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Soviet Journal of Quantum Electronics

Approximate theory of multipass amplification and its application in analytic description of spectral characteristics of amplified radiation

Nikolai N Il'ichev, A V Larikov, A A Malyutin

Published Nov 30, 1991• 1 citation
Publisher

Abstract

An approximate theory of a multipass amplifier with an arbitrary degree of overlap of the amplified beams is used to consider the case of pulses with a low degree of temporal coherence. An analytic description is proposed for the energy and spectral characteristics of the radiation amplified under different conditions. It is shown that the relationships obtained can be used to describe the emission spectrum of an oscillator with instantaneous Q switching.

DOI
10.1070/qe1991v021n11abeh004520

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An approximate theory of a multipass amplifier with an arbitrary degree of overlap of the amplified beams is used to consider the case of pulses with a low degree of temporal coherence. An analytic description is proposed for the energy and spectral characteristics of the radiation amplified under different conditions. It is shown that the relationships obtained can be used to describe the emission spectrum of an oscillator with instantaneous Q switching.
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The interaction between coherent subpicosecond ultraviolet radiation and an XeCl molecule was modeled numerically. It was established that a two-level model of the spectrum of a molecule having stable upper and rapidly decaying lower levels can be applied to study the amplification of 0.1–0.7 ps pulses. A numerical analysis was made of the amplification of 0.2–0.5 ps pulses and the influence of coherence effects was assessed. The conditions of validity of the Frantz–Nodvik equation to describe the energy characteristics of the amplified pulses were analyzed.
An analysis is made of the interaction of radiation with a two-level medium at a high particle density. An explicit allowance is made in the interaction Hamiltonian for the interaction between dipoles induced by a resonance field. A semiclassical description is used to obtain a system of equations describing the propagation of a pulse in the active medium. The equations contain nonlinear terms allowing for the interaction in question. An approximate solution of the equations is obtained for the case of amplification of a weak noncoherent pulse describing pulse compression in the amplifying medium. The theory is compared with experimental results.
Coherent amplification of short-pulse XeCl lasers is studied theoretically by using multilevel Maxwell–Bloch equations in which the vibrational–rotational structures of a XeCl gain spectrum are included. The model used considers 100 transitions each in the P and R branches for six different vibrational transitions of XeCl(B, υ = 0) → XeCl(X, υ′ = 0–5). Coherence components between sublevels in the B and X states are also properly calculated. The model can successfully predict coherent effects such as a quantum beat caused by the spectrum overlap of the several vibrational–rotational transitions involved in a short-pulse laser spectrum. During amplification, laser pulses experience some nonlinear effects caused by the complex gain spectrum structure and by the coherent interactions; thus a considerable change in the laser pulse shape and a substantial reduction in the duration of the amplified laser pulse are predicted. The Frantz–Nodvik equation in the rate-equation limit and even single-level Maxwell–Bloch equations are not applicable for short-pulse propagation analyses in inhomogeneously broadened gain media.