  The precise observation of the angle-frequency spectrum of light filaments in
water reveals a scenario incompatible with current models of conical emission
(CE). Its description in terms of linear X-wave modes leads us to understand
filamentation dynamics requiring a phase- and group-matched, Kerr-driven
four-wave-mixing process that involves two highly localized pumps and two
X-waves. CE and temporal splitting arise naturally as two manifestations of
this process.

Optics