Abdolvand A, Nazarkin A, Chugreev AV, Kaminski CF, Russell PStJ, "Solitary Pulse Generation by Backward Raman Scattering in H2-Filled Photonic Crystal Fibers", Phys. Rev. Lett. 103, 183902, 2009, doi:10.1103/PhysRevLett.103.183902, |
Using a hydrogen-ﬁlled hollow-core photonic crystal ﬁber as a nonlinear optical gas cell, we study ampliﬁcation of ns-laser pulses by backward rotational Raman scattering. We ﬁnd that the ampliﬁcation process has two characteristic stages. Initially, the pulse energy grows and its duration shortens due to gain saturation at the trailing edge of the pulse. This phase is followed by formation of a symmetric pulse with a duration signiﬁcantly shorter than the phase relaxation time of the Raman transition. Stabilization of the Stokes pulse proﬁle to a solitonlike hyperbolic secant shape occurs as a result of nonlinear ampliﬁcation at its front edge and nonlinear absorption at its trailing edge (caused by energy conversion back to the pump ﬁeld), leading to a reshaped pulse envelope that travels at superluminal velocity.