1. science
2. physics
3. optics

Decoupled Line System +
Coherent Terminal Optics
Link Design & Commissioning
ECOC 2014 Workshop on “Can we still
trust our test protocols?”
Vijay Vusirikala, Valey Kamalov, Vinayak
Dangui & Tad Hofmeister
Outline
● Architecture : Decoupled Line Systems and Terminal coherent optics
○
Terrestrial and Subsea
● Design, specification and commissioning of a decoupled system
○
Link Budget Tables
● Evolution of test protocols from static to dynamic optical layer
Google Confidential and Proprietary
Decoupled Line System and Coherent Optics - Terrestrial
Vendor A
Vendor A
Vendor B
Vendor B
Terminal Optics
Open Line System (OLS)
Terminal Optics
Amps, Mux/demux, ROADM, Gain equalizer
Benefits:
●
●
●
Best-of-breed technologies
No vendor lock-in
Coherent optics makes this decoupling a lot easier compared to dispersion managed plants for
non-coherent optics
Google Confidential and Proprietary
Decoupling Wet Plant from Dry Plant SLTE Submarine Systems
Vendor A
Vendor A
Photonic
commons
Photonic
commons
Vendor B
Transponders
Vendor B
Mux, WSS, Upgrade
couplers, Noise
loading, Monitoring
Dry Plant
●
●
●
Repeaters, Branching Units, Gain
equalizers
Open Wet Plant
Different expertise needed for wet plant (marine operations, cabling, repeaters) vs. dry plant
(coherent modems)
Faster time to market and better pricing using terrestrial driven coherent optics
Refresh cycles and longevity are different between wet plant and dry plant
Google Confidential and Proprietary
Transitioning from Coupled Systems to Decoupled Systems
Coupled / Closed Line System
Decoupled / Open Line System
System Design:
System Design:
Cost-optimized to close the link for a
specific transponder type / generation
Optimized for maximizing OSNR
Terrestrial - Raman, NF optimized amps
Subsea - Shorter repeater spacing,
Higher repeater power, ULL fibers
System Commissioning:
System Commissioning:
Q based commissioning with validation
of margin
OSNR commissioning for line system +
validation of Q for transponders
Link / Power Budget:
Link / Power Budget:
B2B OSNR performance with Q
penalties for aging, loading,
transmission penalties
Link / Power Budget:
Hybrid OSNR / Q based
power budget tables
SNR / OSNR based power
budget tables with Q
thresholds and penalties
mapped to SNR
Google Confidential and Proprietary
Link / Power Budget Table for Decoupled Line Systems Thoughts
Ideal Approach ?
Currently Used Methodology
SNR / OSNR based power budget tables
1.
Baseline OSNR from simulation or
measurement (factor in line system aging)
2.
Map nonlinear impairments and penalties
to effective OSNR reduction
3.
Derive link margin in OSNR relative to
required Rx OSNR (for specific modulation
format / vendor)
Hybrid OSNR / Q based power budget tables
1.
Baseline OSNR from simulation or
measurement (factor in line system aging)
2.
Extract expected Q from B2B measurement
for that baseline OSNR
3.
Factor in Q propagation penalties,
transponder aging, time variation, loading,
manufacturing variations etc.
4.
Derive margin by comparing to FEC limit
Advantages:
Cleaner decoupling of line system and terminal
optics in a multi-vendor scenario
Enables ability to generate real time OSNR &
power budget tables for dynamic optical layer
Google Confidential and Proprietary
OSNR-Based Link Budgeting
Q
OSNR
Mapping as per back-to-back
●
●
●
Q penalties:
dependent on exact
link details
OSNR penalties: can
be expressed as
absolute noise
quantities
NL penalty has
simple scaling vs.
launch power in the
perturbative regime
Launch OSNR
Link ASE
Mapping as per back-to-back
Transmission
impairments
Received OSNR
Final OSNR
Link Margin
Minimum OSNR
Extra Margin
Required OSNR
Mapping as per distorted Q vs. OSNR
Google Confidential and Proprietary
OSNR Commissioning for New Subsea Systems
Average, Tilt and Gain Shape
Pre-Tilt
OSNR (dB)
Gain shape
ripple
Average
OSNR
Wavelength (nm)
●
Average OSNR : Goal is to maximize this (till nonlinear roll-off) by efficient cable plant engineering (low loss
fiber, large core fiber, shorter repeater spacing, higher power repeaters).
●
Tilt: Some pre-tilt is needed to compensate for ageing effects (additional loss due to fiber repairs). Trade-off
between Q performance penalty from high tilt vs. future proofing for cable repairs
●
Gain shape ripple: Manufacturing variations and imperfect equalization from preset
equalizers. Goal is to
Google Confidential and Proprietary
minimize
Decoupled Systems - Evolution
● Real-time, In-service OSNR
○
○
Are interferometric techniques practical, stable and accurate?
Loading channel based measurement is feasible but becomes impractical at
high link load
● Coherent transponders - Enhanced diagnostics
○
○
Electrical SNR
Self electrical noise loading to generate Q vs. SNR curve
● Real-time power budget table to determine which link can close at which
modulation format
Google Confidential and Proprietary
Transition from Static to Dynamic optical layer
● Value
○
○
Optical restoration for system availability improvement
Faster service provisioning and response to demand variability
● Necessary hardware:
○
○
○
CDC-Flex ROADMs
Coherent Transponders with flexible modulation support
Flexible client layer
● Real-time Power Budget Tables
○
Ability to combine information about coherent transponders and link OSNR
to determine which paths close and at which modulation format
Google Confidential and Proprietary
Questions?
Contact :
vijayvusiri@google.com
vkamalov@google.com
vdangui@google.com
tad@google.com