Optoelectronics.docx

Optoelectronics Optoelectronics is the study and application of electronic devices that source, detect and control light, usually considered a sub-field of photonics. In this context, light often includes invisible forms of radiation such as gamma rays, X-rays, ultraviolet and infrared, in addition to visible light. Optoelectronic devices are electrical-to-optical or optical-to-electrical transducers, or instruments that use such devices in their operation. 212.61 Explain that light energy generates electron-hole pairs in a semiconductor. When a photon of sufficient energy strikes the photo sensitive device, it excites an electron, thereby creating a free electron and a (positively charged electron) hole. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced.

212.62 Describe briefly the construction and operation of a range of photo sensitive devices. Devices: photo resistive cells (light-dependent resistors – LDR), photovoltaic cells (solar cells), photo diodes, photo transistors.

PHOTO RESISTIVE CELLS A photo resistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor. A photo resistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance. Applications Photo resistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks. They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.

The symbol for a photo resistor

PHOTOVOLTAIC CELLS A photovoltaic cell is a solid state device that uses the photovoltaic effect to generate electrical energy using the potential difference that arises between materials when the surface of the cell is exposed to electromagnetic radiation. A photovoltaic cell is commonly used for detecting radiation (for example, infrared detectors), measurement of light intensity (such as in measuring optical density), chemical processes (for example, spectrophotometry), and for conversion of light energy to electricity in conversion photovoltaic cells (when only the solar light is converted they are called solar cells). Photovoltaic cells convert sunlight into DC current and, at best, are about 30% efficient. Large arrays of photovoltaic cells exposed to sunlight are used to produce solar power.

Schematic symbol for a photovoltaic cell PHOTO DIODES

A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation. Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction. Principle of operation A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron and a (positively charged electron) hole. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. Applications Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions.

PHOTOTRANSISTORS Like diodes, all transistors are light-sensitive. Phototransistors are designed specifically to take advantage of this fact. The most-common variant is an NPN bipolar transistor with an exposed base region. Here, light striking the base replaces what would ordinarily be voltage applied to the base -- so, a phototransistor amplifies variations in the light striking it. Note that phototransistors may or may not have a base lead (if they do, the base lead allows you to bias the phototransistor's light response. For phototransistor selection and comparison information, see the phototransistor section of the BEAM Reference Library's BEAM Pieces collection. Note that photodiodes also can provide a similar function, although with much lower gain (i.e., photodiodes allow much less current to flow than do phototransistors). You can use this diagram to help you see the difference (both circuits are equivalent):

212.63 Compare the relative merits of a range of photo sensitive devices. Merits: cost, size, linearity, dynamic range, speed, temperature sensitive Devices: light dependent resistors, solar cells, photo diodes and transistors Note> to be given as a Research Assignment Light dependent Solar cells Photo diodes resistors Merits Cost Size Linearity Dynamic range Speed Temperature sensitive

Photo transistors

212.64 Describe typical applications of photo sensitive devices. Typical applications: light operated relay, power source (solar cell), light meter, light sensitive switch.

Typical application of a photo transistor switching on a relay

Typical application of a power source (solar cell) powering a bulb

This is a typical application of light sensitive switch.

212.65 Describe the emission of light generated by passing current through a forward biased junction (light emitting diode – LED). When a light-emitting diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

212.66 Determine, using manufacturers’ or suppliers’ information, details of commercially available light emitting diodes. Details: emission colour, forward voltage, maximum Current and power ratings, size. Color Wavelength Voltage (V) Semiconductor Material (nm) Infrared

λ > 760

ΔV < 1.9

Red

610 < λ < 760

1.63 < ΔV < 2.03

Orange

590 < λ < 610

2.03 < ΔV < 2.10

Yellow

570 < λ < 590

2.10 < ΔV < 2.18

Green

500 < λ < 570

1.9 < ΔV < 4.0

Blue

450 < λ < 500

2.48 < ΔV < 3.7

Violet

400 < λ < 450

2.76 < ΔV < 4.0

Purple

Multiple types

2.48 < ΔV < 3.7

Dual blue/red LEDs, blue with red phosphor, or white with purple plastic

Ultraviolet λ < 400

3.1 < ΔV < 4.4

White

ΔV = 3.5

Diamond (235 nm), Boron nitride (215 nm) Aluminium nitride (AlN) (210 nm) Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm) Blue/UV diode with yellow phosphor

Broad spectrum

Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs) Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP) Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate — (under development) ndium gallium nitride (InGaN)

212.67 Sketch circuits for the operation of various LEDs and calculate suitable component values from the information obtained in 212.66. Calculating an LED resistor value An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly. The resistor value, R is given by: R = (VS - VL) / I

R1 Vs

+ D1 LED1

VS = supply voltage VL = LED voltage (usually 2V, but 4V for blue and white LEDs) I = LED current (e.g. 10mA = 0.01A, or 20mA = 0.02A) Make sure the LED current you choose is less than the maximum permitted and convert the current to amps (A) so the calculation will give the resistor value in ohms (). To convert mA to A divide the current in mA by 1000 because 1mA = 0.001A. If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright. For example If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A, R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater). Working out the LED resistor formula using Ohm's law Ohm's law says that the resistance of the resistor, R = V/I, where: V = voltage across the resistor (= VS - VL in this case) I = the current through the resistor So R = (VS - VL) / I

Connecting LEDs in series If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED. All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for VL. Example calculations: A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal. VL = 2V + 2V + 2V = 6V (the three LED voltages added up). If the supply voltage VS is 9V and the current I must be 15mA = 0.015A, Resistor R = (VS - VL) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200, so choose R = 220 (the nearest standard value which is greater). Avoid connecting LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor. Avoid connecting LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

212.68 Describe briefly the construction and operation of an opto-isolator and state typical applications. Applications: completely isolated non-electrical coupling, switching high voltage loads from sensitive low voltage sources

Opto-isolator In electronics, an opto-isolator (or optical isolator, optical coupling device, optocoupler, photocoupler, or photoMOS) is a device that uses a short optical transmission path to transfer an electronic signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated—since the electrical signal is converted to a light beam, transferred, then converted back to an electrical signal, there is no need for electrical connection between the source and destination circuits. Isolation between input and output is rated at 7500 Volt peak for 1 second for a typical component costing less than 1 US$ in small quantities. The opto-isolator is simply a package that contains both an infrared light-emitting diode (LED) and a photodetector such as a photosensitive silicon diode, transistor Darlington pair, or silicon controlled rectifier (SCR). The wave-length responses of the two devices are tailored to be as identical as possible to permit the highest measure of coupling possible. Other circuitry—for example an output amplifier— may be integrated into the package. An opto-isolator is usually thought of as a single integrated package, but opto-isolation can also be achieved by using separate devices.

Configurations A common implementation is a LED and a phototransistor in a light-tight housing to exclude ambient light and without common electrical connection, positioned so that light from the LED will impinge on the photodetector. When an electrical signal is applied to the input of the opto-isolator, its LED lights and illuminates the photodetector, producing a corresponding electrical signal in the output circuit. Unlike a transformer the opto-isolator allows DC coupling and can provide any desired degree of electrical isolation and protection from serious overvoltage conditions in one circuit affecting the other. A higher transmission ratio can be obtained by using a Darlington instead of a simple phototransistor, at the cost of reduced noise immunity and higher delay.

Schematic diagram of a very simple opto-isolator with an LED and phototransistor. The dashed line represents the isolation barrier, over which there is no electrical contact.

With a photodiode as the detector, the output current is proportional to the intensity of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode. In photovoltaic mode, the diode acts as a current source in parallel

with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity. In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light. This optocoupler type is significantly faster than the photo transistor type, but the transmission ratio is very low; it is common to integrate an output amplifier circuit into the same package. The optical path may be air or a dielectric waveguide. When high noise immunity is required an optical conductive shield can be integrated into the optical path. The transmitting and receiving elements of an optical isolator may be contained within a single compact module, for mounting, for example, on a circuit board; in this case, the module is often called an optoisolator or opto-isolator. The photosensor may be a photocell, phototransistor, or an optically triggered SCR or TRIAC. This device may in turn operate a power relay or contactor. Analog optoisolators often have two independent, closely matched output phototransistors, one of which is used to linearize the response using negative feedback. Application Among other applications, opto-isolators can help cut down on ground loops, block voltage spikes, and provide electrical isolation. 





Switched-mode power supplies use optocouplers for mains isolation. As they work in an environment with much electrical noise and with signals which are not small, optocouplers with low transmission ratio are preferred. Where electrical safety is paramount, optocouplers can totally isolate circuitry (which may be touched by humans) from mains electricity. o Medical equipment often uses optocouplers. o One of the requirements of the MIDI (Musical Instrument Digital Interface) standard is that input connections be opto-isolated. o Oscilloscopes and digital multimeters with computer interfaces. Optocouplers are used to isolate low-current control or signal circuitry from transients generated or transmitted by power supply and high-current control circuits. The latter are used within motor and machine control function blocks.

A simple circuit with an opto-isolator. When switch S1 is closed, LED D1 lights, which triggers phototransistor Q1, which pulls the output pin low. This circuit, thus, acts as a NOT gate.

Slotted optical switch The slotted optical switch uses the same basic components as an opto-isolator, but instead of the optical beam being enclosed to protect the photodetector from ambient light, it is arranged so that an object can interrupt the beam. The photoemitter is always active, and the device changes its output state when the beam is blocked. The distinction between this switch and an opto-coupler is that the photoemitter input is fixed and the light path is manipulated, rather than vice versa. It is used to detect, for example, that the carriage of a printer has reached the end of its travel—the carriage has a projection which breaks the beam.

212.69 Describe the principle of light propagation along a fiber optic cable and explain the associated terms. Terms: step index, graded index Optical fiber Fiber optics is the field of applied science and engineering concerned with the design and application of optical fibers. An optical fiber is a thin, flexible, transparent fiber that acts as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication. Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. FIBER OPTIC CABLE ADVANTAGES OVER COPPER: • SPEED: Fiber optic networks operate at high speeds - up into the gigabits • BANDWIDTH: large carrying capacity • DISTANCE: Signals can be transmitted further without needing to be "refreshed" or strengthened. • RESISTANCE: Greater resistance to electromagnetic noise such as radios, motors or other nearby cables. • MAINTENANCE: Fiber optic cables costs much less to maintain. There are three types of fiber optic cable: single mode, multimode and plastic optical fiber (POF). Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns. (One micron is 1/250th the width of a human hair.) Multimode cable is made of multiple strands of glass fibers, with a combined diameter in the 50-to-100 micron range. Each fiber in a multimode cable is capable of carrying a different signal independent from those on the other fibers in the cable bundle. POF is a newer plastic-based cable which promises performance similar to single mode cable, but at a lower cost. While fiber optic cable itself is cheaper than an equivalent length of copper cable, fiber optic cable connectors and the equipment needed to install them are more expensive than their copper counterparts. Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. The light source can either be a lightemitting diode (LED)) or a laser.

The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ones and zeros of the original signals. Even laser light shining through a fiber optic cable is subject to loss of strength, primarily through dispersion and scattering of the light, within the cable itself. The faster the laser fluctuates, the greater the risk of dispersion. Light strengtheners, called repeaters, may be necessary to refresh the signal in certain applications.

Fiber Some 10 billion digital bits can be transmitted per second along an optical fiber link in a commercial network, enough to carry tens of thousands of telephone calls. Hair-thin fibers consist of two concentric layers of high-purity silica glass the core and the cladding, which are enclosed by a protective sheath. Light rays modulated into digital pulses with a laser or a light-emitting diode moves along the core without penetrating the cladding. The light stays confined to the core because the cladding has a lower refractive index—a measure of its ability to bend light. Refinements in optical fibers, along with the development of new lasers and diodes, may one day allow commercial fiber-optic networks to carry trillions of bits of data per second.

Total internal refection confines light within optical fibers (similar to looking down a mirror made in the shape of a long paper towel tube). Because the cladding has a lower refractive index, light rays reflect back into the core if they encounter the cladding at a shallow angle (red lines). A ray that exceeds a certain "critical" angle escapes from the fiber (yellow line).

STEP-INDEX MULTIMODE FIBER has a large core, up to 100 microns in diameter. As a result, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits bandwidth that is, the amount of information that can be sent. Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope, for instance.

GRADED-INDEX MULTIMODE FIBER contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Also, rather than zig-zagging off the cladding, light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: a digital pulse suffers less dispersion. These fibers often become the physical medium for local-area networks

SINGLE-MODE FIBER has a narrow core (eight microns or less), and the index of refraction between the core and the cladding changes less than it does for multimode fibers. Light thus travels parallel to the axis, creating little pulse dispersion. Telephone and cable television networks install millions of kilometers of this fiber every year.

BASIC CABLE DESIGN 1 - Two basic cable designs are: Loose-tube cable, used in the majority of outside-plant installations in North America, and tight-buffered cable, primarily used inside buildings. The modular design of loose-tube cables typically holds up to 12 fibers per buffer tube with a maximum per cable fiber count of more than 200 fibers. Loose-tube cables can be all-dielectric or optionally armored. The modular buffer-tube design permits easy drop-off of groups of fibers at intermediate points, without interfering with other protected buffer tubes being routed to other locations. The loose-tube design also helps in the identification and administration of fibers in the system. Single-fiber tight-buffered cables are used as pigtails, patch cords and jumpers to terminate loose-tube cables directly into opto-electronic transmitters, receivers and other active and passive components. Multi-fiber tight-buffered cables also are available and are used primarily for alternative routing and handling flexibility and ease within buildings. 2 - Loose-Tube Cable In a loose-tube cable design, color-coded plastic buffer tubes house and protect optical fibers. A gel filling compound impedes water penetration. Excess fiber length (relative to buffer tube length) insulates fibers from stresses of

installation and environmental loading. Buffer tubes are stranded around a dielectric or steel central member, which serves as an anti-buckling element. The cable core, typically surrounded by aramid yarn, is the primary tensile strength member. The outer polyethylene jacket is extruded over the core. If armoring is required, a corrugated steel tape is formed around a single jacketed cable with an additional jacket extruded over the armor. Loose-tube cables typically are used for outside-plant installation in aerial, duct and direct-buried applications.

3 - Tight-Buffered Cable With tight-buffered cable designs, the buffering material is in direct contat with the fiber. This design is suited for "jumper cables" which connect outside plant cables to terminal equipment, and also for linking various devices in a premises network. Multi-fiber, tight-buffered cables often are used for intra-building, risers, general building and plenum applications. The tight-buffered design provides a rugged cable structure to protect individual fibers during handling, routing and connectorization. Yarn strength members keep the tensile load away from the fiber. As with loose-tube cables, optical specifications for tight-buffered cables also should include the maximum performance of all fibers over the operating temperature range and life of the cable. Averages should not be acceptable.

Fiber Cable over view by Black Box Over the past few years, fiber optic cable has become more affordable. It's now used for dozens of applications that require complete immunity to electrical interference. Fiber is ideal for high data-rate systems such as FDDI, multimedia, ATM, or any other network that requires the transfer of large, time-consuming data files. Other advantages of fiber optic cable over copper include: • Greater distance-You can run fiber as far as several kilometers. • Low attenuation-The light signals meet little resistance, so data can travel farther. • Security-Taps in fiber optic cable are easy to detect. If tapped, the cable leaks

light, causing the entire system to fail. • Greater bandwidth-Fiber can carry more data than copper. • Immunity-Fiber optics are immune to interference. Single-mode or multimode? Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode fiber-typically 5 to 10 microns. Only a single lightwave can be transmitted at a given time. The small core and single lightwave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type. Multimode fiber gives you high bandwidth at high speeds over long distances. Lightwaves are dispersed into numerous paths, or modes, as they travel through the cable's core. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet [914.4 ml), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission. Testing and certifying fiber optic cable. If you're used to certifying Category 5 cable, you'll be pleasantly surprised at how easy it is to certify fiber optic cable since if s immune to electrical interference. You only need to check a few measurements: • Attenuation (or decibel loss)-Measured in dB/km, this is the decrease of signal strength as it travels through the fiber optic cable. • Return loss-The amount of light reflected from the far end of the cable back to the source. The lower the number, the better. For example, a reading of -60 dB is better than -20 dB. • Graded refractive index-Measures how much light is sent down the fiber. This is commonly measured at wavelengths of 850 and 1300 nanometers. Compared to other operating frequencies, these two ranges yield the lowest intrinsic power loss. (NOTE this is valid for multimode fiber only.) • Propagation delay-This is the time it takes a signal to travel from one point to another over a transmission channel. • Time-domain reflectometry (TDR)-Transmits high-frequency pulses onto a cable so you can examine the reflections along the cable and isolate faults. There are many fiber optic testers on the market today. Basic fiber optic testers function by shining a light down one end of the cable. At the other end, there's a

receiver calibrated to the strength of the light source. With this test, you can measure how much light is going to the other end of the cable. Generally, these testers give you the results in decibels (dB) lost, which you then compare to the loss budget. If the measured loss is less than the number calculated by your loss budget, your installation is good. Newer fiber optic testers have a broad range of capabilities. They can test both 850- and 1300-nm signals at the same time and can even check your Gable for compliance with specific standards. When to choose fiber optic. Although fiber optic cable is still more expensive than other types of cable, it's favored for today's high-speed data communications because it eliminates the problems of twisted-pair cable, such as near-end crosstalk (NEXT), electromagnetic interference (EIVII), and security breaches.

Fiber optic cable consists of a core, cladding, coating, strengthening fibers, and cable jacket (see above).

Core -This is the physical medium that transports optical data signals from an attached light source to a receiving device. The core is a single continuous strand of glass or plastic that's measured (in microns) by the size of its outer diameter. The larger the core, the more light the cable can carry. All fiber optic cable is sized according to its core diameter. The three sizes most commonly available are 50-, 62.5-, and 1 00-micron Gable. Cladding -This is a thin layer that surrounds the fiber core and serves as a boundary that contains the light waves and causes the refraction, enabling data to travel throughout the length of the fiber segment.

Coating -This is a layer of plastic that surrounds the core and cladding to reinforce the fiber core, help absorb shocks, and provide extra protection against excessive cable bends. These buffer coatings are measured in microns (p) and can range from 250 p to 900 p. Strengthening fibers -These components help protect the core against crushing forces and excessive tension during installation. The materials can range from Kevlat4 to wire strands to gel-filled sleeves. Cable jacket -This is the outer layer of any cable. Most fiber optic cables have an orange jacket, although some may be black or yellow. 212.70 Sketch a simple block diagram and describe the operation of a fibre optic communication system. System: transmission, receiver, fibre optic cable Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information.

Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. The light source can either be a light-emitting diode (LED)) or a laser. The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ones and zeros of the original signals.

Applications Optical fiber is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower attenuation and interference, optical fiber has large advantages over existing copper wire in long-distance and high-demand applications. Technology Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typically digital information generated by computers, telephone systems, and cable television companies. 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. Receivers The main component of an optical receiver is a photodetector, which converts light into electricity using the photoelectric effect. The photodetector is typically a semiconductor-based photodiode. Several types of photodiodes include p-n photodiodes, a 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. Optical-electrical converters are typically coupled with a trans impedance amplifier and a limiting amplifier to produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted while passing through the channel. Further signal processing such as clock recovery from data (CDR) performed by a phase-locked loop may also be applied before the data is passed on. Amplifiers The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use

a transmitter to send the signal again at a higher intensity than it was before. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that they had to be installed about once every 20 km), the cost of these repeaters is very high. Bandwidth-distance product Because the effect of dispersion increases with the length of the fiber, a fiber transmission system is often characterized by its bandwidth-distance product, often expressed in units of MHz×km. This value is a product of bandwidth and distance because there is a trade off between the bandwidth of the signal and the distance it can be carried. For example, a common multimode fiber with bandwidth-distance product of 500 MHz×km could carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km. Attenuation Fiber attenuation, which necessitates the use of amplification systems, is caused by a combination of material absorption, Rayleigh scattering, Mie scattering, and connection losses. Although material absorption for pure silica is only around 0.03 dB/km (modern fiber has attenuation around 0.3 dB/km), impurities in the original optical fibers caused attenuation of about 1000 dB/km. Other forms of attenuation are caused by physical stresses to the fiber, microscopic fluctuations in density, and imperfect splicing techniques. Transmission windows Each effect that contributes to attenuation and dispersion depends on the optical wavelength. The wavelength bands (or windows) that exist where these effects are weakest are the most favorable for transmission. These windows have been standardized, and the currently defined bands are the following Band

Description

Wavelength Range

O band E band S band C band L band U band

Original Extended Short wavelengths Conventional ("erbium window") Long wavelengths Ultralong wavelengths

1260 to 1360 nm 1360 to 1460 nm 1460 to 1530 nm 1530 to 1565 nm 1565 to 1625 nm 1625 to 1675 nm