Abstract

Spectral coherence is an important prerequisite for many applications of femtosecond supercontinua, including precision frequency metrology, attosecond science, and high-field physics. These applications often critically depend on measurement of the carrier-envelope phase via $f{-}{2}f$ interferometry. As mode-locked laser sources cannot directly provide octave-spanning spectra, one typically resorts to spectral broadening in highly nonlinear optical fibers to generate the necessary spectral coverage. This process comes with a caveat, as coherence can be severely degraded in the broadening process. This degradation can be mitigated by suitable choice of a fiber dispersion profile. Here we numerically investigate prototypical fiber designs and analyze their susceptibility to coherence degradation. It is found that the generally favored soliton fission process provides the best broadening efficiency at the expense of rather scarce coherence conditions. Sufficient pulse energy provided, all-normally dispersive fiber designs fare far better in terms of spectral coherence. Finally, fiber designs with flattened normal dispersion provide the best compromise between efficiency and resulting spectral coherence. Our study indicates that the spectral width of a supercontinuum may be deceiving. Even when compressible into a short pulse, this does not automatically qualify for sufficient spectral coherence to conduct carrier-envelope-phase-sensitive experiments. The numerical simulations in this study provide a guideline for coherent spectral broadening. In turn, these guidelines may help to improve carrier-envelope phase stabilization and, in particular, to enable stabilization of laser oscillators that have previously proven difficult to stabilize.

© 2020 Optical Society of America

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