Chap_2_Optical_Fiber

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

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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.

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The End