EE 233. LIGHTWAVE SYSTEMS Chapter 2. Optical Fibers Instructor: Ivan P. Kaminow
PLANAR WAVEGUIDE (RAY PICTURE)
Agrawal (2004)
Kogelnik PLANAR WAVEGUIDE a = (n s 2 - n c2 )/ (n f 2 - n s2 ) = asymmetry; n f > n s > n c b = (N 2 - n s2 )/ (n f 2 - n s2 ) = normalized guide index N ~ n s + b(n f - n s ) = effective index [n f ~ n s ] V = kh (n f 2 - n s2 ) 1/2 = normalized frequency h
Waveguide Basics Light is guided by total internal reflection Transverse modes: For a given guide dimension and index profile, there exists an integer number of propagating modes. What is a mode? For every transverse mode, a standing wave is established in the transverse direction The higher the mode number, The sharper the guiding angle The smaller the propagation constant in the z (propagating direction) k k z =! ) = ' ( 2 2 2 2 2# n1 = kx + k y + kz " 0 & $ % 2
WAVE EQUATION
Agrawal (2004)
Kogelnik PLANAR WAVEGUIDE a = (n s 2 - n c2 )/ (n f 2 - n s2 ) = asymmetry b = (N 2 - n s2 )/ (n f 2 - n s2 ) ~ (N - n s ) / (n f - n s ) = normalized guide index
CYLINDRICAL WAVEGUIDE
Structure of Optical Fiber Coating Clad Core Coating Clad Core Optical Fiber Core Clad Coating Material Germanium doped Silica Glass Pure Silica Glass Acrylate Purpose Guiding the light (High-index region) Supporting the light (Low-index region) Protecting the glass region
Propagation of Light Total internal reflection 0 1 0 Input Signal Coating Clad Core 0 1 0 Output Signal Light Source Receiver Optical Fiber The signal light is guided within the core of the optical fibe r by the total internal reflection.
For a fiber with a V nu mber of 4.367, it can support 4 modes as illustrated by the graph V- number Vs propagation constant (Fig 7). Fig 7: V-number Vs Propagation constant ( _) The modes allowed are shown in figure below ( Fig 8). Fig 8: Modes in a slightly multi mode fiber http://www.ee.iitm.ac.in/~skrishna/, Srikrishna Bhashyam,
Modal Dispersion
Modal Dispersion
Graded-Index Fiber
Fiber Attenuation
Harnessing Optical Fiber Bandwidth C L
Power Budget Length = = Plaser " Pdec " PowerPenalty " Potherloss Fiber _ Loss "7dBm " ("35dBm) "10dB 0.2dB /km = 90km! Low insertion loss is very important for all the components along the link.! Power penalty = increase of minimum receiver power for a given SNR (bit error rate) due to the components in the comm. Channel! Caused by dispersion, fiber nonlinearity, etc.! Other power loss: due to multiplexer, connectors, etc.
Power Budget Length = P laser! P dec! PowerPenalty Fiber _ Loss! P otherloss =! 7dBm! (! 35dBm) 0.2dB / km! 10dB = 90km! Low insertion loss is very important for all the components along the link.! Power penalty = increase of minimum receiver power for a given SNR (bit error rate) due to the components in the comm. Channel! Caused by dispersion, fiber nonlinearity, etc.! Other power loss: due to multiplexer, connectors, etc.
Topics of Interest Fundamental limitations Distance Bandwidth Wavelength range Crosstalk Noise Distance Loss-limit Dispersion-limit Bit rate
Fiber Manufacture
Geometry of Preform vs. SMF 1/2 After drawing Core diameter of Fiber ~ 9 µm Clad diameter of Preform ~ 80mm Clad diameter of Fiber ~ 125 µm Core diameter of Preform ~ 5.8mm Core Coating diameter ~ 250 µm Clad Coating Clad Core Clad Core Cross-section of Preform Cross-section of Fiber
Manufacturing of Optical Fiber Growth of preform with specific doping and index profile
Preform Fabrication (MCVD & OJ processes) 36mm 1 st Pure Silica glass tube 27mm Traveling Deposition MCVD Machine (Modified Chemical Vapor Deposition) 22mm Moving Collapsing & Closing (Core rod) 80mm OJ tube Core rod Core Round torch Clad OJ Machine (Over Jacketing) Moving Core rod 2 nd Pure Silica glass tube Preform
Other Manufacturing Processes of Optical Fiber Preform OVD (Outside Vapor Deposition) Deposition VAD (Vapor Axial Deposition) Deposition Seed rod Porous Core (Germanium doped region) Moving Seed rod Traveling Vapors inlet Porous Preform Porous Clad (Pure Silica region) Core soot Clad soot Clad deposition burner Traveling Vapors inlet Core deposition burner Vapors inlet
Fiber Drawing (Draw Tower) Preform Furnace (2200~2300 C) Preform diameter : larger than 80 mm Drawing temperature : 2200~2300 C Fiber diameter measurement Cooler Coater UV lamp Fiber diameter : 125 µm Coated fiber diameter : 245 µm Drawing speed : 1200~2100 m/min Fiber length / preform : 360 km Coated fiber diameter measurement Capstan Fiber take-up
Manufacturing of Optical Fiber Pulling the fiber from a given preform
Optical Fiber Typical Dimension for Silica Fibers: SMF: 8 um core, 125 um cladding MMF: 50, 62.5, 100 um core, 125 um cladding Index profile: Step vs. Graded vs. multi-step
Dispersion Different components of transmitted signal travel at different velocities in the fiber and arrive at different times at the receiver Modal dispersion: different modal components of a pulse travel at different velocities Chromatic dispersion: different spectral components of a pulse travel at different velocities Material dispersion: due to!-dependence in doped silica (or any other core material) Waveguide dispersion: due to waveguide design Polarization dispersion: different polarization components of a pulse travel at different velocities
Single Mode Fiber Define group velocity dispersion (GVD) parameter " 2 and Dispersion parameter D Intersymbol interference (ISI): pulse broadening effect of chromatic dispersion causes the signal of adjacent bit periods to overlap " Cause power penalty W M g g D D c v d d D d d DL L c v d d L d dt T + = =! " " # $ % % & ' = = ( = ( =! ( " " # $ % % & ' = ( = ( 2 2 2 2 2 2 2 2 1 2 1 ) * + *, ) ) * * ) * + * * * *
Origin of Dispersion and Nonlinearities
Polarization Mode Dispersion
Geometry of Preform vs. SMF 2/2 Cross-section of Preform Perfect circle of Preform After Drawing Cross-section of Fiber Perfect circle of fiber Non-circular core of Preform Non-circular core of Fiber Non-circular clad of Preform Non-circular clad of Fiber
Origins of PMD in single-mode fiber (SMF) Intrinsic birefringence Ideal Oval Core Slow axis Form birefringence, characteristic of a non-circular waveguide. Stress birefringence, due to forces acting on a non-circular core. For simple birefringence, fiber PMD is proportional to length. In practice, mode coupling destroys this simple relationship.
POINCARE SPHERE FROM KRAUS AND FLEISCH
Origin of Dispersion and Nonlinearities
Chromatic Dispersion
Group Velocity # / p, N = "! N! ( # ) 1 $ = d" g, N d N /
Ramo, Whinnery & Van Duzer
Types of dispersion in optical fiber Optical frequencies # 1 # 2 Chromatic dispersion Dispersion in Polarization-mode dispersion pulse Polarization modes arrival times Input pulse Output pulse
Chromatic dispersion can be fully compensated Transmission Compensation Input bits Output bits Into receiver Standard fiber: fiber: Dispersion= 17 17 ps/nmkm Maximum length length without compensation ~ 1/(D 1/(D x Bitrate Bitrate 2 2 )) For For 2.5 2.5 Gb/s Gb/s => => 1000km, for for 10 10 Gb/s=> 60 60 km km Methods of of compensation: Fiber Fiber with with inverse dispersion Dispersive filters filters Problems with with compensation: Needs Needs to to be be engineered for for each each link link varies varies with with wavelength (( ~ 1ps/nm 2 2 km) km)
Dispersion Engineering For single mode fiber, chromatic dispersion is the dominant dispersion factor
Dispersion Engineering
(Std SMF) (NZDS Fiber) TrueWave Fiber: Conceived in 1992 Commercially available in 1993 TLI: Dec 2003-16
Dispersion Management Technique Dispersion-compensating fiber TX RX Transmission fiber Total accumulated dispersion, ps/nm 100 0 50 100 150 200 Distance, km Accumulated dispersion at the receiver is close to zero ofcshortcourse.ts.degradation_effects.12
FIBER DISPERSION Profile control allows crafting of fiber dispersion Nominal Disp. (ps/nm/km) C-Band 20 15 10 5 0-5 1530 1550 1570 1590 1610 Wavelength (nm) L-Band USF Large Area TrueWave+ Reduced Slope DSF SMF-LS
Nonlinear Index Impairments n = n 0 + n 2 (P/A eff ) SPM = self-phase modulation XPM = cross-phase modulation FWM = four-wave mixing
Self and Cross Phase Modulation
Four Wave Mixing
TLI: Dec 2003-17
Stimulated Raman Scattering and Stimulated Brillouin Scattering
Spontaneous and stimulated emission! Einstein!s principle of spontaneous and stimulated emission in LASER media Energy pump absorption fast decay fast decay slow decay phonon phonon 2 1 atomic levels noise photon sustained pumping instant decay signal photon emission atom random noise photon OUT spontaneous emission signal photon signal photons IN OUT stimulated emission Desurvire -Campinas
Spontaneous and stimulated scattering! Spontaneous and stimulated scattering are similar processes spontaneous (single) pump photon absorption instant decay fast decay phonon 2 noise photon 1 virtual level sustained pump photon absorption instant decay signal photon emission molecule random noise photon OUT spontaneous scattering signal photon IN stimulated scattering signal photons OUT Desurvire -Campinas
Spontaneous/stimulated emission vs. scattering! Both generate (spontaneous) noise photons and (stimulated) coherent signal photons. More to come next.! Both processes act as laser systems, the first with rare-earth ions as fiber dopants (e.g. erbium, tullium, praseodymium), the second with molecules forming the glass fiber host (e.g. Si-O-Si, or P-O-P), with is the Raman effect.! Both stimulated-emission processes (RE doping and Raman) clone input signal photons (same output polarization and phase); the avalanche effect generates coherent signal wave with power amplified by gain factors between 10 and 10 4 (10dB to 40dB)! With RE-doped amplifiers, the energy levels are fixed, thus determining fixed pump (absorption) and fixed signal (emission) bands; with Raman amplifiers, only the frequency difference between pump and signal is fixed, making the signal band tunable. DesurvireCampinas
The End