CE00038-2 Communications

1. CE00038-2Communications Optical fibre communication Dr Mohammad N Patwary Room C336 (Beacon) Email: m.n.patwary@staffs.ac.uk Phone: 353 557

2. Introduction • Transmission via beams of light traveling over thin glass fibers is a relative newcomer to communications technology, • beginning in the 1970s, • reaching full acceptance in the early 1980s, • and continuing to evolve since then • Fibers now form a major part of the infrastructure for telecommunications information highways around the globe and serve as the transmission media of choice for numerous local area networks. • In addition, short lengths of fiber serve as transmission paths for the control of manufacturing processes and for sensor applications. • The steadily increasing demand for information capacity has driven the search for transmission media capable of delivering the required bandwidths. • Optical carrier transmission has been able to meet the demand and should continue to do so for many years.

3. Fundamentals • Optical communications refers to the transmission of information signals over carrier waves that oscillate at optical frequencies. • Optical fields oscillate at frequencies much higher than radio waves or microwaves, as indicated on the abbreviated chart of the electromagnetic spectrum in following figure. • Frequencies and wavelengths are indicated on the figure.

4. Fundamentals • For historical reasons, optical oscillations are usually described by their wavelengths rather than their frequencies. The two are related by where f is the frequency in hertz,  is the wavelength, and c is the velocity of light in empty space (3×108 m/s). • A frequency of 3×1014 Hz corresponds to a wavelength of 10-6m (a millionth of a meter is often called a micrometer). • Wavelengths of interest for optical communications are on the order of a micrometer.

5. Fundamentals • Glass fibers have low loss in the three regions illustrated in the following figure, covering a range from 0.8 to 1.6 μm (800 to 1600 nm).

6. Fundamentals • This corresponds to a total bandwidth of almost 21014Hz. The loss is specified in decibels, defined by • Where P1 and P2 are the input and output powers. • Typically, fiber transmission components are characterized by their loss or gain in decibels. • The beauty of the decibel scale is that the total decibel value for a series of components is simply the sum of their individual decibel gains and losses.

7. Fundamentals • Losses in the fiber and in other components limit the length over which transmission can occur. • Optical amplification and regeneration are needed to boost the power levels of weak signals for very long paths. • The characteristically high frequencies of optical waves (on the order of 2×1014 Hz) allow vast amounts of information to be carried. • A single optical channel utilizing a bandwidth of just 1% of this center frequency would have an enormous bandwidth of 2×1012 Hz. • As an example of this capacity, consider frequency division multiplexing of commercial television programs. Since each TV channel occupies 6 MHz, over 300,000 television programs could be transmitted over a single optical channel.

8. Multiplexing • In addition to electronic multiplexing schemes, such as frequency-division multiplexing of analog signals and time-division multiplexing of digital signals, numerous optical multiplexing techniques exist for taking advantage of the large bandwidths available in the optical spectrum. • These include • wavelength division multiplexing (WDM) and • optical frequency-division multiplexing (OFDM). • These technologies allow the use of large portions of the optical spectrum. • The total available bandwidth for fibers approaches 2×1014 Hz (corresponding to the 0.8-1.6mm range). • Although atmospheric propagation is possible, the vast majority of optical communications utilizes the waveguiding glass fiber.

9. Optical Communications Systems History • A key element for optical communications, a coherent source of light, became available in 1960 with the demonstration of the first laser. • This discovery was quickly followed by plans for numerous laser applications, including atmospheric optical communications. • Developments on empty space optical systems in the 1960s laid the groundwork for fiber communications in the 1970s. • The first low-loss optical waveguide, the glass fiber, was fabricated in 1970. Soon after, fiber transmission systems were being designed, tested, and installed. • Fibers have proven to be practical for path lengths of under a meter to distances as long as needed on the Earth’s surface and under its oceans (for example, almost 10,000 km for transpacific links).

10. Optical Communications Systems History • Fiber communications are now common for telephone, local area, and cable television networks. • Fibers are also found in short data links (such as required in manufacturing plants), closed-circuit video links, and sensor information generation and transmission.

11. Optical Communications Systems • A block diagram of a point-to-point fiber optical communications system shown in the figure below. This is the structure typical of the telephone network.

12. Optical Communications Systems • The fiber telephone network is digital, operating at data rates from a few megabits per second up to 2.5 Gb/s and beyond. • At the 2.5-Gb/s rate, several thousand digitized voice channels (each operating at 64 kb/s) can be transmitted along a single fiber using time-division multiplexing (TDM). • Because cables may contain more than one fiber (in fact, some cables contain hundreds of fibers), a single cable may be carrying hundreds of thousands of voice channels. • Rates in the tens of gigabit per second are attainable, further increasing the potential capacity of a single fiber.

13. Optical Communications Systems • Telephone applications may be broken down into several distinctly different areas: transmission between telephone exchanges, long-distance links, undersea links, and distribution in the local loop (that is, to subscribers). • Although similarities exist among these systems, the requirements are somewhat different. • Between telephone exchanges, large numbers of calls must be transferred over moderate distances. • Because of the moderate path lengths, optical amplifiers or regenerators are not required. • On the other hand, long-distance links (such as between major cities) require signal boosting of some sort (either regenerators or optical amplifiers). • Undersea links (such as transatlantic or transpacific) require multiple boosts in the signal because of the long path lengths involved

14. Optical Communication Networks • One architecture for the subscriber distribution network, called fiber-to-the-curb (FTTC), is depicted in following figure. Signals are transmitted over fibers through distribution hubs into the neighborhoods.

15. Optical Communication Networks • The fibers terminate at optical network units(ONUs) located close to the subscriber. • The ONU converts the optical signal into an electrical one for transmission over copper cables for the remaining short distance to the subscriber. • Because of the power division at the hubs, optical amplifiers are needed to keep the signal levels high enough for proper signal reception. • Cable television distribution remained totally conducting for many years. This was due to the distortion produced by optical analog transmitters. • Production of highly linear laser diodes [such as the distributed feedback (DFB) laser diode] in the late 1980s allowed the design of practical analog television fiber distribution links.

16. Optical Communication Networks • Conversion from analog to digital cable television transmission is facilitated by the vast bandwidths that fibers make available and by signal compression techniques that reduce the required bandwidths for digital video signals. • Applications such as local area networks (LANs) require distribution of the signals over shared transmission fiber. • Possible topologies include • the passive star, • the active star, and • the ring network

17. Passive star network FIGURE :Passive star network:T represents an optical transmitter and R represents an optical receiver

18. Active star Network

19. Ring Network Fibers connect the nodes together, while the terminals and nodes are connected electronically Ring network: T represents an optical transmitter and R represents an optical receiver. The nodes act as optical regenerators.

20. Components for Optical Communications Systems • The major components found in optical communications systems are: • Modulators, • Light sources, • Fibers, • Photo-detectors, • Connectors, • Splices, • Directional couplers, Star couplers, • Regenerators, • Optical amplifiers. • They are briefly described in the remainder of this Lecture.

21. Fibers • Fiber links spanning more than a kilometer typically use silica glass fibers, as they have lower losses than either plastic or plastic cladded silica fibers. • The loss properties of silica fibers were indicated in slide 5 . • Material and waveguide dispersion cause pulse spreading, leading to inter-symbol interference. • This limits the fiber’s bandwidth and, subsequently, its data-carrying capability. The amount of pulse spreading is given by

22. Fibers • Where M is the material dispersion factor and Mg is the waveguide dispersion factor, L is the fiber length, and Dλ is the spectral width of the emitting light source. • Because dispersion is wavelength dependent, the spreading depends on the chosen wavelength (λ) and on the spectral width (D) of the light source. • The total dispersion (M+Mg) has values near 120, 0, and 15 ps/(nm  km) at wavelengths 850, 1300, and 1550 nm, respectively.

23. Multimode fibers • Allow more than one mode to simultaneously traverse the fiber. This produces distortion in the form of widened pulses because the energy in different modes travels at different velocities. Again, intersymbol interference occurs. For this reason, multimode fibers are only used for applications where the bandwidth (or data rate) and path length are not large.

24. Single-mode fibers • Limit the propagation to a single mode, thus eliminating multimode spreading. • Since they suffer only material and waveguide dispersive pulse spreading, these fibers (when operating close to the zero dispersion wavelength) have greater bandwidths than multimode fibers and are used for the longest and highest data rate systems.

25. Fibers and Bandwidth • Tables given below list bandwidth limits for several types of fibers and illustrates typical fiber sizes.

26. Fiber’s Properties • Step index fibers (SI) have a core having one value of refractive index and a cladding of another value. • Graded-index (GRIN) fibers have a core index whose refractive index decreases with distance from the axis and is constant in the cladding. • As noted, single-mode fibers have the greatest bandwidths. To limit the number of modes to just one, the cores of single-mode fibers must be much smaller than those of multimode fibers. • Because of the relatively high loss and large dispersion in the 800-nm first window, applications there are restricted to moderately short path lengths (typically less than a kilometer). Because of the limited length, multimode fiber is practical in the first window. • Light sources and photo detectors operating in this window tend to be cheaper than those operating at the longer wavelength second and third windows

27. Fiber’s Properties • The 1300-nm second window, having moderately low losses and nearly zero dispersion, is utilized for moderate to long path lengths. • Non-repeatered paths up to 70 km or so are attainable in this window. In this window, both single-mode and multimode applications exist. • Multimode is feasible for short lengths required by LANs (up to a few kilometer) and single-mode for longer point-to-point links. • Fiber systems operating in the 1550-nm third window cover the highest rates and longest unamplified, unrepeated distances. • Lengths on the order of 200 km are possible. Single-mode fibers are typically used in this window. Erbium-doped optical amplifiers operate in the third window, boosting the signal levels for very long systems (such as those traversing the oceans).

28. Other Components • Semiconductor laser diodes (LD) or light-emitting diodes (LED) serve as the light sources for most fiber systems. These sources are typically modulated by electronic driving circuits. The conversion from signal current I to optical power P is given by • Where a0 and a1 are constants. Thus, the optical power waveform is a replica of the modulation current. • For very high-rate modulation, external integrated optic devices are available to modulate the light beam after its generation by the source.

29. Other Components • Laser diodes are more coherent (they have smaller spectral widths) than LEDs and thus produce less dispersive pulse spreading, according to Eq. : • In addition, laser diodes can be modulated at higher rates (tens of gigabit per second) than LEDs (which are limited to rates of just a few hundred megabit per second). • LEDs have the advantage of lower cost and simpler driving electronics. • Photodetectors convert the optical beam back into an electrical current. Semiconductor PIN photodiodes and avalanche photodiodes (APD) are normally used. The conversion for the PIN diode is given by the linear equation

30. Other Components • The conversion for the PIN diode is given by the linear equation • Where I is the detected current, P is the incident optical power, and ρ is the photodetector’s responsivity. • Typical values of the responsivity are on the order of 0.5 mA/mW. • The receiver current is a replica of the optical power waveform (which is itself a replica of the modulating current). Thus, the receiver current is a replica of the original modulating signal current, as desired.

31. An optical regenerator • An optical regenerator (or repeater) consists of an optical receiver, electronic processor, and an optical transmitter. • Regenerators detect (that is, convert to electrical signals) pulse streams that have weakened because of travel over long fiber paths, electronically determine the value of each binary pulse, and transmit a new optical pulse stream replicating the one originally transmitted. • Using a series of regenerators spaced at distances of tens to hundreds of kilometers, total link lengths of thousands of kilometers are produced. Regenerators can only be used in digital systems. • Optical amplifiers simply boost the optical signal level without conversion to the electrical domain. This simplifies the system compared to the use of regenerators. • In addition, optical amplifiers work with both analog and digital signals.

32. Splices and connectors • Splices and connectors are required in all fiber systems. Many types are available. Losses tend to be less than 0.1 dB for good splices and just a few tenths of a decibel for good connectors. • Fibers are spliced either mechanically or by actually fusing the fibers together. • Directional couplers split an optical beam traveling along a single fiber into two parts, each traveling along a separate fiber. • The splitting ratio is determined by the coupler design. In a star coupler the beam entering the star is evenly divided among all of the output ports of the star. • Typical stars operate as 88, 1616, or 3232 couplers. As an example, a 3232 port star can accommodate 32 terminals on a LAN.

33. Signal Quality • Signal quality is measured by the signal-to-noise ratio (S/N) in analog systems and by the bit error rate (BER) in digital links. The signal-to-noise ratio in a digital network determines the error rate and is given by: • Where P is the received optical power, ρ is the detector’s un-amplified responsivity, M is the detector gain if an APD is used, n (usually between 2 and 3) accounts for the excess noise of the APD, B is the receiver’s bandwidth, k is the Boltzmann constant (k= 1.381023 J/K), e is the magnitude of the charge on an electron (1.61019 C), T is the receiver’s temperature in kelvin, IDis the detector’s dark current, and RLis the resistance of the load resistor that follows the photodetector.

34. Signal Quality • The first term in the denominator of Eq. (previous slide) is caused by shot noise, and the second term is attributed to thermal noise in the receiver. If the shot noise term dominates (and the APD excess loss and dark current are negligible), the system is shot-noise limited. If the second term dominates, the system is thermal-noise limited In a thermal-noise limited system, the probability of error Pe (which is the same as the bit error rate) is • where erfis the error function, tabulated in many references • An error rate of 10-9requires a signal-to-noise ratio of nearly 22 dB (S/N = 158.5).

35. System Design • System design involves ensuring that the signal level at the receiver is sufficient to produce the desired signal quality. • The difference between the power available from the transmitting light source (e.g., Pt in dBm) and the receiver’s sensitivity (e.g., Pr in dBm) defines the system power budget L. • Thus, the power budget is the allowed accumulated loss for all system components and is given (in decibels) by

36. System Design • In addition to ensuring sufficient available power, the system must meet the bandwidth requirements for the given information rate. • This requires that the bandwidths of the transmitter, the fiber, and the receiver are sufficient for transmission of the message

37. Defining Terms • Avalanche photodiode: Semiconductor photodetector that has internal gain caused by avalanche breakdown. • Bit error rate: Fractional rate at which errors occur in the detection of a digital pulse stream. It is equal to the probability of error. • Dispersion: Wavelength-dependent phase velocity commonly caused by the glass material and the structure of the fiber. It leads to pulse spreading because all available sources emit light covering a (small) range of wavelengths. That is, the emissions have a nonzero spectral width. • Integrated optics: Technology for constructing one or more optical devices on a common waveguiding substrate. • Laser: A source of coherent light, that is, a source of light having a small spectral width. • Laser diode: A semiconductor laser. Typical spectral widths are on the order of 1–5 nm.

38. Defining Terms • Light-emitting diode: A semiconductor emitter whose radiation typically is not as coherent as that of a laser. Typical spectral widths are on the order of 20–100 nm. • Multimode fiber: A fiber that allows the propagation of many modes. • Optical frequency-division multiplexing: Multiplexing many closely spaced optical carriers onto a single fiber. Theoretically, hundreds (and even thousands) of channels can be simultaneously transmitted using this technology. • PIN photodiode: Semiconductor photodetector converting the optical radiation into an electrical current. • Receiver sensitivity: The optical power required at the receiver to obtain the desired performance (either the desired signal-to-noise ratio or bit error rate). • Responsivity: The current produced per unit of incident optical power by a photodetector.

39. Defining Terms • Signal-to-noise ratio: Ratio of signal power to noise power. • Single-mode fiber: Fiber that restricts propagation to a single mode. This eliminates modal pulse spreading, increasing the fiber’s bandwidth. • Wavelength-division multiplexing: Multiplexing several optical channels onto a single fiber. The channels tend to be widely spaced (e.g., a two-channel system operating at 1300 nm and 1550 n