a952_1.pdf JTuA63.pdf
Optimizing Raman/EDFA hybrid amplifier based on dualorder stimulated Raman scattering of a single pump Zhaohui Li1, Yang Jing Wen2, Changyuan Yu3, Weifeng Rong1, Yixin Wang2 and Tee Hiang Cheng1 1
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, 639798,
[email protected] 2 Lightwave Department, Institute for Infocomm Research (I2R), A-Star, Singapore 3 Department of Electrical & Computer Engineering, National University of Singapore, Singapore, 117576
Abstract: Based on dual-order stimulated-Raman-scattering of single pump laser, hybrid Raman/EDFA is realized by introducing Erbium doped fiber (EDF) within the span. Gain and noise performance can be improved by optimizing the position of EDF. ©2006 Optical Society of America
OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.2410) Fibers, erbium
1. Introduction Compared with the first-order Raman amplification, second-order Raman amplification has been proposed to improve the signal-to-noise ratio (OSNR) in transmission system [1]. However, low pumping efficiency of the second-order Raman scattering and waste of the first-order pump power are still problems for their practical application [1]. Effort has been made to address the issue about low pump efficiency of first-order Raman amplification [2] ~[4]. However, no attention has been paid to low pumping efficiency or pump recycling issue in second-order Raman amplifications till now. In this paper, we propose a Raman/EDFA hybrid amplifier based on dual-order stimulated Raman scattering (SRS) of a single-pump. -10
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WDM Span configuration
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Fig. 1 (a) Experimental setup; (b) Span configurations; (c) Dual-order SRS of 1395nm RFL after 75km SMF+DCF
2. Experimental setup Fig.1 (a) shows the experimental setup for studying the noise and gain characteristics of Raman/EDFA hybrid amplifier based on dual-order SRS of a single pump. The transmission span is composed of 75km single mode fiber (SMF), a segment of EDF with 1000ppm Er3+ and 12.5km dispersion compensation fiber (DCF), which can provide both Raman amplification and corresponding dispersion compensation simultaneously. The Raman pump laser is a depolarized Raman fiber laser (RFL) with maximum output power of 1W at 1395nm. In order to study the effect of the EDF on the hybrid amplifier, EDF is placed at different positions along the span. 20m EDF is chosen according to the optimizing result of noise and gain performance. Four cases of span configurations are illustrated in Fig.1 (b). Configuration I is the conventional amplification span configuration without incorporating EDF. In configuration II, EDF is placed just before the DCF module. In the third configuration, EDF is placed after 50km SMF from the span input end, and in the last configuration, EDF is placed between 25km SMF and 50km SMF plus DCF module. Fig.1 (c) shows the dual-order SRS of a 1395nm RFL generated in the 75km SMF and DCF module. In hybrid amplifiers, the net gain is obtained from the second-order Raman and EDF amplification. 3. Experimental Result and Discussions Fig.2 (a) shows the net gain plotted against the wavelength for four cases of span configurations with 20m EDF. The trends of net gain in four configurations are not different obviously, but net gain values exhibit large variations
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a952_1.pdf JTuA63.pdf
for different configurations. This is because 20m EDF is long enough to generate gain shift effect and thus provide secondary amplification in L-band from 1570nm to 1610nm by recycling the residual first-order SRS. As shown in Fig.2 (a), the net gain in configuration II and III are increased more than 4dB and 10dB respectively compared with that in configuration I. The increase of the net gain in configuration II and III can be explained as the residual firstorder SRS after SMF+DCF is reused to pump EDF in configuration II and III. In addition, EDF amplification exhibits higher pumping efficiency than Raman amplification. Comparing configurations II, III and IV, the net gain obtained under certain pump power is strongly dependent on the position of the EDF. The net gain of configuration II is the highest and is about 5dB higher than that of configuration III, and about 16dB higher than that of configuration IV. When the first-order SRS injected into the EDF is too low and not enough to pump the EDF, the EDF becomes an absorption medium rather than a gain medium. As a result, the net gain in configuration IV is even 7dB lower than that in configuration I. Therefore, there is an optimal location of EDF in achieving maximal net gain.
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Fig.2 (a) Net gain and (b) OSNR versus wavelength under different span configurations with 20m EDF
Fig.2 (b) shows the OSNR against wavelength for four cases of span configuration with 20m EDF. According to recent studies on minimizing the combined effect of noise and nonlinearity, an improved noise performance can be achieved by balancing the gain and loss at every point in the transmission span as analyzed in [5] ~[7]. In a pure backward pumped Raman amplifier, signal gain near the beginning of the transmission span is the lowest and increases toward the end of span. However, when EDF is introduced into the transmission span, the distribution of the signal power along the span will change. The gain of the signal in EDF increases due to the higher pumping efficiency of EDFA compared with that of Raman amplifier. By optimizing the position of EDF, the overall nonuniformity of the signal power distribution along the fiber link can be alleviated and thus the noise performance can be improved. As shown in Fig.2 (b), the best noise performance is obtained in configuration III among the four cases of span configuration. Comparing configuration I with II, OSNR in the latter is about 4dB worse than that in the former. This is because configuration II induces more imbalance of signal power distribution along the span due to larger gain coefficient of the EDF [6]. The worst noise performance is in configuration IV, which is mainly due to too large fiber attenuation to the first-order SRS, leading to too low pump power to obtain gain in EDF. 5. Conclusions Based on dual-order SRS of a single 1395nm RFL, L-band Raman/EDFA hybrid amplifier is realized by incorporating a piece of EDF. Experimental results indicate that both gain and noise performance can be improved with 20m EDF placed in an optimal position along the span. References [1] J. C. Bouteiller, K. Brar, J. Bromage, S. Radic, and C. Headley, “Dual-order Raman pump,” IEEE Photon. Technol. Lett., 15, 212-214, (2003) [2] J. W. Nicholson, “Dispersion compensating Raman amplifiers with pump reflectors for increased efficiency,” J. Lightwave Technol., 21, 1758-1762, (2003) [3] T. Amano, K. Okamoto, T. Tsuzki, M. Kakui, and M. Shigematsu, “Hybrid dispersion compensating Raman amplifier module employing highly nonlinear fiber,” OFC, WB3, 2003 [4] J. H. Lee, Y. M. Chang, Y.-G. Han, S. H. Kim, H. C. and S. B. Lee, “Dispersion-Compensating Raman/EDFA hybrid amplifier recycling residual Raman pump for efficiency enhancement”, IEEE Photon. Technol. Lett., 17, 43-45, 2005 [5] A. Kobyakov, M. Vasilyev, S. Tsuda, G. Giudice, and S. Ten, “Analytical model for Raman noise figure in dispersion-managed fibers,” IEEE Photon. Technol. Lett., 15, 30-32, 2003 [6] R. Hainberger, T. Hoshida, T. Terahara, and H. Onaka, “Comparision of span configurations of Raman-amplified dispersion-managed fibers,” IEEE Photon. Technol. Lett., 14, 471-473, 2002 [7] C. –L. Zhao, Z. Li, X. Yang, C. Lu, W. Jin and M. S. Demokan, “Effect of a nonlinear photonic crystal fiber on the noise characterization of a distributed Raman amplifier,” IEEE Photon. Technol. Lett., 17, 561-563, 2005
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