Generation of Optical Frequency Combs in Fibres

We numerically investigated the possibility of generating high-quality ultra-short optical pulses with broad frequency-combs spectra in a system consisting of three optical fibres. In this system, the first fibre is a conventional single-mode fibre, the second one is erbium-doped, and the last one is a low-dispersion fibre. The system is pumped with a modulated sine-wave generated by two equally intense lasers with the wavelengths lambda(1) and lambda(2) such that their central wavelength is at lambda(c) = (lambda(1) + lambda(2))/2 = 1531 nm. The modelling was performed using the generalised nonlinear Schrodinger equation which includes the Kerr and Raman effects, as well as the higher-order dispersion and gain. We took a close look at the pulse evolution in the first two stages and studied the pulse behaviour depending on the group-velocity dispersion and the nonlinear parameter of first fibre, as well as the initial laser frequency separation. For these parameters, the optimum lengths of fibre 1 and 2 were found that provide low-noise pulses. To characterise the pulse energy content, we introduced a figure of merit that was dependent on the group-velocity dispersion, the nonlinearity of fibre 1, and the laser separation.