Improved OSNR With Optical Amplifier Inisde the ROADM Node

IEICE Communications Express, Vol.2, No.4, 148–153

OSNR improvement byintroducing intra-nodearrayed optical amplifiersinto multi-degree ROADM

Yohei Sakamaki1a), Takeshi Kawai2, Tomoyoshi Kataoka2,and Mitsunori Fukutoku2

1 NTT Photonics Laboratories, NTT Corporation,

3–1, Morinosato-Wakamiya, Atsugi, Kanagawa, 243–0198, Japan2 NTT Network Innovation Laboratories, NTT Corporation,

1–1, Hikarinooka, Yokosuka, Kanagawa, 239–0847, Japan

a) [email protected]

Abstract: We propose an intra-node arrayed optical amplifier (AOA)to improve the OSNR of received signals by reducing the insertionloss of multi-degree ROADM nodes. Our AOA shares a pump laserbetween several EDFAs with the aim of reducing the module size andmanufacturing cost compared with the size and cost when only arrayingdiscrete EDFA modules. We confirmed the feasibility of our fabricatedAOA experimentally. The experimental results show that the pumppower was properly distributed to each EDFA and the OSNR of thereceived signals was successfully improved for 128-Gbit/s PDM-QPSKsignal transmission systems.Keywords: reconfigurable optical add/drop multiplexing, optical am-plifierClassification: Fiber-Optic Transmission for Communications

References

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[2] S. Gringeri, B. Basch, V. Shukla, R. Egorov, and T. J. Xia, “Flexiblearchitectures for optical transport nodes and networks,” IEEE Commun.Mag., vol. 48, no. 7, pp. 40–50, July 2010.

[3] S. Yamamoto, T. Inui, H. Kawakami, S. Yamanaka, T. Kawai, T.Ono, K. Mori, M. Suzuki, A. Iwaki, T. Kataoka, M. Fukutoku, T.Nakagawa, T. Sakano, M. Tomizawa, Y. Miyamoto, A. Suzuki, K. Murata,T. Kotanigawa, and A. Maeda, “Hybrid 40-Gb/s and 100-Gb/s PDM-QPSK DWDM transmission using real-time DSP in field testbed,” Proc.OFC/NFOEC, Los Angeles, CA, paper JW2A.4, March 2012.

[4] F. Buchali, K. Schuh, D. Rosener, E. Lach, R. Dischler, W. Idler, L.Schmalen, A. Leven, R. P. Braun, A. Ehrhardt, C. Gerlach, and L.Schurer, “512-Gb/s DP-16-QAM field trial over 734 km installed SSMF

c© IEICE 2013DOI: 10.1587/comex.2.148Received March 12, 2013Accepted March 29, 2013Published April 15, 2013

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[6] Y. Sakamaki, T. Kawai, M. Fukutoku, T. Kataoka, and K. Suzuki, “Full-add/drop C/D/C-less ROADM achieved by developing arrayed opticalamplifiers with a shared pump laser,” Proc. ECOC, Amsterdam, Nether-lands, paper P3.03, Sept. 2012.

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1 Introduction

To deal with increasing traffic demand driven by new applications such ascloud computing while maintaining or lowering network costs, optical trans-port networks are evolving to achieve more flexibility as regards wavelengthpath routing in addition to supporting bit rates of 100G and beyond. Re-configurable optical add/drop multiplexing (ROADM) brought new flexibil-ity and scalability to conventional static optical transport networks. Afterthe introduction of basic 2-degree ROADM, multi-degree ROADM was de-veloped to realize mesh-based network topologies [1]. Recently, colorless,directionless and contentionless (C/D/C-less) ROADM has been attractingconsiderable attention for realizing dynamic capacity allocation [2]. How-ever, the insertion loss of ROADM nodes with these sophisticated functionsis generally higher than that for the basic 2-degree node (for the reason de-tailed in Section 2). This loss increase leads to a degradation in the opticalsignal-to-noise ratio (OSNR) of received signals and becomes a more criticallimiting factor for higher bit rate signals. For example, while the requiredminimum OSNR with a 0.1-nm resolution is on the order of 13 ∼ 14 dB for128-Gbit/s polarization-division-multiplexing quadrature phase-shift keying(PDM-QPSK) signal transmission [3], that for 256-Gbit/s PDM 16 quadra-ture amplitude modulation signal transmission is closer to 20 dB [4]. Thus,if we are to upgrade the capacity of current optical transport networks, it iscrucial to reduce the insertion loss of multi-degree ROADM nodes. In thispaper, we describe the design concept of an arrayed optical amplifier (AOA)that we propose for reducing the node loss, and report experimental resultsshowing its feasibility and OSNR improvement effect.


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2 Design of intra-node arrayed optical amplifier

2.1 PurposeFirst, we describe the multi-degree ROADM node configuration and theneed for intra-node amplifiers. Figure 1 (a) shows the configuration of a4-degree colorless node. This node consists of pre/post amplifiers, color-less switches for connecting input and output express paths (hereafter re-ferred to as wavelength cross-connects; WXC), multiplexers/demultiplexers(Mux/Demux) and transmitters/receivers (Tx/Rx). Since it is preferableto use passive components such as optical couplers/splitters rather thanwavelength-selective switches (WSS) in terms of equipment size and cost,we assume a broadcast and select architecture using the combination of oneoptical splitter and one WSS in the WXC.

Here, let us discuss the insertion loss of signals that pass through thisnode. Signals broadcast to other WXCs pass through an express path consist-ing of one optical splitter, one WSS and several power monitors where about

Fig. 1. (a) Configuration of 4-degree colorless ROADMnode, (b) OSNR of received signals as a func-tion of the number of spans and level diagram ofsignal power in ROADM node, and (c) those forROADM node with intra-node amplifiers.


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10% of the signal power is tapped. The losses of the WSS and the powermonitor are independent of the node function, that is, the number of degrees.In contrast, the splitter has an intrinsic splitting loss of −10·log(1/M), whereM denotes the number of degrees. Assuming that the WSS loss is 7 dB, thesplitter has an excess loss (such as fiber coupling loss) of 1 dB in addition tothe intrinsic splitting loss, and the total loss caused at several power mon-itoring points is 2 dB. The express-path loss is 16 dB for a 4-degree node.Of course, the express-path loss increases for a node with a larger M . Fig-ure 1 (b) shows the OSNR of received optical signals as a function of thenumber of spans and a level diagram of the signal power in the node. Wecalculated the OSNR based on the assumption that the span loss is 12.5 dB,the OSNR of the output signal from the Tx is 25 dB, the noise figure ofthe amplifiers is 8 dB, and the signal powers output from the pre- and post-amplifiers in the node are +2 and −2 dBm/ch, respectively. This result showsthat an increase in the number of degrees significantly degrades the OSNRof the received signals.

To suppress the OSNR degradation resulting from the upgrade of thenode function from basic 2-degree to multi-degree, we investigated the intro-duction of intra-node amplifiers instead of increasing the gain of the pre/postamplifier, because the existing pre/post amplifiers have very little reserve tocompensate for the increased splitting loss. Figure 1 (c) shows the OSNRof the received signals and the level diagram of the signal power when weinsert an amplifier with a gain of 10 dB after the splitter in the WXC. Thisresult indicates that intra-node amplifiers improve the OSNR and relax thelimitation on transmission distance as in the example below: the maximumtransmission distance can be extended from 10 to 20 spans by introducingintra-node amplifiers for the required minimum OSNR of 20 dB as regards the8-degree node. Thus, the introduction of intra-node amplifiers is a promisingway to ensure that we obtain the OSNR required for higher bit rate signalsor extend the maximum transmission distance.

2.2 ConfigurationIn this section, we describe the configuration of our developed intra-nodeAOA. To reduce the size and manufacturing cost of the intra-node amplifiers,we used the concept of pump sharing between several amplifiers instead ofonly arraying discrete erbium-doped fiber amplifier (EDFA) modules. Thisconcept has already been proposed in [5], and we experimentally demon-strated its feasibility to compensate for the loss of the transponder aggre-gator used to provide the contentionless function with C/D/C-less ROADMnodes [6]. However, the AOA we propose in this paper differs substantiallyfrom the previously reported AOA in terms of the hardware configurationand the pump power control system.

Figure 2 (a) shows the AOA configuration designed for 8-degree nodes.The AOA consists of input/output power monitors, a shared pump laser,signal/pump couplers, EDFAs, isolators and gain-flattening filters (GFF).Since each EDFA amplifies the same WDM signals broadcast by the splitter


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Fig. 2. (a) Configuration and (b) photograph of fabri-cated AOA.

at the WXC, we only have to distribute the pump power to the EDFAsevenly. This equipartition rule makes the pump power control simpler thanwith the AOA for reducing the add/drop-path loss in C/D/C-less nodes [6],and makes it possible to realize fast automatic gain control (AGC). Thenumber of EDFAs sharing an identical pump laser depends on a balance ofthe designed gain with the output power of the shared laser. In this work,the designed gain value is 10 dB for 88-channel WDM systems, and the pumplaser is shared between two EDFAs.

Next, we describe the fabrication details. We used a 976-nm laser with anoutput power of 850 mW as the shared pump laser, and AGC is achieved bycontrolling its output power. The pump power is distributed to two EDFAsthrough a 1× 2 splitter fabricated using silica-based planar lightwave circuit(PLC) technology. The tap couplers for the power monitor and signal/pumpcouplers are integrated in one PLC chip [7, 8]. Figure 2 (b) shows the appear-ance of the fabricated AOA module, which is 200×220×44 mm in size. Thismodule includes eight EDFAs, four shared pump lasers, photodiode arrays,passive optical components (PLC chips, isolators, GFF) and the controllerboard.

3 Experimental results

We confirmed the feasibility of our proposed AOA experimentally. First,we ensured that the pump power was properly distributed to each EDFA.Figure 3 (a) and (b) show the measured gain value and noise figure of eachEDFA. The power level diagram of this experiment corresponds to that ofthe express path in the 8-degree node. One continuous-wave (CW) lightwith the power of −9 dBm and fifteen other CW lights in the C-band werelaunched simultaneously into the AOA. To emulate the input power of 88-channel WDM systems, we adjusted the power of the fifteen CW lights to−1.4 dBm/ch. Then, we measured the output power and noise figure for oneCW light with an input power of −9 dBm. The frequency of the measuredCW light changed from 192.2 to 195.5 THz. As shown in Fig. 3 (a), the gaindeviation from the designed value of 10 dB and the noise figure were less than0.5 dB and 7.6 dB, respectively, for all the EDFAs (Amp. 1 ∼ 8).

Next, we report the results of a transmission experiment designed todemonstrate the OSNR improvement effect. We launched a 128-Gbit/s PDM-


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Fig. 3. Measured (a) gain and (b) noise figure for CWlight. (c) Experimental setup and (d) measuredQ-factors for 128-Gbit/s PDM-QPSK signal.

QPSK signal instead of the CW lights and measured the Q-factors. Fig-ure 3 (c) shows the experimental setup. The recirculating loop consisted ofa 40-km SMF and the optical node including pre/post-amplifiers, one 1 × 8splitter, one WSS and the fabricated AOA. Note that we set the span loss at22.5 dB, which is 10 dB higher than that for the OSNR estimation exampledescribed in Section 2.1. Figure 3 (d) shows the measured Q-factors againstthe span number. The Q-factors obtained in the experiment with the fab-ricated AOA were higher than those for the experiment without the AOA.This result indicates that the OSNR of the received signals was successfullyimproved by introducing our proposed intra-node AOA.

4 Conclusion

We proposed an intra-node AOA with a shared pump laser to reduce a lossof multi-degree ROADM nodes, while suppressing increases in module sizeand manufacturing cost. The experimental results show that as expectedthe pump power was properly distributed to each EDFA and the OSNRof the received 128-Gbit/s PDM-QPSK signals was improved by using ourfabricated AOA.


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Improved OSNR With Optical Amplifier Inisde the ROADM Node