Steven Searcy and Sorin TibuleacADVA Optical Networking, Norcross, GA

Cross-Phase Modulation (XPM) Reduction in Hybrid Systems with 100G QPSK and 10G OOK

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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

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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

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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)• (PNLT is launch power for 1 dB OSNR penalty @ BER=1e-2)

• Higher NLT better tolerance to nonlinearity

(Recirculating Loop)

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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

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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

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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)

SSearcy@advaoptical.com

Thank you

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