Abstract

The shortest to date optical pulse length of 6 fs using pulse compression technique was achieved in [1]. Sub-femtosecond pulses were attained by using a Fourier synthesizer [2] of separate lasers synchronized by nonlinear phase-locking [3]. Here, we propose a new approach based on multi-frequency cascade Raman stimulated scattering (CSRS) whose components are mode-locked within 2π soliton reminiscent to the self-induced transparency solitons [4]. 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 "full-swing" 2π-nutation of population at the Raman transition with the frequency ω₀ ≪ ω_L. Similarly to "bright-bright" 2π-solitons in CSRS with two [5] and three [6] 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 the so called bright solitons (in contrast to well known bright+dark soliton combination in SRS [7]) locked to each other and propagating with the same group velocity. The high-order CSRS required to observe the proposed effect, has first been observed experimentally in [8] 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 has never been observed in experiment; its feasibility is discussed here.

© 1994 Optical Society of America