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Home , Dispersion (optics), Guided ray

Components for Optical Networks Optical Transmission System

• – Propagation in fiber –Fiber modes – Attenuation – – Non-linear effects •Optical transmitters – 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 • converters • Multiplexers/filters

Optical Fiber of Fiber

• and cladding – silica (SiO2) 2 2 •n1: 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 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 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 corresponds to vibrational resonant 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 • 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 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 • dispersion – refractive index depends on distribution of power in core and cladding which depends on wavelength – 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 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 – changes in index lead to phase and frequency variations () – Shifts signal power to lower 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 – 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 of frequencies • Tuning time – low output power

4 Laser Structures Tunable

• 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 –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 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 several wavelengths simultaneously – noise also amplified

5 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 • Uneven gain spectrum – 25-40 dB – Population inversion – most ions at E2 • Gain equalization • Data signal in 1525-1570 nm range causes stimulated emission – Adjust input power – Notch filters after each amplification stage

Other Amplifiers Amplifier Characteristics

• Praseodymium Doped Fiber Amplifier – Similar to EDFA • Gain: Ratio of output power to input power – Amplifies signals in the 1300 nm region • Raman Amplifiers • Gain efficiency: Measure of output power as a function of pump – Uses stimulated Raman scattering power in dB/mW – Pump laser at a given wavelength transfers power to longer wavelengths • Gain bandwidth: Range of frequencies over which the amplifier – Can be used for any wavelength range is effective • e.g. pumps in 1460-1480 nm range will amplify signals in the 1550- 1600 nm range • Gain saturation: Value of output power at which the output – Requires high power pump laser (> 500 mW to 1 W) power no longer increases with the input power • Semiconductor • Crosstalk: Measure of interference between different channels – Similar in structure to semiconductor laser – forward biased p-n junction – Not as useful for amplifying signals over long distances – • Polarization sensitivity: Dependence of gain on the polarization • high crosstalk of the signal • lower gain than EDFAs (25 dB) • Amplified spontaneous emission: Source of noise in amplifiers – Wider gain bandwidth – on order of 100 nm caused by spontaneous emission – Useful as components in optical switches

Couplers Multiplexers and Demultiplexers

• Couplers - Passive devices that combine and split optical signals •2 x 2 coupler • Multiplexer – Passive device that combines different α= power splitting ratio wavelengths onto a single output fiber – Possible implementation – two fused fibers • Demultiplexer – Separates wavelengths from a single fiber onto • 1 x 2 splitter different fibers •2 x 1 combiner

• 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 Filters Tunable Filters • Fabry Perot filter – Mechanically tuned by changing distance between mirrors • Used to implement demultiplexers – Tuning range: 500 nm – Tuning time: 1-10 ms • Grating filters • Acoustooptic Tunable Filter – Transmission grating – Acoustic waves creating periodic variations in refractive index – Reflective grating (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 • Tuning range – Adjustable delay elements introduces β ΔL phase shift • Tuning time β= , e.g. 2πn /λ eff • Free spectral range (FSR) – the distance between two –For βΔL = kπ, k odd neighboring resonant frequencies (2L = nλ) • upper output out of phase • lower output in phase • Finesse – ratio of FSR to 3-dB bandwidth of peak –For βΔL = kπ, k even • upper output in phase • lower output out of phase – Tuning time: several ms

Arrayed Waveguide Grating Optical Switches

• Optical 2 x 2 crossconnects

• 4 x 4 crossbar switch

7 Optical Switch Technologies Optical Switch Technologies

•Opto-mechanical • Semiconductor optical amplifier switch – Mechanical motors align fibers – Amplifier acts as on-off gate – Used for restoration purposes – Fast switching (~ 1 ns) • Electro-optic – Allows multicast – Coupler with voltage applied to coupling region – High polarization sensitivity – Change in voltage changes refractive index which changes coupling • Micro-electromechanical (MEM) switch ratio – Mechanically move mirrors – Fast switching time (< 1 ns) – Slow switching (~ 50 ms) –High loss – Large switching arrays: 1152 x 1152 • Thermo-optic – 2 x 2 Mach-Zhender interferometer – Refractive index of waveguide is function of temperature – Slow switching time (~ 2 ms)

Optical Switch Technologies Wavelength Conversion

• Thermocapillary • Wavelength continuity constraint – connections may be blocked – Waveguide filled with liquid even if capacity is available – 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 • Wavelength converter enables conversion from one wavelength – Switching time: ~ 4 ms to another – Small switching arrays: 2 x 2 – Opto-electronic conversion – All-optical conversion

Wavelength Conversion Wavelength Conversion Techniques Techniques

• Opto-electronic conversion • Cross-gain modulation – Convert signal to electronics – Utilizes crosstalk in semiconductor optical amplifier – Retransmit signal on new wavelength – Requires high input power for data – May not be transparent to bit-rate and modulation format – Results in low extinction ratio (ratio of power for “0” and “1”)

8 Wavelength Conversion Wavelength Conversion Techniques Techniques

• Cross-phase modulation •Four-wave mixing – Mach Zehnder interferometer and SOAs – Waves at frequencies f1 and f2 creates wave at 2f1-f2 – Change in input to SOA causes change in refractive index

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 • Optical Add-Drop Multiplexer – Only a subset of network nodes have conversion capability – Static – Issue: where to place conversion nodes? – Reconfigurable • Limited range wavelength conversion – Converters able to convert to wavelengths in a limited range – Low conversion range is usually sufficient

9 Network Elements Network Elements

• All-Optical Crossconnect • Opaque Crossconnects

Opaque vs. Transparent

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