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Metro Network Utilizing 10-Gb/s Directly Modulated Lasers and Negative Dispersion Fiber Ioannis Tomkos, Member IEEE, Robert Hesse, Rich Vodhanel, Senior Member IEEE, and Aleksandra Boskovic
ECENTLY, there has been an increasing demand for introduction of wavelength-division–multiplexing (WDM) in metro networks [1]–[3]. A shift in the demand for 2.5–10 Gb/s per channel bit rates has been also observed. In the metro networks market, cost effectiveness is a very important factor [2]. Transparent WDM networks can eliminate expensive transponders needed for the optoelectronic conversions in an opaque approach. In transparent metro networks, the overall cost is dominated by the cost of the terminal equipment [2]. Since the transmitters’ cost is a significant fraction of the terminal cost, then cost-effective 1550-nm transmitters is a necessity for WDM metro networks. 10-Gb/s directly modulated lasers (DMLs) have the advantages of low cost, small size, low driving voltage, and high output power, so they could be considered as appropriate transmitters for metro networks. However, the very large frequency chirp of 1550-nm 10-Gb/s DMLs, significantly limits the uncompensated reach of transparent connections to less than 10 km over standard SMFs with high positive dispersion (i.e., SMF-28 fiber) [4]. In addition to dispersion induced limitations, the large chirp of DMLs results in a broad optical spectrum that will affect the performance of transparent networks due to filter concatenation effects [5]. Therefore, the feasibility of transparent metro area networking with 10-Gb/s DMLs is questionable. To our knowledge, no network demonstration of the performance of a WDM metropolitan area network utilizing 1550-nm DMLs operating at 10 Gb/s has been presented. In this work, we demonstrate the feasibility of a WDM metropolitan area network utilizing cost-effective 10-Gb/s DMLs as transmitter sources. The demonstration is based on a ring network testbed consisting of four network nodes
interconnected with 25.5-km spans of uncompensated negative dispersion fiber (MetroCor fiber). The network is designed to support 32 directly modulated channels operating at 10 Gb/s (320-Gb/s capacity). In the nodes of the network, we used optical layer network elements (amplifiers, mutliplexers/demultiplexers, variable optical attenuators) optimized for metropolitan/regional area optical networks. It is shown that all received signals after the longest transmission path in the network, including four nodes (eight amplifiers, eight filters) and 102 km of fiber, have a very good transmission performance with -factors greater than 9 dB. To our knowledge, the capacity length product of our uncompensated network utilizing cost-effective 10-Gb/s DMLs is the highest ever reported. Recently, an optical networking experiment based on optimized components and fiber was reported [6]. That demonstration presented the performance of 2.5 Gb/s directly modulated signals over a regional size ring network with a circumference of 605 km [6]. The network node architecture is illustrated in Fig. 1. Details of the design are presented in [6]. Each node supports 32 channels at 200-GHz spacing distributed across the - and -EDFA bands. The channels were organized in wavelength bands of four wavelengths each. The network-node equipment shown in Fig. 1 was also used for characterizing the performance of metropolitan area networks utilizing 1550-nm 10-Gb/s commercially available DMLs. The metropolitan area network that we build consists of four interconnected nodes in a ring topology.1 The design of the network nodes is such that allows signal conditioning in the form of amplification, channels/bands multiplexing/demultiplexing (MUX/DMUX), power equalization, reconfigurable add–drop functionality, switching, signal monitoring, and optionally dispersion compensation for networks based on standard SMF. In the node, the channels of each wavelength-band undergo amplification by two erbium-doped fiber amplifiers (EDFAs) dedicated to amplify only one band (Fig. 1). Typical characteristics of our amplifiers include gain up to 25 dB, a noise figure of about 5 dB, saturation power of about 17 dBm, and the ability for static and transient gain control within 1 ms. The characteristics of these amplifiers are well suited for reconfigurable metropolitan area networks. In the event of a signal add–drop they are able to maintain constant gain for the surviving channels. It is also worth mentioning that power ripple generated at an earlier stage in the
Manuscript received September 19, 2001; revised November 5, 2001. The authors are with Corning Inc., Photonics Research and Test Center, Somerset, NJ 08873 USA (e-mail:
[email protected]). Publisher Item Identifier S 1041-1135(02)01214-4.
1As described in [6], the actual nodes used in the experiments consisted of the equipment enclosed in the dotted line in Fig. 1 (one of the bands). No switches were used. Instead their losses was emulated as a fixed bias-loss that was added by the variable optical attenuators (VOAs) [6].
Abstract—In this letter, we present the feasibility of metro area transparent wavelength-division-multiplexing networking with cost-effective 1550-nm 10-Gb/s directly modulated transmitters. 9 dB) for We report excellent performance results ( -factor a transparent short-reach metropolitan area ring network based on application-optimized, optical layer components and fiber. The demonstration is based on a network consisting of four nodes interconnected with 25.5-km spans of uncompensated negative dispersion fiber. The network is designed to support 32 directly modulated channels operating at 10 Gb/s (320-Gb/s capacity). Index Terms—Directly modulated lasers, filter concatenation, frequency chirp, metropolitan area optical networks.
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TOMKOS et al.: METRO NETWORK UTILIZING 10-Gb/s DIRECTLY MODULATED LASERS AND NEGATIVE DISPERSION FIBER
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Fig. 1. The full WDM network node design (for simplicity, only the C -band portion is shown with full functionality).
network can be equalized at the node through the use of VOAs. An additional feature of our VOAs is that they provide dynamic output power control within 0.5 ms of a power change event. Finally, we would like to point out that the channel multiplexing/demultiplexing is achieved with a combination of bandpass filters (BPFs) and channel DMUXs/MUXs (Fig. 1) based on multilayer interference filter technology. The BPFs drop the desired band and pass-through the rest of the bands or combine one band with the others. The individual channels’ DMUX’s and MUX’s devices consists of four multilayer interference filters cascaded together to separate or combine the four wavelengths of each band. Typical insertion loss numbers are 1.4 and 0.6 dB for the drop-port and pass-through port, respectively, of the BPFs and the MUX/DMUX filters. The 3-dB bandwidth of the BPFs is about 1 THz, and that of the MUX/DMUX filters about 170 GHz. The typical isolation for the BPFs in the transmission path is larger than 20 dB, and in the reflection path is larger than 12 dB. The typical adjacent channel isolation for the MUX/DMUX filters is larger than 25 dB, and the nonadjacent channel isolation is larger than 45 dB. Based on the characteristics of the filters and the node architecture, we estimated that each node contributes two inband crosstalk terms of about 50 dB each. The number of crosstalk terms at that level will be equal to twice the number of network nodes. In that crosstalk level, we have to add the crosstalk originating from the switches that will be used in the network. However, the total crosstalk level is not expected to degrade significantly the network performance in case of a moderate number of nodes [3].
The four network nodes were interconnected with 25.5-km uncompensated spans of MetroCor fiber, a fiber with negative dispersion across the entire useable fiber bandwidth (1280–1620 nm) [3]– [4][6]. In the experiments, the signals were added at one node and were detected with a 10-Gb/s PIN receiver at the drop-site of each one of the other nodes. The launched power in each fiber span (with an average padded loss of 0.25 dB/km) was about 0 dBm per channel. The signals at the input of each node were attenuated to adjust the total input power to the first amplifier at about 10 dBm (optimum condition in terms of minimum noise figure). The filters in the different nodes of the network testbed were randomly aligned in relation to each other. The signal performance was evaluated with -factor measurements. In Fig. 2, we present the spectrum of four channels of one wavelength band at the output of the fourth node. The DMLs of channels 2, 3, 4 were modulated at 10 Gb/s, while channel 1 was modulated at 2.5 Gb/s. The length of the pseudorandom bit . The broad spectrum of the 10-Gb/s chansequence was nels is due to the large frequency chirp of the 10-Gb/s DMLs. The power equalization feature due to the combined action of the VOAs and EDFAs is clearly demonstrated. The power of all channels after the total path is the same and the accumulated ripple was limited to less than 0.3–0.4 dB mainly due to the use of the VOAs. The optical signal-to-noise ratio (OSNR) of the received signals after the four nodes (total path of 102 km, eight amplifiers) was higher than 27 dB (at 0.1-nm resolution). The -factor performance for the 10-Gb/s signal of channel 3 (worst-performing channel) after the second, third, and forth
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Fig. 2. Optical spectra after the worst path in the network (eight filters, eight amplifiers, 102-km fiber).
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Fig. 4. -factors as a function of the transmitted extinction ratio for the 10-Gb/s signal of channels 2, 3, 4 at the output of node-4.
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Fig. 3. -factors as a function of the transmitted extinction ratio for the 10-Gb/s signal of channel 3 at the output of each node.
Fig. 5. -factors as a function of the transmitted extinction ratio for the 10-Gb/s signal of channel 4 at the output of node-4 and -5.
node was measured at a received power of 6 dBm for various transmitted extinction ratios (ER). The results are presented in Fig. 3. From the results, we can see that there is an optimum extinction ratio for networks with different sizes. At low extinction ratios the performance is mainly limited due to the small eye opening, while at high extinction ratios the performance is limited by dispersion and filter concatenation induced penalties caused by the large frequency chirp of the 10-Gb/s DML. However, for the optimum extinction ratio, the -factor can remain ) even after a path conhigher that 9 dB ( sisting of 102 km of fiber and four network nodes. It is worth mentioning that the -factor performance of channels 2 and 4 were even better (up to 10.5, 11.2 dBQ at an ER of 6–7 dB, respectively), mainly due to the better chirp characteristics of the corresponding 10-Gb/s DMLs. Results for all channels at the output of node-4 are shown in Fig. 4. The DML corresponding to channel 4, impressively, was able for transmission over a fifth node (127.5 km, ten amplifiers and filters). The corresponding -factor is shown in Fig. 5. In conclusion, we demonstrated excellent performance for a metropolitan area WDM ring network utilizing 10-Gb/s 1550-nm DML transmitters. The network is designed to support 10 Gb/s). Negative dispersion a capacity of 320 Gb/s (32
fiber was used for interconnecting the nodes, and thus, no dispersion compensation was required at any point in the network. In the worst path (eight amplifiers, eight filters, and 102 km of uncompensated fiber) the was higher than 9 dB. REFERENCES [1] A. A. M. Saleh and J. M. Simmons, “Architectural principles of optical, regional, and metropolitan access networks,” J. Lightwave Technol., vol. 17, pp. 2431–2448, Dec. 1999. [2] D. A. Cooperson, “Short-haul WDM optical networking: Is it finally for real,” in Proc. NFOEC’00, 2000. [3] N. Antoniades, A. Boskovic, J. K. Rhee, J. Downie, D. Pastel, I. Tomkos, I. Roudas, N. Madamopoulos, and M. Yadlowsky, “Engineering the performance of DWDM Metro networks,” in NFOEC’00, 2000, pp. 204–211. [4] I. Tomkos, B. Hallock, J. Roudas, R. Hesse, Nakano, A. Boskovic, and R. Vodhanel, “Transmission of 1.55 m 10 Gb/s directly modulated signal over 100 km of negative dispersion fiber without any dispersion compensation,” in OFC’01, 2001, Paper TuU6. [5] I. Tomkos, J.-K. Rhee, P. Iydrose, R. Hesse, A. Boskovic, and R. Vodhanel, “Filter concatenation penalties for 10 Gb/s sources suitable for short-reach cost-effective WDM metropolitan area networks,” IEEE Photon. Technol. Lett., to be published. [6] I. Tomkos, R. Hesse, C. Friedman, N. Antoniades, N. Madamopoulos, B. Hallock, R. Vodhanel, and A. Boskovic, “Transport performance of a transparent WDM regional area ring network utilizing optimized components/fiber,” in Proc. OFC’01, 2001, Postdeadline Paper PD-35.
