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IEEE Journal of Quantum Electronics

Dynamic Gain Profiling and Lateral Saturation Effects in Centrally Tuned Inhomogeneously Broadened Lasers Operating Under Unstable Conditions

Belkacem Meziane

Published Oct 15, 2007• 1 citation
Publisher

Abstract

This paper aims at demonstrating that the dynamic gain contour of an inhomogeneously broadened laser tuned to line center undergoes a lateral saturation effect in its unstable regime of operation. Such an effect is shown to be contained in the numerical solutions of the integro-differential Maxwell-Bloch equations that describe the system. After delimiting the domain of applicability of the usual small-sideband approach in describing dynamic gain profiling, we apply a strong harmonic-expansion method to these same equations to obtain an analytical expression that shows an excellent fit with the results of the numerical simulations.

DOI
10.1109/jqe.2007.905290

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IEEE Journal of Quantum Electronics•Apr 1, 1966•2 citations
This paper aims at demonstrating that the dynamic gain contour of an inhomogeneously broadened laser tuned to line center undergoes a lateral saturation effect in its unstable regime of operation. Such an effect is shown to be contained in the numerical solutions of the integro-differential Maxwell-Bloch equations that describe the system. After delimiting the domain of applicability of the usual small-sideband approach in describing dynamic gain profiling, we apply a strong harmonic-expansion method to these same equations to obtain an analytical expression that shows an excellent fit with the results of the numerical simulations.
Summary form only given. Despite the large number of reports that were devoted to the investigation of the dynamic properties of single-mode inhomogeneously broadened (SMIB) lasers, some of their operative characteristics still demand further investigation. In connection with physical understanding of laser instabilities, we demonstrate that the sideband approach is insufficient to explain the distortion that occurs in the (initially) Gaussian profile when the output field undergoes large departures from stable operation. The report focuses on both numerical and analytical handling of the integro-differential 'Maxwell-Bloch' equations. An original approach, presented for the first time, consists in tracking the gain contour when a self-pulsing regime sets in. We demonstrate the occurrence of a lateral hole-burning effect that has never been revealed before. These findings lend themselves to the recognition of the fact that the unstable state initiates lateral saturation (an effect that has always been ignored in the treatment of SMIB laser dynamics). We show that lateral hole-burning cannot be described with the usual small-sideband analysis. In addition, we prove that, to first order, the role of population pulsations is masked by a DC component, distinct from the steady-state value, and responsible for the gain distortion. Leaning on obvious hints related to the occurrence of an amplitude field-switching around a zero mean-value, we derive analytical expressions that give a remarkably good account of the dynamic profiles, obtained likewise by numerical integration of the integro-differential equations.
We consider the dynamics of an inhomogeneously broadened unidirectional ring laser formulated in the traditional Maxwell-Bloch configuration of a two-level atom with transverse modes. We numerically analyze the model for a single-longitudinal mode in the paraxial approximation, using the uniform field limit with cylindrical symmetry and a Gaussian profile for inhomogeneous broadening. Several domains of solutions are found for the resonant case in the good cavity limit. The laser threshold is found to be dependent on the mode spacing as well as on the gain and scaled Doppler broadening. Hysteresis between stable and unstable solutions is obtained in realistic operating conditions for both increasing (decreasing) gain and increasing (decreasing) mode spacing (i.e., for changes in the initial conditions).
We study numerically the effects of local-field corrections originating from near dipole—dipole interactions on the dynamics of a single mode, inhomogeneously broadened laser. Our analysis is based on a set of generalized Maxwell–Bloch equations in which the inhomogeneous nature of gain broadening is accounted for by introducing two new dynamical variables and a single parameter that governs the extent of inhomogeneous broadening. Our results show that local-field effects occurring in a dense gain medium reduce the range of continuous wave operation and lead to instabilities and chaos at much lower pumping levels.
This paper gives a full account of the dynamical properties of single-mode inhomogeneously broadened lasers. The study divides itself into two parts. First, the integro-differential ``Maxwell-Bloch'' equations, which describe such systems, are numerically solved for a large scan of values of the various control parameters that govern the dynamical evolution of self-pulsing lasers. The instability hierarchies, delimited with respect to the excitation parameter, reveal that, along with the various decay rates of the interacting variables, the chosen number of velocity components, which divides the polarization integral into discrete intervals, also plays an important role in the dynamical evolution of the system, especially in the self-pulsing regime of operation. The second part of this paper concerns another approach to modeling single-mode inhomogeneously broadened laser dynamics that largely circumvents the infinite-dimensional complexity of the medium's polarization. A hierarchy of low-dimensional models, with increasing accuracy with respect to the infinite-dimensional ``Maxwell-Bloch'' equations, is constructed and adapted to describe correctly most of the properties of single-mode inhomogeneously broadened lasers.
Using Lamb's model, we analyze the effects of gain saturation by strong traveling-wave fields in dilute laser media. Using approximate solutions of an integral equation for the population inversion density (PID), the index of refraction and incremental gain are studied for arbitrarily strong fields. Effects of atomic motion are included for a Maxwellian velocity distribution, but pressure effects are neglected. The case of a monochromatic field leads to the saturation results of Gordon, White, and Rigden, which are studied as a function of frequency. For a small ratio of natural to Doppler linewidths, there is a transition from inhomogeneous to homogeneous broadening for sufficiently strong fields. An effect of particular interest is the generation of waves at $2{\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$, $2{\ensuremath{\omega}}_{2}\ensuremath{-}{\ensuremath{\omega}}_{1}$, and higher order sidebands by two strong input signals at ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$. The source of the parametric gain at these frequencies is the time-dependent gain saturation due to the presence of multiple strong fields. For ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$ small compared to the decay rates ${\ensuremath{\gamma}}_{a}$ and ${\ensuremath{\gamma}}_{b}$ of the laser levels, the gain at these intermodulation sidebands is computed as a function of the field strengths. The limiting cases of homogeneous and inhomogeneous (due to atomic motion) broadening are studied in detail. These two cases give essentially the same results, for a given unsaturated gain. Numerical results indicate that a first-order side-band intensity at least 10% as large as that of the inducing fields can be easily observed in practice. The integral equation for the PID is converted into an infinite set of linear algebraic equations for a typical solid-state laser. The conditions under which this set of equations can be limited to a finite number is discussed, and the dependence of the sideband gain on ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$ is calculated. For large ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$, the gain at the mthm\mathrm{th}mth sideband decreases as ${({\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2})}^{\ensuremath{-}m}$.
Using Lamb's model, we analyze the effects of gain saturation by strong traveling-wave fields in dilute laser media. Using approximate solutions of an integral equation for the population inversion density (PID), the index of refraction and incremental gain are studied for arbitrarily strong fields. Effects of atomic motion are included for a Maxwellian velocity distribution, but pressure effects are neglected. The case of a monochromatic field leads to the saturation results of Gordon, White, and Rigden, which are studied as a function of frequency. For a small ratio of natural to Doppler linewidths, there is a transition from inhomogeneous to homogeneous broadening for sufficiently strong fields. An effect of particular interest is the generation of waves at $2{\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$, $2{\ensuremath{\omega}}_{2}\ensuremath{-}{\ensuremath{\omega}}_{1}$, and higher order sidebands by two strong input signals at ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$. The source of the parametric gain at these frequencies is the time-dependent gain saturation due to the presence of multiple strong fields. For ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$ small compared to the decay rates ${\ensuremath{\gamma}}_{a}$ and ${\ensuremath{\gamma}}_{b}$ of the laser levels, the gain at these intermodulation sidebands is computed as a function of the field strengths. The limiting cases of homogeneous and inhomogeneous (due to atomic motion) broadening are studied in detail. These two cases give essentially the same results, for a given unsaturated gain. Numerical results indicate that a first-order side-band intensity at least 10% as large as that of the inducing fields can be easily observed in practice. The integral equation for the PID is converted into an infinite set of linear algebraic equations for a typical solid-state laser. The conditions under which this set of equations can be limited to a finite number is discussed, and the dependence of the sideband gain on ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$ is calculated. For large ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$, the gain at the mthm\mathrm{th}mth sideband decreases as ${({\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2})}^{\ensuremath{-}m}$.