Influence of Phase Coherence on Seeded Supercontinuum Generation

The noise properties of supercontinuum (SC) generation have attracted a lot of attention due to a large application demand for low noise SC sources and in the more fundamental context of clarifying links with instabilities in other systems. In typical commercial SC sources the pulse break-up is initiated by noise-driven modulational instability (MI).

The resulting large spectral shot-to-shot fluctuations can be significantly reduced by modulating the pump with a phase coherent seed [1-2], which leads to a coherent pulse break-up through the amplification of a cascade of four-wave mixing (FWM) side-bands. We demonstrate that the noise properties of the generated SC are highly sensitive to the degree of phase noise of the seed and that a nearly coherent seed pulse is needed to achieve a coherent pulse break-up and low noise SC [3].

This limits the mechanisms that can be used to generate the seed, which is important in designing next-generation low-noise SC sources. We performed numerical simulations in which the phase noise of the seed was modelled by a physically justified phase-diffusion model. The cross-spectral density (CSD) relative to the pump, i.e. the coherence of the seed relative to the pump, is shown in Fig. 1(a) for varying seed noise linewidths; the seed becomes increasingly incoherent with the pump when the linewidth is increased.

Simulation results including ensemble calculated coherence for unseeded and seeded SC generation with varying seed noise linewidths are shown in Fig. 1(b). In the absence of a seed the spectral broadening is initiated by noise-induced MI, where a single set of MI sidebands evolves into solitons and dispersive waves (DWs) with low spectral coherence.

By introducing a coherent seed near the pump, the spectral broadening is initiated by the coherent amplification of a cascaded FWM comb, leading to a high spectral coherence over most of the spectral bandwidth. When the seed noise linewidth is increased, the broadening is still initiated by a FWM cascade, but the contrast of the comb is gradually washed out.

This leads to a significant reduction of the coherence. For a seed noise linewidth in the GHz range, the noise properties are only marginally better than for an unseeded SC. A closer inspection reveals that the fringe contrast of the FWM comb decreases with increasing seed noise linewidth. This causes an increasingly incoherent amplification of the comb, where only the FWM sidebands closest to the pump are coherently amplified, and results in a significant coherence degradation of the resulting SC.

Our results thus show that there is a stringent requirement on the linewidth of the seed laser for seeding to be effective in reducing the noise of an SC source. Further results show that the specific maximum allowable linewidth decreases with increasing pump power, which means that the noise of high-power SC sources will be difficult to control by seeding.