The Photonics Is The Propagation Engineering Essay

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Introduction

The Word Photonics came from photon which is the smallest unit of light. The Photonics is the propagation, working and Handling of photon to achieve a certain function.

The data in this kind of network is transmitted in the form of visible light energy i.e. Infrared (IR) signals. The IR data transmission has various advantages over electrical transmission, as the light travels more faster than electrons so the optical transmission will work out thousands of times faster than any electronic transmission. Like this the bandwidth provided by photon signals are very much more than as compare to the other electrical transmission.

Historical Development

The word "Photonics" come in 1960 with the invention of laser, when the research on the light start for the telecommunication and information processing purpose. The history can be trace back to the 800 BC to 150 BC when the Use of fire signal by the Greeks was most famous. As the time was passing trend was coming and then in 1880 then Invention of the photo phone by Alexander Graham Bell was the first big step taken in the field of photonic and from then onward the experiments start in the field of light then in 1917 the mega trend come by the Stimulated Emission which was first populated by Albert Einstein, then in 1930 experiments with silica fibres was done by Lamb (Germany), and in 1950-55 The clad for optical fibre develop, by Kapany et al (USA). going on and in 1960 the Line of sight optical infection by using laser was done, then in 1962 the semiconductor laser was introduce by Natan, Holynal et al(USA). the process continue and in 1966 a paper by C K Kao and Hockham (UL) was a trend in this field they use the Glass fibre rather than crystal because of high viscosity having Strength of 14000 kg /m2 and Loss decrease to 20 dB/km. Then in 1970 Low attenuation fibre from 1000 dB/km to 20 dB/km was introduce by Apron and Keck (USA), this was achieve by adding do pant to the silica to increase the fibre refractive index, then in late 1976 the multi-mode fibre was introduce by Japanese whose Bandwidth was 20 GHz, then in 1976 the 800 nm Graded multimode fibre develop having bandwidth of 2 Gbps/km. Then in 1980's the 1300 nm Single mode fibre come with the bandwidth of 100 Gbps/km and then the 1500 nm Single mode fibre with the bandwidth of 1000 Gbps/km, then in 1990's the use of optical amplifier make the transmission up to 10 Gbps over 106 km with no error, then 2000 and beyond the Optical Network starts and the using the Dense WDM which has the 40 Gbps/channel, 10 channels also using the Hybrid DWDM/OTDM which has up to 50 THz transmission window and having more than 1000 Channels WDM with transmission rate more than 100 Gbps OTDM.

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Now the existing system of photonic in the telecommunication field has 1.2 Tbps on WDM and over the DWDM which has Typical bit rate of 40 Gbps / channel and about 8 THz (or 60 nm) Amplifier bandwidth. It has 32 channels (commercial) with 0.4 nm (50 GHz) spacing and up to 2400 km, it require no regeneration (Alcatel). Just like this the OTDM has typical bit rate of 6.3 Gbps / channel and about 400 Amplifier bandwidth having 16 channels with 1 ps pulse width.

Advantages of Optical

As the demand for faster and efficient communication systems and the Internet traffic is increasing day by day, for this a transmission medium is require which provide a very high bandwidth that meet the growing demand and also increase the transmission length and improved performance etc. so the best choice here is the optical which has the speed of light for the fast transmission rate and a very high bandwidth for the data transfer.

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Fig. 3. Typical Data Bandwidth Requirement.

The Bandwidth is the range of frequencies to transmit a signal. if we have high bandwidth we can transmit more data at a time. As if we look to the figure it shows that in 1990 there was very less demand for the bandwidth as the time pass the demand increases and up to 2010 the demand increase very rapidly because of the typical data bandwidth requirement as the high quality sound and good compressed video demand increase so for that purpose the bandwidth demand also increase.

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From the above discussion it is clear that we need something which has

The High data rate

Low transmission loss

Low bit error rates

High immunity from electromagnetic interference

Bi-directional signal transmission

High temperature capability

High reliability

Avoidance of ground loop

Electrical isolation

Signal security

Small size, light weight, and stronger

All these are the advantages which the optical network over the other networks, it means that the optical is the today word demanding transmission medium.

Fig. 4. Optical Fibers V/S Copper Pairs.

If we compare it by size, weight and streangth the optical fiber has less size as compare to copper and less weight and more stronger than copper wires. If we have 648 optical fibres it has only 21 mm of diameter and has only 363 kg/km which is more less than as compare to that of copper if we have 448 copper pairs which is more less than that of 648 it has 62 mm of diameter which is greater than that of fiber and has very big weight as compare to that with optical fiber.

Chromatic Dispersion

The chromatic dispersion causes the pulse distortion and also the pulse smearing effects. The higher bit-rates and shorter pulses are less robust to Chromatic Dispersion or simply you can say that the Limits "how fast" and "how far" data can travel. The Fig. 1 show the Chromatic Dispersion.page10D:\MS IT 11\second Semestor\Photonic\Chromatic Despresion.png

Fig.1. Chromatic Dispersion

By joining fibers with CD of opposite signs (polarity) and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the spain can be inserted at any point in the link, at the receiver or at the transmitter.

Polarization Mode Dispersion (PMD)

The optical pulse extend as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of some relevance at bit rates of 10Gb/s or more.

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Fig. 2. Polarization Mode Dispersion

Factors contributing to PMD

Bit Rate

Fiber core symmetry

Environmental factors

Bends/stress in fiber

Imperfections in fiber

Solutions for PMD

Improved fibers

Regeneration

Follow manufacturer's recommended installation techniques for the fiber cable

The services provided by Optical Technology are:

The optical fiber is now the back bone of the entire internet and now it is also getting the back bone between cities and in the different network topologies. The basic aim of this is the high data rate and all other factors which we discuss above. The following are the services which is provided by the optical technology.

Next Generation Internet à Internet 2

IPTV

Video Conferencing

Online Gaming

And many more

Network Topology of Optical

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Fig. 5. Topology of Optical Network.

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Fig. 6. Optical Network.

Back Bone

The fiber optic is use as a backbone, it is from continent to continent or from coast to coast, and the distance is greater than 1000 Km with the transmission rate of 10Gbit/s to Tbit/s.

Metro Backbone Area

It is from the city to city when the distance is up to 100 Km. and it has the bandwidth of 1Gbit/s to 40Gbit/s.

Metro Access and Access

The fiber optic use within the city for data transfer here the distance is up to 10 Km. and it has the bandwidth of 100 Kbit/s to 1Gbit/s.

Local Area

Use within offices and homes and having the distance about 100 Meters, it has the bandwidth of 10Mbit/s to 1Gbit/s.

Current Trend

Nearly one terameter (1000 million kilometers) of optical fiber are now deployed around the globe. And still the internet users continue in their demands for newer and broader bandwidth services, now the Provision of Gigabit Ethernet to the home & business is in process simply optical is coming inside the LAN technology.

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Verizon and Nokia Siemens Networks have taken a step closer to being able to transmit commercial traffic at a speed of 100 Gbits/sec. the two companies carried out a successful 100-Gbit/sec transmission on a single wavelength for more than 1,040 kilometers over field fiber.

According to Japan's telecom (2008) powerhouse Nippon Telegraph and Telephone Corp (NTT) the number of people signed up to fiber-optic internet access service topped ten million subscribers. The Spanish cable operator ONO announced that it will launch new broadband packages offerings download speeds of up to 100Mbps and upload speeds of up to 5Mbps.

In Dec 2008 the Swisscom announced commencement of next stage of its fiber-optic broadband network build out Cables laied to neighborhoods (fiber-to-the-node) large companies (fiber-to-the-office) and also to the residential customers and SMEs are next in line. Work has already started in Zurich, Basel, and Geneva, with the aim of connecting 100,000 households with fiber-optic cables by the end of 2009. And over the next six years, Swisscom is planning to invest some CHF 8 billion in the Swiss telecommunications and IT infrastructure 35% of this sum earmarked for fiber-optic expansion.

In Dec 2008 the Ciena Corporation, the network specialist, announced a significant milestone in innovation with the successful demonstration of the industry's first true, single wavelength 100G optical transmission during the Supercomputing Conference 2008 (SC08) the earlier record for 100G tests combined two 40G optical signals or inversely multiplexed ten 10G optical signals 100G of data stream transmitted over 80km of fiber the first true single wavelength 100G transmission.

Architecture of Photonics Network

Figure 1 shows the Architecture of Photonics Network.

Architecture of Photonics Network consists of following components:

Laser Diode (Transmitter)

Optical Fiber

Optical Amplifier

Optical Add-Drop Multiplexer

Optical Cross Connect

Multiplexer

De-Multiplexer

Photodiode (Receiver)

Laser Diode (Transmitter):-

A laser diode acts as a transmitter in photonics network. It is a laser where the active medium is same as that of for Light Emitting Diode (LED) which is semiconductor. The most common form of laser diode is made by use of p-n junction and it is powered by injecting electric current. These types of devices as known as injection laser diode for distinguishing them through optically pumped laser diodes. Figure 2 shows Simple laser diode.

File:Simple laser diode.svg

Different types of devices are used as laser diodes. These devices are:

Double Hetero structure Lasers

Quantum Well Lasers

Quantum Cascade Lasers

Separate Confinement Hetero-Structure Lasers

Distributed Feedback Lasers

Vertical Cavity Surface Emitting Lasers (VCSELs)

Vertical External Cavity Surface Emitting Lasers (VECSELs)

External Cavity Lasers Diode

Optical Fiber:-

An optical fiber is a thin flexible fiber which is used to transmit light signal from one end of the fiber to the other end of the fiber. It is widely used in optical fiber communication which allows transmission over a long distance and at very high bandwidth as compared to other communication methods. Fibers are used instead that of metal wires because there is very less loss of signal while travelling and also immune to electromagnetic interference. These fibers are also used for illumination and these are wrapped in bundles so that images can be carried easily. Some specific designed fibers are used for variety of other applications such as sensors and fiber lasers. It consists of a transparent core which is surrounded by a transparent cladding material with very low index of refraction. Light remains in the internal reflection. Fiber that supports many propagation modes is known as multi mode fiber while fiber that supports only a single mode is known as single mode fiber.

Optical Add-Drop Multiplexer (OADM):-

An Optical Add-Drop Multiplexer is a device which is used WDM for multiplexing the signal as well as routing different channels of light into the Simple Mode Fiber (SMF) or out of the Simple Mode Fiber. It is used to construct optical telecommunication networks. Add and drop terminology is used here for the purpose of referring the capability to add or drop the certain wavelength channels to multi wavelength WDM signals and pass those signals to another network path respectively. An Optical Add-Drop Multiplexer can also be considered as a special type of Optical cross Connect (OXC).

An Optical Add-Drop Multiplexer comprises of three phases. These are; optical multiplexer, optical de-multiplexer, a method for configuring the path between optical multiplexer and optical de-multiplexer, and a set of port for adding and dropping of the wavelength signals. Reconfiguration of a signal is obtained by using optical switch which is used to direct the wavelength signals to the optical multiplexer or to the drop port. The purpose of optical de-multiplexer is used to separate the wavelength in input fiber to the ports. The optical Multiplexer is used to multiplex the wavelength channel and send it to a single output fiber. All the light paths, pass directly through an OADM is known as cut-through lightpaths and the path added or dropped at OADM is known as added / dropped lightpaths. Optical switches are reconfigured remotely by OADM which is known as reconfigurable OADM (ROADM). The OADM that does not have the feature of reconfiguration is known as fixed OADM. Figure 3 shows the working of OADM

http://upload.wikimedia.org/wikipedia/en/5/51/Fbg_oadm.GIF

In this Figure, OADM is shown which is based on Fiber Bragg Grating (FBG), two circulators and 4 channels with 4 different colors. At channel 4, signal is reflected back to the circulator and that signal is dropped where as another signal is added at the same point in the network.

There are few different technologies of multiplexer and de-multiplexer that are able to recognize the OADM. These technologies include thin film, Fiber Bragg Grating with optical circulators, few free space devices for grating and integrated planar as Arrayed waveguide gratings. The reconfiguration function varies from the manual fiber patch panel to certain switching technologies consisting of Micro Electro Mechanical (MEMs), Liquid Crystal and thermo optical switches in waveguide circuits.

OADM is different from Add-Drop Multiplexer. Although both have add/drop feature, but OADM is operated in optical domain under WDM where as Add-Drop Multiplexer is operated in traditional SONET or SDH networks.

Optical Amplifier:-

An optical amplifier is a device that is used to amplify optical signal without converting it into am electrical signal. It may be considered as a laser without having an optical cavity. Optical amplifiers are very important when talking in term of optical communication and laser physics.

Different mechanisms are there to amplify light signals. There are different kinds of optical amplifier which consists of:

Laser Amplifier

Doped Fiber Amplifier

Semiconductor Optical Amplifier

Raman Amplifier

Optical Parametric Amplifier

Laser Amplifier:-

Any laser having active increase medium can be forced to produce increase in light at laser's wavelength with the same material as its increase medium. These types of amplifiers are commonly used to produce high power laser systems. Some special types of laser amplifiers are used to amplify ultra-short pulses. These types of amplifiers are named as regenerative amplifiers and chipped pulse amplifiers.

Doped Fiber Amplifier (DFA):-

In DFA, amplification of incoming light is caused by stimulated emission in the gain medium of amplifier. It is an optical amplifier which uses doped optical fiber as gain medium for amplification of the signal. These are related to the fiber lasers. The signal which is to be amplified is multiplexed with the pump laser into the doped fiber. The signal is amplified with doping icons. Erbium Doped Fiber Amplifier (EDFA) is the most common example of Doped Fiber Amplifier. In EDFA, core of silicon fiber is doped with the help of Erbium icons and it can be pumped efficiently with laser at wavelength of about 980nm or about 1480nm and exhibit gain of about 1550nm in the region. Schematic diagram of simple Doped Fiber Amplifier is shown in Figure 4.

File:Doped fibre amplifier.svg

Semiconductor Optical Amplifier:-

Semiconductor Optical Amplifiers (SOAs) are amplifiers which provide the gain medium by use of semiconductor. The SOA is of very small size and pumped electrically. As compared to EDFA, it is very less expensive. SOAs can be integrated with modulators and semiconductor lasers but it is not still comparable with EDFA in term of performance and SOAs has very high noise as compared to EDFA, it is highly non linear with respect to the passage of time. Its upper state lifetime is very less, in term of nanoseconds. So whenever there is a change in gain, phase also changes which causes distortion in the signal. Due to nonlinearity, optical communication application has to face many severe problems. But it has the possibility to provide gain in different wavelength regions as compared to EDFA. Recently Vertical Cavity SOA is added to the SOA family. These devices are similar to Vertical Cavity Surface Emitting Laser (VCSELs) in structure and also have many common features.

Raman Amplifier:-

In Raman Amplifier, Raman Amplification is used to strengthen the signal. Its amplification affect is achieved by non-linear interaction of signal and a pumping laser with in optical fiber. There are two different types of Raman amplifier, Distributed and lumped. In Distributed Raman Amplifier, transmission fiber is utilized as gain medium by multiplexing a pump wavelength with signal wavelength whereas Lumped Raman Amplifier utilizes a dedicated, shorter length of fiber for providing amplification.

Optical Parametric Amplifier:-

In optical Parametric Amplifier, it allows the amplification of a weak impulse signal in a non linear medium. The main feature of Optical Parametric Amplifier is to expand frequency tunability of ultra-fast solid state lasers. It is capable of extremely broad amplification bandwidth with the help of non-collinear interaction geometry.

Optical Cross Connect:-

Optical Cross Connect is a device which is used by telecommunication carriers which switches the high speed optical signals in optical fiber network such as optical mesh network. Optical cross connect can also be implemented in electronic domain in which all the optical signals are converted into electrical signals after de-multiplexed by using de-multiplexer. These electronic signals are converted back to optical signals after certain processing and the resulting signal is multiplexed by optical multiplexer on the optical fiber. This conversion is known as Optical Electrical Optical (OEO) conversion. Optical cross connects is based on the OEO conversion. One of the key advantages is that it is easy to monitor signal quality in OEO devices because everything is converted back to the electronic format at the switch node. Another advantage is that optical signal can be regenerated.

Multiplexer and De-multiplexer:-

Multiplexer (mux) is a device which is used to select one of several analog or digital input signals and forward the selected input into only single line. If a multiplexer has 2n inputs and has n lines to select which input line is to be sent to the output. It is only possible with the help of electronic multiplexer for several signals to share one device or resource. Figure 5 shows a simple multiplexer structure.

http://upload.wikimedia.org/wikipedia/commons/b/b2/Multiplexer2.png

Whereas De-multiplexer (demux) is a device which takes a single signal as input and select one data output line from many output lines which is connected to the single input. Complement of multiplexer is also referred as de-multiplexer. A multiplexer can be considered as multiple inputs, single-output switch whereas a de-multiplexer is considered as single-input, multiple-output switch. Figure 6 shows a simple de-multiplexer structure.

File:Demultiplexer.png

Photodiode:-

A photodiode is act as a receiver in Photonics network. It is a type of photo-detector which is capable of converting light signal into current or voltages which depends upon the operation being performed. Electric solar power is a large area photodiode which is generated by solar cell. It is same as regular semiconductor diodes. But they are not exposed like X-rays or packaged with a window or optic fiber connection to allow reaching a sensitive part of the device. Many designed diodes use as a purpose of photo diode is also based on PIN junction rather than PN junction. Figure 7 shows symbol for photo diode.

File:Photodiode symbol.svg

Basic Optical Fiber Communication (OFC) Link

Optical fiber communication consist these elements:-

Electro-optical transmitter

It convert incoming electric signals either analog or digital information into a modulated beam of light the hardware which is being used for the transmitter are:

Light Emitting Diode (LED)

Laser Diode (LD)

A light-carrying fiber

Transmission Medium, or channel, is the actual physical path that data follows from the transmitter to the receiver; a fiber is categories as under:

Single Mode Fiber (SMF)

Multi-mode Fiber (MMF)

Dispersion Shifted Fiber

Optoelectronic receiver

It converts detected light into an electric current for useful data. The optical receiver refers as Photodiodes.

Electro-optical transmitter

The role of electro optical transmitter is to convert incoming electrical signal into modulated light so that it can be transmitted over fiber optical cable. There are basically two schemes for the transmitted light, it can be On/off or can be linearly varied in intensity in two predetermined level. These two schemes are shown below.

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The medium or source of light which is used in transmitter is light emitting diode (LED) or laser diodes. These LEDs and Lasers are semiconductors that focus the beam of light right into the optical fiber connector.

LEDs which have a wide spectral frequency are mostly used for large aperture multimode fibers and are used for short to moderate transmission distances. Lasers, on the other hand, feature a narrow band of wavelengths and can couple many times more power into the fiber than LEDs and therefore are useful in applications that require high speed, high bandwidth over long distances. Lasers are not stable over wide operating temperatures, however, with well-designed feedback circuitry; continuous stable output can be achieved.

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Analog modulation takes a number of forms - see figure (3). The simplest is intensity modulation (IM) where the brightness of an LED is varied in direct step with the variations of the transmitted signal. In other methods, an RF carrier is first frequency modulated (FM) with another signal or, in some cases, several RF carriers are separately modulated, then are combined and transmitted as one complex waveform.

Light Emitters

Light emitters refer to a category of semiconductors used to create both visible and infrared light. They are devices that convert electrical energy into light. Originally, lasers were the first to be used to create light pulses, both visible and infrared. Today, lasers for use in optical communications are compressed inside semiconductor microchips and quantum well devices, and are generally referred to as laser diodes or light emitters.

Light emitters are the source of light-based communications and are used to optically represent an electronic, binary bit stream by modulating (switching on and off) an optical pulse stream or by blocking and unblocking the continuous output of a light emitter in a timed sequence. Different light emitters are designed to emit a particular wavelength of photons, injecting the light into the aperture of optical fiber for communications.

Light emitter classifications include the following:

Laser diodes Laser diodes are complex semiconductors used with both single-mode and multimode fiber. Composed of indium gallium arsenide phosphide, they use narrower spectral wavelengths, exhibit higher-power, faster modulation, and support higher-bandwidths, securing their almost exclusive use in long-reach optic communications as well as WDM applications.

Light-emitting diodes (LEDs) LEDs are primarily made of gallium aluminum arsenide, have a wider spectral width, are used with multimode fiber, and exhibit a very low cost. LEDs are less expensive to manufacture than lasers. Common LEDs have a bandwidth limitation of about 622 Mbps. LEDs produce wide-spectrum coverage but have lower power in comparison to the lasers. LEDs are generally used with multimode fiber for optical transmission distances less than 2 km.

Vertical-cavity surface-emitting lasers (VCSELs) VCSELs are a more recent form of LEDs that have emerged as low-cost light sources for multimode fiber. VCSELs are currently designed around the 850 nm wavelength window. Of late, VCSELs have found promising applications in VCSEL arrays that can be mated to optical ribbon cable for short-reach, high-speed parallel optic interconnections.

Optical Fiber

Optical fiber, particularly glass-based fiber, is the physical transmission medium of choice. As a result, optical networking is the ascendant Layer 1 technology on which to build the new era of next-generation optical networks. Optical fiber is possible because of a rather complex integration of physics, minerals, and the resultant technologies.

Optical fiber is composed primarily of silica (SiO2), a silicon dioxide chemical compound that results in very pure glass. Optical fiber is constructed into the three Cs:

Core The core is where the light travels.

Cladding The cladding surrounds the core and reflects the light to keep it moving within the core.

Coating The coating protects the glass fiber as insulation from the elements and against machine and human handling.

The two general classifications of fiber are multimode fiber (MMF) and single-mode fiber (SMF). MMF uses a larger core through which to propagate light pulses. It is called multimode because the size of the core allows multiple "modes" of the exact same wavelength of light to travel the core simultaneously. SMF uses a smaller core, having the effect of allowing only a single mode or instance of any injected wavelength. MMF is less expensive to manufacturer and is often used for short-distance optical communication, and SMF is used for long-distance and multi lambda systems.

Multimode Fiber

Dimensions of optical fiber are traditionally measured in micrometers. In 1976, Corning developed 50-micrometer (measured at the core) MMF. This fiber type was the first to become commonly installed and has been used primarily in Japan and Germany, where 50 micrometer is a data standard.

The United States standardized on 62.5-micrometer core MMF, which was developed in 1986. IBM, at the time, originally endorsed 62.5-micrometer fiber, because the larger size of the aperture compensated somewhat for the immature techniques of connector polishing and alignment. This larger fiber core size was also considered to work well with LEDs. AT&T followed and standardized on the 62.5-micrometer fiber, leading to the acceptance of the 62.5-micrometer size as a MMF optical standard.

For MMF, the claddings diameter typically measures about 125 micrometers and, with the final protective coating, a cross-section of optical fiber measures about 245 micrometers across. The 50- to 62.5-micrometer cores of these fibers are wide enough to allow multiple modes of light propagation, each taking a slightly different path through the fiber core, hence, the designation of MMF. Figure 5 depicts multiple modes of the same wavelength traveling through the core of multimode fiber.

http://www.iphelp.ru/doc/3/Cisco.Press,.Next-Generation.Network.Services.(2005).BBL.LotB/1587051591/images/05fig03.jpg

Source: Cisco Systems, Inc.

Figure 5. Multimode Fiber Light Propagation

While 62.5-micrometer fiber is the typical U.S. standard for multimode, the 50-micrometer fiber is becoming increasingly important as low-cost 850 nm LEDs are being developed. Using 850 nm LEDs with 50-micrometer fiber allows for longer-link distances and higher-speed transmission than using 62.5-micrometer fiber with the same 850 nm LEDs. For example, carrying Gigabit Ethernet (GE) over 850 nm, 62.5-micrometer multimode fiber, and the usable transmission range is about 275 meters. Using 50-micrometer fiber, the range can be extended to 500 meters. For the 10 Gigabit Ethernet standards at 850 nm wavelength, 62.5-micrometer fiber will reach about 35 meters, current 50-micrometer fiber will extend that to 86 meters, and next-generation/premium 50-micrometer optical fiber will more than triple the distance to about 300 meters.

MMF fiber applications are useful for short transmission distances such as fiber patch cords, local area networks, and campus backbone applications less than 2 km. These multimode fiber cables are generally coated with an orange insulation to distinguish them from single-mode fiber, often wrapped in a yellow plastic coating. 

Single-Mode Fiber

SMF uses a much smaller core diameter, about 8.2 micrometers, for example, in Corning's SMF-28 fiber. This core diameter is five to seven times less than MMF and only allows a single mode of propagated light. When combined with narrow beam width laser diodes operating at 1310 nm and 1550 nm windows, SMF allows for long-distance and high-bandwidth optical applications.

Just as in MMF, the cladding diameter of SMF reaches 125 micrometers, and the outer protective coating diameter usually measures about 245 micrometers; but that's where the similarities end. It is this difference in core diameters that technically distinguishes SMF's optical characteristics from MMF. SMF is the fiber classification of choice for long-distance optical communications when using a single lambda, as in pre-WDM optical systems, and for using multilambdas in WDM, coarse wavelength division multiplexing (CWDM), and dense wavelength division multiplexing (DWDM). Figure 6 depicts a single wavelength traveling through the core of single-mode fiber.

http://www.iphelp.ru/doc/3/Cisco.Press,.Next-Generation.Network.Services.(2005).BBL.LotB/1587051591/images/05fig05.jpg

Figure 7. SMF Light Propagation

Source: Cisco Systems, Inc.

Not all SMF is alike. Since the 1980s, there have been a number of application-specific SMFs developed, each purposely designed for a particular optical installation. To illustrate some of these differences, it is useful to itemize some specific fibers from a particular fiber manufacturer, such as those developed by Corning. Some of these single-mode, application-specific fibers are

Corning SMF-28 Often considered the SMF standard, this is perhaps the world's best-selling fiber. It is an unshifted fiber that is optimized for time division multiplexing (TDM) transmission at 1310 nm. It is also useful for TDM at 1550 nm and WDM at 1550 nm, although not the best for those wavelengths.

Corning SMF-28e This photonic fiber is targeted at optical connectorization and optical component manufacturers with versatility between the 1280 nm and 1625 nm range.

Corning SMF-DS A single-mode, fiber dispersion manufactured to specifically shift the dispersion peak to one side of the range of a 1550 nm laser source. This optimizes the fiber for TDM at 1550 nm, which is a single lambda source, but not for WDM use at 1550 nm, because WDM is multilambda, operating on both sides of the 1550 nm center point of the ITU-G.692 optical wavelength grid.

Corning SMF-NZ-DSF This nonzero dispersion-shifted fiber is particularly optimized for both TDM and WDM use at 1550 nm wavelengths. There is both positive dispersion-shifted fiber (+ NZ-DSF) and negative dispersion-shifted fiber (-NZ-DSF).

Corning LEAF This is a fiber principally optimized for DWDM use in the 1550 nm band. LEAF stands for Large Effective Area Fiber, meaning that among other things, it is especially optimized to support maximum DWDM channel plan flexibility. LEAF is a nonzero, dispersion-shifted fiber with industry-leading polarization mode dispersion specifications, supporting immediate upgrades to 40 Gbps optical transmission systems and ultra-long-haul network distances.

Corning VASCADE A family of optical fibers used for submarine applications in harsh undersea environments. Submarine optical fiber cable is important because of its ability to quickly globalize the Internet and support traffic rates of intercontinental data. Not long ago, geosynchronous (GEOS) satellite systems carried the burden of international voice and data traffic, but submarine optical fiber has mounted a storm surge of capacity in the last few years. If you're going to build across or along continents, whether using amplified or nonamplified designs, you might be using some of the following:

The optical attenuation loss per kilometer of today's optical SMF, expressed as decibels per kilometer (dB/km), is 100 times better than Corning's original 20 dB loss per kilometer, considered the benchmark in 1970. At typical .20 to .22 dB/km of signal attenuation, premium SMF at 1550 nm wavelengths achieves longer distances before amplification is necessary, which is of extreme importance to long-haul fiber applications.

Optical fiber might be carrying a digital signal, but the light-propagating carrier is still an analog transmission. As a result, there are impairments that affect light propagation such as attenuation, dispersion, nonlinearity, and distortion. These impairments are present due to the nature of analog transmission, the presence of impurities in the glass core, and nonlinearities in the circumference of fiber. A large part of optical fiber transmission science is devoted to managing these impairments through shaping and compensation devices and sometimes through the use of application-specific fiber. However, that doesn't negate optical fiber's usefulness, as fiber offers error rates ten billion times lower and bandwidth rates ten billion times higher than copper wiring ever will.

Optical fiber is the quintessential carrier for optical light communications. A whole scientific industry has grown up around optical fiber, and fiber is moving ever closer to the business and residential interface of LANs and WANs. Providers use different types of fiber with different light emitters and light detectors to form optical communication networks for specific applications and for future-proofing network capacity and scalability.

Fiber Optic Receivers

The fiber optic receiver converts modulated light coming from an optical fiber back into the original electronic signal applied to the transmitter.

The detector is a photodiode of either the PIN or the Avalanche type and is mounted in a similar package to the one used for the LED or Laser. Sensitivity of the receiver is specified as the minimum signal that it can receive (in dBm). Dynamic Range is the difference between the minimum and maximum acceptance levels. Receivers usually employ high gain internal amplifiers and require special circuitry to avoid saturation or distortion. When the optical dynamic range of the system is equal to the optical power budget, no saturation of the receiver can occur. Quality of signal transmission is equally good at short or long distances.

As in the case of fiber optc transmitters, fiber optic receivers are available in both analog and digital versions. Figure (4) is a functional diagram of a simple analog optical receiver.

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Light Receivers

Optical light receivers, also known as photonic detectors or, more colloquially, photo detectors, perform just the opposite of light emitters. They detect light pulses at the receiving end of the optical fiber, converting it into an electrical energy representation that is proportional to that of the received photonic signal. These photo detectors based on semiconductor technology are the main component of light receivers. Called photodiodes with respect to optical communications, these semiconductors are primarily PIN photodiodes and avalanche photodiodes (APDs). PIN is an abbreviation that refers to a three-layer diode, where the P-type (positive layer material) is separated from the N-type (negative layer material) by an intrinsic layer of material, therefore, a PIN diode.

The basic premise of photo detectors is that upon receiving photons, they convert this photon energy into electrons or electrical energy referred to as photocurrent. Generally, the photocurrent is then amplified for signal reuse beyond the photo detector. APDs have a statistical multiplication phenomenon known as the avalanche effect, which generates a larger number of electrons for every photon received.

Low-speed, low-cost optical systems favor the price point of PIN photodiodes, while multigigabit systems prefer the higher-power conversion of the APDs.

Light receivers usually have a wider wavelength sensitivity to appropriately compensate for wavelength drift as affected by optical impairments. One of the keys to light receiver design and operation is the proper balance of receiver sensitivity, to distinguish between an optical signal and optical noise. Figure 8 depicts a photo detector receiving laser-originated photons through an optical fiber.

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Figure 8. Photo detectors as Light Receivers

Source: Cisco Systems, Inc.