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Optoelectronic IntegrationBergur GudbergssonZach WhitneyMarcus HaleAs the data transfer limits of conventional electric interconnects are approached, emerging on-chip optoelectric solutions look promising as means of

keeping up with increased processing power, efficiency, and bandwidth requirements. This presentation will explore fiber optics, vertical-cavity surface emitting

lasers (VCSEL), optical interconnects, and photodiodes.

05/05/14

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OutlineFiber Optics

– Basics of Fiber– Fiber types– Optical Power– Transmission Bands– Wave Division Mux

PIN Photodiode– Absorption– Energy Band Diagrams– Applications

VCSEL– Basic Operation– Structure– VCSEL-PIN TRx function & fabrication

Optical Interconnects– Basic operation

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The Basics of Fiber• A fiber cable consists of:

1. Core2. Cladding3. Buffer4. Jacket

• “Total Internal Reflection”

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Cladding has lower refractive index than the core which causes total internal reflection within the core

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Fiber Types• Two main types of fiber optics cables– Single Mode Fiber (SMF) (9μM)– Multi Mode Fiber (MMF) (62.5μM or 50μM)

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Single Mode Fiber• Small core carries single mode of light• No modal dispersion• Long-haul data transmission• Requires expensive coherent laser light source• Requires specific connector alignment• Operates in 1.3μM -1.5μM Region

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Multi Mode Fiber• Multiple modes of light can propagate• Modal dispersion limits distance (500 meters)• Uses cheaper light sources– LED– VCSEL

• Larger alignment tolerances• Typically operates at 0.85μM

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Optical Power• Light follows “inverse square law”– inversely proportional to distance squared– Attenuation = loss of intensity

• Measured in Decibel-milliWatts (dBm /dBmW)– 0dBm is 1 mW– 3dBm is 2 mW– -50dBm is 10 nW

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Transmission BandsSplit into four windows– 850nM• High attenuation

– 1310nM• Zero modal dispersion for SMF• Up to 10kM reach

– 1550nM (Conventional-band)• Amplified via erbium doped fibers

– 1570-1610nm (Long-band)

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Typical Mux/Demux System

• Multiple signals are generated• Multiplexer combines the lights into a signal carrier signal• Signal is transmitted• λν=c• Signal is re-separated• Signal is received

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PIN Photodiodes• Photodiodes with an Intrinsic (undoped)

region between highly doped P and N junctions.

• Anti-reflection (1/4 wavelength)

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Absorption• Photons Absorbed in the intrinsic region• Creates Carriers• Increases Photocurrent (Light into Current)• Si: infrared(700nm) up to 1μm• InGaAS: up to 1.7μm (Longer wavelengths)

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Electron-Hole Pair Generation

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Energy band Diagram InGaAs

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Energy band Diagram PIN-Si

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Applications• Optical fiber communications

• Security Systems

• Cameras

• Light Controls

http://www.hamamatsu.com/cs/Satellite?blobcol=urldata&blobheadername1=content-disposition&blobheadervalue1=inline%3Bfilename%3D149%2F656%2Fk_s1226-44bq_bk_-5bq_bk_pp_xx.jpg&blobkey=id&blobtable=MungoBlobs&blobwhere=1328686465431&ssbinary=true

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Fiber Optic Link

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Camera Brightness Metering

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Smoke Detector

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Introduction to VCSELs• Vertical Cavity Surface Emitting Lasers• Different from typical Laser Diodes– Laser is perpendicular to the surface.

• P and N doped regions act as parallel DBR mirrors, also forming a diode junction.

• Multiple quantum wells• Able to arrange in dense 1 or 2D, on-chip

arrays.

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History of VCSELs– 1979 first device

using GaInAsP/InP– 1988 first continuous

wave using GaAs– Today, GaAs-AlGaAs

material is favored, 850nm wavelength used for short-haul data communication (monolithic TRx)

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VCSELs Basic StructureTypical Laser Diode VCSEL

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VCSEL Array

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VCSEL technology

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VCSEL technology

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VCSEL-PIN TRx• Monolithic transceiver chips coupled with

MMF• Miniaturization only possible with PIN PD– Why?

• Data Rates in the 10-Gb/s range• Arrays allow Optical Interconnects even higher

rates

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VCSEL-PIN TRx Fabrication• Two stacks of MBE

layers• PIN PD grown in the

same run of the VCSEL layers.

• Order is important

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VCSEL-PIN TRx

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VCSEL-PIN TRx

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VCSELsPROS

• Easy testing throughout fabrication

• High reflectivity mirrors• Reduced threshold

current (down to the 10’s of uA)

• Low power consumption

CONS• Poor thermal

characteristics at high-power (980nm+)

• Increased heat increases threshold current

• Reduced output at high-power

• Reduced output at longer wavelengths

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Correcting Thermal Issues in VCSELs

• Experiments with various passivation layers– SiO2 (dated)• High resistivity and insulation• Poor heat conductivity (1W/mK)

– AlN (new fabrication)• High resistivity• High heat conductivity (300W/mK)• Increased temp distribution, reduces thermal

resistance

– Carbon Nanotubes (future?)

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Correcting Thermal Issues in VCSELs

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Correcting Thermal Issues in VCSELs

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Final Thoughts on VCSELs

• VCSLEs experiencing huge growth as electrical interconnects slowly become obsolete

• VCSELs are attractive for short-haul, large data transfers

• Can densely back in 1D & 2D array allowing for increased output as well as easy packaging

• Great, and basically only choice for optical interconnects

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Brief Overview of Optical Interconnects

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Brief Overview of Optical Interconnects

• Advantages– Keep up with Moore’s Law– Higher carrier frequency– Less crosstalk– Lower power consumption– Data ranges in the range of Tb/s

• http://www.youtube.com/watch?v=0U4Af2qmgFA (similar but using silicon based lasers)

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Conclusion• All of these optoelectrical innovations

contribute to the growing field of optical interconnection technology

• Immensely complex, research still underway

• Huge growth potential

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References• Arshad, T. S., Othman, M. A., & Yasin, N. Y. Comparison on IV Characteristics Analysis

between Silicon and InGaAs PIN Photodiode.IEEE (ICICI-BME), 71-75. Retrieved May 1, 2014, from http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6698467

• Introduction to DWDM For Metropolitan Networks. (2000). San Jose, CA: Cisco Systems, Inc.

• Kenichi, I. VCSEL -Its Conception, Development, and Future-. IEEE (MOC' 13), 1-2. Retrieved May 1, 2014, from http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6715057

• Kern, A., Al-Samaneh, A., Wahl, D., & Michalzik, R. Monolithic VCSEL–PIN Photodiode Integration for Bidirectional Optical Data Transmission. IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 19, 1-13.

• Lifeng, H., Yongfeng, M., & Yuan, F. Fabrication and Testing of 980nm High-Power VCSEL with AlN Film Passivation Layer. IEEE (ICOM), 45-48. Retrieved May 1, 2014, from http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6316212

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References• Mishra, S., Chaudhary, N., & Singh, K. Overview of Optical Interconnect

Technology. International Journal of Scientific & Engineering Research, 3, 1-7. Retrieved May 1, 2014, from http://arxiv.org/abs/1303.3954

• Muramoto, Y., & Ishibashi, T. InP=InGaAs pin photodiode structure maximising bandwidth and efficiency. ELECTRONICS LETTERS, 29.

• Paschotta, D. R. (n.d.). p–i–n Photodiodes. Encyclopedia of Laser Physics and Technology. Retrieved May 1, 2014, from http://www.rp-photonics.com/p_i_n_photodiodes.html

• Paschotta, R. (n.d.). Passive Fiber Optics. Tutorial “”: multimode fibers, number of modes, core diameter, numerical aperture, graded-index fiber. Retrieved May 1, 2014, from http://www.rp-photonics.com/passive_fiber_optics4.html

• Single mode optical fiber. (2014, April 22). Wikipedia. Retrieved May 2, 2014, from https://en.wikipedia.org/wiki/Single_mode_optical_fiber

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References• Steenbergen, R. (Director) (2013, February 4). Everything You Always Wanted to Know About

Optical Networking - But Were Afraid to Ask. NANOG57. Lecture conducted from GTT, Orlando, Florida.

• Technologies. (n.d.). . . Retrieved May 1, 2014, from http://www.pacer.co.uk/Assets/Pacer/User/Photodiode%20Typical%20Applications.pdf

• Total internal reflection. (2014, April 28). Wikipedia. Retrieved May 2, 2014, from https://en.wikipedia.org/wiki/Total_internal_reflection

• Zeghbroeck., B. V. (2011, January 1). Chapter 4: p-n Junctions. Optoelectronic devices. Retrieved May 1, 2014, from http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_6.htm

• http://en.wikipedia.org/wiki/Vertical-cavity_surface-emitting_laser • http://en.wikipedia.org/wiki/Laser_diode

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Key Points• Single Mode fibers are used in long data

transmission. Multimode Fibers are cheaper and are used for short distances

• Light signal intensity is measured in dBmW• PIN PDs create one electron-hole pair per entering

photon.• VCSELs allow for minimal power use and densely

packed on chip integration• Monolithic VCSEL-PIN based transceivers allow for

short-haul data transfer in the 10Gb/s range.