Understanding Fiber Optic Components

Understanding Fiber Optic Components
Instructor: Jeff Hecht Author: Understanding Fiber Optics, (Prentice Hall, 2002) 4th ed. City of Light: The Story of Fiber Optics (Oxford U Press, 1999) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Course goals Help you understand fiber optics
Learn basics about the technology What the pieces are How they go together Components important for optical networking Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Plan for Course (1): 3. Light sources and receivers
1: Basic ideas of fiber communications 2: Optical fibers Basic concepts of fiber optics Fiber applications and types Fiber attenuation, dispersion, & nonlinear effects Special-purpose fibers 3. Light sources and receivers Light sources: LEDs and lasers Transmitters Receivers and detectors Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Plan for Course (2) 4: Passive optical components 5: Active components
Couplers & taps Planar waveguides Attenuators and filters Wavelength-division multiplexers 5: Active components Repeaters and regenerators Optical amplifiers Modulators Optical switches Wavelength converters Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

1. The Very Basics of fiber optics
Goal is to communicate information Transmitter sends signal Signal modulates a carrier Signal has a data rate and format Signal goes through a length of fiber Attenuation, dispersion, and noise limit distance Amplifiers, switches etc modify signal Receiver decodes signal at end Error rate or signal to noise ratio Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Bandwidth Information capacity Bits per second or megahertz/gigahertz
Increased by Increasing raw data rate Increasing number of optical channels in fiber Limited by Fiber dispersion and attenuation Noise and crosstalk Transmitter speed, receiver sensitivity Range of wavelengths available Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Dispersion Pulse spreading with distance
Degrades bandwidth - pulses overlap Depends on wavelength Amount depends on type of fiber Compensation possible Regeneration Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Wavelength and WDM Fiber transmission depends on wavelength
(can be expressed as frequency - GHz, THz) Usual operating bands 850, 1310, 1550 nm Wavelength-division multiplexing Separate transmitters for each wavelength (l) Optical channels nominally independent but not completely - crosstalk can exist Analogy: radio or TV channels Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fiber tradeoffs Advantages Limits Low attenuation High bandwidth
Compact size EMI immunity Cost effective hardware Limits Not perfect medium Dispersion Attenuation Noise Crosstalk Must be installed Costs Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Part 1: Fibers Concept of optical fiber Fiber applications and types
Multimode Single-mode Fiber properties Attenuation Dispersion Nonlinear effects Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Light guiding in bare fibers
Bare glass rod or fiber in air Total internal reflection traps light in fiber Critical angle Bare glass fiber Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Drawbacks of bare fibers
Light-guiding surface exposed, surface damage Light can leak from fingerprints Light can leak between fibers at contact points in bundles, causing crosstalk Very small diameter for single-mode Because of large index difference between glass and air Light leaks out at fingerprint Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Clad fibers Low-index cladding allows total internal reflection
Reflecting surface safe inside fiber Small index difference still guides light Early fibers with large core, thin cladding (bundles) Unconfined light Total internal reflection Confines light Core Cladding Fingerprint doesn't affect transmission Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Clad Communication Fibers
Smaller core, thicker cladding, single mode Single-mode core size ~ kl/index difference Light guided by core-cladding boundary Most light inside core Cladding Wave guided in core and along core boundary 5l Core Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fiber types Single-mode fiber 50/125 Graded-index fiber
250-µm plastic 250-µm plastic coating coating 125-µm 125-µm glass glass cladding cladding Core Core 50 µm 9 µm 140-µm cladding Core 100 µm 100/140 Step-index multimode Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Two viewpoints of fiber
Optical view: (more intuitive) Total internal reflection of rays Useful for 'highly' multimoded fibers Waveguide model More complex and more accurate Derived from microwave theory Predicts transverse modes Needed to design sophisticated single-mode fibers Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical View Cladding (low-index) Total internal reflection Core
Normal Refracted light Input light Reflected ray Total internal reflection Critical angle Core (high index) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Waveguide view Based on electromagnetic wave propagation
Boundary conditions set by core-cladding interface Properties depend on frequency/wavelength Solve Maxwell's equations for cylindrical boundary conditions Details won't be covered here Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Important Fiber Properties
Attenuation Light coupling efficiency Core diameter Numerical aperture Mode structure Pulse dispersion Depends on modal properties of fiber Determined by fiber type & composition Nonlinear effects Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fiber Attenuation Measured in dB/km dB=-10 log(Pout/Pin)
Sum of scattering and absorption Proportional to distance Using dB Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Attenuation and composition
Absorption depends on composition Silica has little absorption at l<1.7 µm Trace metals have high absorption OH bonds have peak at 1.38 µm Scattering proportional to l-4 Sets lower limit on short-wavelength attenuation Sum of minimum absorption and scattering sets theoretical limits Minimum silica loss near 1.55 µm Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Communication Fiber Loss
window 1.3 µm window 850 nm window Main OH absorption Infrared Absorption Of silica Rayleigh Scattering minimum Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

'Dry' Silica Fibers Fibers made with very low water content
Virtually eliminates 1.38-µm water peak Intended to open entire transmission window from 1.28 to 1.65 µm Allows high optical channel counts Now aimed at short-distance 'metro' market Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Glass fiber mechanical properties
Plastic coating protects surface Standard coating 250 µm on 125 µm fiber Surface damage is main cause of failure Flaws spotted by proof testing 100,000 lb/in2 Strong per unit cross-section But cross-section is small Fiber breaks with very little stretching Bend into 5-cm loop Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Plastic fibers Advantages of plastic Disadvantages of plastic
Cheap More flexible than glass Some types easier to terminate Disadvantages of plastic Much higher attenuation Limited temperature range Several compositions Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Plastic fiber loss Courtesy Takaaki Ishigure
Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Light Coupling Efficiency
Must match emitting area to core diameter Numerical Aperture (NA) Defines range of angles over which fiber collects light NA is 0.21 for ncore=1.50 and nclad=1.485 Typical core-cladding difference around 0.3-2% Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Acceptance angle Confinement angle qc Core Cladding
Whole Acceptance angle 2q (Light here is guided in fiber) Confinement angle qc Core Cladding Half Acceptance angle q NA=sinq=ncore sinqc Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Pulse dispersion Successive pulses overlap as they spread
Initial instantaneous pulses Successive pulses overlap as they spread Spreading increases with distance Degree of dispersion depends on fiber type Unintelligible Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Types of fiber dispersion
Modal (nanoseconds/fiber km) Largest, depends on number of modes Chromatic (ps/nm bandwidth, fiber km) Increases with source bandwidth Equals material + waveguide dispersion Material (ps/nm bandwidth, fiber km) Waveguide (ps/nm bandwidth, fiber km) Polarization mode dispersion Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fiber Types Step-index multimode (large-core) Graded-index multimode
Step-index single-mode Dispersion-modified single-mode Polarization controlling Fiber Bragg gratings Doped fiber for amplifiers Sensing fibers Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Modal Dispersion Largest in magnitude
Equals characteristic dispersion (ns/km) x fiber length (km) Arises from differences in mode velocity Graded-index fibers reduce Smaller in 50/125 fiber than 62.5/125 Single-mode fibers eliminate Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Modes and Modal dispersion
Modes are oscillation/propagation paths Mode velocities differ in step-index multimode fiber Visualize as difference in ray paths Red ray goes shorter distance than blue Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Step-index multimode fiber
Large high-index core, thin cladding Abrupt core-cladding boundary Modal dispersion is large Communications limited to short distance Main uses: imaging and illumination Well described by total internal reflection of rays Unconfined light Total internal reflection Confines light Core Cladding Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Graded-Index Multimode
Refractive index grades from center of core to edge of cladding Index profile Step-index profile Refractive index Graded-index profile Distance from fiber axis Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Graded-Index Guiding Change in index refracts rays toward axis
Rays go fastest in lowest index zones Evening out difference in ray paths Refractive index Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Graded-index fibers Parabolic refractive index profile
Shape of refractive-index profile important Index gradient compensates for modal dispersion 50/125 and 62.5/125 used up to 2 km 50/125 has higher bandwidth Large core eases coupling into fiber Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Limits of graded-index fibers
Ideal index profile hard to realize Dispersion higher than single-mode Modal noise with laser sources Laser source generates speckle pattern in multimode fiber; random drift of speckles within small fiber core generates modal intensity noise. Speckle pattern Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Single-mode Fiber Small core transmits only one mode
9-µm core for l=1.3 µm (0.6% Dn) Light spreads over larger mode field diameter Light intensity Mode field Diameter High intensity At center Core Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Cutoff wavelength Shortest wavelength for single-mode transmission
Typically 1250 to 1280 nm Multimode transmission at shorter wavelengths Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Features of single-mode fiber
Single-mode transmission is simple No modal dispersion No modal noise Transmission distance limited by chromatic dispersion Several types available, differ in dispersion properties Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Types of Single-mode Fiber
Step-Index single-mode fiber (simple) Matched cladding Depressed cladding Reduces core doping required Dopes cladding to reduce index Dispersion-shifted fiber More complex core-cladding design Larger waveguide dispersion Shifts zero chromatic dispersion Higher dispersion slope, larger variation with l Minimum dispersion needed to prevent four-wave mixing (more later) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Material Dispersion Arises because refractive index n varies with wavelength Inherent in material chosen For silica, near zero at 1.28 µm Higher at 0.85 µm Doping causes little change in zero dispersion Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Material Dispersion-2 Derived from group delay
Group delay measures change in transit time with wavelength through L km fiber Refractive index varies with l: n(l) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Material Dispersion-3 Material dispersion is derivative of group delay with respect to wavelength Sign is significant Total dispersion depends on fiber length and source bandwidth Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Refractive index, group delay, and dispersion
Inflection point a) Refractive index vs. l 1.447 Wavelength b)Group delay vs.. l Delay changes Zero slope point faster here delay changes little at peak (inflection point) Wavelength Zero dispersion wavelength c)Dispersion vs.. l + - Wavelength Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Waveguide Dispersion Arises from dependence of waveguide 'size' on wavelength Causing light distribution between core and cladding to change with l Light distribution and dispersion depend on core-cladding design Proportional to source bandwidth and fiber length Same dimensions as material dispersion Can cancel material dispersion if signs are opposite Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Zero chromatic dispersion
Sum of material and waveguide dispersion Step-index single-mode dispersion Chromatic Dispersion Material Dispersion + Wavelength Dispersion Waveguide Dispersion - Zero chromatic dispersion at 1310 nm Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Dispersion shifting Dispersion-shifted single-mode
Changing waveguide dispersion shifts zero dispersion point Dispersion-shifted single-mode Material Dispersion + Chromatic Dispersion Dispersion Wavelength Zero dispersion near 1550 nm - Waveguide Dispersion Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Refractive index profiles
0.9% 0.2% Non-zero dispersion-shifted fiber Large effective area fiber 1.5% Step-index single-mode fiber 0.6% -0.4% 0.4% Dispersion-compensating fiber Fiber with flattened dispersion slope Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Chromatic dispersion impact
Depends on bandwidth of light source Narrow-line light sources limit effect Chromatic dispersion large at 850 nm Can be shifted or compensated at 1550 nm Fiber design Concatenating different fibers Generally dominates for single-mode fiber Reducible below polarization mode Dispersion slope affects WDM systems Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Zero Dispersion-shifted fiber
Zero crossing near 1.55 µm Developed in 1980s for single-channel links at 1.55 µm Vulnerable to 4-wave mixing in DWDM WDM possible at longer wavelengths Now obsolete Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Nonzero dispersion shifted fiber
Large waveguide dispersion Shifts zero dispersion out of Erbium band below 1500 nm or above 1620 nm Nominal dispersion ps/nm-km Reduces 4-wave mixing in 1550 nm band Preferred for DWDM Dispersion slope important Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Chromatic dispersion in single-mode fibers
Nonzero dispersion-shifted Standard single-mode +10 Reduced dispersion slope Dispersion (ps/nm-km) 1500 1300 1400 1600 (wavelength-nm) Nonzero dispersion-shifted -10 Zero dispersion shifted Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Static Dispersion Compensation
Uses fibers with different dispersions Compensates chromatic dispersion only Assumes constant conditions Compensation is cumulative over fiber span Concatenates fibers of opposite signs WDM requires compensating over a range of wavelengths Balances values at different wavelengths Dispersion slope important Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fibers used for compensation
Dispersion-compensating fiber Very high waveguide dispersion Designed to compensate for dispersion Higher attenuation than standard fiber Small core diameter Used in shorter lengths Combinations of other fibers Particularly for balancing dispersion slope Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Dynamic Dispersion Compensation
Can deal with changing conditions Changing environments Temperature cycle effects Normally not needed at 10 Gbit/s Likely to be needed at 40 Gbit/s Can deal with polarization mode dispersion Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Polarization mode dispersion (PMD)
Single-mode fibers transmit Light in two polarizations Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Birefringence in fiber affects each polarization differently, delaying one relative to the other Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

How PMD works Birefringence in fiber is random
In magnitude and orientation Depends on environment; varies with time Light shifts between polarization modes Strong coupling between modes Combination of effects causes pulse spreading that increases with square root of fiber length Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Significance of PMD Small, significant only at high speeds
Does not increase as fast with distance as chromatic dispersion Poorly quantified for older fibers Can limit transmission to 2.5 Gbit/s Limits many present systems to 10 Gbit/s May limit 40 Gbit/s distance Compensation being explored Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical fiber nonlinearities
Weak in glass, signal power is low But fiber cross-section very small Interaction length very long in long-haul Little problem in metro (<100 km) Brillouin scattering Stimulated Raman scattering Self-phase modulation Beam intensity modulates refractive index, spreading signal bandwidth Cross-phase modulation Other optical channels modulate refractive index Four-wave mixing Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Stimulated Raman scattering
Shifts wavelength of scattered light Transfers energy from high-power channel to lower power channel Higher threshold than Brillouin scattering Can be used for amplification Can limit performance Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Four-wave mixing + - Adjacent Adjacent Channel Channel (affected)
(not affected) + - These three uniformly spaced channels mix to generate a fourth frequency Fourth channel Overlaps another signal Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Four-wave mixing Major problem in early DWDM systems
Mixes three channels to give a fourth n1+n2-n3=n4 Uniform channel spacing encourages Worst at low dispersion because the optical channels remain in phase over long distances Sets minimum for chromatic dispersion Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Large-Effective Area Fiber
Design increases single-mode core size Larger area reduces signal intensity Nonlinear effects lower at lower intensity Increases power handling Not compatible with reduced dispersion slope Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Special Transmission Fibers
Polarization Maintaining High internal birefringence Prevents light shifting between polarizations Single-polarization fiber Transmits only one polarization Other polarization attenuated dB within meters Must be aligned with respect to source Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Holey fibers Photonic bandgap -- Hollow core Photonic crystal Core
Jacket Cladding (photonic bandgap) Core defect -light guided along Cladding Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Part 2: Light sources, transmitters & receivers
Light sources for fiber-optic systems LEDs The laser principle Semiconductor & fiber lasers Transmitters Receivers Detectors Receiver sensitivity Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Light Source Considerations
Output power Transmission distance, fiber span Center wavelength Should match fiber windows Attenuation, dispersion, amplifiers Spacing of WDM channels Spectral bandwidth Affects chromatic dispersion, WDM Modulation speed and type Direct or external Cost Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Light source types Surface-emitting LEDs: Cheap and dirty
Edge-emitting LEDs: Not quite so cheap Edge-emitting Fabry-Perot diode lasers VCSELs Single-frequency lasers (DFB, DBR, etc.) Fiber lasers and fiber amplifiers Internal (direct) vs. external modulation Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Light Emitting Diodes Voltage draws carriers to junction
Recombination produces spontaneous emission Light output modulated directly by current flow + voltage Junction layer P type + + + + + + - + - - - N type - - - - voltage Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Surface-emitting LEDs
Light emerges from top Directed by device structure and packaging Common for illumination LEDs Cheap & dirty + - + - Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Edge-emitting LEDs Light in junction plane Emerges from side facet
Faster Smaller emitting area Not quite so cheap + - + - Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

LED vs. lasers LEDs Low cost Long lifetime Lasers Higher power
Higher speed Narrower linewidth Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

LED vs. laser spectral width
Single-frequency laser (<0.04 nm) Laser output is many times higher than LED output; they would not show on same scale Standard laser (1-3 nm wide) LED (30-50 nm wide) Wavelength Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

The Laser Principle Light Amplification by the Stimulated Emission of Radiation Extracts energy from excited species Each stimulated wave is in phase with the wave that stimulates it Laser has resonant cavity Optical amplifiers are single-pass Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Stimulated emission in laser
Partly Transparent Output Mirror Fully Reflecting Mirror Stimulated Emission Laser beam All waves in phase Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Laser emission Output rises rapidly above a threshold current
Laser Output Output rises rapidly above a threshold current Once laser emission starts, line width narrows Laser emits as LED below threshold LED curve Drive Current Laser threshold Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Diode Laser Materials Composition determines wavelength range
InGaAlAs 630 nm GaAlAs nm InGaAs 980 nm Pump for erbium-doped fiber amplifier InGaAsP nm Composition adjusted to pick wavelength Polymers in development Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Edge-emitting Diode Lasers
Resemble LEDs Higher drive current than LEDs End facets reflect light, forming resonator Generate stimulated emission Laser action in narrow stripe Current flow Light out + - - + Reflective facet Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Edge emitting lasers-1 Light confined in narrow stripe in junction layer -- ~5 µm wide Gain along length of chip, µm Edge facets define Fabry-Perot cavity Surface reflection high because of high refractive index (3.5 for GaAs) Rear facet typically coated Some light may be coupled out for monitoring Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Edge emitting lasers -2 Active layers very thin
Light emitting area ~ 0.5 µm x 5 µm Diffraction causes rapid beam spread Laser action in Narrow stripe Current flow + - - + Reflective facet Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fiber coupling Fiber butt coupled to light-emitting spot
Light fits in core Current flow Fiber Light in core + - - + Reflective facet Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

VCSEL Vertical cavity surface emitting laser
Mirrors above and below junction Top partly reflective Bottom totally reflecting Laser output metal contact n-type substrate (transparent) Output mirror (partly transparent) Resonant cavity spacers Junction layer p-type layers blocking layer metal contact Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Advantages of VCSEL Emit from surface Low threshold current
Can test before packaging chip Does not require cleaving, simpler packaging Low threshold current Larger circular emitting area 5-30 µm Size controllable by design of device Lower beam divergence, better quality beam Directly modulatable at higher speeds Less modulated area, so less chirp Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

VCSEL limitations Limited output power (low gain/pass)
Requirement for depositing many thin layers to make mirrors GaAlAs can be deposited with different refractive indexes for mirrors Multilayer mirrors hard to make in InGaAsP/InP Long-wavelength technology is emerging e.g. InGaAsN on GaAs Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

VCSEL applications Good for short, high-speed links
Gigabit Ethernet at 850 nm Kilometer distances Emerging for longer wavelengths Speeds to 2.5 Gbit/s directly modulated Novel designs can be tuned in wavelength Some are pumped optically rather than electrically Need is for WDM systems Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Spectral bandwidth Range of wavelengths from source
Narrow linewidth desirable for High-speed transmission to limit chromatic dispersion WDM to pack more optical channels in available space Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Laser wavelength selection
Gain depends on laser material Oscillation amplifies wavelength of highest gain, narrowing linewidth Fabry-Perot cavity has resonances Round trip must equal integral number of wavelengths: 2D=Nnl (N is integer) N is longitudinal mode Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Laser Bandwidth Fabry-Perot edge emitters: 1-3 nm
Resonance depends on cavity length µm cavity corresponds to circa 1000 wavelengths in n=3.5 material Multiple resonant wavelengths fall within gain curve (longitudinal modes) Spacing circa 0.6 nm at 1300 nm Each resonance is narrow (but not necessarily stable at that wavelength) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fabry-Perot Laser Bandwidth
This is instantaneous snapshot Power on each line varies Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Direct laser modulation
Speed depends on Rise and fall time of laser emission Device structure Materials Effects of direct modulation Chirp in wavelength Refractive index depends on electron density Resonant wavelength depends on refractive index More serious for edge emitters than VCSELs Can limit wavelength stability Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Single-Frequency Diode Lasers
Oscillate on one longitudinal mode Needed for speeds above Gbit/s Temperature stabilization needed Wavelength is function of temperature Distributed feedback laser (DFB) Distributed Bragg reflection laser (DBR) External cavity laser Tunable lasers Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Distributed Feedback Laser
Light scatters from grating in laser substrate Limits oscillation to one wavelength Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Distributed Bragg reflection
Grating etched in substrate In plane of active layer, but outside laser zone Feedback limits oscillation to one wavelength Semiconductor covers grating layer In unpumped region Light scattered From here into active layer Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

External cavity laser-1
Fabry-Perot laser One facet anti-reflection coated Dispersive element turned included in cavity Turning it tunes resonant wavelength Limits oscillation to narrow range Same principle as tunable lab lasers Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

External cavity laser-2

Tunable lasers for WDM Multitude of channels
Easier to have one tunable laser than fixed lasers for 100 channels In plant For spares Must be stable, narrow-line Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Tunable diode lasers Tunable distributed bragg reflection
Several variations Micromirror cavity VCSEL External cavity edge-emitter VCSEL Array containing selectable laser stripes Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Tunable bragg laser Bragg grating selects laser wavelength
Sampled grating has series of peaks Gratings can be on both ends Temperature or current changes Bragg grating spacing Thermal expansion of material Thermal change of refractive index These combine to change wavelength Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Vernier tuning of grating peaks
Reflection where resonances coincide Causes large output wavelengths shift Small shift in this peak Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Micromirror cavity VCSEL
Output beam External partly transparent micromirror moves vertically, changing cavity length and thus tuning wavelength Mirror support Short resonant cavity Active layer Rear reflector Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Selectable laser stripes
Each laser emits different wavelength Tuning is by picking one laser Supplemental tuning possible Output is amplified to overcome mixing loss Waveguide Mixing coupler Output fiber Laser Semiconductor Amplifier Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Transmitters Light sources plus accessories (some optional)
Electronic pre-processing (e.g. voltage-current) Bias current generator Modulator driver (for laser or external) Optical monitor Cooler External modulator Attenuator Optical and electronic interfaces Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Generic Transmitter Electronic interface Electronic signals Optical
preprocessor Modulator driver Optical monitor Laser External Feedback Bias current Modulator Attenuator generator Output signal Bias current Temperature Cooler monitor Light signal Housing Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Direct modulation Steady bias current near laser threshold
output Steady bias current near laser threshold Separate driver adds signal current Bias current (steady) Drive current (varies) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

External modulation Laser operates continuously
Stable single-frequency output Avoids 'chirp' in diode lasers Change in drive current modulates refractive index, affecting wavelength Separate modulator changes light intensity External electro-optic modulator Electro-absorption semiconductor modulator Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Modulation formats Amplitude modulation of carrier signal Analog
Pulse Code Modulation (digital) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Multiplexing Time-division multiplexing
Interleaves signals at slower rates Frequency-division multiplexing Combines signals at different frequencies electronically to make composite signal Wavelength-division multiplexing Sends signals at different wavelengths Like frequency multiplexing of radio spectrum Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Wavelength-division multiplexing
TDM input Individual optical channels Channel 1 Optical l 1 transmitter 1 Channel 2 l 2 l1, l2, l3, l4 Optical transmitter 2 Optical multiplexer WDM Output In one fiber Channel 3 l 3 Optical transmitter 3 Whole unit can be put in One box as WDM transmitter Channel 4 l 4 Optical transmitter 4 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Receivers Convert optical signals to electronic signals
Stages (not counting preamplifiers, switches) Wavelength-division demultiplexing Detection: optical-to-electronic signals Thresholding, retiming (electronic regeneration) Time-division demultiplexing WDM: must be done before detectors Detectors can't discriminate close wavelengths Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

WDM stage of receiver Amplified Optical signal To separate receivers
Individual Amplified Optical signal optical To separate receivers Channels Input signal l 1 (multi-channel) Wavelength- division l , l , l , ... l , l , l , ... demultiplexer 1 2 3 1 2 3 l , l , l , ... 2 3 4 Optical amplifier Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Electro-optic receiver
Digital receiver Analog receiver Thresholding and retiming for digital output Detector (light to electronic) Analog electronic amplifier Digital Electronic output Light input Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Electro-optic detectors:
Convert light to electric signal Cannot discriminate among close optical channels Semiconductor photodiodes Light produces carriers at junction PIN photodiodes Avalanche photodiodes Internal amplification Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Wavelength response Silicon 400-1100 nm Germanium 800-1600 nm
GaAs nm InGaAs nm InGaAsP nm Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

PIN detector operation
Light produces electron-hole pairs in intrinsic region Bias voltage draws carriers Producing current signal Negative voltage + + + + + Intrinsic region + - + - - - - - - Positive voltage Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Detector bandwidth Response is not instant Pulse is blurred
Loss of high frequencies Input optical pulse Output electrical pulse 90% level Fall time Rise time Long, slow tail For some devices 10% level Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Discrimination and timing
Original digital signal Degraded by transmission Discrimination threshold Jitter Discrimination finds high points in signal Retiming with new clock signal Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Transceivers Transceiver is transmitter and receiver at one point, serving the same terminal equipment. One for the input fiber, one for the output Output fiber Transmitter Receiver Input fiber Transceiver Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Part 3: Passive optics Couplers & taps Planar waveguides
Attenuators and filters Wavelength-division multiplexing Types of multiplexing Optics for WDM Other passive components Optical isolators Optical circulators Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Couplers & Taps Couplers split or combine signals going to or coming from two or more fibers Splitting inevitably reduces signal intensity Photons only go one way Divide in half, get 3-dB loss Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Coupler loss Number of ports Fraction of signal Loss (dB) 2 0.5 3.0 3
0.333 4.8 0.25 4 6.0 0.167 6 7.8 10 0.1 10.0 15 0.0667 11.8 20 0.05 13.0 0.0333 30 14.8 50 0.02 17.0 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Functional types of couplers
T and Y (3-port) 1-N or Tree Star Separate inputs and outputs All ports equivalent Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Coupler Technologies Bulk optics Fused fibers Planar waveguides
Also micro optics Fused fibers Planar waveguides Active couplers Repeater with two or more outputs Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fused fiber couplers Couple light between parallel fibers
Often fibers twisted together Cladding removed partly or totally Light leaks between by evanescent wave coupling Two or multiple fibers Details depend on fiber type Single vs. multimode Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Multimode evanescent coupling
Output split 50-50 Light leaks between fiber cores Via evanescent wave coupling Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Single-mode evanescent coupling
Light leaks between fiber cores via evanescent wave coupling until all shifts, then shifts back All output Out top Fraction of light in each core depends on length Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Planar Waveguide Technology
Thin, flat rectangular waveguide Formed by diffusing dopant into substrate Waveguide has elevated refractive index Or by depositing added layer on surface Produced by standard photolithographic methods Silica/silicon, Lithium niobate, semiconductors, polymers Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Planar waveguide coupler-1
Pair of parallel waveguides for coupler Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Planar waveguide coupler-2
Y-shaped splitter - divides light 50-50 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Limitations of planar waveguides
Interaction lengths are relatively long Millimeters or centimeters Planar guide doesn't match fiber Loss is high Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Attenuators Transmit only a fraction of incident light
Nominally uniform across spectrum Used to limit intensity At receivers or transmitters Some attenuators are variable Generally used in instruments Fixed attenuators in systems Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Line and band filters Block undesired wavelengths Wavelength selection
(e.g., pump laser in fiber amplifier) Light that may be background noise Wavelength selection Red filter transmits only red light Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical Isolators Transmit light in only one direction
Block transmission toward laser Important for controlling noise Based on polarizer and faraday rotator Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical Isolators-2 Polarizer 1 Oriented Faraday rotator Polarizer 2
No light -- Crossed polarizer Blocks transmission Polarizer 1 Oriented Faraday rotator Polarizer 2 Oriented Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Wavelength-division multiplexing
Transmits signals at many wavelengths through one fiber Multiplies fiber capacity Distinct from time-division multiplexing TDM interleaves slow signals to make one fast one WDM sends multiple signals at similar speeds to increase capacity DWDM = dense WDM Each wavelength is an optical channel Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

WDM and optical networks
Wavelengths are carriers, as in radio Capacity is data rate X number of channels Dense WDM is closely spaced Capacity sold as optical channels Total WDM band limited by Transmission system Bandwidth of erbium-doped fiber amplifiers Optical channels managed through system Switched and Routed Adds 'granularity' Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

WDM system Receivers Transmitters Demultiplexer Multiplexer Add/drop
1 l 1 l 1 l 2 l 2 l 2 l 3 l 3 l 3 Demultiplexer Multiplexer Add/drop l 1 ,l 2 ,l 3 ,l 4 , l 1 ,l 2 ,l 3 ,l 4* , l 4 l Multiplexer 4 l 4 l 5 ,l 6 ,l 7 ,l 8 l 5 ,l 6 ,l 7 ,l 8 l 5 l 5 l 5 l 6 l 6 l 6 Drop Add l 4 l 4* l 7 l 7 l 7 l 8 l 8 Local Local l 8 receiver transmitter Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

WDM Optics Multiplexing: combine optical channels
Demultiplexing: separating optical channels Detectors can't tell wavelengths apart Separate optical channels When closely spaced Maintain low crosstalk Separated channels may be switched Wavelengths may be converted Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Channel Spacing Close spacing for dense WDM (40 channels @ 100 GHz)
Wavelength Wide spacing for wide WDM ( GHz) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Modulation bandwidth Well known from electronics
50 GHz spacing of carriers Well known from electronics Increases with data rate Limits channel spacing 2.5 Gbit/s 10 Gbit/s 10 Gbit/s Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

WDM channel standards International Telecommunications Union defined carrier grid in frequency units. Frequencies stepped from THz Based on the erbium-fiber amplifier window Original grid specified 100-GHz channels (0.8 nm at 1550 nm) ITU allows spacing on other grids 50, 200, 400, 500, 600, 1000 GHz Standards in evolution Especially outside erbium-fiber C band Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

WDM technologies Interference filters Fiber Bragg gratings
Interference filters for DWDM Fiber Bragg gratings Diffraction gratings Mach-Zehnder interferometers Planar waveguide arrays Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Interference filters Also called dielectric filters
Series of thin layers 2 materials with different refractive index Materials alternate Refractive index difference causes reflection Boundaries create resonances Nl=2nD cosq (D is thickness) Resonant wavelengths transmitted Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Interference filter operation
Other Wavelengths reflected Resonant wavelengths Transmitted (note Angle dependence) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Interference filters for WDM
Multiple filters needed for WDM Pick-off one wavelength at a time Pick bands, then individual wavelengths High resolution possible Many components Scalable in increments Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

One wavelength at a time
Input l 1 , l 2 , l 3 , l 4 , l 5 , l 6 l 1 l 2 , l 3 , l 4 , l 5 , l 6 l , l , l , l l 3 4 5 6 3 l 2 l 4 , l 5 , l 6 l 5 , l 6 l 5 l4 l 6 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

40-channel filter demux High- or low-pass filters l 1-l 40
Reflects wavelengths High- or low-pass filters longer than l9 Demultiplexes l 1 -l 16 l 1 -l 8 l 1 -l 8 Reflects wavelengths shorter than l 17 l 17 -l 40 l 9 -l 16 Reflects wavelengths Reflects wavelengths longer than l shorter than l 24 32 Demultiplexes l 25 -l 40 l 33 -l 40 l 33 -l 40 Demultiplexes l 9 -l 16 l 17 -l 24 l 25 -l 32 Demultiplexes Demultiplexes l 17 -l 24 l 25 -l 32 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Add/drop multiplexer Selects one wavelength (or more) from a transmission line, and drops that wavelength, replacing it with another optical signal. Add/drop multiplexer New signal uses the same optical channel as dropped signal Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fiber Bragg Gratings Function analogous to interference filters
Zones of high refractive index scatter light Selectively reflect one wavelength Transmit other wavelengths Like filters, can be grouped to drop one wavelength at a time Require an optical circulator Fiber components Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fiber grating operation
Red matches grating Period and is reflected Green and blue transmitted Because their wavelengths Don't match grating period Cladding Core High-index zones in fiber core Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Add/drop with fiber grating
Transmits l1-l7 Fiber Bragg grating Reflects l8 l1-l8 l8 Circulator directs reflected light to third port l1-l8 l8 l8 Multichannel input Optical circulator Reflected channel only l1-l8 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Bragg grating WDM systems
Like interference filters, drop one wavelength at a time Transmit rather than reflect unselected wavelengths Grating with segments having different line spacing is equivalent of band-pass filter Input Grating reflects different wavelengths Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical Circulators Needed to separate reflected light
Filters can reflect light at an angle Serve as optical one-way streets Should have low insertion loss Direct light from port 1 to port 2, port 2 to port 3, with none going 2-1 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical circulator structure
Faraday rotators shift polarization +45° when light goes in either direction Waveplates rotate polarization -45° when light goes one way, Faraday rotators +45° when light goes other way Faraday rotators Waveplates Waveplates +45° Beam displacer Beam displacer -45° +45° Beam displacer -45° Port 4 (output only) Port 3 +45° +45° (input/ +45° +45° output) -45° +45° +45° +45° +45° +45° +45° +45° Port 2 (input/ output) -45° +45° Port 1 (input only) -45° +45° -45° +45° +45° +45° Vertical polarization Horizontal Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved. Polarization

Diffraction gratings Diffraction spreads light at different angles
Prism-like effect Widely used in instruments, spectrometers Packaged with GRIN lens Major applications in instruments Optical spectrum analyzers Crosstalk and resolution issues Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Fused Fiber Couplers Single-mode inherently wavelength selective
Coupling distance depends on wavelength Low resolution Used for widely separated wavelengths Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Mach-Zehnder interferometer
More general effect Interference between waves in two parallel arms between a pair of couplers As wavelength changes, light is switched between outputs, interleaving wavelengths Offers high resolution Interleaves wavelengths Easy to make in fiber form Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Mach-Zehnder Interleaving
Interferometer arms Coupler Coupler Light from top output Light from bottom output 100% 0% l l l l 2 3 4 1 l 5 l 6 l 7 l 8 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Mach-Zehnder spacing Output phase determines light distribution
Output phase shifts with wavelength Depends on path difference Easier to express as frequency shift Dn Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Cascaded Mach-Zehnder WDM
400-GHz interleaver l 1 200-GHz l 1 , l 5 interleaver l 5 Input l 1 , l 400-GHz 3 , l 5 , l 7 100-GHz l 3 , l 7 interleaver l 3 interleaver l 1 -l 8 l 7 400-GHz l 2 , l 4 , l 6 , l 8 interleaver l 2 200-GHz l 2 , l 6 interleaver l 6 400-GHz l 4 , l 8 interleaver l 4 400-GHz interleaver l 8 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Arrayed Planar Waveguides
Curved planar waveguides connect two mixing regions Waveguide lengths differ Generating interference at 2nd mixing zone Interference causes diffraction Spreading out wavelengths at different angles Output waveguides collect optical channels Good for high channel counts Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

WDM Summary Several viable technologies Can be used in conjunction
Interleavers for fine resolution Filters for coarser resolution Each has attractions for some applications Costs hard to pin down A real technology horse race Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Part 4: Active components
Repeaters and optical amplifiers Repeaters and regenerators Optical amplifiers Types Functions Accessories Modulators Optical switches Wavelength converters Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Repeaters, Regenerators, and Optical Amplifiers
Boost signal after it fades with distance Needed to span long distances more than km terrestrial often shorter distances in networks Repeater: receiver-transmitter pair Regenerator: Repeater plus signal clean-up Optical amplifiers: amplify signal as light Current state of the art at nm Optical regenerators: would be nice Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Repeaters and regenerators
Detector Electronic amplifier Transmitter Repeater Optional Detector Electronic amplifier Thresholding & retiming Forward error correction Transmitter Regenerator Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Electro-optic repeaters
Receiver converts signal to electronic form Electronics amplify signal, drive transmitter Became obsolete Limited to one transmission format Designed for particular data rate One optical channel per repeater Erbium-doped fiber amplifiers are better NOT obsolete for wavelength conversion Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Why regenerators are still used
Optical amplifiers are analog devices Cannot remove noise or dispersion Contribute amplified spontaneous emission Dispersion accumulates over long distances Regeneration used at termination points Most terrestrial systems <1000 km Terminate in switches or routers Signals redistributed Regeneration is within the switch Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical amplifiers Directly amplify weak optical signal
Stimulated emission from excited material Laser without a resonant cavity Optical signal makes single pass Amplify all wavelengths in their range Compatible with WDM Purely analog devices Require fine tuning to limit noise Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Types of optical amplifiers
Erbium-doped fiber amplifiers: C band nm-most widely used L band nm Thulium doped fibers, S band nm Raman fiber amplifiers: broadband Praseodymium-doped fiber amplifiers 1310 nm range Semiconductor optical amplifiers Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Erbium fiber operation
Single pass produces gain Optical isolators prevent feedback to laser Noise from amplified spontaneous emission Erbium gain is broad: nm Depends on erbium host Designs differ for different wavelength bands Long fibers for low-gain L band nm Short fibers for high-gain C band nm Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Erbium-fiber gain 1480 nm pump L band C band

Multi-wavelength amplification
Enables long-haul WDM Fiber simultaneously amplifies all optical channels in its gain band. But gain must be balanced (varies with l) Gain equalization Balances amplification over all optical channels Gain varies with input power Saturation effects Series of amplifiers magnifies effects Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Power saturation Output Input signal 13 dB gain 22 dB gain 10 dBm

Spontaneous emission noise
Amplification based on stimulated emission Spontaneous emission occurs in fiber Excited erbium atoms release energy As in erbium-doped fiber laser Adds to input signal as background noise Noise spectrum is continuous background Between channels as well as on channels Most severe when input weak Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Spontaneous emission noise builds up
Cuts signal/noise ratio of optical channels Amplified noise from last amplifier Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Chains of amplifiers Noise builds up Differential gain builds up
Later amplifiers boost noise as well as signal The higher the amplification, the more noise Because weaker signal gives smaller signal/noise Like weak analog tape recording Differential gain builds up If one channel 1 dB weaker each amplifier 30 dB after 30 identical amplifiers Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Initial WDM signal All channels roughly equal power

Final WDM signal After a series of amplifiers Signal to noise reduced
Some channels stronger than others Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Amplifier trade-offs Spacing Longer systems amplify problems
Longer spacing reduces costs But gives weaker input signals, thus higher noise Shorter spacing improves performance, reduces noise but increases costs Longer systems amplify problems Long amplifier chains require closer spacing 50 km in submarine cables 100 km for terrestrial (600 km run) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Gain equalization Goal is to balance gain across spectrum
Equalizing filter Attenuates stronger channels Simple, but costs power Raman amplification Amplifies weaker channels Conserves power, but more complex and costly Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Equalizing filters + = Filter transmission Final output Amplifier gain

Raman amplification Stimulated Raman scattering
Excites vibrational modes in atoms in fiber Transfers power from strong pump to weak signal Uses standard transmission fiber Can pump back from receiver/amplifier Peaks at longer wavelengths Requires about 1 W of pump power Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Hybrid Raman amplification
Composite gain curve Erbium fiber gain Gain Raman amplifier gain Wavelength 1600 nm 1530 nm Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Raman amplifier set-up
Fiber Transmitter Signal Raman pump Coupler Raman amplification here Fiber amplifier Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Dual-band erbium amplifiers
Erbium gain varies widely with wavelength Early types nm Became C band at nm Gain is high in this region Reasonably uniform Gain lower at longer wavelengths Reasonably uniform nm Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Dual-band amplifier set-up
Amplified output 1530- 1565 nm C band amplifier Combined input Channels combined Signals split here nm L band amplifier Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Stand-alone Raman amplifiers
Operate over broad range Broadly applicable in principle Peak gain ~13 THz from pump wavelength 1.45 µm pump to amplify 1.55 µm 1.24 µm pump to amplify 1.31 µm No special fiber needed Pump power near 1 W in single-mode fiber Gain curve uneven Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Semiconductor optical amplifiers
Diode lasers without resonant cavities Facets antireflection coated Light makes single pass Can be integrated into planar waveguides Wide range of wavelengths Gain wherever semiconductors have gain Amplification requires drive current Like a diode laser Diode amplifier can double as modulator Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Semiconductor amplifier overview
Limitations Higher noise than fiber amplifiers Polarization sensitive Planar structure does not couple well to fiber Advantages Wide wavelength range Easily integrated into planar devices Can compensate for coupler or component loss Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

External modulators Adjust light transmission to control transmitter output Used only in high-performance systems Electro-optic waveguide modulator Electroabsorption semiconductor Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Electro-optic waveguide modulator
Lithium niobate integrated optics Applied voltage changes refractive index Light passes through two parallel guides Voltage applied across one or both Refractive index change causes phase shift Interference modulates output Phase shift Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Electro-optic modulator

Electroabsorption modulator
Reverse-biased semiconductor diode Transparent when 'off' Can be made on same substrate as laser Integrated with diode lasers Not a semiconductor amplifier Turning voltage on causes absorption Creating states that absorb laser light Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical switching Essential for optical networking
Manages signals as optical channels Switch one channel or whole fiber load Functions Protection switching Provisioning Add/drop switches Cross-connects Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Transparent and opaque switches
Transparent switches Let light go through unchanged Handle any optical signal format Example -- tilting mirrors Opaque switches Convert signals into other formats Light does not go straight through Example: regenerator at switch Both have advocates and applications Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Add-drop switch San Mateo From To San Jose San Francisco Add-drop
l 1 ,l 2 ,l 3 ,l 4 , l 1 ,l 2 ,l 3 ,l 4* , l 5 ,l 6 l 5 ,l 6 Add-drop switch can change which wavelengths are dropped and added. Drop Add l 4 l 4* San Mateo transmitter San Mateo receiver Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Cross-connect example
Outputs Inputs Node l l l l l l l l l l l l l l l l Signal Path l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l Signal Path l l l l l l l l Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Optical switching technologies
Bulk opto-mechanical switches Micromirror/MEMS Bubble-jet waveguide switches Electro-optical switches Liquid crystal switches Others in development Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Bulk-opto-mechanical switches
Moving fibers Moving bulk lenses or mirrors Old, established technology Used for protection and provisioning Little automation Switches all signals on fiber (transparent) Drawbacks: Slow and mechanical Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

MEMS/micromirrors MEMS- micro-electro-mechanical systems
Microstructures etched photolithographically in semiconductors Flex and bend when voltages are applied Fast, very promising for switching Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

MEMS types-1 Continuously tilting mirrors
Direct beams through free space Can switch among many possible outputs High port counts Mirror tilts back and forth, scans continuously Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

MEMS types-2 Pop-up mirrors Mirror down-light goes through
Two discrete latched positions Two possible outputs Mirror up - light reflected Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Bubble-jet waveguide switches
Array of intersecting high-index waveguides Crossed by channels filled with index-matching fluid Bubbles move back and forth in channels Bubble at junction Total internal reflection diverts beam No bubble Light goes through junction Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Bubble-jet waveguide switch
Liquid channel Total internal reflection No bubble Light goes straight through Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Electro- or thermo-optic switches
Like modulator with two outputs Solid-state technology Thermal or electro-optic 2x2 switches easiest Others are cascaded Planar waveguide devices Can be integrated Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Liquid crystal switch Voltage changes polarization properties of liquid crystal. Used with polarizers to switch light. Like liquid-crystal displays. Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Wavelength conversion needs
Switching optical channels from one wavelength to another Same wavelength can't be guaranteed across entire system Like switching lanes on a freeway Cleveland 1540 nm Pittsburgh Philadelphia Harrisburg 1542 nm Wavelength Wavelength 1538 nm Converter (1540 not available) Converter (1540, 1542 Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved. not available)

Wavelength conversion technology
Opto-electro-optic (OEO) conversion Receiver back to back with transmitter at another wavelength Tunable laser transmitter would be nice Nonlinear wavelength conversion Tame four-wave mixing Raman shift Optically modulate diode laser emitting another wavelength Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Resources-1 (main source)
This presentation is based upon Jeff Hecht, Understanding Fiber Optics 4th edition (Prentice Hall, 2002). Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Resources-2 (other books)
Gerd Keiser, Optical Fiber Communications 3rd ed. (McGraw Hill, 2000) Luc B. Jeunhomme, Single-Mode Fiber Optics: Principles and Applications (Marcel Dekker) Donald B. Keck, ed., Selected Papers on Optical Fiber Technology (SPIE Milestone Series Vol. MS38, 1992) Jeff Hecht, Understanding Lasers (IEEE Press, 1994) Govind Agrawal Fiber-Optic Communication Systems (Wiley-Interscience) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Resources-3 (other books)
P.C. Becker et al Erbium Fiber Amplifiers: Fundamentals and Technology (Academic Press, 1999) Bob Chomycz, Fiber Optic Installer’s Field Manual (McGraw Hill, 2000) Jim Hayes, Fiber Optics Technician’s Manual (Delmar Publishers, 1996) Govind P. Agrawal, Semiconductor Lasers: Past, Present and Future (AIP Press, 1995) Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Resources-4 (trade journals)
Laser Focus World Lightwave Fiberoptic Product News Telephony Photonics Spectra Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Resources-5 (scholarly journals)
Journal of Lightwave Technology IEEE Photonics Technology Letters Electronics Letters IEEE Communications Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

Understanding Fiber Optic Components

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Understanding Fiber Optic Components

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