering may occur, recommending the use of OH-free silica fibers in case the wavelengths between second and third transmission window shall also be used. Avoiding this region, the channels 31,49,51,53,55,57,59,61 remain and these are the most commonly used.

CWDM is also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310 nm while the upstream signal is at 1550 nm.