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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Short pulse generation due to coherent population trapping

Roman L. Kolesov, Olga A. Kocharovskaya

Published May 30, 2002• 0 citations
Publisher

Abstract

An efficient broadband optical continua generation due to stimulated Raman scattering originated from relaxation processes from the excited electronic states is predicted. It is shown that, even when the populations of Raman levels are equal, Raman coherence can be efficiently excited due to coherent population trapping process (CPT). Scattering of the electromagnetic field due to the excited coherence gives rise to a comb of equally spaced spectral components of optical radiation. The generated spectrum can be compressed into a sequence of ultrashort pulses. Estimates for realistic parameters of the medium show that it is possible to produce a sequence of femtosecond and even subfemtosecond pulses with repetition rate several GHz.

DOI
10.1117/12.468985

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An efficient broadband optical continua generation due to stimulated Raman scattering originated from relaxation processes from the excited electronic states is predicted. It is shown that, even when the populations of Raman levels are equal, Raman coherence can be efficiently excited due to coherent population trapping process (CPT). Scattering of the electromagnetic field due to the excited coherence gives rise to a comb of equally spaced spectral components of optical radiation. The generated spectrum can be compressed into a sequence of ultrashort pulses. Estimates for realistic parameters of the medium show that it is possible to produce a sequence of femtosecond and even subfemtosecond pulses with repetition rate several GHz.
It is shown that broadband optical continua may be generated as light propagates through a coherently prepared medium. Stimulated Raman scattering is a parametric process responsible for the spectral broadening. Even though the generation of spectral components is limited by the dispersion of the medium, numerical estimates for realistic parameters show feasibility of generating coherent optical continua with total bandwidth of several hundred terahertz without implementation of any dispersion compensating technique. Finally, a technique for producing broadband coherent optical spectra and, thus, ultrashort pulses of radiation is proposed.
Subfemtosecond (sub-fs) pulses can be generated by cascade-stimulated Raman scattering in a Raman medium with modulated Raman excitations, driven by two sufficiently intense laser beams, one of which is amplitude modulated. The nonadiabatic Raman interaction establishes a strong modulated Raman coherence, which supports compression of the generated broadband Raman sidebands to a train of sub-fs pulses regardless of whether the carrier frequencies of the driving lasers are tuned above, below or on two-photon Raman resonance.
We show that utmost possible Raman coherence between equally populated levels can be excited by two-photon resonant bichromatic field far from one photon resonance by means of relaxation processes. On this basis we suggest a mechanism for generation of extremely short pulses with high pulse duty factor and large energy in the pulse. It can be realized in solid materials, such as the rare-earth doped dielectrics with long-lived spin coherence or nitrogen-vacancy $(N\ensuremath{-}V)$ centers in diamond and provide a simple, reliable, and convenient technique for many practical applications.
An analysis is made of parametric excitation of anti-Stokes components of stimulated Raman scattering in an optical resonator filled with a medium whose refractive index is dispersion-free. The case of steady generation of the first Stokes component is considered. Expressions are obtained for the amplitudes of steady-state fluctuations of the anti-Stokes components in the case of a weak reaction of these components on the pump radiation and on the first Stokes component. Various cases of frequency dependences of the intrinsic resonator losses are considered. Conditions are found under which a large number of anti-Stokes components with similar intensities is excited so that the anti-Stokes radiation leaving the resonator is a periodic sequence of ultrashort (femtosecond) pulses with one pulse per period.
Subfemtosecond pulses can be generated in a Raman generator by using electromagnetically induced transparency if ultrabroad Raman sidebands are generated and then propagate in such a way that the generated sidebands at higher frequencies are temporally advanced with regard to those at lower frequencies. However, it was recently demonstrated by experiments [1] that above-resonance Raman excitations ensured the establishment of anti-phased Raman coherence for pulse compression but only very limited number of Raman sidebands could be generated due to Raman self-defocusing [2], while broadband sidebands could be generated under below-resonance Raman excitations, which supported no pulse compression in the same Raman media. In order to establish anti-phased Raman coherence and simultaneously overcome Raman self-defocusing to benefit efficient generation of broadband sidebands, we suggest in this report a new technique to generate sub-fs pulse train in a Raman generator driven by three sufficiently intense single-mode laser beams whose carrier frequencies are tuned off the Raman resonance with two different two-photon detunings. The Raman interaction establishes a molecular modulation through nonadibatic processes. The simulation on the basis of the fundamental rotational Raman transitions in H2 shows that sub-fs pulse can be generated by above-resonance or below-resonance Raman excitations, or even Raman excitations with opposite detunings, as is shown in Fig.1. This opens a new way for experiments to observe enhanced generation of broadband Raman sidebands, together with compression of the phase-locked Raman sidebands into sub-fs pulse trains in the same Raman media