28 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 1, JANUARY 2001 Compensation of Raman-Induced Crosstalk Using a Lumped Germanosilicate Fiber Raman Amplifier in the 1.571–1.591- m Region H. S. Seo, K. Oh, Member, IEEE, W. Shin, U. C. Ryu, and U. C. Paek, Senior Member, IEEE Abstract—A new method to equalize power imbalance caused by Raman-induced crosstalk among optical channels is proposed using a lumped germanosilicate fiber Raman amplifier. Evolution of optical channels through the Raman amplifier was simulated using Raman frequency modeling, which theoretically predicted si- multaneous amplification and power equalization. Experimentally, a gain band with negative slope in the range of 1.571–1.591 m was achieved in a lumped Raman amplifier pumped by a broad-band laser diode centered at 1.467 m. We demonstrated compensation of the Raman-induced crosstalk of 5 dB accumulated along 330 km of conventional single-mode fiber. Index Terms—Channels, equalization, optical fiber, Raman am- plifier, wavelength division multiplexing. I. INTRODUCTION R ECENTLY, wide-band amplifiers have been intensively researched to increase the capacity of wavelength-divi- sion-multiplexing (WDM) optical communication systems. The gain band of erbium-doped fiber amplifiers (EDFA) has been ex- tended into L-band, 1570–1610 nm, with inherent gain flatness [1]. Raman amplifiers also have been of recent research focus due to their capability to synthesize a gain spectrum with mul- tiple pump sources. GeO -doped silica fiber Raman amplifiers especially have shown significant potential with a signal gain over 30 dB and flat bandwidth over 120 nm [2], [3]. Even though optical amplifiers show flat spectral responses, WDM optical channels could accumulate power imbalance over several deci- bels as they propagate through conventional single-mode fibers (SMFs) by Raman-induced crosstalk among channels [4]. To prevent this nonlinear penalty, equalization methods based on external filters [5] and the spectral inversion [6] have been pro- posed. In this letter, we report a new method to compensate the WDM channel power imbalance caused by Raman crosstalk, using a GeO -doped silica fiber Raman amplifier whose gain band has a negative slope in the 1.570–1.591 m region. Simul- taneous equalization and amplification through a single lumped Raman amplifier without external filter is demonstrated both theoretically and experimentally, for the first time, to the best knowledge of the authors. Manuscript received August 14, 2000. This work was supported in part by UFON, an ERC program sponsored by KOSEF, by the BK21 program, and by MOE in Korea. The authors are with the Department of Information and Communications, Kwangju Institute of Science and Technology, Puk-gu Kwangju, 500-712, South Korea (e-mail: koh@kjist.ac.kr). Publisher Item Identifier S 1041-1135(01)00512-2. Fig. 1. Schematic diagram of channel equalization and amplification using a lumped Raman amplifier with a negative gain slope. Fig. 2. Raman gain distribution of germanium-doped fiber pumped at 1.472 m. n is defined by , where and are core and cladding refractive index, respectively. Here, n is assumed to be from the germanium contribution only. II. THEORETICAL ANALYSIS The principal idea of this study is schematically shown in Fig. 1. WDM channels suffer from Raman-induced crosstalk as they propagate along conventional SMF to result in the power transfer from the shorter wavelength channels to the longer wavelengths. An amplifier with a proper negative gain slope could achieve channel power equalization as well as amplification. Fig. 2 shows the Raman gain spectrum for germanium-doped silica at a given pump wavelength, 1.472 m, the wavelength of pump laser diodes used in the experiments. Note that in conventional SMF with typical n , Raman gain shows over all positive slope and negative gain slope region is only confined to the longer wavelength boundary. As germanium concentration increases, however, the peak shifts toward 440 cm to result in a gain band with a negative slope in the range of 440 cm to 505 cm . This 1041–1135/01$10.00 © 2001 IEEE