Analysis of Raman amplification in DWDM communication systems (original) (raw)

Fiber Raman Amplifier has become one of the key technologies in long-haul and ultra long-haul fiber optic transmission systems. Distributed Raman amplifier offers improvement in noise figure and at the same time, reduction in nonlinear penalties allowing longer amplifier spans, higher traffic capacity (higher bit rates and close channel spacing) as well as operation near zero-dispersion wavelength. Meanwhile, discrete Raman amplifier helps to increase the capacity of fiber optic networks by opening up new windows for Wavelength Division Multiplexing (WDM), such as Sband lumped Raman amplifier. The application of Raman amplifier in Dense Wavelength Division Multiplexing (DWDM) introduces some challenges such as poor gain uniformity within the wide transmission bandwidth, poor pumping efficiency and also additional noise and crosstalk generated due to some nonlinear effects. We address these issues and propose solutions to optimize the use of fiber Raman amplifier in DWDM communication. We start with briefly discussing some fundamental properties of fiber Raman amplifier, including the origin of Raman amplifier, the gain and noise properties. The amplification process can be explained by both classical and quantum theory of the stimulated Raman scattering. In addition, we also briefly discuss the polarization dependence of Raman amplifier. We present two possible methods to flatten Raman gain profile. First, we introduce the use of optical filter based on fiber loop mirror. High birefringence fiber loop mirror using Sagnac interferometer has some advantages such as its resistance to environmental changes, strong noise rejection ability and more importantly, input polarization independent. We present the detail characterization of this filter and-ii ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library experimentally demonstrate the use of this particular filter to flatten Raman gain for signal wavelengths within C-band. The filter is located at the end of the distributed Raman amplifier using single mode fiber of 70 km length, generating average 15 dB On-Off Raman gains. We show that by employing two sections of high birefringence fiber loop mirror, we obtain about 38 nm flat gain with gain ripple ±0.5 dB. We also report the application of multi-wavelength Raman pump to generate flat Raman gain. We present a new scheme called Alternating Time-Division Multiplexed Raman pump scheme. This method involves the use of multi pump sources with different wavelengths, which are modulated and temporally separated from each other in order to improve gain and noise properties due to avoidance of pump-to-pump interaction effect. We present the necessary operating condition of this new scheme to generate flat Raman gain for signals with C-band wavelength range. By using 100 km Non-Zero Dispersion Shifted Fiber (NZDF), we obtain flat gain with gain ripple ±0.5 dB and amplifier noise figure-5 dB. We also propose the use of Gaussian pump pulses as a better alternative solution in this TDM Raman pump scheme. Finally, we report a new method to describe the Raman scattering between channels in DWDM based on Volterra Series. Basically the optical signal in fiber transmission can be represented by the Volterra series containing first order kernel, third order kernel, and fifth or even higher order kernels for the case of high nonlinear system. As part of the nonlinear effects, Raman scattering can also be described by the Volterra Kernels. It was proven that the accuracy of this series for optical signal transmission in single mode fiber is good, compared to the existing split-step Fourier method. Therefore, we propose to use this series to identify the Raman kernels' component within the DWDM iii-ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library transmission. The model derived can become a method to measure the interference between DWDM channels due to the stimulated Raman scattering, i.e. Raman crosstalk. We investigate the identification of Raman crosstalk between two channels for On-Off Keying Non-Return to Zero (OOK-NRZ) input modulation format. We use the principal of mean square error algorithm to identify the Raman kernels numerically. By correlating input and output signals, we obtain the third order Volterra kernels due to Raman scattering. Our results show good agreement with the one resulting from the split step Fourier method.