Optical Interconnect and Sensing

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1 Topics Optical Interconnect and Sensing Dr. How T. Lin Endicott Interconnect Technologies 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 Disadvantages of Electrical Interconnects/Sensors Physical Problems (at high frequencies/high noise environments) Cross-talk Signal Distortion Electromagnetic Interference Reflections High Power Consumption High Latency (RC Delay) Limited Bandwidth Why Optics? Advantages: Capable to provide high bandwidths Free from electrical short-circuits Low-loss transmission at high frequencies Immune to electromagnetic interference Essentially no crosstalk between adjacent signals No impedance matching required Successful long-haul telecommunication system based on fiber optics

Optical Interconnect Fundamentals Transmitter Basic Optical Interconnect λ 1 λ 1 Transmission Medium Receiver Transmitter: LED or

2 Using Lightwave to Transmit Information 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! Optical Interconnect Fundamentals Transmitter Basic Optical Interconnect λ 1 λ 1 Transmission Medium Receiver Transmitter: LED or Laser Transmission Medium: Fiber optics (MM/SM), Polymer Waveguide or Free Space Receiver: Photo Diode or Transistor EM Spectrum EM Spectrum (Visible) UV....Visible IR

$Waves Interference n 0 n 1 n 0 Rays Refraction Reflection A little Quantum Theory Definition: Optical power watt (W) - a rate of energy of one joule (J) per second.$ Optical power is a function of both the number of photons and the wavelength.

Optical power is a function of both the number of photons and the wavelength.

3 Particles Conduction band Absorption Emission Bandgap Valence band What is Light? Waves Interference n 0 n 1 n 0 Rays Refraction Reflection A little Quantum Theory Definition: 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 s s equation: Q = hc /λ where Q= photon energy in J h = Planck s s constant (6.623 x Js) c = speed of light (2.998 X x 10 8 m/s) λ = wavelength in meters Basic Optical Principles 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. Interface Losses : Fresnel s s Law r λ = reflection loss (normal incidence) n λ = n /n r λ = n λ -1/ n λ +1 Transmission through an optical filter Interface Losses Refraction : Snell s s Law n sin(θ) ) = n n sin(θ ) Index of refraction: n = 1.0 for air n = 1.5 for glass Transmission through an optical filter

$Diffraction Basic Optical Principles Diffraction: Lightwave bends when pass by small aperture θ = λ/d where θ is the$ nature of light causes interference patterns: Interference filter for wavelength selection - D Basic Optical

nature of light causes interference patterns: Interference filter for wavelength selection - D Basic Optical

(parallel light beam) Basic Optical Principles Wavelength Selection: Prisms: with high n, select λ with narrow slit

4 Diffraction Basic Optical Principles Diffraction: Lightwave bends when pass by small aperture θ = λ/d where θ is the diffraction angle λ is the wavelength D is the aperture width Basic Optical Principles Interference Interference: Wave nature of light causes interference patterns: Interference filter for wavelength selection - 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: disperse light into spectrum with ruled lines where m is an integer (order) Slit Collimation with lens and parabolic mirror

Light Sources Lasers Lasers Gas Liquid Solid State Semiconductor (diodes) Light Emitting Diodes (LED) Light Sources (Light ight Amplification by Stimulated Emission of

Relative narrow spectra Low divergence after collimation. Difficult to modulate (gas, liquid). High cost.

5 Light Sources Lasers Lasers Gas Liquid Solid State Semiconductor (diodes) Light Emitting Diodes (LED) Light Sources (Light ight Amplification by Stimulated Emission of Radiation) Gas Solid State Liquid Semiconductor (diode) Characteristics: Coherence - Photons have fixed phase relationship. Relative narrow spectra Low divergence after collimation. Difficult to modulate (gas, liquid). High cost. LED (Light ight Emitting Diodes) Characteristics: Incoherence -Photons with random phase Relative broad spectra. Low cost. Easy modulation. Small size Light Sources : Semiconductor Lasers Light Sources : LEDs Edge emitting LED p-dbr active n-dbr Surface emitting LED VCSEL

6 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.. 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 s 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 is read as a change in the voltage drop across the resistor. Light 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 R d x hv = 100 x R d (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 = IAV N / V S ( f) 1/2 where I is the irradiance incident on the detector of area A, V N is the root mean square noise voltage within the measurement bandwidth f, and V S is the root mean square signal voltage. Materials Silicon (Si) Least expensive Germanium (Ge( Ge) Classic detector Indium gallium arsenide (InGaAs) Highest speed Light Detection 1.0 Responsivity (A/W) Quantum Efficiency = 1 Silicon Germanium InGaAs Wavelength nm

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.

Dielectric Waveguides and Optical Fibers Step Index Fiber Optical fiber structure The cladding is the layer completely surrounding the core.

$difference in refractive index between the core and cladding is < 0.5%.$

7 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 Step Index Fiber Schematic diagram of Step Index 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. y n 2 n 1 n Cladding Core φ r y z Fiber axis n1 n2 = n Normalized index difference 1 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.

8 The Graded Index (GRIN) Optical Fiber The Graded Index (GRIN) Optical Fiber O n 2 n 1 Multimode Step Index Fiber n Ray paths are different so that rays arrive at different times. TIR TIR O O' O'' n 2 n n 1 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. n decreases step by step from one layer to next upper layer; very thin layers. 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. n decrease in continuous gives a ray path changing continuously. In a medium where n decreases continuously the path of the ray bends continuously. n 2 Attenuation Light Absorption and Scattering 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. Light Absorption and Scattering Absorption Lattice absorption through a crystal A solid with ions E Medium k z E x Light direction k z Attenuation = Absorption + Scattering + 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.

9 Light Absorption and Scattering Rayleigh scattering Incident wave A dielectric particle smaller than wavelength Displacing electron with respect to positive nuclei. Through wave Attenuation in Optical Fibers Optical Fiber Attenuation vs. wavelength Oscillating charge = Alternating current Scattered waves Radiates EM 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 Micro-bending 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. 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. Fiber Loss Stretching of Si-O bonds in ionic polarization induced by EM wave, which is around 9 µm. Field distribution Cladding Core θ θ Microbending Escaping wave θ < θ θ θ > θc θ R combination of Si-O & 1.4 µm 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 n 1 and n 2, 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.

10 Fiber Fabrication Fiber materials 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. SiO2:GeO2 core; SiO2 cladding SiO2:P2O5 core; SiO2 cladding 1.48 SiO2 core; SiO2:B2O3 cladding GeO 2 SiO2:GeO2/B2O3 core; SiO2:B2O3 cladding Refractive index P 2O 5 SiO 850 nm B 2O Dopant addition (mol %) Halide Glass Fibers Extremely low transmission losses at mid-ir (@0.2 8 µm) db/km) ZrF 4, BaF 2, LaF 3, AlF 3, NaF Fabricating long lengths of fibers is difficult. Active Glass Fibers Amplification, Attenuation, Phase retardation Rare earth elements are doped ( mole%): atomic no , Er, Pr Chalgenide Glass Fibers High non-linearity optical properties for all optical switch or fiber lasers Chalcogen elements are doped: S, Se, Te Plastic Optical Fibers: POF Short distance ( 100 m), very flexible, relaxation of connector tolerance, low cost polymethylmethacrylate (PMMA) or perifluorinated polymer (PFP) Fiber Fabrication Fiber Fabrication Outside Vapor-Phase Oxidization Vapor-Phase Axial Deposition Modified Chemical Vapor Deposition Plasma-Activated Chemical Vapor Deposition Double-Crucible Method Fiber Drawing Schematic illustration of a fiber drawing tower. Preform feed Thickness monitoring gauge Furnace 2000 C Polymer coater Ultraviolet light or furnace for curing Preform Take-up drum Capstan

Outside Vapor Deposition (OVD) Outside Vapor Deposition (OVD) Schematic illustration of OVD and the preform preparation for fiber drawing SiCl 4 (gas) + O 2 (gas) SiO 2 (solid) + 2Cl 2 (gas) GeCl 4

11 Outside Vapor Deposition (OVD) Outside Vapor Deposition (OVD) Schematic illustration of OVD and the preform preparation for fiber drawing SiCl 4 (gas) + O 2 (gas) SiO 2 (solid) + 2Cl 2 (gas) GeCl 4 (gas) + O 2 (gas) GeO 2 (solid) + 2Cl 2 (gas) Vapors: SiCl 4 + GeCl 4 + O 2 Fuel: H 2 Burner Deposited soot Drying gases Porous soot preform with hole Furnace Preform Furnace Target rod Deposited Ge doped SiO 2 Rotate mandrel (a) Reaction of gases in the burner flame produces glass soot that deposits on to the outside surface of the mandrel. (b) Clear solid glass preform 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. (c) Drawn fiber The consolidated glass rod is used as a preform in fiber drawing. The soot rod fed into the consolidation furnace for sintering. Glass preform fed into the fiber drawing furnace 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 Duplex LC Single Fiber ST - Multimode SC - Multimode FC Single mode MU Single Mode E2000 Multimode

12 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

13 Polymer Optical Waveguides Processing Steps Polymer Optical Waveguides Samples

Optical Backplanes Speed Data Free-Space Interconnects Pack in Data Channels In DaimlerChrysler's optical backplane, the beam from a laser diode passes through one set of lenses and reflects off a

An experimental module from the University of California, San Diego, just 2 cm high, connects stacks of CMOS chips.

14 Optical Backplanes Speed Data Free-Space Interconnects Pack in Data Channels 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. 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 Sensing Typical sensing system configuration using photons Optical detector signal + noise Ambient (light): noise source Ambient (light): noise source Electronics Subject of interest Optional optical detector Light source Operating medium Photon 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

15 Fiber Optics For Measurement Applications Temperature Measurement Example: Temp. Technology - λ abs = f(t) λ abs Light absorption/transmission properties of gallium arsenide (GaAs) Semiconductor Teflon Crystal Light Dielectric Fiber Mirror Fiber Optics For Measurement Applications Fiber Optic Chemical Sensors (FOCS): Light Teflon Fiber Chemical Escape light Dielectric Mirror Fiber Optic Temperature Probe Technology - Fluoresence-decay of phosphor. Jacket Phosphor Light Fiber Time decay = f(temp.) Fiber Optic Temperature Probe Mirror Cladding removed substituted by suitable chemical Amount of light loss is proportional to the amount of chemical present FBG (Fiber Bragg Grating) FBG (Fiber Bragg _ Grating) I λ I λ Λ= Grating Period I λ

Operation Principle of FBG Sensor Mounting block that attaches fiber optic sensor to the structure When the fiber optic sensor is initially mounted

.., λ x Reflection Without Strain Reflection Without Strain λ n Structure starts to pull mounting blocks apart, which stretches the fiber optic sensor.

.., λ x λ n + λ FBG Sensor Temperature Response Utilization of FBG Characteristics for measurement Wavelength, nm 1551.3 Athermal, max shift: 21.

16 Operation Principle of FBG Sensor Mounting block that attaches fiber optic sensor to the structure When the fiber optic sensor is initially mounted to a structure, it's in resonance with laser wavelength l n. FBG Sensing λ 1,λ 2,..., λ x λ n λ 1,λ 2,..., λ n,..., λ x Reflection Without Strain Reflection Without Strain λ n Structure starts to pull mounting blocks apart, which stretches the fiber optic sensor. The resonance of fiber optic sensor is now shifted. λ n + λ λ 1,λ 2,..., λ n,..., λ x λ n + λ FBG Sensor Temperature Response Utilization of FBG Characteristics for measurement Wavelength, nm Athermal, max shift: 21.6 pm (2.7 GHz) from 24 o c to 70 o C Athermal FBG Sensor Temperature Response Standard FBG Sensor Temperature Response Accelerometer Accelerometer Conventional, 10.4 pm/ o C (1.3 GHz/ o C) Temperature, o C

Other FBG Sensors FBG For Structure Health Monitoring Wavelength (nm) FBG

01 sec) Typical Structure Health Monitoring System Broadband Source λ 1 λ

λ 1 λ 2 λ 3 λ 3 λ 2 λ 1 Reflected Light Tunable Filter Detection

17 Other FBG Sensors FBG For Structure Health Monitoring Wavelength (nm) FBG Railway Sensing Time (0.01 sec) Typical Structure Health Monitoring System Broadband Source λ 1 λ 2 λ 3 λ 3 λ 2 λ 1 Reflected Light Tunable Filter Detection Tunable Source λ 1 λ 2 λ 3 λ 3 λ 2 λ 1 Reflected Light Tunable Filter Detection Broadband coupler λ 1 λ λ 2 λ 3 2 λ 3 λ 3 Optical Subsystem FBGs Broadband coupler λ 1 λ λ 2 λ 2 λ 3 λ 3 3 Optical Subsystem FBGs

18 Pulsed Broadband light Low Contrast Fabry-Perot Filter SLED or Laser Light Source Trigger Module Timing Generator FBG-LTDM Structure Monitoring System λ 1 λ 2 λ 3 λ 3 λ 2 λ 1 Reflected Light Wavelength Locker Broadband coupler Interrogation Unit (High Speed Signal Conditioning, Sampling and ADC) Microcontroller Internal Optical Subsystem λ 1 λ λ 2 λ 2 λ 3 λ 3 3 Electrical Subsystem Ethernet Interface FBGs External PC FBG-LTDM Structure Monitoring System Timing Example λ 1 λ 2 λ 3 λ 1 λ λ 2 λ 2 λ 3 λ meters 10 meters 10 meters FBGs Light Pulse λ1 λ 2 λ 3 1st. Reflected Wavelength T p λ 1 2nd. Reflected Wavelength T fr λ 2 T sw 3rd. Reflected Wavelength λ Time (ns) λ 1 λ 2 λ 3 λ 1 λ 2 λ 3 Light Pulse Light Pulse T sl 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 λ 1 λ 1 λ 2 λ n Tunable Laser Source or DFB Laser Dense WDM (DWDM): Transmission of many closely spaced channels λ Tunable Filter 1 λ 1 MUX Add/Drop Channel MUX =Multiplexer DEMUX =Demultiplexer EDFA =Erbium Doped Amplifier EDFA Optical Fiber (Single fiber, multiple wavelengths) DEMUX λ 2 λ n Detector

19 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 MEL-ARI: Optoelectronic interconnects for Integrated Circuits Achievements Linking with light - IEEE Spectrum Optically Interconnected Computing Group Optoelectronics-VLSI system integration Technological challenges SPOEC/MSEB2000/MSEB2000.pdf

DIELECTRIC WAVEGUIDES and OPTICAL FIBERS

DIELECTRIC WAVEGUIDES and OPTICAL FIBERS Light Light Light n 2 n 2 Light n 1 > n 2 A planar dielectric waveguide has a central rectangular region of higher refractive index n 1 than the surrounding region