Investigations on the effects of fourwave-mixing in dwdm fiber channels

Optical fiber cables have become ideal for achieving the challenge of reaching high speed data rates and terabit transmission, due to its low attenuation characteristics and high bandwidth capability such as 50 THz. Multiplexing of numerous channels on the same fibre requires higher transmit power or sufficiently lower fibre losses to utilize the available bandwidth having high bit rate with a span of thousands of kilometer.It is required to increase the number of optical channels in dense WDM systems in order to meet the extreme capacity demands on data transmission networks. This necessity of increment in number of channels can be provided only with small channel spacing. In order to reach required amount of signal channels, new frequency standards such as 25 GHz and 12.5 GHz were recently specified by ITU. In such narrower channel spacing with large number of WDM channels, the non-linear effects of the optical fibre can induce serious system impairments. In modern WDM systems, the primary nonlinear effects are cross phase modulation (XPM), and the four-wave mixing (FWM). The FWM characteristics are especially related to frequency allocation of channels. Throughout this thesis, we will be mainly dealing with the FWM impairments.The occurrence of FWM depends on several factors, such as frequency spacing between channels, the input power per channel, the dispersion characteristics of the optical fiber, and the distance along which the channels interact.Four wave mixing results from changes in the refractive index with optical power called optical Kerr effect. In FWM, two co-propagating waves produce two new optical sideband waves at different frequencies. The generation of these beat signals which fall into original signal wavelengths causes a channel energy loss, which induces a crosstalk effect. In long haul transmission links, the deployment of optical amplifiers makes the problem even worse, as not only the transmitted signal is amplified, but also the generated FWM products, which mix again with the signals, causing new products. Also when intense incident signal power launched into a fiber, linearity of optical response is lost. Moreover, in the usage of dispersion-shifted fibers (DSF), the FWM mechanism is enhanced, due to a depletion of the phase mismatch associated to the fiber's chromatic dispersion. In consequence, the detected signal power will fluctuate considerably.Numerous techniques have been proposed to minimize the detrimental limiting effects of FWM such as: the spectral allocation of the channels, the spectral assignment of the channels as far as possible from the zero-dispersion wavelength (?ZD), and the spectral distribution of the unequally spaced channels, which requires a complex system design.In this study, the impact of the channel spacing (positioning of the DWDM channels), phase mismatching, changing of channel input power and fiber length on FWM efficiency were analyzed based on represented algorithm. For various types of fibers such as G.652 (Single- Mode Fiber - SMF), G.653 (Dispersion-Shifted Fiber - DSF), and G.655 (Non-Zero Dispersion-Shifted Fiber - NZDSF) compliant fibers, considering the DWDM grids suggested by the ITU-T Recommendations G.692, and G.694.1, with uniform channel spacing of 100, 50, 25, and 12.5 GHz were compared with simulations. Split Step Fourier Method (SSFM) numerical technique has been used to model nonlinear Schrödinger (NLS) equation in order to investigate pulse propagation in optical fibers.