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Soviet Journal of Quantum Electronics

Estimate of the gain of an active medium from the superradiance decay kinetics

Aleksander A. Makovetskii

Published May 31, 1980• 0 citations
Publisher

Abstract

An analysis is made of a one-dimensional problem involving superradiance of four-level active media. The rate equations for the populations and the transport equations for the superradiance fluxes are used to derive differential equations for the description of the behavior of the integrated population and the observed superradiance flux in the active medium excited by the pump flux. A detailed analysis is made of the superradiance decay. Analytic dependences are obtained to describe the decay of the normalized superradiance flux. The gain of the active medium at the onset of decay occurs as the parameter in these dependences and they are used as the basis of a technique for obtaining upper and lower estimates of this parameter.

DOI
10.1070/qe1980v010n05abeh010153

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An analysis is made of a one-dimensional problem involving superradiance of four-level active media. The rate equations for the populations and the transport equations for the superradiance fluxes are used to derive differential equations for the description of the behavior of the integrated population and the observed superradiance flux in the active medium excited by the pump flux. A detailed analysis is made of the superradiance decay. Analytic dependences are obtained to describe the decay of the normalized superradiance flux. The gain of the active medium at the onset of decay occurs as the parameter in these dependences and they are used as the basis of a technique for obtaining upper and lower estimates of this parameter.
An analysis is made of the problem of kinetics of the superradiance decay in the case of nonlinear quenching of excited centers in four-level active media. A method is proposed for estimating the initial axial gain and of the stimulated emission cross sections of active media: it is based on a simultaneous analysis of the normalized superradiant flux and integrated population of the active medium. The proposed method is not sensitive to nonlinear quenching. A qualitative analysis is made of the decay of excitation in neodymium pentaphosphate crystals after pulsed excitation with laser radiation of the 0.538 μ wavelength and stimulated emission cross sections are estimated for the 4 F 3/2 → 4 I 11/2 transition in Nd 3+ resulting in lasing at 1.051 μ and at some other wavelengths of this transition.
Previous numerical work on the spatiotemporal distributions of the atomic polarization and of the difference in levels population of a one-dimensional system of two-level atoms initially inverted strongly suggested that the dynamics of the slab atomic ensemble in the linear regime of superradiance may best be treated by an eigenmode analysis. This method proved effective in obtaining both the time dependence of the atomic polarization and the spectral distribution of the emitted radiation from the slab in a number of physically interesting problems in the linear regime. In the present paper, I use the eigenfunction expansion technique to reduce the Maxwell-Bloch system of partial differential equations into a system of coupled first-order ordinary differential equations whose solutions clearly detail the contributions of the different modes to the system dynamics in the nonlinear regime of superradiant emission. I compute also the delay in time of the peak of the superradiant burst and its magnitude as functions of the ratio of the longitudinal decay rate to the transverse decay rate of the system.
We conduct a theoretical study to understand the periodic superradiance observed in an Er:YSO crystal. First, we construct a model based on the Maxwell-Bloch equations for a reduced level system, a pair of superradiance states and a population reservoir state. Analysis of the eigenvalues of the linearized differential equations shows that periodic superradiance can be realized only for certain parameters. We also derive two-variable equations consisting of the coherence and population difference between the two superradiance states, which contain the essential feature of the periodic superradiance. The two-variable equations clarify a mathematical structure of this periodic phenomenon and give analytical forms of the period, pulse duration, and number of emitted photons. Our model successfully reproduces the periodic behavior, but the actual experimental parameters are found to be outside the parameter region for the periodic superradiance. This result implies that some other mechanism(s) is required. As one example, assuming that the field decay rate varies with the electric field, the periodic superradiance can be reproduced even under the actual experimental condition.
A one-dimensional radiative transfer model is used in a study of the amplification of spontaneous radiation as a result of a population-inverted ionic transition pumped by recombination. The problem of such transfer is solved analytically in the steady-state case and it is shown that the intensities of the radiation emitted in both directions are the same irrespective of the distribution of the unsaturated gain over the length of the active medium. Numerical calculations are reported of the stage of establishment of light fluxes. It is shown that during this transient stage a nonlinear interaction between opposite light fluxes is responsible for nonmonotonic behavior of the output radiation intensity. The problem of amplification in a traveling pump wave is treated analytically.
POPULATION RATE EQUATIONS From the discussion in the preceding chapter, it is clear that population inversion is the basic condition for an optical gain. For any system in its normal state in thermal equilibrium, a low-energy level is always more populated than a high-energy level, hence there is no population inversion. Population inversion in a system can only be accomplished through a process called pumping by actively exciting the atoms in a low-energy level to a high-energy level. If left alone, the atoms in a system relax to thermal equilibrium. Therefore, population inversion is a nonequilibrium state that cannot be sustained without active pumping. To keep a constant optical gain, continuous pumping is required to maintain population inversion. This condition is clearly consistent with the law of conservation of energy: amplification of an optical wave leads to an increase in optical energy, which is possible only if the required energy is supplied by a source. Pumping is the process that supplies energy to the gain medium for the amplification of an optical wave. There are many different pumping techniques, including optical excitation, electric current injection, electric discharge, chemical reaction, and excitation with particle beams. The use of a specific pumping technique depends on the properties of the gain medium being pumped. The lasers and optical amplifiers of particular interest in photonic systems are made of either dielectric solid-state media doped with active ions, such as Nd:YAG and Er:glass fiber, or direct-gap semiconductors, such as GaAs and InP. For a dielectric gain medium, the most commonly used pumping technique is optical pumping using either an incoherent light source, such as a flashlamp or a light-emitting diode, or a coherent light source from another laser. A semiconductor gain medium can also be optically pumped, but it is usually pumped by electric current injection. In this section, we consider the general conditions for pumping to achieve population inversion. Detailed pumping mechanisms and physical setups are not addressed here because they depend on the specific gain medium used in a particular application. The net rate of increase of the population density in a given energy level is described by a rate equation. As we shall see below, pumping for population inversion in any practical gain medium always requires the participation of more than two energy levels.