The document discusses recent trends in optical networking for data center interconnects (DCI). It compares coherent and direct detection modulation formats. While coherent transmission offers higher spectral efficiency using complex modulation formats, direct detection has simpler implementations and lower power consumption, making it suitable for short-reach DCI applications. The document outlines new direct detection technologies that could expand its role, such as pluggable 100G QSFP28 optics using dual-wavelength PAM4 modulation to deliver up to 4Tb/s capacity per fiber.

1. Recent trends in DCI optical networking
February 2018
APRICOT 2018
Niall Robinson, VP Global Business Development

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Modulation format comparison
Fiber Capacity
Size, Power and Cost
Applications
Evolution
Coherent
Detection
Direct
Detection
Local-oscillator (laser) at receiver, for optical phase detection.
Higher-Order-Modulation.
No local oscillator (laser) at the receiver.
Higher-Order-Modulation.
I
Q
Maximum spectral efficiency Reduced spectral efficiency
Complex Tx/Rx, high speed ASICs and DSPs Simpler, lower cost, lower power
Higher order QAM modulation More carriers, low cost modulation formats
Regional and long distance Shorter reach point to point metro

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Spectral efficiency comparison
Coherent flexibility vs direct detect
0
1
2
3
4
5
6
7
8
9
0 100 200 300 400 500 600 700
Bit rate vs Spectral Efficiency
Coherent
Increasing baud rate
Increasing QAM levels
Direct detect
Bit rate (Gb/s)
Spectral efficiency
(bits/sec/Hz)

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Power consumption comparison
Current state of the art coherent solutions
currently @ 0.2W/Gb/s
100Gb/s direct detect @ 0.05W/Gb/s
BUT next-gen coherent plugs (2020
timeframe) moving towards similar power
dissipation per Gb/s
Coherent
Direct detect

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Coherent has dominated DCI
• Customers with minimal personnel with
sufficient optical knowledge levels
• Customers with small operations teams
• Needs to be plug-and-play
Direct detect has its challenges
New approach to direct detect solutions needed

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• QSFP28 DWDM (non-tunable) pluggable optics (directly into switch/router)
• Dual-wavelength 2x 56Gbit/s PAM4 (non-tunable)
• Up to 4Tbit/s capacity in C-band
• Low power solution: ~4.5W
Direct Detect Pluggable
Colored 100Gbit/s QSFP28 pluggable
100GHz channel spacing 100GHz channel spacing
Each channel, made up of two wavelengths,
is centered on the ITU-T 100GHz channel grid
- One wavelength is offset from the
grid center frequency by -25GHz
- The other wavelength is offset by +25GHz
è 100G of capacity in 100GHz of bandwidth

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Dispersion tolerance
A dispersion tolerance of ~200ps/nm (+/- 5km) for the DWDM QSFP28:
§ Active CD compensation required because of direct detection
§ Amplification required with integrated tunable dispersion compensator
Active dispersion compensation required for direct detect solutions
Switc
h
QSFP
QSFP
Switc
h
QSFP
QSFP
DCM
1
2N
100G per QSFP28
AWG
AWG
Tunable DCM
Adapted from: M. Filer, S. Searcy, Y. Fu, R. Nagarajan, and S. Tibuleac, Presented at W4D.4 OFC 2017

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• 100Gbit/s PAM4 optimized link solution
• Integrates high-power amplification, DWDM
multiplexing and precise chromatic dispersion control
• Fully automated solution for single span
• Managed via open APIs
Smart Optical Layer
Smarter optics helping direct detect
+ + =

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Is 100G direct detect real?
Fully Loaded, 4Tb/s: 80x 28Gbaud PAM4

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Does direct detect have a future?
Yes!!!* (some caveats apply J)
1. In applications were large capacities (>4Tb/s/fiber) aren’t required
2. Where low power is critical
• 4.5W / 100G QSFP28, not currently possible with coherent and will take some time to come along
3. Where network is homogeneous
• Perfect for metro/campus DC to DC and intra-DC where cabling is an issue
Ongoing investigations into capacity upgrades
• E.g. PAM4 with 50Gbaud, DMT, or PAM8

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Summary
100G coherent optics has dominated the DCI market
• Using electronic compensation techniques to achieve plug-and-play
Direct Detect has been a niche play but now has an expanding role
• Using optical compensation techniques to achieve plug-and-play
As long as Direct Detect offers lower power and lower cost it will continue to be a
valuable (and viable) technology choice
• Application space may shift over time
Doesn’t scale as well, but hey, you can’t have everything!

12. Thank you
nrobinson@advaoptical.com
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Niall Robinson
Vice President
Global Business Development
Phone: +1 972 759 1262
Fax: +1 972 759 1201
Mobile: +1 978 933 1081
nrobinson@advaoptical.com
ADVA Optical Networking NA. Inc.
2301 N. Greenville Avenue
Suite 300
Richardson, TX 75082
USA
www.advaoptical.com