Incremental Capacity Upgrade of Exisitng Fiber Optic Links - Mitre

Incremental Capacity
Upgrade Solutions for
Existing Fiber Optic Links
Dr. Hatem Abdelkader
703-983-3131 abdelkader@mitre.org
Army-Contract MOIE
© 2006, The MITRE Corporation

Problem
� As demand grows current fiber optic links will
need to be re-engineered to increase capacity
� Increasing the capacity using the current
modulation is challenging due to increased
linear/ nonlinear propagation impairments
� Proposed Solution: Advanced optical
modulation techniques may allow:
– Exploitation of the investment in legacy
optical infrastructure (amplifiers and fiber)
– Incremental funding of link upgrades
© 2006, The MITRE Corporation

Background
� In order to ensure control of its critical
communications infrastructure, the U.S.
government has acquired long-term
possession of a global fiber optic backbone.
� To fully exploit this backbone, the optical
layer design and optimization must be
actively managed; an effort complicated by:
– Fiber topology heterogeneity (different fiber types)
– Unique architectural and operational requirements
(Network Operations differ from Telcos)
– Churn in traffic termination points (due to dynamic
BW demand and procurement process)
© 2006, The MITRE Corporation

Objective
� Evaluate the benefits of advanced optical
modulation formats, with focus on optical
Differential Phase Shift Keying (DPSK)
� Demonstrate DPSK feasibility and quantify
the resulting improvement in optical link
aggregate bandwidth, length, and/or margin
� Assist industry in understanding the potential
government market for advanced modulation
– DPSK/NRZ modulation formats interoperability
– D-QPSK feasibility (spectral efficiency & tolerance)
– Experimental studies on GIG-BE-like links
© 2006, The MITRE Corporation

Activities
� Design and build a 10 Gb/s DPSK transceiver
(XCVR) unit on a laboratory breadboard,
using discrete off-the-shelf components
� Characterize the XCVR unit to establish its
back-to-back performance envelope
� Experimentally quantify the optical transport
performance improvement when DPSK is
used – instead of on/off keying – on optical
test bed representative of GIG-BE links
� Evaluate the NRZ/DPSK modulation format’s
interoperability (on the same optical link)
© 2006, The MITRE Corporation

Highlight: How Does DPSK Work?
• Transmitted data is encoded on the laser optical phase
differentially between adjacent bits rather than in the amplitude.
• Decoding at the receiver is done via an optical delay-and-add
Mach-Zehnder Interferometer. Here’s how it’s done:
Input DATA
Binary ON/OFF keyed NRZ Data
0 1 0 1 1 0 1 0 0 1 1 0 0 0 1 0
Electrically Pre-Coded Data Out(0) 1 1 0 1 1 0 0 0 1 0 0 0 0 1 1
DPSK Encoded Optical
Signal
Shifted by Demodulator 1-bit
Delay
Demodulator DATA Out
—
ϕ=0
ϕ=π
© 2006, The MITRE Corporation

Highlight: DPSK XCVR Diagram
PPG
Broadband
Divider
Tunable
Laser
10 GHz
Clock
DPSK Encoder
DATA
Data
Amp
(Phase Locked
Ref. Oscillators)
(Manual
Gain Control)
5 GHz
½Clock
Clock
Amp
MZ Data Modulator (DM)
DATA
Bias
Tee
(null)
Bias
Controller
Athermal demodulator or (Manual wavelength control in lieu
of demodulator temperature control)
DPSK Receiver
Instrumentation
Optical Components, Connections
Control Components, Connections
Microwave Components, Connections
For NRZ, the DPSK RX is replaced by a standard
PIN-TIA/limiting amplifier front end.
Demodulator,
100 ps delay
Microwave
Var. Delay
MZ Pulse Carving Modulator (PC)
(null/peak)
Tunable
Filter
DATA
DATA-
BAR
Matched
delays
DCA
Error
Detector
OSA
TIA
Differential
Receiver
© 2006, The MITRE Corporation

Highlight: DPSK XCVR Testbed Evaluation
SONET
TX
WDM
Source
(40 DFB
lasers)
Data
Data Modulator Driver
PM-AWG
5
Optical
Modulator
Current Optical Link Configuration at the MITRE
Optical Networking Laboratory:
• 1,200 km linear test bed (TW-RS & LEAF fibers)
• 40ch x10Gb/s = 400 Gb/s link aggregate bit rate
• 12 state-of–the-art Onetta EDFAs
• 40 ch on 100 GHz ITU grid
6
1
6
7
9
7
1
8
17
1
8
2
15
9
Optical
Amplifier
5
2
9
3
12
10
13
3
10
4
Span
7
11
20
4
Optical Receiver
11
5
8
12
DCM
6
SONET
RX
© 2006, The MITRE Corporation

Impacts
� Compared to NRZ, DPSK allows optical link
operation with a 3-dB lower OSNR at the RX.
This can potentially translate into a factor of
two increase in link capacity
� Our sponsors can benefit from this work to:
– Significantly increase the aggregate
capacity of exhausted deployed links
– Markedly extend the reach on these links
– Improve the BER performance on marginal
links (that are susceptible to outages due
to transients caused by unplanned events)
© 2006, The MITRE Corporation

Future Plans
� Upgrade the DPSK XCVR to D-QPSK.
Compared to binary on/off Keying, D-QPSK:
– is 2 times more spectrally efficient, and 4 times
more tolerant to CD & PMD distortions
– has a narrow optical spectrum that can enable
closer channel spacing (more capacity)
� Investigate a coherent based DPSK RX:
– Effective electrical dispersion compensation
– Higher RX sensitivity, and
– Compatibility with several modulation formats
© 2006, The MITRE Corporation