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The generation of picosecond pulse trains has become of great interest for many scientific applications. However, even though different techniques of nonlinear compression have been developed for optical fibers in the anomalous dispersion regime, only a few exist for normally dispersive fibers. Here, we describe a new method based on the generation of a strong nonlinear focusing effect induced by the cross phase modulation of a high power 40-GHz beat-signal on its orthogonally polarized interleaved weak replica. More precisely, while the normally dispersive defocusing regime induced a nonlinear reshaping of a high power 40-GHz sinusoidal signal into successively parabolic then broad and sharp square pulses, it also progressively close a singularity at its null point characterized by steeper and steeper edges. Here we show that the cross phase modulation induced by this nonlinear dark structure on a weak interleaved orthogonally polarized replica then turns out the normally dispersive regime into a focusing dynamics. This phenomenon is similar to the polarization domain wall effect for which the energy of a domain is strongly localized and bounded by the commutation of both orthogonally polarized waves. In other words, since a particle in a gradually collapsing potential, the energy contained in the weak interleaved component is found to be more and more bounded and is thus forced to temporally compress along the fiber length, thus reshaping the initial beat-signal into a train of well-separated short pulses.
We have experimentally validated the present method by demonstrating the temporal compression of an initial 40-GHz beat-signal into a train of well separated pulses in different types of normally dispersive fibers. To this aim, an initial 40-GHz beat-signal is first split into 2 replica for which one is half-period delayed and 10-dB attenuated before polarization multiplexing in such a way to generate a strongly-unbalanced orthogonally-polarized interleaved signal. The resulting signal is then amplified and injected into the fiber under-test. In first fibers of 1 and 2 km (D = -15 ps.km-1.nm-1, γ = 2.3 W-1.km-1, α = 0.2 dB.km-1), we have observed the nonlinear focusing of the initial 40-GHz sinusoidal signal input into a train of 5.5-ps pulses. By decreasing the dispersion coefficient down to D = -2.5 ps.km-1.nm-1 in such a way to exacerbate the nonlinear defocusing effect of the strongest component far beyond the wave breaking, we have successfully compressed the orthogonally polarized 40-GHz beat-signal into well-separated 2.5-ps pulses after 5 km of propagation for a total input power of 28 dBm. We then studied the effect of total power on the compression ratio, and showed that compression is more efficient with higher total power, even after the wave breaking phenomenon. We followed by showing that the power ratio between the two polarization axes is closely linked to the compression factor, as the higher the power difference between the two axes, the better compression. Finally, our experimental results are in excellent agreement with our numerical predictions.
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