Recent developments in high bandwidth optical interconnects. Brian Corbett.

Recent developments in

high bandwidth optical

interconnects

Brian Corbett

Outline

• Introduction to photonics for interconnections

• Polymeric waveguides and the Firefly project

• Silicon on insulator (SOI) platform for photonics

• Transfer Print (TP) technology

• Laser on silicon using TP

• Conclusions

Tyndall National Institute

• Dedicated Research Institute

– Initiated in 1985

– 460 people (120 PhD students) – €32M pa turnover

– €140M Irish government investment

• Unique combination of III-V, Si, MEMS • Photonics:

– Epitaxy

– Process technology – Device characterisation – Systems

• Why optics for communications?

– Frequency of 1550nm light is 200 THz

• 40 GHz << baseband

– Optical fibre waveguides

• No ‘skin’ effect

• 10 mm core for optical isolation • Loss ~ 0.2 dB / km

– Multiplexing

• Weak interaction of light with itself (especially at low intensity) • Dense wavelength division multiplexing to fill the spectrum • Factor of 2 increase in bandwidth by adding polarization

• The result

– Long haul / metro communications all based on light – Fibre migrating (slowly) to the home

Services Time L o g (T ra ffi c )

• Bandwidth is increasing at every level

– Data centres and data management – Supercomputing

– All following exponential curves (which may saturate)

– Requires increased bandwidth over internet, on board and at chip level

• Increase bits per symbol ~1 b/s/Hz

• 60nm gain bandwidth of Fibre Amplifiers – 7.5 Tb/s per fibre • Coherent techniques

• Decrease loss (to allow higher threshold for nonlinear effects) and allow higher Signal/Noise

• Increase number of spatial modes

http://mphotonics.mit.edu/

Reach of photonics

• Active optical cable for bidirectional connection between peripherals • From 5 to 100 Gbps

– 850nm surface emitting laser arrays and 1550nm edge emitting arrays – Example from Molex: 0.78W for 10 Gbps (78mW/Gb/s)

– Luxtera: 4x10 Gb/s at 0.78W

• Targetting 4 x 25 Gb/s cables for 100 GbE

Optics for data centres < 1km

Supercomputing needs

• Exascale (1018_{) computing}

– Factor of 10 increase every 4 years:→ x 100 by 2020

– IBM Roadrunner system (1 PetaFlop): 48,000 fibres to the boards – IBM Power P775 system: Optical fibres at the board level

– Need bandwidth to scale at all levels (inter-rack, backplane, card chip)

• Low energy per bit

– <1mW/Gbps

• Scaling manufacturing

– Including the ‘wires’

• Light source

– Edge/Surface emitting 850, 1300, 1550nm

• Detector

– Si, InGaAs, InP

• Interconnect medium

– Fibres (single/ multimode), polymer, silicon, glass like dielectrics,.

• Drive and receive circuits

• Need decisions where integration is essential

– Eg wavelength, waveguide, light source,..

Optics for backplanes

http://www.fp7-firefly.eu/pages/about.htm

Firefly project: Multilayer Photonic Circuits made by Nano-Imprinting of Waveguides and Photonic Crystals

Fibre output

Single mode polymer waveguides 5 – 10 cm

• Choice of wavelength:

– 1300 / 1550nm compatible with internet communications and with on chip Silicon On Insulator (SOI) platform

• Choice of source:

– Surface emitting laser

• Choice of waveguide:

– Single mode

– Imprintable, photo-processable

• Add photonics crystal structures for more compact

functions

– Wavelength controlled scattering from high index TiO₂ spheres

• Silicon on insulator (SOI)

– Silicon 0.22 mm thick; Oxide > 1mm thick

– Transparent waveguide for wavelengths > 1100nm – Single mode waveguides with width of 0.2 mm

– High index (3.5) permitting bends with radius of curvature <2mm – Precision fabrication and manufacturing scale of silicon

– Carrier depletion effects permit modulation at >25 GHz – Integrate detectors by depositing and crystallising Ge – Optical circuits eg Luxtera using 0.13 µm SOI process

– Foundry model for development now established (ePIXfab, OpSiS)

Photonic Integrated Circuits

Layout using PICDraw: F. Peters et al (Tyndall/UCC)

• Packaging results (ePIXnet)

Connecting to SOI circuits

Ring Resonator performance Packaged SOI ring resonator

• Silicon Optoelectronics Technology Work Group

To provide a truly compelling solution, silicon microphotonics will likely need to achieve a high degree of monolithic integration with at most a small degree of hybrid integration, (e.g. laser sources) in

order to offer low cost and increased functionality.

• Sparse array of lasers are required in optical circuits

– Laser patch desired

– Mismatch between dimensions of III-V and of silicon wafers – Coupling to external lasers challenging

• Options

– Hybridisation of pre-fabricated devices – Wafer bonding (of patch of material) – Transfer print and post processing

Transfer print concept

B

A

D

A

B

C

D

GaAs Laser Wafer with pre-etched coupons

Transfer printing stamp

Pick-up of GaAs coupons

Silicon Wafer

Silicon Wafer Populated with GaAs coupons

A 100mm epi wafer will have > 130,000 printable 400x100mm2 _coupons

C

P-Cladding Guide Active Guide N-Cladding Release layer Substrate

350 mm

840 mm

Laser epitaxial material

Silicon substrate

Transfer print: examples

Laser fabrication

Laser Ridge P-metal Contact P-AlGaAs Active Region N-AlGaAs Silicon Substrate N-GaAs N-Metal Contact Laser Ridge P-metal Contact P-AlGaAs Active Region N-AlGaAs Silicon Substrate N-GaAs N-Metal Contact 100 mm

5 mm

Silicon substrate

N-GaAs N-AlGaAs P-AlGaAs P-GaAs 2 x Quantum wells

Laser performance

0 20 40 60 80 100 0 5 10 15 20 25 30 1000C 200 C 300C 400 C 500C 600 C 700 C 800C 900 C 1000C P o w e r p e r fa c e t (m W ) Current (mA) 200C 820 822 824 826 -70 -60 -50 P o w e r (d B ) Wavelength (nm)

Above threshold Below threshold

Coupling to waveguides

Laser Diode Dielectric Waveguide Dielectric waveguide Laser Fabry-Perot cavity Retro-reflector cavity

• Can optics meet the dimensions of electronics?

– SOI waveguides 200 nm x 200 nm

• Plasmonics

– Collective oscillations of electrons at optical frequencies on a metal surface

– Fundamental dimensions in the 10s of nm

• Photonics is making a difference to the upcoming

generations of computing systems

– Optical interconnect now competitive at distances of 10 m – Next is the 10 cm – 1 m range

• Future:

– IC sitting on a silicon photonics chip – Signals to optical circuit boards

– Then to the internet

• Enormous new opportunities

– Ambient intelligence platform

– Environmental sensing (spectrometer on chip + sensing) – Information signalling in buildings

Acknowledgements