DWDM transmission at 10Gb/s and 40Gb/s using 25GHz

grid and flexible-bandwidth ROADM

M. Filer and S. Tibuleac ADVA Optical Networking. 5755 Peachtree Industrial Blvd., Norcross, GA 30092

mfiler@advaoptical.com, stibuleac@advaoptical.com

Abstract: DWDM transmission with flexible-bandwidth ROADM supporting 25GHz-spaced 10G

channels and co-propagating 50GHz-spaced 40G channels is studied experimentally. Nonlinear

and ROADM-induced OSNR penalties are measured, and XPM mitigation is demonstrated. OCIS codes: 060.2330 Fiber optics communications; 060.2360 Fiber optics links and subsystems; 060.4230 Multiplexing

1. Introduction

The capacity offered by the today’s DWDM networks using 10Gb/s and 40Gb/s modulated wavelengths with

50GHz spacing is considered insufficient to meet the rapidly growing bandwidth requirement. In response to this

market demand, DWDM systems are being developed for 100Gb/s transmission per wavelength operating on the

50GHz grid, using polarization multiplexed quadrature phase shift keying (PM-QPSK) modulation format. This

modulation format has been proven to offer the optimum combination of features: highest bandwidth density with

commercially available components, transmission distances comparable to 40G differential phase-shift keying

(DPSK) and 10G NRZ-OOK, compatibility with existing ROADMs, and the ability to upgrade existing networks.

These features come at a higher cost, increased power consumption, and increased latency compared to the

modulation formats used for lower data rates. Hence, there is a need to explore other options, specifically for

metro/regional networks, where cost, power consumption and latency are as important as the need for higher

transmission capacity.

An alternative approach to increasing capacity while maintaining the low cost points of 10Gb/s transmission

employs a channel plan on the 25GHz grid. Although the spectral density and the total transmission capacity does

not match that offered by 100G PM-QPSK operating on the 50GHz grid, a system with lower data rate operating on

the 25GHz grid with the option of concatenating multiple 25GHz bandwidths to support higher bandwidth signals

offers a viable alternative to the 50GHz PM-QPSK solution for metro/regional networks.

Transmission on the 25GHz grid has been reported previously for long-haul [1] and submarine applications [2].

A significant drawback of implementing 25GHz-spaced DWDM transmission in metro networks has been the lack

of an adequate reconfigurable optical add/drop multiplexer for wavelength add/drop and equalization. This

roadblock is removed with recently-developed flexible bandwidth wavelength-selective switches (FB-WSS) [3],

which are able to adjust power continuously across the C-band with no gaps in the spectrum. This feature enables

ROADMs to operate with 25GHz add/drop granularity and up to 160 channels in a single module.

This paper presents transmission experiments over an 8x85km link with 10G NRZ, 40G DQPSK and 40G DPSK

operating on the 25GHz grid with different bandwidth allocations for different data rates provided by a FB-WSS. A

ROADM based on FB-WSS is shown to induce no measureable bandpass penalty for add/drop and equalization of

25GHz-spaced 10G channels, while enabling a reduction in cross-phase modulation (XPM) penalty. The contiguous

bandwidth control on FB-WSS allows efficient use of available bandwidth for traffic with different data rates and

modulation formats. This is demonstrated in transmission experiments with 40G DPSK using 50GHz bandwidth and

10G NRZ using 25GHz bandwidth allocations on the same FB-WSS.

2. System Configuration

The experimental configuration consisted of a primary channel with 10G NRZ-OOK, 40G RZ-DQPSK or 40G

NRZ-DPSK modulation followed by an appropriate amount of pre-compensating fiber (-340 ps/nm for the

dispersion map used). The primary signal was coupled together with externally-modulated 10G NRZ-OOK

aggressor channels at ±25, ±50, ±75, and/or ±100 GHz from the primary signal. External channels were

polarization-scrambled to ensure an averaging of cross-phase modulation (XPM) impact over all polarization states.

All channels were launched into an 8-span link comprised of 80-90 km standard single-mode fiber (SSMF) with

fiber-based dispersion compensation (80-km) in the line amp mid-stages. A 2x1 flexible-grid ROADM was placed

after the 4th

span to provide power equalization and add/drop functionality.

NThB3.pdf   

OSA/OFC/NFOEC 2011 NThB3.pdf

©Optical Society of America

Fig. 1. System configuration

Optical ASE noise was coupled with the signal at the link output before demultiplexing through a 25GHz interleaver

and AWG demux for 10G OOK and 40G DQPSK measurements, or through a tunable-bandwidth filter in 40G

DPSK measurements. Residual dispersion was optimized using an etalon-based tunable dispersion compensator.

3. Results with 25GHz-spaced 10G

Measurements were performed using a 10G NRZ-OOK primary signal in the presence of aggressor channels spaced

at ±25 and ±50 GHz away. A 25GHz (de)interleaver pair was used to (de)multiplex the signals at link ingress and

egress. In a back-to-back configuration (no fiber spans, no adjacent channels), there was no measureable penalty due

to the insertion of the ROADM in the optical path, even when configured to drop the adjacent 25GHz neighbors

(i.e., tightest filtering condition indicated no passband penalty). Turning on the ±50GHz neighbors yielded

negligible penalty, but the addition of the ±25GHz neighbors caused an OSNR penalty of 0.5 dB at 1E-4 BER. This

was likely caused by adjacent-channel crosstalk from the imperfect isolation of the 25GHz interleavers.

Configuring as the full multi-span system shown in Fig. 1, nonlinearities such as SPM and XPM become the

dominant impairments. For this case, the primary channel was launched at a nominal power (for a 25GHz spaced

system) of -1 dBm, and the aggressor channels were launched at a higher power of +4 dBm/ch in order to emphasize

the impact of nonlinear effects. Results can be seen in Fig. 2a, where XPM penalties of 0.5dB due to the 50GHz

neighbors (“50 GHz”) increases to 1.5 dB when the 25GHz neighbors are present (“25 GHz”).

Fig. 2. 10G OOK (a) multi-span with ROADM and SPM + XPM, (b) reduction of XPM with diverse routing at ROADM node

It is important to point out that a reduction in the XPM effect is observed when the neighboring channels are

dropped and added back at the same ROADM node though a different port on the FB-WSS than the channel under

test. This ROADM-enabled XPM reduction, demonstrated previously in simulations [4], is explained by the group

delay introduced between adjacent channels through propagation along different paths within the ROADM node.

This explanation is confirmed by the further decrease in XPM penalty with increased group delay between the

adjacent channels generated by splitting the adjacent channels at the transmit end of the link and recombining them

with the test channel at the ROADM node (Fig. 2b “drop/add (source)”). However, the short delay introduced by a

typical length of a patchcord connecting the drop splitter with the Nx1 WSS in a typical ROADM provides most of

the benefits in XPM reduction (Fig. 2b “drop/add (ROADM)”).

4. Flexible-grid with 10G and 40G

The transmission capacity was increased while maintaining the 25GHz grid spacing, using a common modulation

format, DQPSK at 40Gb/s. The OSNR penalty induced by 25GHz interleaver filtering was ~1dB, and with optimum

pre-compensation, a nonlinear penalty of ~2dB was measured over 8 spans with adjacent 10G channels. The FB-

ROADM generates high penalties in filtering the wide-bandwidth 40G signal, but improves performance in point-

point applications by equalizing power levels without measureable penalty, as illustrated in Fig.3.

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

1E-2

10 11 12 13 14 15 16 17 18 19 20

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OSNR [dB]

single ch

50 GHz

25 GHz

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

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BE

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OSNR [dB]

drop/add (ROADM)

drop/add (source)

all pass

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Fig. 3. Power equalization with FB-WSS of 158 wavelengths with 25GHz spacing after 8 spans including one 40G DQPSK channel at 1554 nm

and 4 adjacent 10G channels

Support for mixed channel plan with optimized bandwidth allocation per channel was demonstrated in a hybrid

10G/40G transmission system using the wider bandwidth NRZ-DPSK modulation at 40Gb/s. The setup is illustrated

in Fig. 1, where 10G NRZ-OOK aggressor channels were placed ±50, ±75, and ±100 GHz away from the primary

signal, with a full 50GHz channel slot allocated to the 40G wavelength. The 25GHz (de)interleaver pair was

replaced by a passive combiner on the transmit side, and a tunable-bandwidth filter on the receive side.

Measurements were made in the back-to-back configuration, both with and without the ROADM in place, as well as

over the multi-span link. Fiber launch powers were set to +1 dBm for the primary channel and +4 dBm/ch for the

neighboring channels in order to exaggerate the XPM impact.

In the back-to-back configuration, there was no observable penalty due to the addition of adjacent channels or

from the ROADM device itself, even when configured to drop the adjacent 50GHz neighboring channels.

Measurable penalties were observed over the 8-span link, in which SPM and XPM become the dominating

impairments (Fig. 4). For the case when the 40G signal is propagating with no aggressor channels, the penalty due to

SPM alone was 1.3 dB at BER = 1E-4

(“single ch”). Adding in the aggressor

channels at ±50, ±75, and ±100 GHz, the

penalty increased to approximately 2.5 dB

(“all pass”). However, as in the 10G case,

the XPM penalty could be mitigated

somewhat by the dropping/adding of the

neighboring channels. In this case, only the

±50GHz neighbors were dropped and added

back at the ROADM (“drop/add (source)”).

This resulted in a reduction of the XPM

penalty by about 0.5dB, for an overall

nonlinear penalty of 2 dB.

5. Conclusions

Flexible bandwidth WSS offer a low-cost alternative to bandwidth increase by utilizing 10G channels on the 25GHz

grid without sacrificing add/drop capabilities required in today’s networks. These devices also allow upgrades to

higher-bandwidth 40G signals, as demonstrated in this paper. The ability to allocate the minimum bandwidth

required for a given data rate and modulation format makes efficient use of the available bandwidth in the C-band.

This feature can be exploited to provide transmission at 100G, or higher data rates using either a wider-bandwidth

modulation on a single wavelength, or multiple carriers with narrow-bandwidth modulation formats.

6. References

[1] C. Hullin, et al. “Ultra long haul 2500 km terrestrial transmission of 320 channels at 10Gbit/s over C+L bands with 25GHz wavelength

spacing,” Proceedings ECOC 2002, paper 1.1.3.

[2] G. Vareille et al. “3Tbit/s (300x11.6Gbit/s) transmission over 7380km using C+L band with 25GHz channel spacing and NRZ format,”

Proceedings OFC 2001, paper PD22-1.

[3] T. A. Strasser and J. L. Wagener, "Wavelength-Selective Switches for ROADM Applications," IEEE J. Selec. Topics in Quant. Elec. 16,

1150-1157 (2010)

[4] O. Vassilieva et al. “Suppression of XPM penalty in dispersion managed hybrid 10G/40G/100G DWDM networks using group delay

managaement,” ECOC 2009 Proceedings, paper P4.04.

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1528.5 1531 1533.5 1536 1538.5 1541 1543.5 1546 1548.5 1551 1553.5 1556 1558.5 1561 1563.5

Fig. 4. 40G DPSK measurements on hybrid 40G/10G flexible-grid link

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

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13 14 15 16 17 18 19 20 21 22

BE

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OSNR [dB]

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single ch

drop/add (source)

all pass

NThB3.pdf   

OSA/OFC/NFOEC 2011 NThB3.pdf