Check out this slide set that outlines the research ADVA Optical Networking's Steven Searcy and Sorin Tibuleac have been doing. They have successfully demonstrated how we can improve the performance of a DWDM system operating with mixed 10G and 100G wavelengths.

1. Steven Searcy and Sorin Tibuleac
ADVA Optical Networking, Norcross, GA
Cross-Phase Modulation (XPM)
Reduction in Hybrid Systems
with 100G QPSK and 10G OOK

2. 22
Introduction & Motivation
• Coherent Pol-Mux Quadrature Phase Shift Keying (PM-QPSK) is
considered the best modulation format for 100G in regional and
long-haul networks. However, many legacy networks with traffic
at bit rates below 100G are being upgraded gradually, resulting in
“hybrid” systems with mixed data rates.
• In this work we focus on a very common type of hybrid DWDM
system, which offers significant challenges in terms of nonlinear
performance: Coherent 100G PM-QPSK co-propagating with
legacy 10G OOK channels and in-line optical CD compensation
(DCMs)
• Hybrid 100G-10G systems are limited by nonlinear
interactions between channels, known as Cross-Phase
Modulation (XPM)
• Simple and cost-effective solutions for XPM reduction are
desired in these upgrade scenarios

3. 33
XPM Basics
• Cross-Phase Modulation (XPM) is an inter-channel nonlinear effect in
which intensity variations in 1 channel modulate the phase of another
• For Pol-Mux signals this also includes XPM-induced polarization scattering
• XPM is the dominant nonlinearity in most DWDM systems, and is
typically the limiting factor for performance in hybrid networks
• Methods of XPM reduction explored in this work:
• Higher fiber dispersion
• Wider channel spacing/guard band(s)
• Type of DCMs and frequency of CD compensation
• Receiver DSP: carrier phase recovery window size

4. 44
Experimental Setup
• Test channel: 100G PM-QPSK, surrounded by 10G OOK (50 GHz grid)
• Transmission over 12 spans SSMF or TrueWave-RS, with variable CD
map and type of DCMs (fiber-based DCF or 50 GHz Fiber-Bragg gratings)
• Quantify nonlinear performance with Nonlinear Threshold (NLT)
• 𝑁𝐿𝑇 = 𝑁𝑠𝑝𝑎𝑛𝑠 ∙ 𝑃 𝑁𝐿𝑇 (PNLT is launch power for 1 dB OSNR penalty @ BER=1e-2)
• Higher NLT  better tolerance to nonlinearity
(Recirculating Loop)

5. 55
Experimental Results
• Performance without DCMs
provides the best-case nonlinear
performance (but unrealistic for
most legacy 10G traffic)
• With DCF, nonlinear performance
improves with increasing residual
dispersion per span (RDPS)
• With FBGs, nonlinear performance
is nearly independent of RDPS and
roughly equivalent to the
uncompensated case (no DCMs)
• FBGs offer a significant advantage
in nonlinear tolerance over DCF
when RDPS is very low
• Same general performance trends
are also seen with SSMF
Nonlinear Threshold (NLT) results vs average residual
dispersion per span (RDPS) for 12 spans TrueWave-RS
fiber with CD compensation placed every 2 spans

6. 66
Experimental Results
• With less-frequent in-line compensation (DCMs placed every 4 spans vs
every 2 spans), nonlinear performance becomes less dependent on RDPS
and closer to performance of uncompensated case
• Addition of 50 GHz guard band (one empty 50 GHz channel slot on either
side of test channel) can improve NLT by as much as 1 dB
TrueWave-RS results
With DCMs every 4 spans
TrueWave-RS results
with DCMs every 2 spans

7. 77
Summary & Conclusions
• A number of simple and practical solutions exist to reduce XPM in
hybrid 100G-10G systems
• Use of fiber Bragg gratings can be very beneficial in hybrid systems,
providing particular benefit over DCF on links with low RDPS
• Other system conditions can also have a significant impact on nonlinear
performance (≥1 dB): Average RDPS, frequency of DCM placement, use
of guard bands, and carrier phase recovery window size
• When XPM is reduced significantly by any one of the above methods, the
others tend to lose their effectiveness  must properly account for
combined effect of multiple methods when assessing nonlinear tolerance
• Complete details and results from this work have been published in
Optics Express (available online):
• “Experimental study of cross-phase modulation reduction in hybrid
systems with co-propagating 100G PM-QPSK and 10G OOK,” Optics
Express, Vol. 21, Iss. 25, pp. 31483-31491 (2013)

8. SSearcy@advaoptical.com
Thank you
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