Directly modulated CWDM/DWDM system using negative dispersion fiber for metro network application

Transcription

1 Optics Communications 245 (2005) Directly modulated /DWDM system using negative dispersion fiber for metro network application H.S. Chung, Y.C. Chung * Korea Advanced Institute of Science and Technology, Department of Electrical Engineering, Guseong-dong, Yuseong-gu, Daejeon , Republic of Korea Received 23 April 2004; received in revised form 19 August 2004; accepted 7 October 2004 Abstract We demonstrate the error-free transmission of directly modulated 2.5, 10, and 40-Gb/s coarse wavelength-divisionmultiplexing/dense wavelength-division-multiplexing signals over negative dispersion fiber (dispersion: nm) without dispersion compensation. The results indicate that the metro core/access network could be implemented cost-effectively by using the proposed negative dispersion fiber and directly modulated lasers. Ó 2004 Elsevier B.V. All rights reserved. PACS: Sz Keywords: Directly modulated laser; Negative dispersion fiber; Metro network 1. Introduction Recently, there have been growing interests in coarse wavelength-division-multiplexed () systems using directly modulated lasers (DMLÕs), with the potential upgrade with dense wavelength-division-multiplexing (DWDM) overlay, for the metro core and access applications [1 6]. For example, it has been demonstrated that the signals (directly modulated at 2.5 Gb/s) * Corresponding author. Tel.: ; fax: address: ychung@ee.kaist.ac.kr (Y.C. Chung). could be transmitted over 125 km of single mode fiber by using a linear optical amplifier [1]. The possibility of using DWDM overlay in such a system has also been demonstrated [2]. The capacity of this /DWDM system could be further increased by increasing the operating speeds of directly modulated lasers to >10 Gb/s. However, when the bit rate of the directly modulated signal was increased to 40 Gb/s, the maximum transmission distance was limited to be less than 2 km due to the laser chirp and fiber dispersion [3]. To increase the capacity and transmission distance of /DWDM systems, both the fiber /$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi: /j.optcom

2 172 H.S. Chung, Y.C. Chung / Optics Communications 245 (2005) loss and dispersion become critically important. To accommodate the extremely wide bandwidth without the prohibitively large loss (caused by water absorption peak), a single-mode fiber with low water peak (LWP-SMF) has been used [1,2]. However, the large dispersion of LWP-SMF would force the use of external modulators instead of direct modulation and/or dispersion compensators [2]. In this paper, we propose to use negative dispersion fiber (NDF) with a small dispersion of 2.5 ps/km/nm at 1550 nm for metro core and access networks. The NDF could support the directly modulated /DWDM channels operating at >10 Gb/s without dispersion compensation for most metro applications. For the demonstration, we used the proposed fiber for the error-free transmission of directly modulated signals (2.5 Gb/s 125 km) with DWDM overlay (10 Gb/s 320 km). We also demonstrate that, for the first time to our knowledge, directly modulated 40-Gb/s signals can be transmitted over 40 km of NDF without dispersion compensation. The performance of NDF was compared with LWP-SMF. directly modulated the 11th channel by using a commercial DML, while the other channels were externally modulated by using a LiNbO 3 modulator. The bias and modulation currents of DML were set to be 60 ma and 40 ma p p, respectively. Under these conditions, the output power and extinction ratio were measured to be 6 dbm and 5.1 db, respectively. The combined / DWDM signals were then traversed through either LWP-SMF or NDF link. We measured the power penalties at various transmission distances. The channels were demultiplexed after transmission of up to 120 km, and then sent to a 2.5-Gb/s avalanche photodiode (APD) receiver. On the other hand, the DWDM channels were amplified by using an erbium-doped fiber amplifier (EDFA) after passing through the demultiplexer, and then sent to the 240-km long NDF link (three 80-km long spans). A Gaussian-shaped arrayed waveguide grating (AWG) was used in front of the receiver to separate WDM channels. After transmission, we measured the Q-factor of each DWDM channel by changing the decision threshold of the receiver [7]. 2. Experiment setup Fig. 1 shows the experimental setup. According to ITU-T G.694.2, systems can operate in the wavelength range of nm with 20-nm spacing. However, the most commonly used are 8 wavelengths in the range of nm. In this experiment, we used two uncooled DML transmitters operating at 1470 and 1510 nm for channels. These transmitters were modulated with 2.5-Gb/s NRZ signal (pattern length: ). Their measured output powers and extinction ratios were 0.5 dbm and 9.8 db, respectively. The modulated signals were combined by using a passive multiplexer (insertion loss: 2 db, passband: 16 nm). In the 1550-nm band, however, we used 19 DWDM channels operating at nm with 100-GHz channel spacing. Thus, the passband of multiplexer at 1550 nm was fully utilized with DWDM channels. All the DWDM channels were modulated at 10 Gb/s (pattern length: ). We 3. Results and discussion Fig. 2(a) shows the power penalties for 2.5-Gb/ s channels measured at BER = 10 9 as a function of transmission length. When we used NDF, negative power penalty was obtained even after the transmission over 125 km. This was because the optical pulse, broadened by the positive chirp of DML, was compressed by the negative dispersion of the transmission fiber. On the other hand, when we used LWP-SMF, the power penalty became larger as the transmission length was increased. In the future, systems could be easily upgraded by utilizing directly modulated 10-Gb/s channels. Fig. 2(b) shows the performance of directly modulated channels operating at 10 Gb/s. In this simulation, we used the measured chirp parameter of 10-Gb/s DML shown in Fig. 2(c). The linewidth enhancement factor was measured to be The results show that, when we used LWP-SMF, the power penalty was 6.26 db even for the channel

3 H.S. Chung, Y.C. Chung / Optics Communications 245 (2005) C-band DWDM 2.5 Gb/s DML 10 Gb/s DML 1470 λ 1 λ APDs 40 ~ 120 km km 1530 λ 18 λ ~ nm C-band DWDM Rx (a) 1570 LWP-SMF or NDF λ 1 λ 2 λ λ km NDF EDFA 80 km NDF EDFA Dispersion (ps/nm/km) LWP-SMF NDF 2.5 Gb/ s 10 Gb/ s DWDM (b) Wavelength (nm) Fig. 1. /DWDM overlay network for metro application (, coarse wavelength-division-multiplexing; DWDM, dense wavelength-division-multiplexing; DML, directly modulated laser; APD, avalanche photodiode; LWP-SMF, low water peak-single mode fiber; NDF, negative dispersion fiber; EDFA, erbium doped fiber amplifier; Rx, receiver). (a) Experimental setup. (b) Dispersion characteristics and channel assignment. operating at 1470 nm (where the dispersion is the lowest) after transmission through merely 15 km. On the other hand, when we used NDF, the power penalty was less than 0.28 db after the transmission of 80 km for all the channels except the one operating at the longest wavelength of 1610 nm (power penalty: 2.76 db). These results clearly indicate that NDF has performance superior to LWP-SMF for future metro networks. Fig. 2(d) shows the spectrum and Q-factors for the DWDM channels measured after the transmission over 320 km of NDF. In this experiment, we used a temperature controller for the 10-Gb/s DML for the stabilization of its wavelength and chirp. The Q-factor was measured to be db for the directly modulated channel (channel 11), and db for the externally modulated channels. Thus, the total capacity of these DWDM channels was 190 Gb/ s (19 10 Gb/s). In this experiment, no dispersion compensating fiber module was used. Thus, it should be possible to implement a cost-effective metro network by using NDF and directly modulated WDM lasers.

4 174 H.S. Chung, Y.C. Chung / Optics Communications 245 (2005) Fig. 2. Performance of and DWDM channels (, coarse wavelength-division-multiplexing; LWP-SMF, low water peak-single mode fiber; NDF, negative dispersion fiber). (a) Directly modulated 2.5-Gb/s channels (measured). (b) Directly modulated 10-Gb/s channels (simulation). (c) Measured chirp characteristic of 10-Gb/s DML operating at 1550 nm Gb/s DWDM channels in the C-band after transmission through 320 km of NDF. The spectrum and measured Q-factors are shown together with the transmission passband of the coarse wavelength-division-multiplexer (arbitrary offset). The uncooled DML (used in 2.5-Gb/s channels) and the DML (used in 10-Gb/s channel) had transient dominated chirp characteristics to take advantage of the negative dispersion of NDF. However, it has been reported that the chirp characteristics of DML is very sensitive to temperature [8]. Thus, when the channels were upgraded from 2.5 to 10 Gb/s, the use of uncooled DML could reduce the maximum achievable transmission distance. In addition, to utilize a large number of DMLs for DWDM channels, the use of temperature control would be necessary for the stabilization of their chirps as well as wavelengths. Nevertheless, due to the small negative dispersion, we could still achieve longer transmission distance without dispersion compensation by using NDF instead of LWP-SMF. We have also evaluated the possibility of using directly modulated 40-Gb/s signals in the future metro network using NDF. Fig. 3 shows the measured Q-factors and eye diagrams. The Q-factor was measured by changing the decision threshold of receiver [7]. The experimental setup is shown

5 H.S. Chung, Y.C. Chung / Optics Communications 245 (2005) Fig. 3. Directly modulated 40-Gb/s signal transmission over NDF (EDFA, erbium doped fiber amplifier; Rx, receiver; EA, electroabsorption modulator; NRZ, nonreturn-to-zero; NDF, negative dispersion fiber; DFB-LD, distributed feedback-laser diode). (a) Experimental setup and measured Q-factors. (b) Measured eye diagrams over (i) 40 km of NDF and (ii) and 1 km. in the inset of Fig. 3. The laser was directly modulated with a 40-Gb/s NRZ signal electronically multiplexed from four copies of 10-Gb/s signals. We increased the bias and modulation current of DML to 130 ma and 100 ma p p, respectively. In these conditions, the output power and the extinction ratio were measured to be 10.6 dbm and 3.4 db, respectively. After transmission, we optically demultiplexed the 40-Gb/s signal into 20-Gb/s signals and sent them to the 20-Gb/s receiver for BER measurement. For the short reach application in metro network, the 40-Gb/s optically time-division-multiplexed (OTDM) receiver used in this experiment could be replaced with a small electrically time-division-multiplexed (ETDM) receiver. After 40-km transmission, the Q-factor was measured to be 18.3 db (<BER = ) without any dispersion compensation or spectral filtering. To our knowledge, this represents the longest transmission distance reported for the directly modulated 40-Gb/s signals. The dispersion-length product of 100 ps/nm ( 2.5 ps/nm/km 40 km), obtained in this work, exceeded the dispersion tolerance of ±62.5 ps/nm required for the transmission of 40-Gb/s signal. This was because pulse broadening was suppressed by the interaction between the positive chirp of DML and negative dispersion of NDF. Thus, we conclude that it should be possible to implement a short reach metro-network by using NDF and 40-Gb/s DMLÕs.

6 176 H.S. Chung, Y.C. Chung / Optics Communications 245 (2005) On the other hand, when LWP-SMF was used, directly modulated 40-Gb/s signal could not be transmitted even over 1-km long fiber, as shown in Fig. 3(b). 4. Summary We have demonstrated error-free transmission of directly modulated /DWDM signals operating at 2.5 and 10 Gb/s using NDF (dispersion: nm) without dispersion compensation. We also show that NDF can be used for the transmission of directly modulated 40-Gb/s signals up to 40 km. To our knowledge, this represents the longest transmission distance reported for a directly modulated 40-Gb/s signal. These results indicate that it should be possible to implement a metro core and access network cost-effectively by using NDF and directly modulated lasers. Acknowledgment This work was supported in part by KISTEP. References [1] H. Thiele, L. Nelson, J. Thomas, B. Eichenbaum, L. Spiekman, G. Hoven, Linear optical amplifier for extended reach in transmission systems, in: Tech. Digest of OFC2003, Atlanta, USA, March, Paper MF21, pp , [2] P. Iannone, K. Reichmann, L. Spiekman, In-service upgrade of an amplified 130-km metro transmission system using a single LOA with 140-nm bandwidth, in: Tech. Digest of OFC2003, Atlanta, USA, March, Paper ThQ3, pp , [3] K. Sato, S. Kuwahara, Y. Miyamoto, N. Shimizu, Electron Lett. 38 (July) (2002) 816. [4] H.S. Chung, Y.G. Jang, Y.C. Chung, IEEE Photon. Technol. Lett. 15 (September) (2003) [5] I. Tomkos, R. Hesse, N. Madamopoulos, C. Friedman, N. Antoniades, B. Hallock, R. Vodhanel, A. Boskovic, J. Lightwave Technol. 20 (April) (2002) 562. [6] I. Tomkos, D. Chowdhury, J. Conradi, D. Culverhouse, K. Ennser, C. Giroux, B. Hallock, T. Kennedy, A. Kruse, S. Kummar, N. Lascar, I. Roudas, M. Sharma, R.S. Vodhanel, C. C. Wang, IEEE J. Select. Top. Quant. Electron. 7 (May/ June) (2001) 439. [7] N.S. Bergano, F.W. Kerfoot, C.R. Davidson, IEEE Photon. Technol. Lett. 5 (March) (1993) 304. [8] H.-J. Thiele, R. Yatsu, M. Morshed, M. Funabashi, Metro transmission performance of uncooled 1.55 lm DFB lasers operating up to 85 C, in: Proc. 28th European Conference on Optical Communication, vol. 3, Paper P2.27, 2002.