Download Optical Interconnect and Sensing

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Optical Interconnect and
Sensing
Dr. How T. Lin
Endicott Interconnect
Technologies
Disadvantages of Electrical
Interconnects/Sensors
• Physical Problems (at high frequencies/high
noise environments)
ƒ
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ƒ
ƒ
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CrossCross-talk
Signal Distortion
Electromagnetic Interference
Reflections
High Power Consumption
High Latency (RC Delay)
Limited Bandwidth
• Light Fundamentals
• Common Optical Components for Light Emission and
Detection and Transmission
• Optical Interconnect Principle
• Optical Interconnects
• Fiber Optics
• Optical Waveguides
• Optical Sensing with FBG (Fiber Bragg Grating Sensing)
• Principle
• Applications
Why Optics ?
• Advantages:
ƒ Capable to provide high bandwidths
ƒ Free from electrical shortshort-circuits
ƒ LowLow-loss transmission at high frequencies
ƒ Immune to electromagnetic interference
ƒ Essentially no crosstalk between adjacent signals
ƒ No impedance matching required
• Successful longlong-haul telecommunication system based
on fiber optics
1
Using Lightwave to Transmit
Information
Optical Interconnect Fundamentals
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!
EM Spectrum
Basic Optical Interconnect
λ1
λ1
Transmission Medium
Transmitter
Receiver
Transmitter: LED or Laser
Transmission Medium: Fiber optics (MM/SM), Polymer Waveguide or Free Space
Receiver: Photo Diode or Transistor
EM Spectrum (Visible)
UV…..……………..Visible…………………IR
2
What is Light?
Particles
A little Quantum Theory
Waves
Conduction band
Rays
n0
Bandgap
n1
n0
• Optical power watt (W) - a rate of energy of one
joule (J) per second.
• Optical power is a function of both the number of
photons and the wavelength. Each photon carries
an energy that is described by Planck’
Planck’s equation:
Q = hc /λ
Valence band
Absorption
Emission
• Definition:
Interference
Refraction
Reflection
where Q= photon energy in J
h = Planck’
Planck’s constant (6.623 x 10-34 Js)
c = speed of light (2.998 X x 108 m/s)
m/s)
λ = wavelength in meters
Basic Optical Principles
• Optical Filter :
• Absorption by filter glass varies
with λ and thickness (d) of
substrate
• At each interface, part of the
incident light will be reflected
and the rest will pass through.
Basic Optical Principles
• Refraction :
Snell’
Snell’s Law –
Transmission through an optical filter
n sin(θ
sin(θ) = n’
n’ sin(θ
sin(θ’)
Index of refraction:
n = 1.0 for air
n = 1.5 for glass
Transmission through an optical filter
• Interface Losses :
• Fresnel’
Fresnel’s Law
rλ = reflection loss (normal
incidence)
nλ = n’/n
rλ = nλ -1/ nλ +1
Interface Losses
3
Basic Optical Principles
• Diffraction:
• Interference:
• Lightwave bends when pass by small aperture
θ = λ/D
λ/D
where
Basic Optical Principles
• Wave nature of light causes interference patterns:
Interference filter for wavelength selection -
θ is the diffraction angle
λ is the wavelength
D is the aperture width
D
Basic Optical Principles
• Collimation:
• Place point source at focal point of lens or parabolic mirror
can produce collimated light (parallel light beam)
Basic Optical Principles
• Wavelength Selection:
• Prisms:
• with high n, select λ with
narrow slit
• Gratings:
Slit
• disperse light into
spectrum with ruled lines
where m is an integer
(order)
Collimation with lens and parabolic mirror
4
Light Sources
•
•
•
•
•
• Lasers
•
•
•
•
Light Sources
Lasers
(Light Amplification by Stimulated Emission of Radiation)
Gas
Liquid
Solid State
Semiconductor (diodes)
Gas
Solid State
Liquid
Semiconductor (diode)
Characteristics:
Coherence - Photons have fixed phase relationship.
•
•
•
•
•
• Light Emitting Diodes (LED)
•
Relative narrow spectra
Low divergence after collimation.
Difficult to modulate (gas, liquid).
High cost.
LED
(Light Emitting Diodes)
Characteristics:
•
•
•
•
•
Light Sources : Semiconductor Lasers
Incoherence -Photons with random phase
Relative broad spectra.
Low cost.
Easy modulation.
Small size
Light Sources : LEDs
Edge emitting LED
p-DBR
active
n-DBR
Surface emitting LED
VCSEL
5
Light Detection
• Two broad classes of optical detectors:
• Photon detectors – interactions of quanta of light energy with electrons in
the detector material and generating free electrons (wavelength
dependent).
• Thermal detectors - respond to the heat energy delivered by the light
(wavelength independent).
Light Detection
• Photon detectors:
• Photoemissive.
Photoemissive. These detectors use the photoelectric effect, in
which incident photons free electrons from the surface of the
detector material. These devices include vacuum photodiodes,
CCD camera, bipolar phototubes, and photomultiplier tubes.
• Photoconductive. The electrical conductivity of the material
changes as a function of the intensity of the incident light.
Photoconductive detectors are semiconductor materials. They
have an external electrical bias voltage.
• Photovoltaic. These detectors contain a p-n semiconductor
junction and are often called photodiodes. A voltage is self
generated as radiant energy strikes the device. The photovoltaic
detector may operate without external bias voltage. A good
example is the solar cell used on spacecraft and satellites to
convert the sun’
sun’s light into useful electrical power.
Photoconductive and photovoltaic detectors are commonly used in circuits in which
there is a load resistance in series with the detector. The output
output is read as a
change in the voltage drop across the resistor.
Light Detection : Detector characteristics
•Quantum
- Defined as the detector output per unit of input power.
The units of responsivity are either amperes/watt
(alternatively milliamperes/milliwatt or
microamperes/microwatt.
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.
Light Detection
• Materials –
• Silicon (Si)
• Least expensive
• Germanium (Ge
(Ge))
• “Classic”
Classic” detector
• Indium gallium
arsenide (InGaAs)
• Highest speed
1.0
Responsivity (A/W)
•Responsivity
Quantum
Efficiency = 1
0.5
Germanium
InGaAs
Silicon
0.1
500
1500
1000
Wavelength nm
6
Optical Fiber
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 Fibers
Step Index Fiber
‰ Optical fiber structure
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.
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)
•
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.
Dielectric Waveguides and Optical Fibers
• Multimode vs. Single-mode
•
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.
•
Step Index Fiber
• Schematic diagram of Step Index Fiber
y
y
∆=
Cladding
φ
Core
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.
n2 n1
•
•
•
n
r
z
Fiber axis
n1 − n2
n1
Normalized
index difference
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.
7
The Graded Index (GRIN) Optical Fiber
The Graded Index (GRIN) Optical Fiber
n2
TIR
n1
3
2
1
O
n
Ray paths are different so that
rays arrive at different times.
n2
Graded Index Fiber
O
O'
O''
3
2
1
2
3
n1
n
TIR
Multimode Step Index Fiber
Ray paths are different but
so are the velocities along
the paths so that all the rays
arrive at the same time.
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.
A ray in thinly stratified medium
becomes refracted as it passes from one
layer to the next upper layer with lower n
and eventually its angle satisfies TIR.
In a medium where n decreases
continuously the path of the ray
bends continuously.
n2
Light Absorption and Scattering
•
Light Absorption and Scattering
Attenuation
• Absorption
•
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.
E
Medium
=
Absorption
+
A solid with ions
Ex
k
z
Attenuation
• Lattice absorption through a crystal
Scattering
Light direction
k
z
+
Extrinsic factor
(
fib
b
di
• 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.
)
8
Attenuation in Optical Fibers
Light Absorption and Scattering
•
Rayleigh scattering
• Optical Fiber Attenuation vs. wavelength
A dielectric particle smaller than wavelength
Displacing electron with
respect to positive nuclei.
Through wave
Incident wave
Oscillating charge = Alternating current
ª
Radiates EM waves
Scattered waves
•
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.
Attenuation in Optical Fibers
Attenuation in Optical Fibers
• Micro-bending loss
• Attenuation vs. wavelength
Microbending
Field distribution
Escaping wave
Cladding
Stretching of Si-O bonds
in ionic polarization
induced by EM wave,
which is around 9 µm.
θ θ
Fiber Loss
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.
Core
θ′ < θ
θ
θ > θc
θ′
Stretching of Si-O bonds
in ionic polarization
induced by EM wave,
which is around 9 µm.
R
•
•
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.
•
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.
combination
of Si-O & 1.4 µm
9
Fiber Fabrication
•
• 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.
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
• Active Glass Fibers
Silica Glass Fibers
• Amplification, Attenuation, Phase retardation
• Rare earth elements are doped (0.005-0.05 mole%): atomic no. 57-71, Er, Pr…
•
Glass do not have well defined melting point. The glass become to soften at high
temperature (>1000°C), it became viscous liquid.
• SiO2:GeO2 core; SiO2 cladding
SiO2:P2O5 core; SiO2 cladding
1.48
SiO2 core; SiO2:B2O3 cladding
GeO2
SiO2:GeO2/B2O3 core; SiO2:B2O3 cladding
• Chalgenide Glass Fibers
• High non-linearity optical properties for all optical switch or fiber lasers
• Chalcogen elements are doped: S, Se, Te…
P2O5
1.46
SiO2 @ 850 nm
• Plastic Optical Fibers: POF
B2O3
1.44
5
10
15
Dopant addition (mol %)
Fiber Fabrication
20
• Short distance (∼100 m), very flexible, relaxation of connector tolerance, low cost
• polymethylmethacrylate (PMMA) or perifluorinated polymer (PFP)
Fiber Drawing
• Schematic illustration of a fiber drawing tower.
• Fiber Fabrication
Preform feed
• Outside Vapor-Phase Oxidization
Preform
•
•
•
• Halide Glass Fibers
Fiber Materials ¼ Glasses and Plastics
Refractive index
•
Fiber materials
• Vapor-Phase Axial Deposition
• Modified Chemical Vapor Deposition
• Plasma-Activated Chemical Vapor Deposition
• Double-Crucible Method
Furnace
2000°C
Thickness
monitoring gauge
Polymer coater
Ultraviolet light or furnace
for curing
Take-up drum
Capstan
10
Outside Vapor Deposition (OVD)
Outside Vapor Deposition (OVD)
• Schematic illustration of OVD and the preform preparation for fiber drawing
SiCl4 (gas) + O2 (gas) ¼ SiO2 (solid) + 2Cl2 (gas)
GeCl4 (gas) + O2 (gas) ¼ GeO2 (solid) + 2Cl2 (gas)
Drying gases
Porous soot
preform with hole
Vapors: SiCl4+ GeCl4 + O2
Fuel: H 2
Burner
Furnace
Preform
Furnace
Deposited soot
Target rod
Deposited Ge doped SiO2
Rotate mandrel
(b)
(a)
Reaction of gases in the burner
flame produces glass soot that
deposits on to the outside surface
of the mandrel.
Clear solid
glass preform
(c)
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.
Drawn fiber
The soot rod fed into the
consolidation furnace for sintering.
The consolidated
glass rod is used as
a preform in fiber
drawing.
Optical Cables
•
•
•
•
•
Single mode and Multimode
Single fiber and Fiber arrays
Polished face
Strain relief
Parameters: Insertion Loss, Attenuation,
min bend radius, Face angle
• Expensive
Glass preform fed into the fiber
drawing furnace
Single Fiber
Duplex LC
FC – Single mode
ST - Multimode
MU – Single Mode
SC - Multimode
E2000 Multimode
11
Fiber Arrays
Multilayer Arrays
MTP test from Mipox
XMP from Xanoptix
Polymer Optical Waveguides
• Requirements:
•
•
•
•
•
Compatible with standard PWB Technologies
High performance (low optical loss)
Robust (>230 degrees C, >10 sec.)
Dense (<60 micron Line and space)
Standard tooling
12
Polymer Optical Waveguides
• Processing Steps
Polymer Optical Waveguides
Samples
http://matlib.kjst.ac.kr/~optoelec/research/waveguide/p-waveguide.html
13
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.
Optical Sensing
Free-Space Interconnects Pack in
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.
Photon Sensing System Issues
Typical sensing system configuration using photons
Ambient (light):
noise source
Optional optical
detector
Optical detector
signal + noise
Light source
Operating medium
Ambient (light):
noise source
Electronics
Subject of
interest
• Selection of Light Sources
• Selection Light Detectors
• Minimizing effect of background noise
resulting from ambient light sources
• System Performance
• Resolution
• Speed
• Accuracy
14
Fiber Optics For Measurement Applications
Fiber Optics For Measurement Applications
Temperature Measurement Example:
Technology -
Teflon
λabs = f(T)
Temp.
Fiber Optic Chemical Sensors (FOCS):
Light absorption/transmission properties of gallium
arsenide (GaAs)
Light
λabs
Semiconductor
Crystal
Teflon
Dielectric
Mirror
Fiber
Chemical
Escape light
Fiber
Light
Dielectric
Mirror
Fiber Optic Temperature Probe
Technology -
Cladding removed substituted by suitable
chemical
Fluoresence-decay of phosphor.
Jacket
Light
Timedecay = f(temp.)
Phosphor
Fiber
Amount of light loss is proportional to the amount of chemical present
Mirror
Fiber Optic Temperature Probe
FBG (Fiber Bragg Grating)
_
FBG (Fiber Bragg
Grating)
I
I
λ
Λ= Grating Period
λ
I
λ
15
Operation Principle of FBG Sensor
Mounting block that
attaches fiber optic
sensor to the structure
FBG Sensing
When the fiber optic sensor is initially mounted to a
structure, it's in resonance with laser wavelength ln.
λn
λ1,λ2, ......, λ x
λ1,λ2, ..., λn, ..., λx
Reflection
Without Strain
Reflection
Without Strain
λn
Structure starts to pull mounting blocks apart ,
which stretches the fiber optic sensor. The
resonance of fiber optic sensor is now shifted.
λn+∆λ
λ1,λ2, ..., λn, ..., λx
λn+∆λ
FBG Sensor Temperature Response
1551.3
Wavelength, nm
1551.2
1551.1
Athermal, max shift: 21.6 pm (2.7 GHz) from 24oc to 70oC
Accelerometer
Athermal FBG Sensor
Temperature Response
1551
Accelerometer
Standard FBG Sensor
Temperature Response
1550.9
Conventional, 10.4 pm/oC (1.3 GHz/oC)
1550.8
1550.7
Utilization of FBG Characteristics for measurement
30
40
50
Temperature, oC
60
70
16
Other FBG Sensors
FBG For Structure Health Monitoring
FBG Railway Sensing
Typical Structure Health Monitoring
System
Broadband
coupler
λ
λ λ
λ
λλ λ
λλ
Broadband
Source
1 2
λ3
λ2
λ1
1
3……
2 3…
2
3…
3
…
FBGs
Reflected
Light
Tunable Filter
Wavelength (nm)
Detection
Tunable
Optical Subsystem
λ1λ2 λ3……
Broadband coupler
λ1
λ2λ3…
Source
λ3
λ2
λ1
λ2
λ3…
λ3
…
FBGs
Reflected
Light
Tunable Filter
Time (0.01 sec)
Detection
Optical Subsystem
17
FBG-LTDM Structure Monitoring System
Low Contrast
Fabry-Perot
Filter
Pulsed
Broadband
light
SLED or Laser
λ1λ2 λ3……
λ3
λ2
λ1
Broadband coupler
λ1
λ2λ3…
FBG-LTDM Structure Monitoring System Timing Example
λ2
λ3…
λ3
…
λ3
λ3…
10 meters
…
10 meters
FBGs
Light Pulse
Internal
λ2
λ2λ3…
10 meters
Reflected
Light
Wavelength
Locker
λ1
λ1λ2 λ3……
FBGs
λ1λ2 λ3……
External
1st. Reflected Wavelength
Tp
Optical Subsystem
λ1
2nd. Reflected Wavelength
Light Source
Trigger Module
Interrogation Unit (High
Speed Signal Conditioning,
Sampling and ADC)
Tfr
Electrical Subsystem
50
Timing
Generator
Microcontroller
λ2
3rd. Reflected Wavelength
Tsw
Ethernet
Interface
100
λ3
150
PC
200
Time (ns)
250
λ1λ2 λ3……
150
200
λ1λ2 λ3……
Light Pulse
Light Pulse
Tsl
Conclusions
• Interconnect problem significant in ultra high
speed data communication
• Performance of Electrical lnterconnects will limit
high performance system throughput
• OIs will provide significant performance boost
but not completely replace EIs
• Optical Sensing will be deployed in new areas
that were not feasible with electrical sensors
Wavelength Division Multiplexing
WDM enables transmission of multiple communication channels
through a single fiber using various colors of light
Coarse WDM (CWDM): Transmission of a few widely spaced channels
λ1
λ1
λ2
λ2
Dense WDM (DWDM): Transmission of many closely spaced channels
λ1
λn
Tunable
Laser
Source or
DFB
Laser
λ1
Tunable Filter
MUX
MUX
Add/Drop
Channel
EDFA
Optical Fiber (Single
fiber, multiple
wavelengths)
DEMUX
λn
Detector
=Multiplexer
DEMUX =Demultiplexer
EDFA
=Erbium Doped Amplifier
18
References
•
•
•
•
•
•
•
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
Optoelectronics 11, 1997, pp.155-168.
MEL-ARI: Optoelectronic interconnects for Integrated Circuits –
Achievements 1996-2000
Linking with light - IEEE Spectrum
http://www.spectrum.ieee.org/WEBONLY/publicfeature/aug02/opti.html
Optically Interconnected Computing Group
http://www.phy.hw.ac.uk/~phykjs/OIC/index.html
Optoelectronics-VLSI system integration Technological challenges
www.phy.hw.ac.uk/~phykjs/OIC/Projects/
SPOEC/MSEB2000/MSEB2000.pdf
19