EE 233. LIGHTWAVE SYSTEMS Chapter 2. Optical Fibers Instructor: Ivan P. Kaminow
PLANAR WAVEGUIDE (RAY PICTURE) !
Agrawal (2004)
Kogelnik PLANAR WAVEGUIDE h a = (ns2 - nc2)/ (nf2 - ns2) = asymmetry; nf > ns > nc b = (N2 - ns2)/ (nf2 - ns2) = normalized guide index N ~ ns + b(nf - ns) = effective index [nf ~ ns] V = kh (nf2 - ns2) 1/2 = normalized frequency
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)
) 2#n1 & $$ k 2 = k x2 + k y2 + k z2 = '' " ( 0 % kz = !
2
WAVE EQUATION !
Agrawal (2004)
Kogelnik PLANAR WAVEGUIDE
a = (ns2 - nc2)/ (nf2 - ns2) = asymmetry b = (N2 - ns2)/ (nf2 - ns2) ~ (N - ns) / (nf - ns) = normalized guide index
CYLINDRICAL WAVEGUIDE !
Structure of Optical Fiber
Coating Clad
Coating
Clad
Core
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 Receiver
Light Source
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 Plaser " Pdec " PowerPenalty " Potherloss Length = Fiber _ Loss "7dBm " ("35dBm) "10dB = = 90km 0.2dB /km !
!
! !
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 Plaser ! Pdec ! PowerPenalty ! Potherloss Length = Fiber _ Loss ! 7 dBm ! (!35dBm) ! 10dB = = 90km 0.2dB / km !
!
!
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 After drawing Clad diameter of Preform ~ 80mm
1/2 Core diameter of Fiber ~ 9 µm
Clad diameter of Fiber ~ 125 µm
Core diameter of Preform ~ 5.8mm
Core
Coating diameter ~ 250 µm
Clad Coating Clad
Core Cross-section of Preform
Clad
Core
Cross-section of Fiber
Manufacturing of Optical Fiber
Growth of preform with specific doping and index profile
Preform Fabrication (MCVD & OJ processes)
1st Pure Silica glass tube
36mm
27mm
MCVD Machine (Modified Chemical Vapor Deposition)
Traveling
22mm
Moving
Deposition
Collapsing & Closing (Core rod)
80mm OJ tube Core rod
Core
Round torch Clad
OJ Machine (Over Jacketing)
Moving Core rod
Preform
2nd Pure Silica glass tube
Other Manufacturing Processes of Optical Fiber Preform
OVD (Outside Vapor Deposition) Deposition
VAD (Vapor Axial Deposition) Deposition
Seed rod Porous Core (Germanium doped region)
Traveling
Moving
Seed rod
Porous Preform
Vapors inlet
Clad soot Porous Clad (Pure Silica region) Core soot
Traveling
Vapors inlet
Core deposition burner
Clad 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
Fiber diameter : 125 µm
Cooler
Coated fiber diameter : 245 µm
Coater
Drawing speed : 1200~2100 m/min
UV lamp
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 dT d '% 1 (T = (* = L d* d* %& v g
)2 =
d 2) d, 2
$ "(* = ! 2+c ) 2 L(* = DL(* 2 " * # d '% 1 $" 2+c D= = ! 2 ) 2 = DM + DW d* %& v g "# *
• Intersymbol interference (ISI): pulse broadening effect of chromatic dispersion causes the signal of adjacent bit periods to overlap " Cause power penalty
Origin of Dispersion and Nonlinearities Four Wave Mixing (FWM) Cross-phase Modulation (XPM) A fiber nonlinearity caused by the nonlinear index of refraction of glass. The index of refraction varies with optical power level which causes different optical s ignals to interact
A nonlinearity common in DWDM systems where multiple wavelengths mix together to form new wavelengths, called interfering products. Interfering products that fall on the original signal wavelength become mixed with the signal, mudding the signal , and causing attenuation. Interfering products on either side of the original wavelength can be filtered out. FWM is most prevalent near the zero-dispersion wavelength and at close wavelength spacings. Four Wave Mixing (FWM)
Polarization Mode Dispersion
Geometry of Preform vs. SMF Cross-section of Preform Perfect circle of Preform
2/2 Cross-section of Fiber
After Drawing
Non-circular core of Preform
Non-circular clad of Preform
Perfect circle of fiber
Non-circular core of Fiber
Non-circular clad of Fiber
polarity mode dispersion
Origins of PMD in single-mode fiber (SMF)
Intrinsic birefringence Core
Slow
Form birefringence, characteristic
axis
of a non-circular waveguide. Stress birefringence, due to forces acting on a non-circular core.
Ideal Oval
• 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
$ g ,N
!1 ( ) = d# N / d"
Ramo, Whinnery & Van Duzer
Types of dispersion in optical fiber
Chromatic dispersion Optical frequencies
Dispersion
#1 #2
in Polarization-mode dispersion
Polarization modes
Input pulse
pulse arrival times
Output pulse
Chromatic dispersion can be fully compensated
Compensation
Transmission Input bits
Output bits
Into receiver
Standard Standardfiber: fiber:Dispersion= Dispersion=17 17ps/nmkm ps/nmkm 2 Maximum Maximumlength lengthwithout withoutcompensation compensation~~1/(D 1/(DxxBitrate Bitrate2)) For For2.5 2.5Gb/s Gb/s=> =>1000km, 1000km,for for10 10Gb/s=> Gb/s=>60 60km km
Methods Methodsof ofcompensation: compensation: ••Fiber Fiberwith withinverse inversedispersion dispersion ••Dispersive Dispersivefilters filters
Problems Problemswith withcompensation: compensation: ••Needs Needsto tobe beengineered engineeredfor foreach eachlink link 2 ••varies varieswith withwavelength wavelength((~~1ps/nm 1ps/nm2km) km)
Dispersion Engineering • For single mode fiber, chromatic dispersion is the dominant dispersion factor
Dispersion Engineering
(NZDS (Std SMF)
TrueWaveâ„¢ Fiber: Conceived in 1992 Commercially available in 1993
TLI: Dec 2003- 16
Fiber)
Dispersion Management Technique Dispersion-compensating fiber
TX
RX
Total accumulated dispersion, ps/nm
Transmission fiber 100
0 50
100
150
Distance, km
Accumulated dispersion at the receiver is close to zero ofcshortcourse.ts.degradation_effects.12
200
FIBER DISPERSION Profile control allows crafting of fiber dispersion “L-Band”
“C-Band”
Nominal Disp. (ps/nm/km)
20
USF
15 Large Area
10
TrueWave+
Reduced Slope DSF
5
SMF-LS
0 -5 1530
1550
1570
1590
Wavelength (nm)
1610
Nonlinear Index Impairments • • • •
n = n0 + n2(P/Aeff) 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
!
fast decay pump absorption
atom
phonon
2
slow decay
fast decay
atomic levels
noise photon
sustained pumping
1
instant decay
signal photon emission
phonon
random noise photon OUT
spontaneous emission
signal photon IN
signal photons
stimulated emission
Desurvire -Campinas
OUT
Spontaneous and stimulated scattering
!
Spontaneous and stimulated scattering are similar processes 2
virtual level
noise spontaneous instant photon decay (single) pump 1 photon absorption fast phonon decay
molecule
random noise photon OUT
spontaneous scattering
sustained pump photon absorption
signal photon IN
instant decay
signal photon emission
signal photons
stimulated scattering
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OUT
!
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 104 (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