Optical Interconnect and Optical Interconnect and Sensing Sensing Dr. How T. Lin Dr. How T. Lin Endicott Interconnect Endicott Interconnect Technologies Technologies Topics Topics • Light Fundamentals Light Fundamentals • Common Optical Components for Light Emission and Common Optical Components for Light Emission and Detection and Transmission Detection and Transmission • Optical Interconnect Principle Optical Interconnect Principle • Optical Interconnects Optical Interconnects • Fiber Optics Fiber Optics • Optical Waveguides Optical Waveguides • Optical Sensing with FBG (Fiber Bragg Grating Sensing) Optical Sensing with FBG (Fiber Bragg Grating Sensing) • Principle Principle • Applications Applications Disadvantages of Electrical Disadvantages of Electrical Interconnects/Sensors Interconnects/Sensors • Physical Problems (at high frequencies/high Physical Problems (at high frequencies/high noise environments) noise environments) Cross Cross-talk talk Signal Distortion Signal Distortion Electromagnetic Interference Electromagnetic Interference Reflections Reflections High Power Consumption High Power Consumption High Latency (RC Delay) High Latency (RC Delay) Limited Bandwidth Limited Bandwidth Why Optics ? Why Optics ? • Advantages: Advantages: Capable to provide high bandwidths Capable to provide high bandwidths Free from electrical short Free from electrical short-circuits circuits Low Low- loss transmission at high frequencies loss transmission at high frequencies Immune to electromagnetic interference Immune to electromagnetic interference Essentially no crosstalk between adjacent signals Essentially no crosstalk between adjacent signals No impedance matching required No impedance matching required • Successful long Successful long-haul telecommunication system based haul telecommunication system based on fiber optics on fiber optics
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Optical Interconnect and Optical Interconnect and SensingSensing
Dr. How T. LinDr. How T. LinEndicott Interconnect Endicott Interconnect
TechnologiesTechnologies
TopicsTopics
•• Light FundamentalsLight Fundamentals•• Common Optical Components for Light Emission and Common Optical Components for Light Emission and
Detection and TransmissionDetection and Transmission•• Optical Interconnect PrincipleOptical Interconnect Principle•• Optical InterconnectsOptical Interconnects
•• Optical Sensing with FBG (Fiber Bragg Grating Sensing)Optical Sensing with FBG (Fiber Bragg Grating Sensing)•• PrinciplePrinciple•• ApplicationsApplications
Disadvantages of Electrical Disadvantages of Electrical Interconnects/SensorsInterconnects/Sensors
•• Physical Problems (at high frequencies/high Physical Problems (at high frequencies/high noise environments)noise environments)
CrossCross--talktalkSignal DistortionSignal DistortionElectromagnetic InterferenceElectromagnetic InterferenceReflectionsReflectionsHigh Power ConsumptionHigh Power ConsumptionHigh Latency (RC Delay)High Latency (RC Delay)Limited BandwidthLimited Bandwidth
Why Optics ?Why Optics ?•• Advantages:Advantages:
Capable to provide high bandwidthsCapable to provide high bandwidthsFree from electrical shortFree from electrical short--circuitscircuitsLowLow--loss transmission at high frequenciesloss transmission at high frequenciesImmune to electromagnetic interferenceImmune to electromagnetic interferenceEssentially no crosstalk between adjacent signalsEssentially no crosstalk between adjacent signalsNo impedance matching requiredNo impedance matching required
•• Successful longSuccessful long--haul telecommunication system based haul telecommunication system based on fiber opticson fiber optics
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Using Lightwave to Transmit Using Lightwave to Transmit InformationInformation
Simplified phasor representation of EM wave
E(t) cos(ωt+θ)
Amplitude frequency phase
Device a method to detect change in any one of the three variables listed above……….we have a data transmitter!
Transmission Medium: Fiber optics (MM/SM), Polymer Waveguide or Free Space
Receiver: Photo Diode or Transistor
EM Spectrum EM Spectrum EM Spectrum (Visible)EM Spectrum (Visible)
UV…..……………..Visible…………………IR
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What is Light?What is Light?RaysWavesParticles
AbsorptionEmission
Interference RefractionReflection
Bandgap
Conduction band
Valence band
n0
n1
n0
A little Quantum TheoryA little Quantum Theory•• Definition:Definition:
•• Optical powerOptical power watt (W) watt (W) -- a rate of energy of one a rate of energy of one joule (J) per second.joule (J) per second.
•• Optical power is a function of both the number of Optical power is a function of both the number of photons and the wavelength. Each photon carries photons and the wavelength. Each photon carries an energy that is described by Planckan energy that is described by Planck’’s equation: s equation: Q = Q = hchc //λλwhere where Q= photon energy in JQ= photon energy in J
h = Planckh = Planck’’s constant (6.623 x 10s constant (6.623 x 10--3434 Js)Js)c = speed of light (2.998 X x 10c = speed of light (2.998 X x 108 8 m/sm/s))λλ = wavelength in meters= wavelength in meters
Basic Optical PrinciplesBasic Optical Principles
•• Optical FilterOptical Filter ::•• Absorption by filter glass varies Absorption by filter glass varies
with with λλ and thickness (d) of and thickness (d) of substratesubstrate
•• At each interface, part of the At each interface, part of the incident light will be reflected incident light will be reflected and the rest will pass throughand the rest will pass through..
•• Interface LossesInterface Losses ::•• FresnelFresnel’’s Law s Law
rrλλ = reflection loss (normal = reflection loss (normal incidence)incidence)
nnλλ = = nn’’/n/n
rrλλ = = nnλλ --1/ 1/ nnλλ +1+1
Transmission through an optical filter
Interface Losses
Basic Optical PrinciplesBasic Optical Principles
•• RefractionRefraction ::SnellSnell’’s Law s Law ––
n sin(n sin(θθ) = n) = n’’ sin(sin(θθ’’))Index of refraction: Index of refraction: n = 1.0 for airn = 1.0 for air
n = 1.5 for glassn = 1.5 for glass
Transmission through an optical filter
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Basic Optical PrinciplesBasic Optical Principles
•• DiffractionDiffraction::•• Lightwave bends when pass by small apertureLightwave bends when pass by small aperture
θ = λ/θ = λ/DDwherewhere θ θ is the diffraction angleis the diffraction angle
λλ is the wavelengthis the wavelengthD is the aperture widthD is the aperture width
D
Basic Optical PrinciplesBasic Optical Principles
•• InterferenceInterference::•• Wave nature of light causes interference patterns:Wave nature of light causes interference patterns:
Interference filter for wavelength selection Interference filter for wavelength selection --
Basic Optical PrinciplesBasic Optical Principles
•• CollimationCollimation::•• Place point source at focal point of lens or parabolic mirror Place point source at focal point of lens or parabolic mirror
can produce collimated light (parallel light beam)can produce collimated light (parallel light beam)
Light DetectionLight Detection•• Two broad classes of optical detectors:Two broad classes of optical detectors:
•• Photon detectors Photon detectors –– interactions of quanta of light energy with electrons in interactions of quanta of light energy with electrons in the detector material and generating free electrons (wavelength the detector material and generating free electrons (wavelength dependent).dependent).
•• Thermal detectors Thermal detectors -- respond to the heat energy delivered by the light respond to the heat energy delivered by the light (wavelength independent).(wavelength independent).
•• PhotoemissivePhotoemissive. These detectors use the photoelectric effect, in . These detectors use the photoelectric effect, in which incident photons free electrons from the surface of the which incident photons free electrons from the surface of the detector material. These devices include vacuum photodiodes, detector material. These devices include vacuum photodiodes, CCD camera, bipolar phototubes, and photomultiplier tubes.CCD camera, bipolar phototubes, and photomultiplier tubes.
•• Photoconductive. The electrical conductivity of the material Photoconductive. The electrical conductivity of the material changes as a function of the intensity of the incident light. changes as a function of the intensity of the incident light. Photoconductive detectors are semiconductor materials. They Photoconductive detectors are semiconductor materials. They have an external electrical bias voltage.have an external electrical bias voltage.
•• Photovoltaic. These detectors contain a Photovoltaic. These detectors contain a pp--nn semiconductor semiconductor junction and are often called photodiodes. A voltage is self junction and are often called photodiodes. A voltage is self generated as radiant energy strikes the device. The photovoltaicgenerated as radiant energy strikes the device. The photovoltaicdetector may operate without external bias voltage. A good detector may operate without external bias voltage. A good example is the solar cell used on spacecraft and satellites to example is the solar cell used on spacecraft and satellites to convert the sunconvert the sun’’s light into useful electrical power.s light into useful electrical power.
Photoconductive and photovoltaic detectors are commonly used in Photoconductive and photovoltaic detectors are commonly used in circuits in which circuits in which there is a load resistance in series with the detector. The outpthere is a load resistance in series with the detector. The output is read as a ut is read as a change in the voltage drop across the resistorchange in the voltage drop across the resistor..
Light Detection : Detector characteristicsLight Detection : Detector characteristics•Responsivity - Defined as the detector output per unit of input power.
The units of responsivity are either amperes/watt (alternatively milliamperes/milliwatt or microamperes/microwatt.
•Quantum efficiency – Defined as the effectiveness of the incident radiant energy for producing electrical current in a circuit. It may be related to the responsivity by the equation:
Q = 100 x Rd x hv = 100 x Rd (1.2395/λ ).
•Noise equivalent power (NEP) - Defined as the radiant power that produces a signal voltage (current) equal to the noise voltage (current) of the detector.
NEP = IAVN / VS(∆ f)1/2
where I is the irradiance incident on the detector of area A, VN is the root mean square noise voltage within the measurement bandwidth ∆ f, and VS is the root mean square signal voltage.
Professor Charles Kao who has been recognized as the inventor of fiber optics is receiving an IEE prize from Professor John Midwinter (1998 at IEE Savoy Place, London, UK; courtesy of IEE)
Optical Fiber An optical fiber is a flexible filament of very clear glass and is capable of carrying information in the form of light. This filament of glass is a little thicker than a human hair.
Dielectric Waveguides and Optical FibersStep Index Fiber
Optical fiber structure
• The difference in refractive index between the core and cladding is < 0.5%.• The refractive index of the core is higher than that of the cladding, so that
light in the core strikes the interface with the cladding at a bouncing angle and is trapped in the core by total internal reflection.
The cladding is the layer completely surrounding the core.
The core, or the axial part of the optical fiber, is the light transmission area of the fiber.
• A mode is a defined path in which light travels.
• A light signal can propagate through the core of the optical fiber on a single path (single-mode fiber) or on many paths (multimode fiber). The mode in which light travels depends on geometry, the index profile of the fiber, and the wavelength of the light.
• Single-mode fiber has the advantage of high information-carrying capacity, low attenuation and low fiber cost, but multimode fiber has the advantage of low connection and electronics cost that may lead to lower system cost.
Dielectric Waveguides and Optical Fibers
• Multimode vs. Single-mode
Step Index Fiber• Schematic diagram of Step Index Fiber
n
y
n2 n1
Cladding
Core z
y
rφ
Fiber axis
Normalized index difference
1
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nnn −
=∆
Typically∆ << 1
• The core has greater refractive index than the cladding.• The fiber has cylindrical symmetry. r, φ, z to represent any point in the fiber. • Cladding is normally much thicker than shown.
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n1
n2
21
3
nO
n1
21
3
n
n2
OO' O''
n2
Multimode Step Index Fiber Ray paths are different so thatrays arrive at different times.
Graded Index Fiber
Ray paths are different but so are the velocities along the paths so that all the rays arrive at the same time.
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The Graded Index (GRIN) Optical Fiber
n decreases step by step from one layer to next upper layer; very thin layers.
n decrease in continuous gives a ray path changing continuously.
TIR
A ray in thinly stratified medium becomes refracted as it passes from one layer to the next upper layer with lower nand eventually its angle satisfies TIR.
In a medium where n decreases continuously the path of the ray bends continuously.
The Graded Index (GRIN) Optical Fiber
TIR
Light Absorption and Scattering• Attenuation
• The reduction in signal strength is measured as attenuation.
• Attenuation measurements are made in decibels (dB). The decibel is a logarithmic unit that indicates the ratio of output power to input power.
• Each optical fiber has a characteristic attenuation that is normally measured in decibels per kilometer (dB/km).
• Optical fibers are distinctive in that they allow high-speed transmission with low attenuation.
• The field in the wave oscillates the ions which consequently generate "mechanical“ waves in the crystal; energy is thereby transferred from the wave to lattice vibrations.
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Scattered waves
Incident wave Through wave
A dielectric particle smaller than wavelength
• Rayleigh scattering involves the polarization of a small dielectric particle or a region that is much smaller than the light wavelength.
• The field forces dipole oscillations in the particle (by polarizing it) which leads to the emission of EM waves in "many" directions so that a portion of the light energy is directed away from the incident beam.
• Rayleigh scattering
Displacing electron with respect to positive nuclei.
Oscillating charge = Alternating current
Radiates EM waves
Light Absorption and Scattering Attenuation in Optical Fibers
• Optical Fiber Attenuation vs. wavelength
Fiber LossFiber Loss
Attenuation in Optical Fibers
• Attenuation vs. wavelength
Stretching of Si-O bonds in ionic polarization induced by EM wave, which is around 9 µm.
Stretching of Si-O bonds in ionic polarization induced by EM wave, which is around 9 µm. Presence of hydroxyl ions (water) as
an impurity.Stretching vibration of OH- bonds at 2.7 µm. Its overtones at 1.0 & 1.4 µm.
combinationof Si-O & 1.4 µm
• Micro-bending loss
Attenuation in Optical Fibers
• Sharp bends change the local waveguide geometry that can lead to waves escaping.• The zigzagging ray suddenly finds itself with an incident angle θ’ that gives rise to either a
transmitted wave, or to greater cladding penetration; the field reaches the outside medium and some light energy is lost.
Escaping wave
θ θ
θ′ < θ
θθ > θc θ′
Microbending
R
Cladding
Core
Field distribution
• Small changes in the refractive index of the fiber due to induced strains when it is bent during its use, e.g., when it is cabled and laid.
• Induced strains change n1 and n2, and hence affect the mode field diameter, that is field penetration into the cladding.
• Macrobending loss crosses over into microbending loss when the radius of curvature becomes less than a few centimeters.
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Fiber Fabrication
• Fiber Materials Glasses and Plastics• It must be possible to make long, thin flexible fibers from the materials.• The material must be transparent at a particular optical wavelength in order
for the fiber to guide light efficiently.• Physically compatible materials that have slightly different refractive
indices for the core and cladding must be available• Silica Glass Fibers
• Glass do not have well defined melting point. The glass become to soften at high temperature (>1000°C), it became viscous liquid.
• Plastic Optical Fibers: POF• Short distance (∼100 m), very flexible, relaxation of connector tolerance, low cost• polymethylmethacrylate (PMMA) or perifluorinated polymer (PFP)
• High non-linearity optical properties for all optical switch or fiber lasers• Chalcogen elements are doped: S, Se, Te…
• Amplification, Attenuation, Phase retardation• Rare earth elements are doped (0.005-0.05 mole%): atomic no. 57-71, Er, Pr…
• Extremely low transmission losses at mid-IR (@0.2∼8 µm) 0.01∼0.001 dB/km)• ZrF4, BaF2, LaF3, AlF3, NaF• Fabricating long lengths of fibers is difficult.
• Outside Vapor-Phase Oxidization
• Vapor-Phase Axial Deposition
• Modified Chemical Vapor Deposition
• Plasma-Activated Chemical Vapor Deposition
• Double-Crucible Method
• Fiber Fabrication
Fiber Fabrication• Schematic illustration of a fiber drawing tower.
Fiber Drawing
Preform feed
Furnace 2000°C
Thicknessmonitoring gauge
Take-up drum
Polymer coater
Ultraviolet light or furnacefor curing
Capstan
Pref
orm
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Vapors: SiCl4+ GeCl4+ O2
Rotate mandrel
(a)
Deposited sootBurner
Fuel: H2
Target rod
Deposited Ge doped SiO2
(b)
Furnace
Porous sootpreform with hole
Clear solidglass preform
Drying gases
(c)
Furnace
Drawn fiber
Preform
Reaction of gases in the burner flame produces glass soot that deposits on to the outside surface of the mandrel.
The mandrel is removed and the hollow porous soot preform is consolidated; the soot particles are sintered, fused, together to form a clear glass rod.
The consolidated glass rod is used as a preform in fiber drawing.
Outside Vapor Deposition (OVD)
• Schematic illustration of OVD and the preform preparation for fiber drawing
•• Requirements:Requirements:•• Compatible with standard PWB TechnologiesCompatible with standard PWB Technologies•• High performance (low optical loss)High performance (low optical loss)•• Robust (>230 degrees C, >10 sec.)Robust (>230 degrees C, >10 sec.)•• Dense (<60 micron Line and space) Dense (<60 micron Line and space) •• Standard toolingStandard tooling
Optical Backplanes Speed Data Optical Backplanes Speed Data
In DaimlerChrysler's optical backplane, the beam from a laser diode passes through one set of lenses and reflects off a micromirror before reaching a polymer waveguide, then does the converse before arriving at a photodiode and changing back into an electrical signal. A prototype operates at 1 Gb/s.
FreeFree--Space Interconnects Pack in Space Interconnects Pack in Data Channels Data Channels
An experimental module from the University of California, San Diego, just 2 cm high, connects stacks of CMOS chips. Each stack is topped with an optics chip [below center] consisting of 256 lasers (VCSELs) and photodiodes. Light from the VCSELs makes a vertical exit from one stack [below, left] and a vertical entry into the other. In between it is redirected via a diffraction grating, lenses, an alignment mirror [center], and another grating. Each of the device's 256 channels operates at 1 Gb/s.
Optical SensingOptical SensingTypical sensing system configuration using photons
Light source
Optical detector
Optional optical detector
Ambient (light): noise source
Ambient (light): noise source
Electronics
Subject of interest
sign
al +
noi
se Operating medium
Photon Sensing System IssuesPhoton Sensing System Issues
• Selection of Light Sources• Selection Light Detectors• Minimizing effect of background noise
resulting from ambient light sources• System Performance
• Resolution• Speed• Accuracy
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Fiber Optics For Measurement ApplicationsFiber Optics For Measurement ApplicationsTemperature Measurement Example:Technology - Light absorption/transmission properties of gallium
arsenide (GaAs)
Fiber
Semiconductor Crystal
Dielectric Mirror
Teflon
Technology - Fluoresence-decay of phosphor.
Temp. λabs
λabs = f(T)
Fiber Optic Temperature Probe
Fiber Mirror
Jacket
Timedecay = f(temp.)Fiber Optic Temperature Probe
Phosphor
Light
Light
Fiber Optics For Measurement ApplicationsFiber Optics For Measurement Applications
Fiber Optic Chemical Sensors (FOCS):
Fiber Dielectric
Mirror
TeflonChemical
Cladding removed substituted by suitable chemical
Light
Escape light
Amount of light loss is proportional to the amount of chemical present
Operation Principle of FBG SensorOperation Principle of FBG Sensor
λn+∆λλ1,λ2, ..., λn, ..., λx
λn+∆λ
λnλ1,λ2, ..., λn, ..., λx
λ1,λ2, ......, λx
λn
When the fiber optic sensor is initially mounted to astructure, it's in resonance with laser wavelength ln.
Mounting block thatattaches fiber optic
sensor to the structure
Structure starts to pull mounting blocks apart ,which stretches the fiber optic sensor. Theresonance of fiber optic sensor is now shifted.
Reflection Without Strain
Reflection Without Strain
FBG SensingFBG Sensing
FBG Sensor Temperature Response
30 40 50 60 701550.7
1550.8
1550.9
1551
1551.1
1551.2
1551.3
Temperature,oC
Wav
elen
gth,
nm
Athermal, max shift: 21.6 pm (2.7 GHz) from 24oc to 70oC
Conventional, 10.4 pm/oC (1.3 GHz/oC)
Standard FBG Sensor Temperature Response
Athermal FBG Sensor Temperature Response
Utilization of FBG Characteristics for measurementUtilization of FBG Characteristics for measurement
Accelerometer
Accelerometer
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Other FBG SensorsOther FBG Sensors FBG For Structure Health MonitoringFBG For Structure Health Monitoring
FBG Railway SensingFBG Railway Sensing
Wav
elen
gth
(nm
)
Time (0.01 sec)
Typical Structure Health Monitoring Typical Structure Health Monitoring SystemSystemBroadband coupler λ1 λ2 λ3Broadband
Sourceλ1λ2 λ3……
Tunable Filter
Optical Subsystem
Reflected Light
λ2λ3… λ3… …
FBGsλ3 λ2λ1
Detection
Broadband coupler λ1 λ2 λ3Tunable
Source
λ1λ2 λ3……
Tunable Filter
Optical Subsystem
Reflected Light
λ2λ3… λ3… …
FBGsλ3 λ2λ1
Detection
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Broadband coupler
SLED or Laser
λ1 λ2 λ3Low Contrast Fabry-Perot
Filter
λ1λ2 λ3……
WavelengthLocker
Light Source Trigger Module
Timing Generator
Interrogation Unit (High Speed Signal Conditioning,
Sampling and ADC)
Microcontroller
Ethernet Interface
PC
Optical Subsystem
Electrical Subsystem
Pulsed Broadband
light
Reflected Light
FBG-LTDM Structure Monitoring System
λ2λ3… λ3… …
FBGs
ExternalInternal
λ3 λ2λ1 λ1λ2 λ3……
FBG-LTDM Structure Monitoring System Timing Example
Time (ns)
λ1
λ2
λ3
λ1 λ2 λ3
FBGs
λ1λ2 λ3…… λ2λ3… λ3… …
10 meters 10 meters 10 meters
Light Pulse
1st. Reflected Wavelength
2nd. Reflected Wavelength
3rd. Reflected Wavelength
50 100 150 200 250 150 200
Tp
Tfr
Tsw
Tsl
Light Pulse
λ1λ2 λ3…… λ1λ2 λ3……
Light Pulse
ConclusionsConclusions
•• Interconnect problem significant in ultra high Interconnect problem significant in ultra high speed data communicationspeed data communication
•• Performance of Electrical lnterconnects will limit Performance of Electrical lnterconnects will limit high performance system throughputhigh performance system throughput
•• OIs will provide significant performance boost OIs will provide significant performance boost but not completely replace but not completely replace EIsEIs
•• Optical Sensing will be deployed in new areas Optical Sensing will be deployed in new areas that were not feasible with electrical sensors that were not feasible with electrical sensors
WWavelength avelength DDivision ivision MMultiplexingultiplexingWDM enables transmission of multiple communication channels
through a single fiber using various colors of light
Detector
MUX =Multiplexer
DEMUX =Demultiplexer
EDFA =Erbium Doped Amplifier
λn
λ1
λn
λ1
λ2
λ1
λ2
λ1
Coarse WDM (CWDM): Transmission of a few widely spaced channels
Dense WDM (DWDM): Transmission of many closely spaced channels
ReferencesReferences• International Technology Roadmap for Semiconductors (ITRS), 2001• R. Havemann and J.A Hutchby, “High-Performance Interconnects: An
integration Overview”, Proc. Of IEEE, Vol.89, May 2001 • D.A.B Miller, “Physical reasons for optical interconnections”, Int. Journal of
