Flat Supercontinuum Generation With High-Order Soliton Pre-Compression

Supercontinuum (SC) generation in fibers offers an effective solution for nonlinear wavelength conversion, significantly broadening the laser spectrum with high spatial coherence [1]. In many applications, for example, in spectrum calibrations, a flat SC is required. Traditional methods for generating femtosecond SC rely on soliton dynamics within nonlinear fibers. These solitons undergo fission and emit dispersive waves, which contribute to spectral broadening. However, this approach often results in SC spectra that lack smoothness, with spectral gaps between the dispersive waves and the pump wavelength. Alternatives, such as noise-like pulse pumping and the use of nonlinear fibers doped with special materials, have been explored to improve spectral flatness [2]. In this work, we experimentally and theoretically demonstrate a flat SC spectrum using higher-order soliton pre-compression. Our results show a spectrum flatness of 9 dB within the wavelength range of 960 nm to 1550 nm without the residual wavelength component of the pump laser. Using a custom-built <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f-to-2 f$</tex> interferometer, this flat SC spectrum enable the detection of the carrier-envelope offset frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(f_{\text{CEO}}\right)$</tex> signal with a signal-to-noise ratio (SNR) exceeding 35 dB, confirming the high coherence of the SC generation.