Sub-Femtosecond Pulses in 2π-Solitons of the Cascade Stimulated Raman Scattering

The shortest to date optical pulse length of 6 fs [1] was achieved by using pulse compression technique. Further shortening could be attained by using a Fourier synthesizer [2] of separate lasers synchronized by nonlinear phase-locking [3]. Here, we propose a new approach [4] based on multi-frequency cascade Raman stimulated scattering (CSRS) whose components are mode-locked within 2π soliton reminiscent to the self-induced transparency solitons [5], We show that Raman active materials can support solitons consisting of pump laser wave with the frequency ω L and many cascade-excited Stocks and anti-Stocks component with their frequencies ω j = ω L + jω0, j =±1, +2, +3…, mode-locked to each other through a fast "full­swing" 2π-nutation of population at the Raman transition with the frequency ω0 ≪ ω L . Similarly to "bright-bright" 2π-solitons in CSRS with two [6] and three [7] components, these solitons have a new, very simple, Lorentzian intensity profile. Due to the engagement of many mode-locked components, however, their total EM field in the time domain consists of the train of ultra-short pulses (separated by the interval 2π/ω0) with their length being of the same order of magnitude or even shorter than the pump cycle, 2π/ω L . The major feature of the proposed effect is that all the frequency components of the new soliton are so called bright solitons (in contrast to the well known bright+dark soliton combination in SRS [8]) locked to each other and propagating with the same group velocity. The high-order CSRS required to observe the proposed effect, was first observed experimentally in [9] and later in many other experiments, with the total number of components up to ~10-15. The lock-in of all these components into "all-bright-SRS" 2π soliton, however, has never been observed in experiment; its feasibility and resulting effects are discussed here.