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MONODIP SINGHA ROY
M.TECH(MODERN COMMUNIATION)
ROLL NO: 12013515004
 Optical communication is a communication at a distance using light to carry
information.
 Optical communications can be...
 Optical Communication is mainly divided into two forms , they are :
 Analog Optical Communication.
 Digital Optical Co...
 Fiber-optic communications is the method of transmitting data from one point to
another by sending pulses of light throu...
 A block diagram of a fiber-optic communication system is
shown below:
 When a light ray is incident on a boundary separating two different media, part of
the ray is reflected back into the fi...
 TRANSMITTER:
An optical transmitter is needed to convert an electrical signal to
a light pulse for transmission in the o...
 Types of Transmission
There are two types of transmission schemes may be
present in fiber-optic transmission system.
 S...
 Used in telecommunication and networking because it is flexible and can be
bundled as cables. Although fibers can be mad...
 Lower cost in the long run.
 Low loss of signal (typically less than 0.3 dB/km), so repeater-less transmission over
lon...
 High investment cost.
 Need for more expensive optical transmitters and receivers.
 More difficult and expensive to sp...
Free-space optics FSO communication uses modulated optical beams, usually generated
by laser sources or light emitting dio...
 FSO can be implemented for portable applications, e.g., movable radar dish
antennas.
 FSO provides a viable transmissio...
Where,
EDFA: Erbium Doped Fiber Amplifier is a optical repeater used to boost the
optical signals carried out through opti...
 The electrical signal is converted into optical power and the transmitted through air.
 After undergoing the influences...
 TRANSMITTER:
One or more Laser Diode (LD) or Laser Emitting Diode (LED) are used. The
choice between LED or LD is determ...
 No licensing required.
 Installation cost is very low as compared to laying Fiber.
 No sunk costs.
 No capital overha...
APPLICATIONS:
 Enterprise communication.
 Enterprise Connectivity.
 Health Care.
 Engineering & Design.
 Voice & Data...
 If the signal is detected by a receiver that requires a minimum average power of bit
rate B, the maximum transmission di...
Where,
• Ptr = Average transmitted power
• Prec = Average received power
• αf = Net loss, fiber connectors, splices
• B=bi...
At 1550nm, wide region of low-loss wavelengths is irresistible for
WDM systems even with high dispersion.
FiberLoss(dB/km)...
LONG-HAUL SYSTEM:
 Long-Haul it means long distance, Long-haul optics refers to the transmission of
visible light signals...
 Dispersion is defined as pulse spreading in an optical fiber. As a pulse of light
propagates through a fiber, elements s...
 Modal Dispersion: Modal dispersion is defined as pulse spreading caused by the time
delay between lower-order modes and ...
After fiber transmission
40 Gb/s optical signal
Transmitter output
25 ps
Transmission fiber
Positive dispersion
(Negative ...
 Large negative dispersion coefficient
 Low attenuation
 Minimal nonlinear contributions
 Wide bandwidth
 Corrects di...
 In order to remove the spreading of the optical or light pulses, the dispersion
compensation is the most important featu...
 Dispersion Compensation Fibers: DCF is a loop of fiber having negative dispersion
equal to the dispersion of the transmi...
 Digital filters: Digital filters using Digital Signal Processing (DSP) can be used for
compensating the chromatic disper...
POWER BUDGET
Power budget :The purpose of the power budget is to ensure that enough power will
reach the receiver to maint...
 Start with BER and bit rate, determine B based on coding method
 B = 1/2RC gives the maximum load resistance R based o...
 Add power penalties, if necessary, for extinction ratio, intensity noise
(includes S/N degradation by amplifiers), timin...
Imagine we want to set up a link operating at 1550 nm with a bit rate of 1 Gb/s using
the RZ format and a BER of 10-9. We ...
 Bit rate and coding format determine upper limit of rise time
 Rise time of transmitter (from manufacturer; laser faste...
For this example, tMD=0, tTR=100 ps, tRC=0.5 ns, and tGVD= 21.8 ps as before.
tr is therefore 510 ps, and the rise time bu...
OPTICAL COMMUNICATION
OPTICAL COMMUNICATION
OPTICAL COMMUNICATION
OPTICAL COMMUNICATION
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OPTICAL COMMUNICATION

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OPTICAL COMMUNICATION

  1. 1. MONODIP SINGHA ROY M.TECH(MODERN COMMUNIATION) ROLL NO: 12013515004
  2. 2.  Optical communication is a communication at a distance using light to carry information.  Optical communications can be said , in combination with microwave and wireless technologies, are enabling the construction of high-capacity networks with global connectivity.  The most common wavelengths used for optical communication fall between 0.83 and 1.55 microns. Other wavelengths are also used but this range encompasses the most popular applications. A wavelength of 1 micron corresponds to a frequency of 300 THz (300,000 GHz).
  3. 3.  Optical Communication is mainly divided into two forms , they are :  Analog Optical Communication.  Digital Optical Communication. Analog Optical Communication: Fiber Optics. Digital Optical Communication: Free Space Optics.
  4. 4.  Fiber-optic communications is the method of transmitting data from one point to another by sending pulses of light through an optical fiber. Optical fiber is a waveguide made of very thin tubes of glass whose diameter is of the order of a few micrometers. Here the optical fiber works on the principle of ‘Total Internal Reflection’.  The process of communicating using an optical fiber involves the following - a) Converting electrical signal to optical signal at the transmitter. b) Transmission of optical signal to the cable. c) Relaying the signal through the optical-fiber. d) Receiving the optical signal at the detector. e) Converting the optical signal back to the electrical signal at the receiver.
  5. 5.  A block diagram of a fiber-optic communication system is shown below:
  6. 6.  When a light ray is incident on a boundary separating two different media, part of the ray is reflected back into the first medium and while the other part is bent and enters into the second medium. The bending in the second medium known as refraction and depends on the refractive index of the two media. By Snell’s Law n1sinφ1 = n2sinφ2 sinφ1/sinφ 2 = n2/n1  where n1 and n2 are the refractive indices of the medium 1 and 2 respectively and φ1 and φ2 are the angle of incidence and angle of refraction respectively.  For n1>n2, if sinφ1=n2/n1 then φ2=90°.(Here, φ1 is known as the ‘Critical Angle’). If we further increase φ1 from this angle there will be no refracted wave and the light ray will be completely reflected into the first medium. This phenomenon is known as the Total Internal Reflection.
  7. 7.  TRANSMITTER: An optical transmitter is needed to convert an electrical signal to a light pulse for transmission in the optical medium i.e. the fiber-optic cable. The most commonly used optical transmitters are semiconductor devices such as Light-Emitting Diodes (LEDs) and Laser Diodes. In our experiment, we have used an LED transmitter.  RECEIVER: An optical receiver is needed to convert light pulses to electrical signals. The main component of an optical receiver is a photo detector, which converts light into electricity using the photoelectric effect. In our experiment the photo detector is a semiconductor based photodiode.
  8. 8.  Types of Transmission There are two types of transmission schemes may be present in fiber-optic transmission system.  Single Mode As the name implies, single mode optical fiber is designed to propagate only one light ray. It is used in high speed long distance communication.  Multi Mode A multi mode optical fiber is designed to propagate more than one light wave at a time. A larger diameter of the core is required to accommodate more light rays facilitate the transmission. It is typically used in short distance communication. In this particular experiment, we have used a multi mode fiber.
  9. 9.  Used in telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either transparent plastic (POF = plastic optical fibers) or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical absorption. The light transmitted through the fiber is confined due to total internal reflection within the material. This is an important property that eliminates signal crosstalk between fibers within the cable and allows the routing of the cable with twists and turns. In telecommunications applications, the light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use.  Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction, however bidirectional communications is possible over one strand by using two different wavelengths (colors) and appropriate coupling/splitting devices.
  10. 10.  Lower cost in the long run.  Low loss of signal (typically less than 0.3 dB/km), so repeater-less transmission over long distances is possible.  Large data-carrying capacity (thousands of times greater, reaching speeds of up to 1.6 Tb/s in field deployed systems and up to 10 Tb/s in lab systems).  Immunity to electromagnetic interference, including nuclear electromagnetic pulses (but can be damaged by alpha and beta radiation).  No electromagnetic radiation; difficult to eavesdrop.  High electrical resistance, so safe to use near high-voltage equipment or between areas with different earth potentials.  Low weight.  Signals contain very little power.  No crosstalk between cables.  No sparks (e.g. in automobile applications).  Difficult to place a tap or listening device on the line, providing better physical network security.
  11. 11.  High investment cost.  Need for more expensive optical transmitters and receivers.  More difficult and expensive to splice than wires.  At higher optical powers, is susceptible to "fiber fuse" wherein a bit too much light meeting with an imperfection can destroy as much as 1.5 kilometers of wire at several meters per second. A "Fiber fuse" protection device at the transmitter can break the circuit to prevent damage, if the extreme conditions for this are deemed possible.  Cannot carry electrical power to operate terminal devices. However, current telecommunication trends greatly reduce this concern: availability of cell phones and wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack of real interest in hybrid metal-fiber cables; and increased use of fiber-based intermediate systems. Almost all these disadvantages have been surmounted or bypassed in contemporary telecommunications usage, and communication systems are now unthinkable without fiber optics. Their cost is much more economic than old coaxial cables because the transmitters and receivers (laser and photodiodes) turn out cheaper than electric circuitry as their capacity is much superior. The cost of regeneration in electrical long distance transmission systems is completely impractical for modern communications.
  12. 12. Free-space optics FSO communication uses modulated optical beams, usually generated by laser sources or light emitting diodes LEDs, to transmit information line of sight through the atmosphere. Recently, there has been an exponential increase in the use of FSO technology, mainly for “last mile” applications, because FSO links provide the transmission capacity to overcome bandwidth bottlenecks. The desire to develop high-speed Internet access has stimulated much of this growth and as a result, the major focus of most FSO research and development has been toward the transmission of digital signaling formats. Fiber optics has been traditionally used for transmission of both digital and analog signals. The transmission of rf intensity modulated signals over optical fibers is well established. FSO can transmit data, voice or video at speeds capable of reaching 2.5 Gbps. The advantages of transmitting modulated rf signals over FSO links are as follows:  FSO transmission links can be deployed quicker, and in some instances more economically, than optical fiber links.  FSO is highly invulnerable to interference from other sources of laser radiation.
  13. 13.  FSO can be implemented for portable applications, e.g., movable radar dish antennas.  FSO provides a viable transmission channel for transporting IS-95 CDMA signals to base stations from macro- and microcell sites and can decrease the setup costs of temporary microcells deployed for particular events, e.g., sporting events, by eliminating the need for installing directional microwave or connecting cable.  FSO introduces a viable transmission medium for the deployment of cable television CATV links in metropolitan areas where installing new fiber infrastructure can be relatively expensive.  Analog FSO can reduce the cost of transmission equipment as compared to a digital implementation.  When compared with wireless rf links, FSO requires no licensing and provides better link security and much higher immunity from electromagnetic interference EMI.
  14. 14. Where, EDFA: Erbium Doped Fiber Amplifier is a optical repeater used to boost the optical signals carried out through optical cables. APD: Avalanche Photodiode used for converting light signals into electrical signals.
  15. 15.  The electrical signal is converted into optical power and the transmitted through air.  After undergoing the influences of time-dispersive channel and ambient light, the optical signal is directly translated into a photocurrent as the detector.  The electrical SNR in the optical links depends on the square of the optical power, which has a deep impact in both the designs and performance of the optical wire system.
  16. 16.  TRANSMITTER: One or more Laser Diode (LD) or Laser Emitting Diode (LED) are used. The choice between LED or LD is determined by various factors that influence price and performance as known from traditional optical communication.  RECEIVER: An optical concentrator (collect and concentrate on incoming radiation) and an optical filter (to reject ambient light) , a photo detector (to convert optical power into photocurrent) and an electrical front-end (performing amplification, filtering, etc).
  17. 17.  No licensing required.  Installation cost is very low as compared to laying Fiber.  No sunk costs.  No capital overhangs.  Highly secure transmission possible.  High data rates, up to 2.5 Gbps at present and 10 Gbps in the near future. DISADVANTAGES:  High launch power represents eye hazards.  Signals scattering results in multiple impairments.  Blockage leads to design challenges.  Low power source requires high sensitive receivers.  Light interference negatively affects systems performance.
  18. 18. APPLICATIONS:  Enterprise communication.  Enterprise Connectivity.  Health Care.  Engineering & Design.  Voice & Data.  Video.  Telco Bypass.  Security.  Disaster Recovery.  MOBILE CARRIER APPLICATIONS  BTS Backhaul Connectivity.
  19. 19.  If the signal is detected by a receiver that requires a minimum average power of bit rate B, the maximum transmission distance is limited.  The system requirements typically specified in advance are the bit rate B and the transmission distance L.  The performance criterion is specified through bit-error rate, a typical requirement being BER < 10-9.
  20. 20. Where, • Ptr = Average transmitted power • Prec = Average received power • αf = Net loss, fiber connectors, splices • B=bit rate • Np = Minimum bits • L = Distance hvBNP prec  rec tr f P P L log 10   The equation for loss limited light wave system is given as:
  21. 21. At 1550nm, wide region of low-loss wavelengths is irresistible for WDM systems even with high dispersion. FiberLoss(dB/km) 1550nm window -30 -20 -10 0 10 20 30 1250 1350 1450 1550 1650 Wavelength (nm) Dispersion(ps/nm) 1300nm
  22. 22. LONG-HAUL SYSTEM:  Long-Haul it means long distance, Long-haul optics refers to the transmission of visible light signals over optical fiber cable for great distances, especially without or with minimal use of repeaters.  In advance in fiber optic technology have made long-haul communications systems reach distances that were once unheard of. Today's fiber optic transmission links transmit multiple channels of video and audio signals over worldwide distances, and can reach high traffic volumes. This distance is made possible by a number of devices that amplify optical signals and combine larger and larger numbers of signals for transmission over a single optical fiber.  Currently, there are three systems in use for long-haul applications, namely, intensity modulation and direct detection (IM-DD), wavelength division multiplexing (WDM), and coherent systems. IM-DD can be enhanced by optical amplification using Er-doped fiber amplifiers (EDFA). To increase the capacity of existing systems, WDM is used. Using 50 wavelengths around 1.55 mm, a fiber can offer an information bandwidth approaching 1 Tb/s. All channels can be simultaneously amplified by EDFA.  The long-distance systems can be divided into terrestrial and submarine applications. For submarine system, reliability is the most important consideration because of the high cost of repair if it is needed. Redundancy often is built into the system. The terrestrial systems are subject to weather conditions, e.g., extreme temperature change.
  23. 23.  Dispersion is defined as pulse spreading in an optical fiber. As a pulse of light propagates through a fiber, elements such as numerical aperture, core diameter, refractive index profile, wavelength, and laser line width cause the pulse to broaden. Dispersion increases along the fiber length. The overall effect of dispersion on the performance of a fiber optic system is known as Intersymbol Interference (ISI). ISI occurs when the pulse spreading caused by dispersion causes the output pulses of a system to overlap, rendering them undetectable.  Dispersion is generally divided into three categories:  Modal dispersion  Polarization mode dispersion.  Chromatic dispersion
  24. 24.  Modal Dispersion: Modal dispersion is defined as pulse spreading caused by the time delay between lower-order modes and higher-order modes. Modal dispersion is problematic in multimode fiber, causing bandwidth limitation.  Polarization Mode Dispersion: Polarization Mode Dispersion (PMD) occurs due to birefringence along the length of the fiber that causes different polarization modes to travel at different speeds which will lead to rotation of polarization orientation along the fiber.  Chromatic Dispersion: Chromatic Dispersion (CD) is pulse spreading due to the fact that different wavelengths of light propagate at slightly different velocities through the fiber because the index of refraction of glass fiber is a wavelength-dependent quantity; different wavelengths propagate at different velocities. Chromatic dispersion consists of two parts:  Material dispersion  Waveguide dispersion.
  25. 25. After fiber transmission 40 Gb/s optical signal Transmitter output 25 ps Transmission fiber Positive dispersion (Negative dispersion) + Dispersion compensating fiber (DCF) After dispersion comp. Negative dispersion (Positive dispersion) Longer wavelength Slow (Fast) Shorter wavelength Fast (Slow) Longer wavelength Fast (Slow) Shorter wavelength Slow (Fast) DISPERSION COMPENSATION EXAMPLE
  26. 26.  Large negative dispersion coefficient  Low attenuation  Minimal nonlinear contributions  Wide bandwidth  Corrects dispersion slope as well  Minimal ripple  Polarization independent  Manufacturable
  27. 27.  In order to remove the spreading of the optical or light pulses, the dispersion compensation is the most important feature required in optical fiber communication system. The most commonly employed techniques for dispersion compensation are as follows:  Dispersion Compensating Fiber.  Electronic Dispersion Compensation.  Fiber Bragg Grating.  Digital Filters.
  28. 28.  Dispersion Compensation Fibers: DCF is a loop of fiber having negative dispersion equal to the dispersion of the transmitting fiber. It can be inserted at either beginning (pre-compensation techniques) or end (post-compensation techniques) between two optical amplifiers. But it gives large footprint and insertion losses.  Electronic Dispersion Compensation: Electronic equalization techniques are used in this method. Since there is direct detection at the receiver, linear distortions in the optical domain, e.g. chromatic dispersion, are translated into non linear distortions after optical-to-electrical conversion. It is due to this reason that the concept of nonlinear cancellation and nonlinear channel modeling is implemented. For this mainly feed forward equalizer (FFE) and decision feedback equalizers (DFE) structures are used. EDC slows down the speed of communication since it slows down the digital to analog conversion  Fiber Bragg Grating: Fiber Bragg Grating (FBG) has recently found a practical application in compensation of dispersion-broadening in long-haul communication. In this, Chirped Fiber Grating (CFG) is preferred. CFG is a small all-fiber passive device with low insertion loss that is compatible with the transmission system and CFG’s dispersion can be easily adjusted. CFG should be located in-line for optimum results. This is a preferred technique because of its advantages including small footprint, low insertion loss, dispersion slope compensation and negligible non-linear effects. But the architectures using FBG is complex.
  29. 29.  Digital filters: Digital filters using Digital Signal Processing (DSP) can be used for compensating the chromatic dispersion. They provide fixed as well as tunable dispersion compensation for wavelength division multiplexed system. Popularly used filter is lossless all-pass optical filters for fiber dispersion compensation, which can approximate any desired phase response while maintaining a constant, unity amplitude response. Other filters used for dispersion compensation are band pass filter, Gaussian filters, Super-Gaussian filters, Butterworth filters and microwave photonic filter.  Various techniques for Dispersion Compensation:
  30. 30. POWER BUDGET Power budget :The purpose of the power budget is to ensure that enough power will reach the receiver to maintain reliable performance during the entire system lifetime system lifetime PB : PRX > PMIN PRX = Received Power PMIN = Minimum Power at a certain BER PRX = PTX – Total Losses + Total Gain - PMARGIN PTX = Transmitted Power PMARGIN ≈ 6 dB
  31. 31.  Start with BER and bit rate, determine B based on coding method  B = 1/2RC gives the maximum load resistance R based on B and C  Based on R and M, determine detector sensitivity (NEP), multiply by B1/2  Add system margin, typically 6 dB, to determine necessary power at receiver
  32. 32.  Add power penalties, if necessary, for extinction ratio, intensity noise (includes S/N degradation by amplifiers), timing jitter  Add loss of fiber based on link distance  Include loss contributions from connections and splices  End up with required power of transmitter, or maximum length of fiber for a given transmitter power
  33. 33. Imagine we want to set up a link operating at 1550 nm with a bit rate of 1 Gb/s using the RZ format and a BER of 10-9. We want to use a PIN photodiode, which at this wavelength should be InGaAs. The R0 for the diode is 0.9 A/W.
  34. 34.  Bit rate and coding format determine upper limit of rise time  Rise time of transmitter (from manufacturer; laser faster than LED)  Pulse spread due to dispersion  Rise time of receiver (from manufacturer; PIN faster than APD) Rise time components are combined by taking the square root of sums of squares.
  35. 35. For this example, tMD=0, tTR=100 ps, tRC=0.5 ns, and tGVD= 21.8 ps as before. tr is therefore 510 ps, and the rise time budget does not meet the limit.  Can use NRZ format  Use faster detector or transmitter  Use graded-index fiber for less dispersion 2222 RCGVDMDTRr ttttt 

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