EKT 465 OPTICAL COMMUNICATION SYSTEM Chapter 2 OPTICAL FIBER COMMUNICATIONS SEMESTER 1-2017/18 3 Credit Hours
222.3 Gbps pada 2017, daripada 6.4Gbps pada 2012 10/3/2017 2
Light Propagation & Transmission Characteristics of Optical Fiber 10/3/2017 3
2.1 A History of Fiber Optic Technology 10/3/2017 4
The Nineteenth Century John Tyndall, 1870 water and light experiment demonstrated light used internal reflection to follow a specific path William Wheeling, 1880 piping light patent never took off Alexander Graham Bell, 1880 optical voice transmission system called a photophone free light space carried voice 200 meters Fiber-scope, 1950 s Light 10/3/2017 5
The Twentieth Century Glass coated fibers developed to reduce optical loss Inner fiber - core Glass coating - cladding Development of laser technology was important to fiber optics Large amounts of light in a tiny spot needed 1960, ruby and helium-neon laser developed 1962, semiconductor laser introduced - most popular type of laser in fiber optics core cladding 10/3/2017 6
The Twentieth Century (cont.) 1966, Charles Kao and Charles Hockman proposed optical fiber could be used to transmit laser light if attenuation could be kept under 20dB/km (optical fiber loss at the time was over 1,000dB/km) 1970, Researchers at Corning developed a glass fiber with less than a 20dB/km loss Attenuation depends on the wavelength of light Today, less than 4dB/km 10/3/2017 7
The Twentieth Century (cont.) Military 1970 s, Fiber optic telephone link installed aboard the U.S.S. Little Rock 1976, Air Force developed Airborne Light Fiber Technology (ALOF) Commercial 1977, AT&T and GTE installed the first fiber optic telephone system Fiber optic telephone networks are common today Research continues to increase the capabilities of fiber optic transmission 10/3/2017 8
Applications of Fiber Optics Military Advanced Computer BioMedical Optometric Optical Sensor Communication Robotics High-tech Industries 10/3/2017 9
Military Application 10/3/2017 10
Military Application 10/3/2017 11
Computer Application 10/3/2017 12
Sensors Gas sensors Chemical sensors Mechanical sensors Fuel sensors Distance sensors Pressure sensors Fluid level sensors Gyro sensors 10/3/2017 13
Medical Application 10/3/2017 14
The Future (Present) Fiber Optics have immense potential bandwidth (over 1 Terahertz, 10 12 Hz) Fiber optics is predicted to bring broadband services to the home interactive video interactive banking and shopping distance learning security and surveillance high-speed data communication digitized video 10/3/2017 15
2.2 Fiber Optic Fundamentals 10/3/2017 16
Advantages of Fiber Optics o Immunity from Electromagnetic (EM) Radiation and Lightning o Lighter Weight o Higher Bandwidth o Better Signal Quality o Lower Cost o Easily Upgraded o Ease of Installation 10/3/2017 17
Immunity from EM Radiation and Lightning o Fiber is made from dielectric (non-conducting) materials, It is un affected by EM radiation. o Immunity from EM radiation and lightning most important to the military and in aircraft design. o The fiber can often be run in same conduits that currently carry power, simplifying installation. 10/3/2017 18
Lighter Weight o Copper cables can often be replaced by fiber optic cables that weight at least ten times less. o For long distances, fiber optic has a significant weight advantage over copper cable. 10/3/2017 19
Higher Bandwidth o Fiber has higher bandwidth than any alternative available. o Cable TV (CATV) industry in USA and Europe required amplifiers every thousand feet, when copper cable was used (due to limited bandwidth of the copper cable). o A modern fiber optic system can carry the signals up 100km without repeater or without amplification. 10/3/2017 20
Better Signal Quality o Because fiber is immune to EM interference, has lower loss per unit distance, and wider bandwidth, signal quality is usually substantially better compared to copper. 10/3/2017 21
Lower Cost o Fiber certainly costs less for long distance applications. o The cost of fiber itself is cheaper per unit distance than copper if bandwidth and transmission distance requirements are high. 10/3/2017 22
Principles of Fiber Optic Transmission o Electronic signals converted to light o Light refers to more than the visible portion of the electromagnetic (EM) spectrum 10/3/2017 23
Principles of Fiber Optic Transmission (cont.) o Light is organized into what is known as the electromagnetic spectrum. o The electromagnetic spectrum is composed of visible and nearinfrared light like that transmitted by fiber and all other wavelengths used to transmit signals such as AM and FM and television. 10/3/2017 24
Principles of Fiber Optic Transmission (cont.) o Wavelength - the distance a single cycle of an EM wave covers o For fiber optics applications, two categories of wavelength are used 1. Visible (400 to 700 nanometers) - limited use 2. Near-infrared (700 to 2000 nanometers) - used almost always in modern fiber optic systems 10/3/2017 25
Principles of Fiber Optic Transmission (cont.) o Fiber optic links contain three basic elements 1. Transmitter 2. Optical fiber 3. Receiver Optical Fiber User Input(s) Transmitter Receiver User Output(s) Electrical-to-Optical Conversion Optical-to-Electrical Conversion 10/3/2017 26
Principles of Fiber Optic Transmission (cont.) Transmitter (TX) o Electrical interface encodes user s information through AM, FM or Digital Modulation o Encoded information transformed into light by means of a light-emitting diode (LED) or laser diode (LD) User Input(s) Electrical Interface Data Encoder/ Modulator Light Emitter Optical Output 10/3/2017 27
Receiver (RX) Principles of Fiber Optic Transmission (cont.) o o Decodes the light signal back into an electrical signal Types of light detectors typically used: 1. PIN photodiode 2. Avalanche photodiode o o Made from silicon (si), indium gallium arsenide (ingaas) or germanium (ge) The data decoder/demodulator converts the signals into the correct format Optical Input Light Detector/ Amplifier Data Decoder/ Demodulator Electrical Interface User Output(s) 10/3/2017 28
Principles of Fiber Optic Transmission (cont.) Transmission comparison metallic: limited information and distance free-space: large bandwidth long distance not private costly to obtain useable spectrum optical fiber: offers best of both 10/3/2017 29
2.3 Fiber Optic Components 10/3/2017 30
Fiber Optics Cable o Extremely thin strands of ultra-pure glass o Three main regions center: core (9 to 100 microns) middle: cladding (125 or 140 microns) outside: coating or buffer (250, 500 and 900 microns) 10/3/2017 31
A FIBER STRUCTURE 10/3/2017 32
2.3.2 Light Emitters Two types Light-emitting diodes (LED s) Surface-emitting (SLED): difficult to focus, low cost Edge-emitting (ELED): easier to focus, faster Laser Diodes (LD s) narrow beam fastest 10/3/2017 33
Two types Optical Detectors Avalanche photodiode internal gain more expensive extensive support electronics required PIN photodiode very economical does not require additional support circuitry used more often 10/3/2017 34
Interconnection Devices Connectors, splices, couplers, splitters, switches, wavelength division multiplexers (WDM s) Examples Interfaces between local area networks and devices Patch panels Network-to-terminal connections 10/3/2017 35
2.4 - Manufacture of Optical Fiber 10/3/2017 36
Introductions o 1970, Corning developed Inside Vapor Deposition (IVD) to first achieve attenuation less than 20dB/km o Later, Corning developed Outside Vapor Deposition (OVD) which increased the purity of fiber o Optical fiber was developed that exhibits losses as low as 0.2dB/km (at 1550nm). This seemed to be adequate for any application. 10/3/2017 37
Modified Chemical Vapor Deposition (MCVD) another term for IVD method vaporized raw materials are deposited into a premade silica tube 10/3/2017 38
Outside Vapor Deposition (OVD) o Vaporized raw materials are deposited on a rotating rod o The rod is removed and the resulting preform is consolidated by heating o Preform later will going into drawing process 10/3/2017 39
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2.5 Principles Operation 10/3/2017 41
Introduction o An optical fiber is a very thin strand of silica glass in geometry quite like a human hair. o In reality it is a very narrow, very long glass cylinder with special characteristics. o When light enters one end of the fiber it travels (confined within the fiber) until it leaves the fiber at the other end. o Two critical factors stand out: o Very little light is lost in its journey along the fiber o Fiber can bend around corners and the light will stay within it and be guided around the corners. 10/3/2017 42
Introduction (cont.) o An optical fiber consists of two parts: the core and the cladding. o The core is a narrow cylindrical strand of glass and the cladding is a tubular jacket surrounding it. o The core has a (slightly) higher refractive index than the cladding. This means that the boundary (interface) between the core and the cladding acts as a perfect mirror. o Light traveling along the core is confined by the mirror to stay within it - even when the fiber bends around a corner. 10/3/2017 43
Basic Principle: Total Internal Reflection 10/3/2017 44
Light Propagation: Refracted o The light waves spread out along its beam. o Speed of light depend on the material used called refractive index. o Speed of light in the material = speed of light in the free space/refractive index o Lower refractive index higher speed o The changing of a light ray s direction (loosely called bending) when it passes through variations in refractive index is called refraction This end travels further than the other hand Lower Index Higher Index 10/3/2017 45
Light Propagation: Total Internal Reflection o Total internal reflection reflects 100% of the light o A typical mirror only reflects about 90% o Fish tank analogy 10/3/2017 46
Refraction o Light entering an optical fiber bends in towards the center of the fiber refraction Refraction LED or LASER Source 10/3/2017 47
Reflection o Light inside an optical fiber bounces off the cladding - reflection Reflection LED or LASER Source 10/3/2017 48
Critical Angle o If light inside an optical fiber strikes the cladding too steeply, the light refracts into the cladding - determined by the critical angle Critical Angle 10/3/2017 49
Angle of Incidence o Also called as Incident Angle o Measured from perpendicular o Exercise: Mark two more incident angles Incident Angles 10/3/2017 50
Angle of Reflection o Also reflection angle o Measured from perpendicular o Exercise: Mark the other reflection angle Reflection Angle 10/3/2017 51
Reflection o Thus light is perfectly reflected at an interface between two materials of different refractive index if: o The light is incident on the interface from the side of higher refractive index. o The angle θ is greater than a specific value called the critical angle. 10/3/2017 52
Angle of Refraction o Also refraction angle o Measured from perpendicular o Exercise: Mark the other refraction angle Refraction Angle 10/3/2017 53
Angle Summary o Three important angles o The reflection angle always equals the incident angle Refraction Angle Incident Angles Reflection Angle 10/3/2017 54
Index of Refraction n = c / v c = velocity of light in a vacuum v = velocity of light in a specific medium Light bends as it passes from one medium to another with a different index of refraction air, n is about 1 glass, n is about 1.4 Light bends away from normal - higher n to lower n Light bends in towards normal - lower n to higher n 10/3/2017 55
Snell s Law The angles of the rays are measured with respect to the normal. n 1 sin 1 = n 2 sin 2 Where n 1 and n 2 are refractive index of two materials 1 and 2 the angle of incident and refraction respectively 10/3/2017 56
Snell s Law (cont.) o The amount light is bent by refraction is given by Snell s Law: n 1 sin 1 = n 2 sin 2 o Light is always refracted into a fiber (although there will be a certain amount of Fresnel reflection) o Light can either bounce off the cladding (TIR) or refract into the cladding 10/3/2017 57
Snell s Law (Example 1) Calculate the angle of refraction at the air/core interface Solution - use Snell s law: n 1 sin 1 = n 2 sin 2 1 sin (30 ) = 1.47 sin ( refraction ) refraction = sin -1 (sin(30 )/1.47) refraction = 19.89 n air = 1 n core = 1.47 n cladding = 1.45 q incident = 30 10/3/2017 58
Snell s Law (Example 2) Calculate the angle of refraction at the core/cladding interface Solution - use Snell s law and the refraction angle from Example 3.1 1.47sin (90-19.89 ) = 1.45 sin( refraction ) refraction = sin -1 (1.47 sin (70.11 )/1.45) refraction = 72.42 n air = 1 n core = 1.47 n cladding = 1.45 incident = 30 10/3/2017 59
Snell s Law (Example 3) Calculate the angle of refraction at the core/cladding interface for the new data below Solution: 1 sin (10 ) = 1.45 sin ( refraction(core) ) refraction(core) = sin -1 (sin(10 )/1.45) = 6.88 1.47 sin (90-6.88 ) = 1.45 sin ( refraction(cladding) ) refraction(cladding) = sin -1 (1.47 sin (83.12 ) / 1.45) = sin -1 (1.0065) = can t do light does not refract into cladding, it reflects back into the core (TIR) n air = 1 n core = 1.47 n cladding = 1.45 incident = 10 10/3/2017 60
Critical Angle Calculation o The angle of incidence that produces an angle of refraction of 90 is the critical angle n 1 sin( c ) = n 2 sin(90 ) n 1 sin( c ) = n 2 c = sin -1 (n 2 /n 1 ) Light at incident angles greater than the critical angle will reflect back into the core n 1 = Refractive index of the core n 2 = Refractive index of the cladding Critical Angle, c 10/3/2017 61
Numerical Aperture o The Numerical Aperture is the sine of the largest angle contained within the cone of acceptance. o NA is related to a number of important fiber characteristics. It is a measure of the ability of the fiber to gather light at the input end. The higher the NA the tighter (smaller radius) we can have bends in the fiber before loss of light becomes a problem. The higher the NA the more modes we have, the greater will be the dispersion of this fiber (in the case of MM fiber). Thus higher the NA of SM fiber the higher will be the attenuation of the fiber o Typical NA for single-mode fiber is 0.1. For multimode, NA is between 0.2 and 0.3 (usually closer to 0.2). 10/3/2017 62
Acceptance Cone o There is an imaginary cone of acceptance with an angle α o The light that enters the fiber at angles within the acceptance cone are guided down the fiber core Acceptance Angle, a Acceptance Cone 10/3/2017 63
Acceptance Angle and NA o The angle of light entering a fiber which follows the critical angle is called the acceptance angle, a a = sin -1 [(n 12 -n 22 ) 1/2 ] Numerical Aperature (NA) describes the lightgathering ability of a fiber NA = sin a = (n 12 -n 22 ) 1/2 Acceptance Angle, a Critical Angle, c n 1 = Refractive index of the core n 2 = Refractive index of the cladding 10/3/2017 64
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Acceptance Cone: Important 10/3/2017 66
Acceptance Cone: Important Light Ray A (green) entered acceptance cone; transmitted through the core by Total Internal Reflection (TIR) Light Ray B (yellow) did not enter acceptance cone; signal lost 10/3/2017 67
Summary: Snell s Law 10/3/2017 68
Formula Summary Index of Refraction n c v Snell s Law n1 sin n sin 1 2 2 Critical Angle c sin 1 n n 2 1 Acceptance Angle a sin 1 n 2 1 n 2 2 Numerical Aperture NA sin a n 2 1 n 2 2 10/3/2017 69
Practice Problems 10/3/2017 70
Practice Problems (1) Calculate: i. angle of refraction at the air/core interface, r ii. critical angle, c iii. incident angle at the core/cladding interface, I Will this light ray propagate down the fiber? air/core interface n air = 1 n core = 1.46 n cladding = 1.43 incident = 12 core/cladding interface Answers: r = 8.2 c = 78.4 i = 81.8 light will propagate 10/3/2017 71
Solution (1) 1 sin (12 ) = 1.46 sin ( r ) r = sin -1 (sin (12 ) / 1.46) = 8.2 c = sin -1 (1.43 / 1.46) = 78.4 i = 90-8.2 = 81.8 The light ray will reflect off the cladding and propagate down the fiber since the incident angle at the cladding is greater than the critical angle. air/core interface core/cladding interface n air = 1 n core = 1.46 n cladding = 1.43 incident = 12 10/3/2017 72
Refractive Indices and Propagation Times 6 5 4 3 2 1 0 Refractive Index Propagation Time (ns/m) Vacuum 1 3.336 Air 1.003 3.346 Water 1.333 4.446 Fused Silica 1.458 4.863 Belden Cable (RG- 59/U) N/A 5.551 10/3/2017 73
Propagation Time Formula Metallic cable propagation delay cable dimensions frequency Optical fiber propagation delay related to the fiber material formula t = Ln/c t = propagation delay in seconds L = fiber length in meters n = refractive index of the fiber core c = speed of light (2.998 x 10 8 meters/second) 10/3/2017 74
Temperature and Wavelength Considerations for detailed analysis Fiber length is slightly dependent on temperature Refractive index is dependent on wavelength 10/3/2017 75
2.6 Types of Optical Fiber 10/3/2017 76
Optical Fiber Types Mutimode(MM) Larger core of MM fiber allows hundreds of rays (modes) of light to move through the fiber simultaneously Step-index: abrupt change in index of refraction from core to cladding Graded-index: light follows a curved path, decreases dispersion 10/3/2017 77
Single Mode Fiber (SMF) Single Mode Fiber (SMF) has a smaller core only one ray (mode) is transmitted better for maintaining the fidelity of each light pulse exhibit less dispersion caused by multiple rays higher bandwidth, lower attenuation Single-mode fiber - disadvantages Smaller core diameter makes coupling light into the core more difficult Tolerances for single-mode connectors and splices are also much more demanding 10/3/2017 78
Single Mode Fiber (SMF)(cont.) Single-mode fiber types being deployed at this time. 1. Non-DSF (NDSF) - oldest type of single-mode fiber 2. Dispersion Shifted Fiber (DSF) - fiber nonlinearities make this fiber undersirable for DWDM applications 3. (+D) NZ-DSF - Zero-dispersion wavelength placed outside of 1550nm window. Has positive dispersion slope. 4. (-D) NZ-DSF - like above, except has negative dispersion slope. Allows alternating segments of (-D) NZ-DSF and (+D) NZ-DSF to yield an overall near zero dispersion 10/3/2017 79
Single Mode Fiber (SMF)(cont.) Polarization-Maintaining Fiber (PM) important variety of single-mode fiber designed to propagate only one polarization of the input light important for components that require a polarized light input (like external modulators) contain stress rods - two additional circles which create stress causing only one polarization plane of light to be favored PM fiber requires more care when spliced - X, Y, Z and rotational alignment must be precise. 10/3/2017 80
2.7 Optical Fiber Attenuation 10/3/2017 81
Po(mW) Optical Fiber Attenuation Logarithmic relationship between the optical output power and the optical input power Measure of the decay of signal strength or light power P ( z ) where: P(z) = Optical Power at distance z from the input P o = Input optical power -a = Fiber attenuation coefficient, [1/km] P o e a ' z 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Optical Attenuation alpha prime = 0.1 alpha prime = 0.3 alpha prime = 0.5 0 5 10 15 20 z (km) 10/3/2017 82
Optical Fiber Attenuation Usually, attenuation is expressed in terms of decibels Attenuation Conversion: a = 4.343a where: P( z) P o 10 az /10 10 log a z P(z) = Optical Power at distance z from the input P o = Input optical power a = Fiber attenuation coefficient, [db/km] a = a scattering + a absorption + a bending P out P in 10/3/2017 83
Optical Fiber Attenuation (cont.) Factors contribute to fiber attenuation: o Intrinsic Factors 1. Scattering and 2. Absorption o Extrinsic Causes 1. Factory 2. Environment 3. Micro and Macro bending 10/3/2017 84
Optical Fiber Attenuation (cont.) o Rayleigh Scattering - most common form of scattering caused by microscopic non-uniformities making light rays partially scatter nearly 90% of total attenuation is attributed to Raleigh Scattering becomes important when wavelengths are short - comparable to size of the structures in the glass: long wavelengths are less affected than short wavelengths 10/3/2017 85
Optical Fiber Attenuation (cont.) o Absorption - can be caused by the molecular structure, impurities such as metal ions, OH - ions (water) and atomic defects optical energy is absorbed and dissipated as a small amount of heat between 1250nm and 1390nm - optical loss is due to OH - ions above 1700nm - glass starts absorbing light energy due to the molecular resonance of the SiO 2 molecule 10/3/2017 86
Optical Fiber Attenuation (cont.) o Extrinsic Causes of Attenuation 1. Cable manufacturing stresses 2. Environmental effects 3. Physical bends microbending - result of microscopic imperfections in the geometry of the fiber macrobending - fiber bending with diameters on the order of centimeters 10/3/2017 87
Attenuation Due to Microbending and Macrobending Microbending - result of microscopic imperfections in the geometry of the fiber Macrobending - fiber bending with diameters on the order of centimeters (usually unoticeable if the radius of the bend is larger than 10 cm) 10/3/2017 88
Optical Fiber Attenuation (cont.) o Attenuation of an optical fiber segment depends on its length and the wavelength of the light traveling through the fiber. 10/3/2017 89
Example 1 Given: Input Power = 1mW Length = 1.3km Attenuation Coefficient, a = 0.6dB/km Find: Output Power Solution: a = 0.6B/km Pin = 1mW Pout =? 1.3km P(z) = Po10 -az/10 = (1m)10 -(0.6)(1.3)/10 = 836 W 10/3/2017 90
Problem 1 Given: Input Power = 1mW Length = 2.6km Attenuation Coefficient, a = 0.6dB/km Find: Output Power a = 0.6B/km Pin = 1mW Pout =? 2.6km 10/3/2017 91
Problem 2 Given: Input Power = 1mW Output Power = 250 W Length = 2km Find: Attenuation Coefficient, a Pin = 1mW a =? 2km Pout = 250 W 10/3/2017 92
2.8 Fiber Dispersions 10/3/2017 93
Dispersion (cont.) o Dispersion - spreading of light pulses in a fiber o It limits the bandwidth o Most important types 1. Multimode Dispersion 2. Chromatic Dispersion material dispersion waveguide dispersion profile dispersion 3. Polarization Mode Dispersion (PMD) 10/3/2017 94
Multimode Dispersion o Also call Modal Dispersion o Caused by different modes traveling at different speeds o Characteristic of multimode fiber only o Can be minimized by: Using a smaller core diameter Using graded-index fiber Use single-mode fiber - single-mode fiber is only single-mode at wavelengths greater than the cutoff wavelength o When multimode dispersion is present, it usually dominates to the point that other types of dispersion can be ignored. 10/3/2017 95
Multimode Dispersion (cont.) o Different modes take a different amount of time to arrive at the receiver. o Result is a s p r e a d - o u t s i g n a l o Graded Index Fiber prior discussion concerned with Step Index Fiber Design to solve modal dispersion GRIN fiber is designed so that all modes travel at nearly the same speed GRIN fiber core has a parabolic index of refraction 10/3/2017 96
Chromatic Dispersion Caused by different wavelengths traveling at different speeds Is the result of material dispersion, waveguide dispersion or profile dispersion For the fiber characteristics shown at right, chromatic dispersion goes to zero at 1550 nm (dispersion-shifted fiber) For a light-source with a narrow spectral emission, the bandwidth of the fiber will be very large. (FWHM = Full Width Half Maximum) 10/3/2017 97
Material Dispersion Caused by the fact that different wavelengths travel at different speeds through a fiber, even in the same mode. Determined by: range of light wavelengths injected into the fiber (spectral width of source) LEDs (35-170 nm) Lasers (< 5 nm) center operating wavelength of the source around 850 nm: longer wavelengths (red) travel faster than shorter wavelengths (blue) around 1550 nm: the situation is reversed - zero dispersion occurs where the wavelengths travel the same speed, around 1310 nm o Material dispersion greatly affects single-mode fibers. In multimode fibers, multimode dispersion usually dominates. 10/3/2017 98
Waveguide and Profile Dispersion o Waveguide Dispersion occurs because optical energy travels in both the core and cladding at slightly different speeds. A greater concern for single-mode fibers than for multimode fibers o Profile Dispersion the refractive indices of the core and cladding are described by a refractive index profile since the refractive index of a graded index fiber varies, it causes a variation in the propagation of different wavelengths profile dispersion is more significant in multimode fibers that in single-mode fibers 10/3/2017 99
Polarization Mode Dispersion o Complex optical effect that occurs in single-mode fibers o Most single-mode fibers support two perpendicular polarizations of the original transmitted signal o Because of imperfections, the two polarizations do not travel at the same speed. o The difference in arrival times is known as PMD (ps/km1/2) 10/3/2017 100
Dispersion: Recap 10/3/2017 101
Dispersion Effect to Bit Rate 10/3/2017 102
2.9 Fiber Comparisons 10/3/2017 103
Fiber Comparisons o Attenuation - smaller number indicates the better fiber o Bandwidth-Distance Product - bandwidth times distance for a fiber is a constant o Power - shows the relative optical power that can into the optical fiber using an LED 10/3/2017 104
Fiber Comparisons 10/3/2017 105
Fiber Comparisons Most popular fiber types (first number is core diameter, second number is cladding diameter) o 9/125 mm (SM): high data rate and long distance o 62.5/125 mm (MM): low-to-moderate data and video links, LANs o 50/125 mm (MM): military and gigabit ethernet o 100/140 mm (MM): used in aircraft 10/3/2017 106
Fiber Selection Criteria 1. Fiber core/cladding size 2. Fiber material and construction Most fibers are all glass Plastic Clad Silica (PCS) - high nuclear radiation and for imaging All Plastic - limited to very short distances 3. Fiber attenuation - depends heavily on the operating wavelength 4. Fiber bandwidth-distance product - gives the attenuation only due to multimode dispersion 5. Environmental considerations temperature is usually the most demanding parameter most optical fibers perform worse at low temperatures 10/3/2017 107
2.10 Fiber Nonlinearities 10/3/2017 108
Fiber Nonlinearities o Nonlinear fiber effects not important a few years ago - only attenuation and dispersion considered. o Because of higher data rates, longer transmission distances, multiple wavelengths and higher power levels, fiber nonlinearities must now be considered. 1. Stimulated Brillouin scattering (SBS) 2. stimulated Raman scattering (SRS) 3. four wave mixing (FWM) 4. Self-phase modulation (SPM) 5. Cross-phase modulation (XPM) 6. Intermodulation (mixing) 10/3/2017 109
Fiber Nonlinearities (cont.) o Two basic mechanisms 1. Kerr Effect; most serious - refractive index is dependent on the optical power (see Graph). Gives rise to SPM, XPM, FWM and intermodulation. Increasing A eff is how these nonlinearities are being minimized. 2. Scattering phenomena gives rise to SBS and SRS. n n n P / 0 2 A eff n 0 = normal refractive index of the core n 2 = nonlinear refractive index coefficient (2.35 x 10-20 m 2 /W for silica P = power in Watts A eff = effective area of the fiber core in square meters 10/3/2017 110
Fiber Nonlinearities (cont.) 10/3/2017 111
Fiber Nonlinearities (cont.) Scattering Phenomena - second set of mechanisms causing nonlinearities Stimulated Brillouin Scattering (SBS) imposes upper limit on power after the SBS threshold a significant fraction of the transmitted light is reflected back toward the transmitter SBS also introduces noise: BER worsens Particularly a problem with externally modulated continuous wave (CW) laser sources. SBS Threshold worsens with narrower linewidth lasers - improved by adding a small AC modulation signal to the DC current source which broadens the spectral linewidth 10/3/2017 112
Fiber Nonlinearities (cont.) Scattering Phenomena (Continued) Stimulated Brillouin Scattering (SBS) (Continued) another way to increase SBS threshold is to phase dither the output of the external modulator - see Graphs below. A high frequency (usually 2 x highest frequency) is imposed at the external modulator. Erbium-Doped Fiber Amplifiers (EDFAs) reduces the SBS threshold (in Watts) by the number of amplifiers. 10/3/2017 113
Fiber Nonlinearities (cont.) Scattering Phenomena (Continued) Stimulated Raman Scattering (SRS) much less of a problem than SBS threshold is close to 1 Watt, nearly a thousand times higher than SBS with an EDFA having an output power of 200mW, SRS threshold will be reached after 5 amplifiers. Recall that threshold drops with each amplifier. Shorter wavelengths are robbed of power and fed to longer wavelengths. (See Graphs below) 10/3/2017 114
Fiber Nonlinearities (cont.) Four Wave Mixing (FWM) usually only shows up with multiple wavelength systems, like DWDM. classified as a third-order distortion generates third harmonics with one channel with multiple wavelengths, generates third-order harmonics and a whole range of cross products. See graph at bottom left. Solid bars are original signals. Interfering cross products rapidly become a large number. See Graph at bottom right. No. of interfering = ½(N 3 N 2 ) 10/3/2017 115
Fiber Nonlinearities (cont.) Conventional four-wave mixing (left) and inverse four-wave mixing (right) 10/3/2017 116
Fiber Nonlinearities (cont.) Four Wave Mixing (FWM) (Continued) Two factors strongly influence magnitude of FWM products (mixing efficiency) channel spacing - wider is better (recommended International Telecommunications Union (ITU) channel spacing is 0.8nm) dispersion - smaller is better 10/3/2017 117
Fiber Nonlinearities (cont.) Self-phase Modulation (SPM) like FWM, SPM is due to the power dependency of the refractive index of the fiber core. Interacts with Chromatic Dispersion Increasing dispersion decreases FWM, but increases SPM leading edge of pulse increases refractive index - blue shift falling edge of pulse decreases refractive index - red shift introduces a frequency chirp which broadens pulse (see Graph at right) Cross-phase Modulation (XPM) very similar to SPM except it refers to two pulses overlapping and causing distortion increasing effective area reduces XPM Intermodulation (mixing) very similar to SPM and XPM deals with the effects of multiple wavelengths causing the index of refraction to change 10/3/2017 118
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