Generalized equation for inverse population in a laser active medium

Jia-Ming Liu

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.

Andrei L. Mikaelyan, M.L. Ter-Mikaelyan

Iliya V. Doronin, A. A. Zyablovsky, E. S. Andrianov +2 more

We propose an approach to obtain lasing without inversion, which does not require preparation of the quantum superposition of states in the active medium. We demonstrate that with periodically variable pumping, lasing is achievable even in the case when the population inversion of the active medium is always negative. This is possible due to the fact that laser in strong-coupling regime is a non-Hermitian system with an exceptional point existing at negative value of population inversion. We show that due to nonorthogonality of eigenstates near the exceptional point, the time variation of pump power can result in parametric instability in the laser and excitation of the electromagnetic field self-oscillations. These oscillations take place even when the population inversion of the active medium is always negative. Our results help to pave the way for the creation of a new class of lasers operating at negative value of population inversion.

Nicolai G. Basov, Rafael V Ambartsumyan, V. S. Zuev +2 more

Vl. V. Kocharovsky, Igor' V Koryukin, E. R. Kocharovskaya +3 more

The recent experiments on a pulsed superfluorescence and prospects of a CW superradiant lasing in various active media, especially semiconductor heterostructures, [1] attract much attention to new coherent dynamical effects in such lasers. These effects take place in the case of very dense active medium and low-Q cavities when a photon (cavity) lifetime is much shorter than a polarization (optical dipole) lifetime of an active center. Here, on the basis of numerical modeling of the Maxwell-Bloch equations, we describe the steady and dynamic spontaneous symmetry breaking of the structure of the field, polarization and population inversion of an active medium with almost homogeneously broadened spectral line placed into a symmetric low-Q combined distributed feedback (DFB) Fabry-Perot cavity.

O. A. Kocharovskaia, Paul Mandel, Ia. I. Khanin