In this paper, we investigate performance improvements in 10 Gbit/s dispersion .... [1] B. Wedding, B. Franz, and B. Junginger, "10-Gb/s optical transmission up to ...
Performance Improvement of Dispersion Supported Transmission at 10 Gbit/s by Partial Chirp Compensation in a Travelling Wave Amplifier Mário M. Freire Department of Mathematics and Computer Science, University of Beira Interior Rua Marquês d'Ávila e Bolama, P-6200 Covilhã, Portugal Henrique J. A. da Silva Department of Electrical Engineering, Pole II of University of Coimbra Pinhal de Marrocos, P-3030 Coimbra, Portugal
Abstract In this paper, we investigate performance improvements in 10 Gbit/s dispersion supported transmission (DST) systems by partial chirp compensation due to self phase modulation in a travelling wave semiconductor optical amplifier (TWA) used as a booster amplifier. For an unsaturated gain of 20 dB, the system performance is improved by 1.14 dB at 204 km, and by 4.13 dB at 253 km SMF. The improvement at 253 km allows a further increase in the maximum transmission distance. I. Introduction The maximum span length of high bit rate optical transmission systems operating in the 1550 nm window can be severely limited by chromatic dispersion if installed fibre routes of standard singlemode fibre (SMF) are used. Using the method of dispersion supported transmission (DST) at 10 Gbit/s link lengths up to 253 km SMF have been reported in laboratory experiments [1], and up to 240 km SMF in field experiments using the fibre cable of the CPRM (Companhia Portuguesa Radio Marconi) network between Sesimbra and Lisbon, Portugal [2]. The next step is the increase of the bit rate to 40 Gbit/s, since such transmission capacity will be needed by the end of the century [3]. However, in accordance with the principle of dispersion supported transmission [1] the dispersion limited link length is reduced by a factor of four for a doubling of the bit rate, and in a DST experiment at the intermediate bit rate of 20 Gbit/s the link length was reduced to 53 km [4]. Since then, several approaches have been proposed to increase the link length: four-level DST [5], DST with duobinary coding [6], and WDM DST [7]-[10]. In [11], it was shown that the chirp reduction in a TWA improves, for long link lengths, the performance of optical transmission systems using directly modulated lasers. Since DST systems use directly modulated lasers, in this paper, we investigate performance improvements in 10 Gbit/s DST systems by partial chirp compensation due to self phase modulation in a travelling wave semiconductor optical amplifier (TWA) used as a booster amplifier. II. Modelling and Simulation Methodology The block diagram of the simulated 10 Gbit/s DST system is shown in Fig. 1. The pseudopattern generator (PPG) provides a maximal-length pseudorandom binary sequence (PRBS) with 27-1 bits at 10 Gbit/s. The optical transmitter includes a laser driver, and a multiple-quantum well (MQW) laser. The dynamic response of the MQW laser has been described by a rate equation model, which takes into account carrier transport effects. This
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model has been presented in [8]. Erbium doped fibre amplifiers (EDFAs) have been considered as linear and we assume they are used in the configurations of in-line and preamplifiers as in DST experiments reported in [1]. An equivalent noise bandwidth of 1 THz and a noise factor of 6 dB have been considered for the optical preamplifier. A TWA is assumed to be used as a booster amplifier, instead of an EDFA, in order to investigate the influence of partial chirp compensation in a TWA on the performance of DST systems. The travelling wave amplifier has been modelled as in [12]-[13], with an unsaturated gain of 20 dB. The complex envelope of the optical field at the output of the TWA has been modelled as in [11]. The standard singlemode fibre (SMF) was modelled using the lowpass transfer function with first order dispersion of 17 ps/(nm.km) at 1532 nm. A PIN photodiode, with a 3-dB cut-off frequency of 9.35 GHz, is assumed to be used to convert the optical signal into an electrical signal. The receiver main amplifier (AMP) and the lowpass filter (LPF) have been jointly modelled as a lowpass RC filter with the 3-dB bandwidth required by the DST method. Optical transmitter DRIVER MQW
TWA
Optical receiver SMF
EDFA
SMF
EDFA
SMF
EDFA
PIN PD
AMP LPF
PPG
2 7-1 bits
1532 nm
Fig. 1. Block diagram of the simulated 10 Gbit/s DST system. For performance evaluation, a pure semi-analytical method has been used, which combines noiseless signal transmission simulation with noise analysis. Employing the Gaussian approximation, the average error probability was estimated as in [9]-[10]. The variance of the noise voltage at the input of the decision circuit includes the contribution of the variances of signal-ASE and ASE-ASE (ASE is the amplified spontaneous emission) beat noise voltages, the variances of shot and thermal noise voltages, and the variance of the voltage due to the laser noise after the receiver filter. The later term has been included since it was shown [14] that the contribution due to frequency-to-intensity conversion of laser phase noise, after propagation via dispersive fibres, is responsible for the BER floor observed in DST experiments for distances around 253 km SMF [1]. III. Performance Assessment For each fibre length, the system parameters, namely the bias current, the modulation current, and the receiver cut-off frequency have been adjusted in order to minimise the EDFA preamplifier input mean optical power for an average error probability (BER) of 10-9 (receiver sensitivity). Fig. 2 shows the eye-diagrams at 0 km, using an EDFA and a TWA as a booster amplifier in a DST system operated at 10 Gbit/s. The gain saturation in the TWA causes a small pulse distortion appearing in the eye diagram as an overshoot. However, no sensitivity degradation is found for an unsaturated gain of 20 dB. Fig. 3(a) shows the average error probability after 204 and 253 km SMF assuming that an EDFA or a TWA is used. As can be seen, the use of a TWA as a booster amplifier improves the system performance by 1.14 dB at 204 km, and by 4.13 dB at 253 km SMF for a BER of 10-9. The improvement at 253 km allows a further increase in the maximum transmission distance, which is limited by the error floor due to frequency-to-intensity conversion of laser phase noise. As shown in Fig. 3(b) the BER at 253 km SMF obtained with an EDFA is close to
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the BER at 260 km obtained with a TWA. Higher gains of the booster amplifier may improve the system performance. This situation is under study for 40 Gbit/s DST systems.
(a) (b) Fig. 1. Eye diagrams at 0 km using (a) an EDFA as a booster amplifier; (b) a TWA as a booster amplifier. -1
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253 km (EDFA) 260 km (TWA)
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log (BER) 10
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log (BER) 10
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253 km (EDFA) 253 km (TWA) 204 km (EDFA) 204 km (TWA)
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Fig. 3. Average error probability (BER) versus mean optical power using an EDFA or a TWA as a booster amplifier. (a) after 204 and 253 km SMF; (b) after 253 and 260 km SMF. IV. Conclusions Performance improvements in dispersion supported transmission (DST) systems operated at 10 Gbit/s are investigated by partial chirp compensation due to self phase modulation in a travelling wave semiconductor optical amplifier (TWA) used as a booster amplifier. For an unsaturated gain of 20 dB, the system performance is improved by 1.14 dB at 204 km, and by 4.13 dB at 253 km SMF for a BER of 10-9. References [1] B. Wedding, B. Franz, and B. Junginger, "10-Gb/s optical transmission up to 253 km via standard single-mode fiber using the method of dispersion supported transmission", IEEE J. Lightwave Tech., Vol. 12, No.10, pp. 1720-1727, 1994.
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[2] B. Franz, W. Pöhlmann, B. Wedding, and A. Jaime. Ramos, "Field Experiments at 10 Gbit/s Over 80, 160 and 240 km of Standard Singlemode Fibre Installed Between Sesimbra and Lisbon Using Dispersion Supported Transmission Technique", Electron. Lett., Vol. 31, No. 21, pp. 1860-1961, 1995 [3] M. A. Newhouse, L. J. Button, D. Q. Chowdhury, Y. Liu, and V. L. da Silva, “Optical Amplifiers and Fibers for Multiwavelength Systems”, in Proc. LEOS’95, San Francisco, E.U.A., October 30 - November 2, 1995, Vol. 2, paper OC 5.1. [4] B. Wedding, K. Köffers, B. Franz, D. Mathoorasing, C. Kazmierski, P. Pereira Monteiro, and J. Nuno Matos, "Dispersion-supported transmission at 20 Gbit/s over 53 km standard singlemode fibre", Electron. Lett., Vol. 31, No. 7, pp. 566-568, 1995 [5] B. Wedding, W. Pöhlmann, B. Franz, H. Geupel, “Multi-level Dispersion Supported Transmission at 20 Gbit/s Over 46 km Installed Standard Singlemode Fibre”, Proc. ECOC’96, Oslo, Norway, 1996, paper MoB4.4. [6] João Correia, and Adolfo Cartaxo, “Duobinary Coding for Dispersion Supported Transmission at 20 Gbit/s”, I Conferência Nacional de Telecomunicações, April 10-11, 1997, Aveiro, Portugal, pp. 241-244. [7] Mário M. Freire, Álvaro M. F. de Carvalho and Henrique J. A. da Silva, "Performance Implications of Three-Mirror Fabry-Perot Demultiplexers for 10-Gb/s WDM DispersionSupported Transmission with 0.5 nm Channel Spacing", IEEE Photon. Technol. Lett. , Vol. 8, No. 9, pp. 1261-1263, 1996. [8] Mário M. Freire, and Henrique J. A. da Silva, "Performance Assessment of High Density Wavelength Division Multiplexing Systems with Dispersion Supported Transmission at 10 Gbit/s", Proc. ISCC'97, Alexandria, Egypt, July 1-3, 1997, pp. 315-319. [9] Mário M. Freire and Henrique J. A. da Silva, “Performance Improvement of 10-Gb/s FourChannel WDM Dispersion-Supported Transmission by Using Multi-Layer Thin Film Interference Filters as Demultiplexers”, Proc. NOC’97 edited by D. W. Faulkner, A. L. Harmer, Vol. III “Photonic Networks, Optical Technology and Infrastructure”, pp. 135-140, IOS Press, 1997. [10] Mário M. Freire and H. J. A. da Silva, "Performance Improvement of 40-Gb/s Capacity Four-Channel WDM Dispersion-Supported Transmission by Using Broadened Passband Arrayed-Waveguide Grating Demultiplexers", Proc. CLEO/Pacific Rim'97, Chiba, Japan, July 14-18, 1997, paper TuD4. [11] C. R. Medeiros, J. J. O’Reilly, “Chirp Compensation Capability of a Semiconductor Laser Amplifier”, Electron. Lett., Vol. 27, No. 8, pp 649-650, 1991. [12] Adel A. M. Saleh, Isam M. I. Habbab, “Effects of Semiconductor-Optical-Amplifier Nonlinearity on the Performance of High-Speed Intensity-Modulation Lightwave Systems”, IEEE Trans. Commun., Vol. 38, No.6, pp. 839-846, 1990. [13] C. Tai, and W. I. Way, “Dynamic Range and Switching Speed Limitations of an NxN Optical Packet Switch Based on Low-Gain Semiconductor Optical Amplifiers”, IEEE J. Lightwave Tech. , Vol. 14, No.4, pp. 525-533, 1996. [14] R. F. S. Ribeiro, J. R. F. da Rocha, and A. V. T. Cartaxo, “Influence of Laser Phase Noise on Dispersive Optical Fiber Communication Systems”, IEEE Photon. Technol. Lett., Vol. 7, No. 12, pp. 1510-1512, 1995.
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