Effect of PMD on the continuous spectrum of nonlinear optical fibre

Abstract

Nonlinear Fourier transform (NFT) can be used as a tool to overcome nonlinearity in optical fibre communication systems [1-5]. Signalling on the continuous spectrum (CS), defined by NFT, can be performed as well as on solitons. The feasibility of this technique is demonstrated by simulation and experiment [2-5]. Amplified spontaneous emission (ASE) noise is considered as the main perturbation that limits the capacity of the CS channel [1-5]. However, it is important to include other optical fibre impairments such as polarization mode dispersion (PMD) as well. Coupled nonlinear Schrodinger equations (CNLSE) should be considered to model the effect of random birefringence on the propagation of light in the fibre. CNLSE can be simplified to scalar NLSE if the input optical field is initialized in a single polarization state and PMD is negligible [6]. However, PMD is usually not negligible in long-haul fibre links. Hence, in this paper, we investigate the effect of PMD, as a perturbation, on the continuous spectrum in systems designed based on NFT in single-polarization NLSE [1-5]. In NFT-based systems, data is mapped on the CS, and the time domain signal obtained from the inverse NFT is launched into one of the orthogonal polarizations of the fibre. Taking into account the CLNSE, almost half of the energy will be coupled into the other polarization at the end of the fibre. Therefore, some kind of PMD compensation is needed. Since CS represents the quasi-linear regime, it is expected that even a linear PMD compensation would be effective.