Optical Bus for Intra and Inter-chip Optical Interconnects

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Spencer Walters
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1 Optical Bus for Intra and Inter-chip Optical Interconnects Xiaolong Wang Omega Optics Inc., Austin, TX Ray T. Chen University of Texas at Austin, Austin, TX

2 Outline Perspective of Optical Backplane Bus Optical Bus Architectures based on Fully Embedded Board Level Optical Interconnects Optical Design of the Waveguide based Bus Structure Molding Fabrication for Large Cross Section Multimode waveguide System Integration Summary and Forecast

3 Perspective of Optical Backplane Bus Electrical Backplane Example

4 Shared Bus vs. Switched Media Switched Media Node Node Shared Bus Node Node Node Switch Fabric Node Node Direct Network Broadcast Aggregation Bandwidth Point-to-Point Additional Latency Additional Complexity

5 Limitation of Electrical Backplane ITRS 2007 report: Electrical interconnects has put a bottleneck upon the roadmap!!!

6 Limitation of Electrical Backplane Future System ( 2010 ~ ) - CPU performance (~ 10 GHz) - Off-chip speed (~ 5 GHz) - Multi-cores - Decreased Feature Size - High Density and High Speed Interconnections Limitation of Cu / low-k Interconnection - Crosstalk / Skin effects / Power Dissipation

7 Optical Interconnects vs. Electrical Interconnects Intel Roadmap for optical interconnects Challenges: Cost Interfaces with electronic components Reliability

8 Optical Backplane Bus Methodologies Optical Waveguide Interconnections Optoelectronic Transceivers Daughter Board Daughter Board Substrate-Guided Interconnections Free-Space Interconnections Waveguiding Substrate Laser Holographic Grating Photodiode

micro-mirror Cu Trace Optical PCB Waveguide

9 Optical Bus Architectures based on Fully Embedded Board Level Optical Interconnects Micro-via 45 micro-mirror Cu Trace Optical PCB Waveguide Photodiode array VCSEL array Cross section view of optical PCB

10 Optical waveguide based Backplane Bus LD D LD D Features - Bidirectional - Multi-points interconnects - No wiring congestion - No loading effect - Compatible fabrication processes

11 Optical Design of the Waveguide based Bus Structure Uniform Optical Signal Distribution Distributor Point L2 Rx Point L1 Rx Point C Tx Point R1 Rx Point R2 Rx Uniform Delivery ηi ηi+ 1 = 1 η i Shared bus waveguide

12 Waveguide Geometry Design Minimize waveguide bending loss: height=50µm Optimized point: width-=25 µm, radius=5mm Transmission>90%

13 Waveguide Geometry Design Coupling between the branch waveguide and the bus waveguide =5mm W 0 =50µm 1 W 1 from 25~50µm, tunable splitting ratio 0.1~0.5

14 Design of the 45 Micro-Mirror Simulation of 45º Micro-mirrors--- M 2 Factor Gaussian Beam VCSEL Cladding (n2) Core (n1) Cladding (n2) 1. VCSEL (λ = 850 nm) - Gaussian Beam Approx. - Aperture = 15 µm - M factor = Bottom cladding - Thickness (h) = 20 µm

15 Mask Layer Design 10mm Features: Eight parallel nodes Bidirectional connection bus structure Equalized power distribution

Molding Fabrication for Large Cross Section Multimode waveguide (A) (B) Mold : Hard mold (Si or metal) PMMA substrate Hard molding (Imprint): Massive producible Good resolution Durable No deformation

16 Molding Fabrication for Large Cross Section Multimode waveguide (A) (B) Mold : Hard mold (Si or metal) PMMA substrate Hard molding (Imprint): Massive producible Good resolution Durable No deformation (C) (D) Squeegee Deposit metal mirror Core Material Physical Dimension of Waveguide Structure (E) Hard pressing & UV curing (F) Bottom cladding - No. of channels : 12 / - Cross-Section : 50 X 50 µm 2 - Channel to channel separation : 250 µm - Total Length : ~ 109 cm - Curvatures : 3.68 cm / 1.72 cm

17 Metal Template Formed by Electroplating

18 Metal Template Formed by Electroplating 45 tilted angle setup water 52.7 Parameters: su softbake: 65 for 2 minutes, 95 for 2minutes exposure angle: 52.7 in DI water exposure time: 100s peb: 65 for 1 minute, 95 for 30 seconds development: 1 minutes Su-8 photoresist with 45 slope at the end surface

19 Silicon Template Formed by DRIE AFM image of surface topology after DRIE AFM image after surface smoothing

20 51-cm Long Molded Polymer Waveguide Array Total waveguide length 51cm, propagation loss ~0.25dB/cm Large area (up to 36 X24 ) molding machine with controllable pressure and temperature The 3-dB optical bandwidth is determined to be 150GHz for the 51cm long waveguide

array with micro-mirrors on both ends Coupling efficiency as high as 92% has been achieved

21 System Integration Experimental Results of 45 Micro-Mirror Image scope Input light Near field images of the 1X12 micro-mirror array co-illuminated by 633nm light source Thin film waveguide array with micro-mirrors on both ends Coupling efficiency as high as 92% has been achieved 2-dimensional optical coupling between the light source and the molded waveguide through 45º micro-mirror

um, 10Gbps VCSEL Photodiode L = 3335 um W

22 System integration with VCSELs and photodiodes Thin film waveguide on flexible substrate L = 3200 um W = 485 um H = 200 um Pitch = 250 um Aperture = 15 um, 10Gbps VCSEL Photodiode L = 3335 um W = 690 um H = 200 um Pitch = 250 um Aperture = 70 um, 2.5Gbps

Microwave Probe 850 nm VCSEL Optical W/G Under Evaluation 850 nm PIN

23 System evaluation with 10Gbps transmission Pulse Pattern Generator Trigger Digital Oscilloscope Bias Source Bias-T Bias Source Bias-T Microwave Probe 850 nm VCSEL Optical W/G Under Evaluation 850 nm PIN Photodiode Measured Q-factor is 7.24 Bit error rate (BER) lower than 10-12

24 Summary and Forecast Investigated the optical bus architecture for fully embedded board level optical interconnects Molded 50umX50um multimode waveguide array by silicon template, with total length up to 51cm and bandwidth of 150GHz Simulated and implemented 45º waveguide micro-mirrors with coupling efficiency of 92% Integrated VCSEL and photodiode array with the molded waveguide, and demonstrated 10Gbps/channel transmission Future work: Fabricate metal templates by electroplating method Fabricate an optical layer with bus architecture Investigate system integration with printed circuit boards

Polymer optical waveguide based bi-directional optical bus architecture for high speed optical backplane

Polymer optical waveguide based bi-directional optical bus architecture for high speed optical backplane Xiaohui Lin a, Xinyuan Dou a, Alan X. Wang b and Ray T. Chen 1,*, Fellow, IEEE a Department of Electrical