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Soviet Journal of Quantum Electronics

Formation of regions of efficient compression of ultrashort pulses in a passive switch

M. I. Demchuk, I. A. Manichev, V. P. Mikhailov

Published Jan 31, 1991• 0 citations
Publisher

Abstract

A system of partial differential rate equations for a population inversion in a medium and for the intensity of radiation propagating this medium is used in a numerical analysis of a single trip of a Gaussian pulse through a two-level two-component saturable absorber. Relationships governing the formation of regions of efficient compression of ultrashort pulses are identified and the possibility of controlling the maximum degree of reduction of pulse duration is considered. Two-component absorbers selected with the proposed recommendations can ensure the maximum efficiency of pulse compression in a trip when the input intensity is fixed and the input durations can vary within a wide range. The last circumstance makes such absorbers very promising as passive switches for solid-state lasers with passive mode locking and stabilization of the intensity of ultrashort pulses in order to minimize the duration of the output radiation.

DOI
10.1070/qe1991v021n01abeh003709

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A system of partial differential rate equations for a population inversion in a medium and for the intensity of radiation propagating this medium is used in a numerical analysis of a single trip of a Gaussian pulse through a two-level two-component saturable absorber. Relationships governing the formation of regions of efficient compression of ultrashort pulses are identified and the possibility of controlling the maximum degree of reduction of pulse duration is considered. Two-component absorbers selected with the proposed recommendations can ensure the maximum efficiency of pulse compression in a trip when the input intensity is fixed and the input durations can vary within a wide range. The last circumstance makes such absorbers very promising as passive switches for solid-state lasers with passive mode locking and stabilization of the intensity of ultrashort pulses in order to minimize the duration of the output radiation.
The generation of solitons is considered in the model of a two-component dense resonance medium having the amplifying and absorbing components when the both components or one of them (amplifying) can exhibit near dipole—dipole interactions depending on the concentration of resonance atoms and dipole transition moments. The features of formation of phase-modulated π- and 2π-pulses are studied. The stability of the obtained solutions to small perturbations is analysed. It is shown that the 2π-pulse is stable, whereas the π-pulse is unstable.
Two-photon absorption of a continuous train of ultrashort pulses in a gas gives rise to resonances when the sum of the frequencies of two locked modes coincides with the transition frequency in the gas. It is shown that the width of these resonances is not governed by the pulse duration but by the width of the twophoton transition. Resonance intensities are of the same order of magnitude as in the single-frequency regime at the same power level. It is worth noting an important feature of the use of ultrashort pulses in studies of two-photon resonances due to the 1S-2S transition in the hydrogen atom: there is a strong increase in the efficiency of conversion of dye laser radiation to the ultraviolet range.
The coupled Maxwell-Schrodinger equations are used to describe propagation of a pulse in a nonlinear absorbing medium. The absorber is assumed to be comprised of a homogeneously broadened system of two-level atoms that are exactly resonant with the field. Numerical calculations have been performed for incident pulses of hyperbolic-secant shape and of duration that is long compared to the atomic phase-memory time. Selected values of input intensity are considered. For intensities below a certain level the calculations nearly agree with rate-equation predictions, but as the incident intensity is increased we find that optical nutation comes into play. The nutation effect leads to pulse breakup as the packet penetrates into the medium. An analytical calculation, for the case of an applied optical field of step-function form, has been performed in order to provide additional insight about the intensity threshold for nutation.
Self-formation of one or several soliton-like pulses from a nonsoliton powerful laser pulse during its propagation through a two-component resonant amplifying—absorbing medium with an energy dissipation channel is studied numerically. Both components of the medium are simulated by two-level particles. Homogeneous mixtures of these components and media with consecutive amplifying and absorbing layers are considered. A qualitative analysis of the results is based on a simplified model of the process of soliton-like pulse propagation.
A generalized formulation is presented for determining the saturation thresholds for optical processes excited by ultrafast pulses based on the pulse area of the excitation pulse. It is demonstrated that the threshold of driving-pulse intensity for absorption and fluorescence saturation in a two-level system is inversely proportional to the square of the duration of the excitation pulse. These results are obtained from both a simplified analytical solution assuming a Gaussian excitation pulse shape and a detailed numerical calculation based on density-matrix equations. The calculation is generalized further to obtain the saturation condition for a two-photon Raman process by defining a two-photon pulse area both analytically and numerically. These results not only provide predictive capabilities for determining thresholds of signal saturation using ultrashort durations with arbitrary pulse shapes and durations but also open up possibilities for predetermining the threshold intensities of various resonant nonlinear processes.
In a laser with a saturable absorber (LSA), two spatially separated cells are placed in the laser cavity as shown in Figure 8.1. The roles of the two cells are quite different: one of them is pumped so that the atoms have a positive population inversion (active or amplifying medium); the other one is left with a negative population inversion (passive or absorbing medium). As these two media are in general different, they saturate at different power levels. The most interesting case corresponds to the situation where the absorber saturates more easily than the active medium, introducing nonlinear losses inside the cavity. This new nonlinearity is responsible for two phenomena. An LSA may exhibit optical bistability, i.e. two distinct stable steady states may coexist for a range of values of a parameter. It may also produce pulsating intensity oscillations which have been called “passive Q-switching” (PQS) in contrast to “active Q-switching” experiments such as the “gain switching” experiments discussed in Section 1.3.2. The interest in LSAs varied very much over time, with peaks in the late 1960s for their large intensity pulses, in the mid 1980s for their chaotic outputs, and in the late 1990s for the design of compact microlasers. Historically, physicists trying to explain the irregular intensity pulses delivered by the ruby laser suspected the possible destabilizing role of a saturable absorber. Shimoda proposed that the nonlinear losses generated by a saturable absorber could explain this phenomenon.