Sub-femtosecond pulses in mode-locked 2π-solitons of the cascade stimulated Raman scattering

The shortest to date optical pulse using the pulse compression technique was achieved as 6 fs. Sub-femtosecond pulses (SFP) were attained with a Fourier synthesizer of separate lasers synchronized by phase-locking. The author proposes a new approach based on cascade Raman stimulated scattering (CSRS) whose components are mode-locked within 2π soliton reminiscent to the self-induced transparency solitons. The author shows that Raman active materials can support solitons consisting of pump laser wave with the frequency ω_{L and many cascade-excited Stokes and anti-Stokes component with their frequencies ωj=ωL+jω0 mode-locked to each other through a fast “full-swing” 2π-nutation of population at the Raman transition with the frequency ω0≪ωL. Similarly to “bright-bright” 2π-solitons in CSRS with two and three components, these solitons have a new, very simple, Lorentzian intensity profile. The multiple mode-locked components, however, give rise to the train of ultra-short pulses (separated by the interval with their length being of the same order of magnitude or even shorter than the pump cycle). The major feature of the proposed effect is that all the frequency components of the new soliton are synchronized bright solitons (in contrast to well known bright+dark soliton combination in SRS) propagating with the same group velocity}