Components for Optical Networks. Optical Fiber Transmission System. Optical Fiber. Numerical Aperture of Fiber. Modal Dispersion.

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1 Components for Optical Networks Optical fiber Propagation in fiber Fiber modes Attenuation Dispersion Non-linear effects Optical transmitters Laser principles Modulation Optical receivers Optical amplifiers Couplers Multiplexers Filters Optical switches and crossconnects Wavelength converters Optical Fiber Transmission System Fiber Transmitter Laser or LED fixed or tunable Modulator Receiver Photodetector Amplifier Multiplexers/filters Core and cladding silica (SiO 2 ) n 1 : refractive index of core n 2 : refractive index of cladding n 1 > n 2 (~1.45) θ 1 : angle of incidence θ 2 : angle of refraction Snell s Law: n 1 sin θ 1 = n 2 sin θ 2 Critical angle: θ crit = sin -1 (n 2 /n 1 ) Total internal reflection: θ 1 > θ crit Optical Fiber Numerical Aperture of Fiber 2 max n1 n2 Acceptance angle: θ 0 = n0 For total internal reflection: θ < Numerical aperture: n sin 2 max 0 θ 0 max 0 θ 0 Fiber Modes Modal Dispersion Modes corresponds to solutions of wave equations Geometric interpretation: A mode is one possible path that a guided ray may take in a fiber Each mode travels at a different speed Dispersion: spreading of signal in the time domain Modal dispersion caused by multiple modes propagating along a fiber Limits bit rate and/or distance that signal can travel 1

2 Multimode vs. Single-Mode Fiber Graded-Index Fiber Fiber will capture only a single mode for wavelength λ if: 2π 2 2 V = Δ a n1 n2 < λ where a = core radius Multimode fiber Core diameter: μm For large V, number of modes ~= V 2 /2 Single mode fiber Core diameter: 8-10 μm Captures only a single mode (fundamental mode) n 1 > n 2 > n 3 > n 4 > n 5 Reduces modal dispersion (number of modes reduced by ~ ½) Attenuation in Fiber Material absorption Absorption by silica and impurities Wavelength of light corresponds to vibrational resonant frequency of molecules Rayleigh scattering Small fluctuations in refractive index cause light to scatter Effect stronger for shorter wavelengths Rayleigh scattering ultraviolet absorption hydroxyl ion (OH-) absorption infrared absorption Attenuation in Fiber Pin = input power Pout = output power L = length of fiber in km A = attenuation constant in db/km Receiver sensitivity: Pr = minimum power required at receiver Loss in db = A x L = - 10 log 10 Pout/Pin Pout = Pin x 10 -AL/10 Find maximum distance L for receiver sensitivity Pr Pout > Pr Pin x 10 -AL/10 > Pr L < 10/A log 10 Pin/Pr e.g. Pin = 0.1 mw, A = 0.2 db/km, Pr = 0.05 mw Lmax < 15 km Wavelength Bands Dispersion in Fiber S-band (short): nm C-band (conventional): nm L-band (long): nm Defined by wavelengths at which specific components, such as amplifiers, can operate Dispersion: broadening of pulse in time domain as it propagates along the fiber Leads to inter-symbol interference Types of dispersion Modal dispersion modes travel at different speeds 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

3 Controlling Dispersion Fiber Types Chromatic dispersion is zero near 1300 nm Dispersion-shifted fiber (DSF) change waveguide dispersion such that zero dispersion at 1550 nm Nonzero dispersion-shifted fiber (NZ-DSF) dispersion of 1-6 ps/nmkm at 1550 nm Dispersion compensating fiber insert fiber with negative dispersion between fibers with normal dispersion Multimode fiber (MMF) Short range, low-cost transmitters, single channel 850 nm or 1300 nm e.g., 100 Base-FX Fast Ethernet (~2 km) or 1000 Base-SX Gb Ethernet (~500 m) 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) refractive index depends on signal intensity changes in index lead to phase and frequency variations (chirp) frequency variations lead to increased chromatic dispersion limits maximum transmit power Cross-phase modulation (XPM) variations of signal intensity on other channels leads to phase shifts and chirp effect decreases with increased channel spacing Four-wave mixing signals at frequencies w 1 and w 2 generate new signals at 2w 1 -w 2 and 2w 2 -w 1 Stimulated Brillouin Scattering (SBS) Interaction between signal and acoustic waves Shifts signal power to lower frequencies propagating in the opposite direction of the original signal Range of frequencies affected: 20 MHz Gain coefficient: 4x10-11 m/w Stimulated Raman Scattering (SRS) Shifts signal power to lower frequencies propagating in the same direction as the original signal Range of frequencies affected: 40 THz Gain coefficient: 6x10-14 m/w Transmission System Parameters Optical Transmitters Maximum transmit power limited by SPM, XPM Maximum propagation distance limited by dispersion, attenuation Maximum data rate 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 Transmitter components Light source Laser LED light emitting diode Modulator 3

4 Laser Principles Laser Principles LASER light amplification by stimulated emission of radiation Particle (atom/molecule) has discrete energy levels determined by state of its electrons Absorption photon incident on particle transfers energy to particle photon is absorbed particle moves from ground state to higher energy state Spontaneous emission particle in high energy state spontaneously drops to ground state photon is released E2 E1 34 frequency of photon: f = h = J s h random direction, polarization, phase Stimulated emission photon incident on particle in state E2 particle falls from E1 to E1 and releases new photon new photon has same frequency, direction, polarization and phase as incident photon Population inversion apply energy such that number of particles in state E 2 > number of particles in state E 1 Laser Principles Cavity laser particles placed in cavity with reflective surfaces Semiconductor Laser Electrons occupy different energy levels Conduction band electron at higher energy level, high mobility Valence band electron at lower energy level, low mobility 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 Forward bias leads to population inversion Photon incident on electron causes electron to recombine with hole to produce stimulated emission Light emitting diode (LED) p-n junction without population inversion primarily spontaneous emission broad spectrum of frequencies low output power Linewidth spectral width of generated light affects channel spacing affects chromatic dispersion 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 wavelengths λ for which nλ=2l (L = cavity length, n=integer) will be amplified Tuning range Tuning time 4

5 Laser Structures Fabry Perot cavity laser has multiple longitudinal modes Distributed feedback (DFB) grating in gain cavity amplifies λ for which nλ=2l and nλ=2lg strongest for λ = 2Lg Distributed Bragg reflector (DBR) grating outside of gain medium can control index of grating independently from gain medium External cavity laser Tunable Lasers Injection current DFB/DBR Electric current changes refractive index of grating Tuning range: 10 nm Tuning speed: 1-10 ns External cavity tunable laser Change length of external cavity mechanically Tuning range: 500 nm Tuning speed: 1-10 ms electro-optically or acousto-optically change refractive index Tuning range: 100 nm Tuning speed: 10 μs Types of Lasers Laser Modulation Gas Helium-neon: 633 nm Nitrogen: nm, nm Carbon dioxide: 9400 nm, nm Semiconductor GaAs: 630 nm 1000 nm Used for some short-reach systems utilizing 850 nm band InP: 1300 nm 2000 nm Used for long-haul systems utilizing 1300 nm and 1550 nm bands Binary amplitude shift keying (on-off keying) 1 laser on 0 laser off Direct modulation directly turn laser on/off leads to chirp External modulation laser always on Encoding NRZ on for entire duration of 1 RZ pulse for 1 Optical Receivers Amplification Photodetector converts photons to electric current Implemented using reverse-biased p-n junction Incident light creates electron-hole pairs Electrons move towards n region Holes move towards p region 3R regeneration, reshaping, reclocking Electrical regeneration 3R bit rate and modulation dependent Optical amplification 1R boosts signal transparent to data format and bit rate amplifies several wavelengths simultaneously noise also amplified 5

6 Erbium-Doped Fiber Amplifier EDFA Gain Spectrum Pump laser raises Erbium ions from E1 to E3 Spontaneous emission from E3 to E2 ~ 1 μs Spontaneous emission from E2 to E1 ~ 10 ms Population inversion most ions at E2 Data signal in nm range causes stimulated emission Uneven gain spectrum db Gain equalization Adjust input power Notch filters after each amplification stage Other Amplifiers Praseodymium Doped Fiber Amplifier Similar to EDFA Amplifies signals in the 1300 nm region Raman Amplifiers Uses stimulated Raman scattering Pump laser at a given wavelength transfers power to longer wavelengths Can be used for any wavelength range e.g. pumps in nm range will amplify signals in the nm range Requires high power pump laser (> 500 mw to 1 W) Semiconductor Optical Amplifier Similar in structure to semiconductor laser forward biased p-n junction Not as useful for amplifying signals over long distances high crosstalk lower gain than EDFAs (25 db) Wider gain bandwidth on order of 100 nm Useful as components in optical switches Amplifier Characteristics Gain: Ratio of output power to input power Gain efficiency: Measure of output power as a function of pump power in db/mw Gain bandwidth: Range of frequencies over which the amplifier is effective Gain saturation: Value of output power at which the output power no longer increases with the input power Crosstalk: Measure of interference between different channels Polarization sensitivity: Dependence of gain on the polarization of the signal Amplified spontaneous emission: Source of noise in amplifiers caused by spontaneous emission Couplers Couplers - Passive devices that combine and split optical signals 2 x 2 coupler α = power splitting ratio Possible implementation two fused fibers 1 x 2 splitter 2 x 1 combiner Multiplexers and Demultiplexers Multiplexer Passive device that combines different wavelengths onto a single output fiber Demultiplexer Separates wavelengths from a single fiber onto different fibers N x N passive star coupler Signal on any input broadcast to all outputs For N x N coupler, output power = (1/N)x(input power) 6

7 Filters Used to implement demultiplexers Grating filters Transmission grating Reflective grating Fiber Bragg grating Tunable Filters Fabry Perot filter Mechanically tuned by changing distance between mirrors Tuning range: 500 nm Tuning time: 1-10 ms Acoustooptic Tunable Filter Acoustic waves creating periodic variations in refractive index (grating) Can be used as a wavelength crossconnect High crosstalk Coarse selectivity (100 GHz passband) Tuning range: 250 nm Tuning time: 10 μs Tunable Filters Tunable Filter Characteristics Mach-Zehnder Interferometer (MZI) Couplers introduce π/2 phase shift Adjustable delay elements introduces β ΔL phase shift β = propagation constant, e.g. 2πn eff /λ For βδl = kπ, k odd upper output out of phase lower output in phase For βδl = kπ, k even upper output in phase lower output out of phase Tuning time: several ms Tuning range Tuning time Free spectral range (FSR) the distance between two neighboring resonant frequencies (2L = nλ) Finesse ratio of FSR to 3-dB bandwidth of peak Arrayed Waveguide Grating Optical Switches Optical 2 x 2 crossconnects 4 x 4 crossbar switch 7

8 Optical Switch Technologies Optical Switch Technologies Opto-mechanical Mechanical motors align fibers Used for restoration purposes Electro-optic Coupler with voltage applied to coupling region Change in voltage changes refractive index which changes coupling ratio Fast switching time (< 1 ns) High loss Thermo-optic 2 x 2 Mach-Zhender interferometer Refractive index of waveguide is function of temperature Slow switching time (~ 2 ms) Semiconductor optical amplifier switch Amplifier acts as on-off gate Fast switching (~ 1 ns) Allows multicast High polarization sensitivity Micro-electromechanical (MEM) switch Mechanically move mirrors Slow switching (~ 50 ms) Large switching arrays: 1152 x 1152 Optical Switch Technologies Wavelength Conversion Thermocapillary Waveguide filled with liquid Liquid heated to form bubble Switching time: < 10 ms Switch arrays: 32 x 32 Liquid crystal Polarizes signal Uses liquid crystal to block/pass polarized light Switching time: ~ 4 ms Small switching arrays: 2 x 2 Wavelength continuity constraint connections may be blocked even if capacity is available Wavelength converter enables conversion from one wavelength to another Opto-electronic conversion All-optical conversion Wavelength Conversion Techniques Wavelength Conversion Techniques Opto-electronic conversion Convert signal to electronics Retransmit signal on new wavelength May not be transparent to bit-rate and modulation format Cross-gain modulation Utilizes crosstalk in semiconductor optical amplifier Requires high input power for data Results in low extinction ratio (ratio of power for 0 and 1 ) 8

9 Wavelength Conversion Techniques Wavelength Conversion Techniques Cross-phase modulation Mach Zehnder interferometer and SOAs Change in input to SOA causes change in refractive index Four-wave mixing Waves at frequencies f1 and f2 creates wave at 2f1-f2 Wavelength Converting Switches Wavelength Converting Switches Full wavelength conversion Shared wavelength converters Per-node Per-link Limited/Sparse Wavelength Conversion Network Elements Sparse wavelength conversion Only a subset of network nodes have conversion capability Issue: where to place conversion nodes? Limited range wavelength conversion Converters able to convert to wavelengths in a limited range Low conversion range is usually sufficient Optical Add-Drop Multiplexer Static Reconfigurable 9

10 Network Elements Network Elements All-Optical Crossconnect Opaque Crossconnects Opaque vs. Transparent 10