In Search of the Elusive All-Optical Packet Buffer

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1 In Search of the Elusive All-Optical Packet Buffer Rod Tucker Centre for Ultra-Broadband Information Networks (CUBIN) Department for Electrical and Electronic Engineering University of Melbourne, Australia

2 Summary Buffers for Optical Packet Switching Delay-line buffers - Fiber and waveguide - Slow light Resonator-based buffers Holographic buffers Baseline comparisons with CMOS Loss Happens! Loss Energy

3 Delay Line Optical Buffers Control Variable Delay Controllable Delay Line Dispersion Compensator Amplifier Recirculating Loop Delay Cross Point Staggered Delay Line Delay Cross Point Cross Point

Some Numbers 40-Tb/s Optical Router (1000 ports) Buffer size: 20 packets (200 kb @ 40 Gb/s per port, 200 Mb total) (5 μs/port) Fiber 1 km/port, 1 Mm total Storage Density: 1 bit / 5 mm

4 Some Numbers 40-Tb/s Optical Router (1000 ports) Buffer size: 20 packets ( Gb/s per port, 200 Mb total) (5 μs/port) Fiber 1 km/port, 1 Mm total Storage Density: 1 bit / 5 mm Practical Slow Light Waveguide Slow-down factor = 5x m/port, 2 km total Storage Density: 1 bit / 10 μm Ideal Slow Light Waveguide 0.2 m/port, 200 m total Storage Density: ~1 bit / μm. Wavelength

5 Size Matters Practical Ideal Slow Slow Light Light Waveguide CMOS (2018) 5λ 1 bit λ 10λ 1 cell 80 nm Minimum bit area ~ 50λ 5λ 22 (λ = ~1 μm) edram cell area 80 nm x 80 nm Storage Density Area 150 Gbit/m 2 per wavelength Capacity 130 cm Mbit m Gbit 150 Tbit/m mm 2 13 cm 2

6 Loss Happens Fibre: ~0.2 db/km In Out 15 km for 3-dB loss e -1 absorption time ~ 100 μs 20 packets (5 μs) 0.2 db 200 packets (50 μs) 2.0 db Low loss Planar WG: 0.1 db/cm In In Out 20 packets (5 μs) 3,300 db 200 packets (50 μs) 33,000 db 30 cm for 3-dB loss e -1 absorption time ~ 2 ns

7 How to Build a Delay Line Stage 1 Stage m Power Power Signal α β g α β g Signal P sat L Noise Slow light waveguide Dispersion compensation Waveguide loss compensation Two key limitations: Output SNR Amplifier Saturation Power (Psat)

8 Buffer Capacity 100 M Contention Resolution Packet Synchronization 20 Packets: Loss < 0.01 db/cm 1 M 20 Packets Capacity, (b) 10 k μw Fiber + crosspoints Slow Light, Planar WG 10 μw 100 μw 0.01 db/cm 0.5 db/cm All delay line buffers 1 mw Amplifier saturation power, P sat 10 mw mw Energy per bit (J) Tucker, JLT, 2006

9 CMOS DRAM Word Line Data In/Out Buffers edram Column Decoder Sense Amps Bit Line Bit Lines. Switch Storage Capacitor Row Decoder..Word Lines.. Memory Array 80 nm R leakage ~ 2x10 16 Ω Retention time = 1/RC C storage ~ 1 ff ITRS 2018: edram Read/Write Energy* 1.6x10-16 J Retention time 64 ms Access time 200 ps *Excludes decoders and buffers International Technology Roadmap for Semiconductors, 2006

10 Resonator Memory Cell Ring Resonator τ c Crosspoint Input K Output Adjustable coupling coefficient Cavity Input/Output Cavity Waveguide, Photonic Crystal, etc. Adjustable coupling region Asano and Noda, OSA Topical Meeting on Slow and Fast Light, Guo et al., LEOS Annual Meeting, Savchenkov et al., LEOS Summer Topical Meeting, 2004.

11 Resonator RAM Input τ b τ b Bit Lines τ b τ b τ b τ b Row Decoder Word Lines Output

12 Coupling Coefficient 1.0 Input Pulse Output Pulse τ Coupling Coefficient K Kwrite ER K store Kread K Write Store Read Time

13 Retention Time 1.0 Input 5-ps pulse Output Pulses Cavity Q = 2x10 6 Normalized Amplitude Retention Time ~ 800 ps Time (ns) Simulation (VPI)

14 Resonator RAM Q store = 2π n λ ( α + K g store / L) Waveguide power loss Switch coupling coefficient Cavity length K store Retention Time Q 2π f store 1 = = = v ( + K / L) o g α store Absorption Time α (db/cm) Switch Extinction (L = 100 μm) 0.1 > 40 db 5x ns Q store Retention Time Access Time > 70 db 5x μs (8 packets) Pulse duration Pulse duration

15 Resonator Bandwidth and Chirp Bandwidth BW (GHz) Resonator Bandwidth τ c Input Pulse = 0.1ps 40 Gb/s RZ 1.0 Normalized Amplitude Time ps Chirp

16 Holographic Buffers Holographic Medium: Photorefractive material (e.g LiNbO 3, polymers) Output image Write speeds up to 1 Gb/s Reference beam Read speeds up to 10 Gb/s Input image Storage density Retention time Access Time Write Time 1/λ 3 ~ 50 μs > 500 μs Orlov et al., Proc IEEE, 2004 Psaltis, CLEO 2002 Ashley et al. IBM J. Res. Dev., May 2000

17 Summary Technology Access Time Retention Time Capacity (Packets) Fiber Planar, Slow Light Structuredependent Structuredependent Resonator Holographic CMOS- O/E/O Small ~ 50 μs 200 ps > 500 μs < 5 μs ns 64 ms > 2,000 < 20 << 1 Energy/bit ~ 1 fj ~ 1 pj ~ 1 pj ~ 1 pj ~ 1 fj Physical Size Chirp Sensitivity Very Large Medium Medium Small Very Small No Small Large Large No Show stopper Challenge

18 Conclusions CMOS buffering is a tough act to follow Non - fibre delay line buffering limited to a few packets Absorption time is a fundamental limitation Chirp and wavelength registration a challenge for resonatorbased buffers Loss Happens! Loss Energy

IBM T. J. Watson Research Center IBM Corporation

IBM T. J. Watson Research Center IBM Corporation Broadband Silicon Photonic Switch Integrated with CMOS Drive Electronics B. G. Lee, J. Van Campenhout, A. V. Rylyakov, C. L. Schow, W. M. J. Green, S. Assefa, M. Yang, F. E. Doany, C. V. Jahnes, R. A.