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Optical Fiber Transmission System Components for Optical Networks Optical Fiber Transmission System • Optical fiber – Propagation in fiber –Fiber modes – Attenuation – Dispersion – Non-linear effects •Optical transmitters – Laser principles –Modulation • Fiber •Optical receivers • Transmitter • Optical amplifiers – Laser or LED – fixed or tunable •Couplers – Modulator • Multiplexers • Receiver • Filters – Photodetector • Optical switches and crossconnects • Amplifier • Wavelength converters • Multiplexers/filters Optical Fiber Numerical Aperture of Fiber • Core and cladding – silica (SiO2) 2 2 •n1: refractive index of core n − n θmax = 1 2 •n2: refractive index of cladding • Acceptance angle: 0 n0 •n1 > n2 (~1.45) • θ1: angle of incidence max • For total internal reflection: θ0 < θ0 • θ2: angle of refraction • Snell’s Law: n1 sin θ1 = n2 sin θ2 max -1 • Numerical aperture: n0 sin θ0 •Critical angle: θcrit = sin (n2/n1) • Total internal reflection: θ1 > θcrit Fiber Modes Modal Dispersion • Modes corresponds to solutions of wave equations • Dispersion: spreading of signal in the time domain • Geometric interpretation: A mode is one possible path that a • Modal dispersion caused by multiple modes propagating along a guided ray may take in a fiber fiber • Each mode travels at a different speed • Limits bit rate and/or distance that signal can travel 1 Multimode vs. Single-Mode Fiber Graded-Index Fiber • Fiber will capture only a single mode for wavelength λ if: Δ 2π 2 2 •n1 > n2 > n3 > n4 > n5 V = a n1 − n2 < 2.405 λ • Reduces modal dispersion (number of modes reduced by ~ ½) –where a = core radius • Multimode fiber – Core diameter: 50-100 μm –For large V, number of modes ~= V2/2 • Single mode fiber – Core diameter: 8-10 μm – Captures only a single mode (fundamental mode) Attenuation in Fiber Attenuation in Fiber • Material absorption – Absorption by silica and impurities – Wavelength of light corresponds to vibrational resonant frequency of • Pin = input power molecules • Rayleigh scattering • Pout = output power – Small fluctuations in refractive index cause light to scatter • L = length of fiber in km – Effect stronger for shorter wavelengths • A = attenuation constant in dB/km • Receiver sensitivity: Pr = minimum power required at receiver • Loss in dB = A x L = - 10 log10 Pout/Pin •Pout = Pin x 10-AL/10 • Find maximum distance L for receiver sensitivity Pr –Pout > Pr -AL/10 hydroxyl ion (OH-) absorption –Pin x 10 > Pr – L < 10/A log10 Pin/Pr – e.g. Pin = 0.1 mW, A = 0.2 dB/km, Pr = 0.05 mW Rayleigh scattering infrared absorption • Lmax < 15 km ultraviolet absorption Wavelength Bands Dispersion in Fiber • S-band (short): 1450-1530 nm • Dispersion: broadening of pulse in time domain as it propagates • C-band (conventional): 1530-1570 nm along the fiber • L-band (long): 1570-1620 nm – Leads to inter-symbol interference • Types of dispersion • Defined by wavelengths at which specific components, such as – Modal dispersion – modes travel at different speeds amplifiers, can operate – Chromatic dispersion – different wavelengths travel at different speeds • material dispersion – refractive index is function of wavelength • waveguide dispersion – refractive index depends on distribution of power in core and cladding which depends on wavelength – Polarization mode dispersion – fundamental mode has two polarization states which travel at different speeds 2 Controlling Dispersion Fiber Types • Chromatic dispersion is zero near 1300 nm • Dispersion-shifted fiber (DSF) – change waveguide dispersion such that • Multimode fiber (MMF) zero dispersion at 1550 nm – Short range, low-cost transmitters, single channel • Nonzero dispersion-shifted fiber (NZ-DSF) – dispersion of 1-6 ps/nm- – 850 nm or 1300 nm km at 1550 nm – e.g., 100 Base-FX Fast Ethernet (~2 km) or 1000 Base-SX Gb Ethernet • Dispersion compensating fiber – insert fiber with negative dispersion (~500 m) between fibers with normal dispersion • Single mode fiber (SMF) – Moderate distance, single channel – 1300 nm – e.g., 1000 Base-LX Gb Ethernet (~5 km) • Dispersion shifted fiber (DSF) – Long distance, single channel – 1550 nm • Non-zero dispersion shifted fiber (NZ-DSF) – Long distance, DWDM systems – 1550 nm Fiber Nonlinearities Fiber Nonlinearities • Self-phase modulation (SPM) • Stimulated Brillouin Scattering (SBS) – refractive index depends on signal intensity – Interaction between signal and acoustic waves – changes in index lead to phase and frequency variations (chirp) – Shifts signal power to lower frequencies propagating in the – frequency variations lead to increased chromatic dispersion opposite direction of the original signal – limits maximum transmit power – Range of frequencies affected: 20 MHz • Cross-phase modulation (XPM) – Gain coefficient: 4x10-11 m/W – variations of signal intensity on other channels leads to phase shifts • Stimulated Raman Scattering (SRS) and chirp – Shifts signal power to lower frequencies propagating in the same – effect decreases with increased channel spacing direction as the original signal •Four-wave mixing – Range of frequencies affected: 40 THz – Gain coefficient: 6x10-14 m/W – signals at frequencies w1 and w2 generate new signals at 2w1-w2 and 2w2-w1 Transmission System Parameters Optical Transmitters •Maximum transmit power • Transmitter components – limited by SPM, XPM – Light source • Maximum propagation distance • Laser – limited by dispersion, attenuation • LED – light emitting diode • Maximum data rate – Modulator – limited by dispersion • Number of WDM channels – limited by low-loss region of fiber – limited by channel spacing • Channel spacing – affected by four-wave mixing, SBS, SRS 3 Laser Principles Laser Principles • LASER – light amplification by stimulated emission of radiation • Stimulated emission • Particle (atom/molecule) has discrete energy levels determined – photon incident on particle in state E2 by state of its electrons – particle falls from E1 to E1 and releases new photon • Absorption • new photon has same frequency, direction, polarization and – photon incident on particle transfers energy to particle phase as incident photon – photon is absorbed • Population inversion – particle moves from ground state to higher energy state – apply energy such that number of particles in state E2 > number of particles in state E • Spontaneous emission 1 – particle in high energy state spontaneously drops to ground state – photon is released E − E • frequency of photon: f = 2 1 h = 6.63×10−34 J ⋅s h • random direction, polarization, phase Laser Principles Semiconductor Laser • Electrons occupy different energy levels • Cavity laser – Conduction band – electron at higher energy level, high mobility – Valence band – electron at lower energy level, low mobility – particles placed in cavity with reflective surfaces • Electron dropping from conduction band to valence band releases photon • n-type semiconductor – excess free electrons • p-type semiconductor – excess holes Semiconductor Laser Laser Characteristics • Laser consists of forward-biased p-n junction • Linewidth – spectral width of generated light – Forward bias leads to population inversion – affects channel spacing – Photon incident on electron causes electron to recombine with hole to – affects chromatic dispersion produce stimulated emission • Frequency instability – mode hopping – jump in frequency caused by change in injection current – mode shifts – change in frequency due to change in temperature – wavelength chirp – variations in frequency due to variations in injection current • Number of longitudinal modes • Light emitting diode (LED) – wavelengths λ for which nλ=2L (L = cavity length, n=integer) will – p-n junction without population inversion be amplified – primarily spontaneous emission • Tuning range – broad spectrum of frequencies • Tuning time – low output power 4 Laser Structures Tunable Lasers • Fabry Perot – cavity laser • Injection current DFB/DBR – has multiple longitudinal modes – Electric current changes refractive index of grating • Distributed feedback (DFB) – Tuning range: 10 nm – grating in gain cavity – Tuning speed: 1-10 ns –amplifies λ for which nλ=2L and nλ=2Lg • External cavity tunable laser –strongest for λ = 2Lg – Change length of external cavity • Distributed Bragg reflector (DBR) • mechanically – grating outside of gain medium – Tuning range: 500 nm – can control index of grating independently from gain medium – Tuning speed: 1-10 ms • External cavity laser • electro-optically or acousto-optically change refractive index – Tuning range: 100 nm – Tuning speed: 10 μs Types of Lasers Laser Modulation •Gas • Binary amplitude shift keying (on-off keying) – Helium-neon: 633 nm –“1”–laser on – Nitrogen: 337.1 nm, 357.6 nm –“0”–laser off – Carbon dioxide: 9400 nm, 10600 nm • Direct modulation – directly turn laser on/off • Semiconductor –leads to chirp – GaAs: 630 nm – 1000 nm • External modulation – laser always on • Used for some short-reach systems utilizing 850 nm band •Encoding – InP: 1300 nm – 2000 nm – NRZ – on for entire duration of “1” • Used for long-haul systems utilizing 1300 nm and 1550 nm –RZ –pulse for “1” bands Optical Receivers Amplification • Photodetector – converts photons to electric current • 3R – regeneration, reshaping, reclocking • Implemented using reverse-biased p-n junction • Electrical regeneration – 3R – Incident light creates electron-hole pairs – bit rate and modulation dependent – Electrons move towards n region • Optical amplification – 1R – Holes move towards p region – boosts signal – transparent to data format and bit rate – amplifies
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