Optical Buffers for HighCapacity Routers

1
Optical Buffers
for
High-Capacity
Routers
Rod Tucker ARC Special Research Centre for
Ultra-Broadband Information
Networks (CUBIN) Department of Electrical and
Electronic Engineering University of Melbourne,
Australia
2
Optical Buffers
Control
Variable Delay
Dispersion Compensator
Waveguide Delay Line
Amplifier
Delay
Recirculating Loop
Cross Point
Staggered Delay Line
Delay
Cross Point
Cross Point
3
Summary
Optical Buffers
Electronic Buffers

• 20-nm CMOS ( 2020)

• Fiber, Slow Light
• Fundamental limitations
• Delay-Bitrate Product
• Storage Density (Bit Size)
• Power/Energy
• Dispersion
• Waveguide attenuation

International Technology Roadmap for
Semiconductors http//public.itrs.net/
More conservative projections than for Optics
Loss Happens!

• Compare key parameters
• Power dissipation
• Physical size

4
100 Tb/s Router Buffer Size
2,500 ports at 40 Gb/s, 250 ms buffering per port
Electronic Router CMOS Buffers
Total buffer capacity 25 Tb 103 RAM chips _at_ 2.5
GB/chip
Optical Router Fibre Delay Line Buffers
.
Sun
Earth
5
Packet Switching with Reduced Buffering
Enachescu et al., ACM/SIGCOMM July 2005
Buffer size can be reduced
Optical Router Fibre Delay Line Buffers
Total fibre length 500 Mm (200
km/port) Distance from Earth to the Moon
1 ms buffering per port (100 Gb total)
Buffering with fiber delay lines is challenging
6
Slow-Light to the Rescue?
Photonic Crystal
Electromagnetically-Induced Transparency in
Semiconductor
Group Velocity
Attenuation
Hilbert Transform
Effective Refractive Index n
Refractive Index n
Optical frequency
C.J Chang-Hasnain et al., Proc IEEE, 9, 2003
Y. Xu et al., QELS, 2000
7
Car Analogy
20
100 km/h Speed Limit
20 km/h Speed Limit
Real World
Reduced Group Velocity Constant Bitrate
Slow Light World
Lbit
8
Some Numbers
100-Tb/s Optical Router 1 ms buffering per port
(100 Gb total)
Fiber 200 km/port, 0.5 Gm total Storage Density
1 bit / 5 mm
Ideal Slow Light
Waveguide 40 m/port, 100 km total Storage
Density 1 bit / mm
Slow Light Waveguide Slow-down factor 5x102
400 m/port, 1 Mm
total Storage Density 1 bit /
10 mm
.
Wavelength
9
Size Matters
Ideal Slow Light Waveguide
CMOS (2018)
Non-Ideal Slow Light Waveguide
5l
l
100l
1 bit
Minimum bit area 5l2 (l 1 mm)
Minimum bit area 500l2 (l 1 mm)
Storage Density
150 Gbit/m2
per wavelength
1.5 Gbit/m2
70 m2
1,700 m2
10
Size required to store 200 IP packets
Slow Light Waveguide (200 m)
CMOS
5l
100l
1 bit
4 cm
0.02 mm2
Fiber (10 km)
10 cm
4 cm
10 cm
5 cm
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Loss
Fibre 0.2 dB/km
In
Out
15 km for 3-dB loss
Photonic Crystal WG 0.1 dB/cm
Out
In
6 mm for 3-dB loss
200 m (200 packets) 33,000 dB
20 m (20 packets) 3,000 dB
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Putting it All Together Delay Line Buffer
Stage 1
Stage m
Power
Power
Signal
Signal
?
?
g
g
b
b
Psat
L
Noise
Two key limitations
Slow light waveguide

• Output SNR

• Amplifier Saturation Power

Dispersion compensation
Waveguide loss compensation
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Energy per Bit in Slow Light Buffers
100 M
Contention Resolution
1 M
Packet Synchronization
0.05 dB/cm
10 k
Slow Light
10-11
Capacity, (b)
100
10-12
0.5 dB/cm
10-13
Energy per bit (J)
1
CMOS eDRAM
10-14
All delay line buffers
10-15
10-16
10 mW
100 mW
100 mW
1 mW
10 mW
1 mW
Saturation power, Psat
International Technology Roadmap for
Semiconductors
Projections for 2018
14
Optical and Electronic Buffers
Optical Buffer
Optical Cross Connect
Optical Inputs
CMOS Buffer
O/E
E/O
5 mW
10-4
7 mW
7 mW
10-2
Buffer
O/E E/O
20 mW
10-2
CMOS
10-2
20 mW
20 mW
2x10-2
10-2
20 mW
Total
Viable buffer solutions
15
Conclusions

• Slow light and fiber delay lines struggle to
  compete with CMOS

• Power dissipation and dispersion are key issues

• Research challenges for optical buffering
• Waveguide losses ltlt 0.1 dB/cm
• Integration of gain and dispersion compensation
• Crosspoint power dissipation and footprint

16
Dispersion Limits in Slow Light Buffers
Maximum (Slow-down factor 1) and fiber
40 Gb/s
10 mm
PC
EIT
1 mm
0.5 dB/cm
0.05 dB/cm
Stored bit length, Lbit
100 mm
CRW
Psat 100 mW
10 mm
1 mm
Minimum (Ideal)
1
10
10 k
0.1
100
1 k
100 k
Number of stored bits, Nbit (b)