302 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 4, APRIL 2001 Eliminating SRS Channel Depletion in Massive WDM Systems via Optical Filtering Techniques Christopher M. McIntosh, Alexandra G. Grandpierre, Demetrios N. Christodoulides, Jean Toulouse, and Jean-Marc P. Delavaux Abstract—We demonstrate that the channel depletion due to stimulated Raman scattering in massive wavelength-division-mul- tiplexed (WDM) systems can be eliminated using high-frequency pass filters. These filters, when inserted appropriately into the transmission link, can effectively suppress the SRS power flow from the WDM channels to lower frequency noise. Numerical simulations carried out on WDM systems indicate that the channel depletion penalties can be kept below 0.25 dB even for a total channel power of 2 W. Index Terms—Optical fibers, stimulated Raman scattering, wavelength division multiplexing. I. INTRODUCTION W AVELENGTH-DIVISION-MULTIPLEXED (WDM) technology is expected to play an ever increasing role in future high-bandwidth networks. To date, transmission systems with more than 100 channels at aggregate rates greater than 1 Tb/s have been successfully demonstrated in laboratory experiments [1], [2]. It has been long recognized, however, that stimulated Raman scattering (SRS) is one of the major non- linear optical processes that can impair the performance of such fiber systems [3]–[6]. The impacts of SRS are particularly acute in the case of massive WDM technologies, where the large number of channels involved leads to high optical power levels in the fiber. In such systems, SRS power penalties can occur in the following two ways: 1) it can limit the maximum allowable launched power, since noise components around the Raman gain peak tend to be amplified at the expense of the WDM channels, which is referred to as channel depletion; 2) at the same time, SRS is known to induce power exchange between channels, which is defined as SRS crosstalk. In principle, SRS crosstalk can be eliminated via spectral inversion techniques [7]–[9]. On the other hand, no methods have yet been suggested to combat channel depletion. Moreover, unlike single channel systems for which the channel depletion due to noise can be theoretically determined [5], [10], [11], calculation of channel depletion in massive WDM systems is a considerably more involved task, due to the presence of multiple “pumps.” Manuscript received July 21, 2000; revised January 11, 2001. C. M. McIntosh and J. Toulouse are with the Department of Physics, Lehigh University, Bethlehem, PA 18015 USA (e-mail: christopher@lehigh.edu). A. G. Grandpierre and D. N. Christodoulides are with the Department of Elec- trical Engineering and Computer Science, Lehigh University, Bethlehem, PA 18015 USA. J.-M. P. Delavaux is with Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 USA. Publisher Item Identifier S 1041-1135(01)03116-0. Fig. 1. Raman gain coefficient of silica glass, where its peak magnitude is scaled to the 1500 nm wavelength. In this letter, we propose a method to combat channel de- pletion effects in a massive WDM fiber transmission system involving the periodic insertion of high-frequency pass filters (HPFs) into the fiber network. These filters can effectively sup- press the SRS power flow from the WDM channels to lower fre- quency noise components. Numerical simulations carried out on WDM systems indicate that the channel depletion penalties can be kept below 0.25 dB even for a total channel power of 2 W. II. THEORETICAL MODEL Let us consider channels co-propagating in a single-mode optical fiber. In addition, we assume that these channels are accompanied by background noise. As a result of SRS, power flows from higher frequency components to lower fre- quency ones. This power transfer includes channel-to-channel, channel-to-noise and noise-to-noise interactions. In general, these SRS interactions can be described by [5] (1) In the above equation, refers to the intensity of beam and is the Raman gain coefficient. The Raman gain profile of a typical silica glass fiber is shown in Fig. 1. In writing (1), we assume . The first sum on the right-hand side of (1) represents the gain that component receives from all components at higher frequencies, whereas the second sum is the loss to light components at lower frequencies. The third term describes attenuation along the optical fiber. The factor of appearing in (1) accounts for polarization randomization effects, while the frequency ratio describes vibrational losses. 1041–1135/01$10.00 © 2001 IEEE