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College Physics

Analysis on width of burning hole in inhomogeneously broadened laser medium

Geguo Du

Published Jan 1, 2005• 1 citation
OpenAlex

Abstract

A detailed calculation about width of the burning hole in saturation behavior of inhomogeneously broadened laser medium is presented.The width of the burning hole is determined by the linewidth of homogeneous broadening because of the finite lifetime of the excited state due to spontaneous emission.

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We have studied microscopic effects leading to mode competition in an inhomogeneously broadened medium of a gas laser with a weakly anisotropic cavity. The polarization bistability observed in some lasers, which is believed to be caused by hole burning, we attribute to microscopic coherent effects induced by two light fields. Due to these effects one field can be coupled with another. Hole burning is found to be important, but not crucial. The exact solution of the equation of motion for field amplitudes is compared with the third order approximation of perturbation theory. The role of four wave mixing, important for bistability of axial modes in a homogeneously broadened medium, is discussed.

Journal of Modern Optics•Mar 1, 2001•0 citations
The instabilities in homogeneously broadened lasers, which we described in the previous chapter, have a paradigmatic importance because they arise in models which allow one to perform the linear-stability analysis in a simple analytic form. The treatment of the corresponding instabilities in the case of in homogeneously broadened lasers is substantially more complex in general. However, it is very interesting because, as we shall see, the instabilities arise in parametric domains that are much more accessible from the experimental viewpoint. A related issue is that of the possibility of multimodal emission in a laser. It is common knowledge that in order to attain multimode emission it is necessary to have a sort of in homogeneity in the gain medium, because in a completely homogeneous configuration all modes interact with all atoms, and the arising competition among modes implies that only one of them survives. There are basically two main kinds of in homogeneity, the first is of spatial nature and the other is of spectral nature. The first kind is realized in Fabry–Perot lasers, because different cavity modes interact with atoms at different spatial positions in the cavity thanks to the standing-wave configuration of the field and to the consequent phenomenon of spatial hole- burning (see Section 14.2 and Fig. 14.1). This circumstance is capable of weakening the mode–mode competition and allowing multimode operation. In unidirectional ring lasers, on the other hand, there is an “inhomogeneous” configuration in the case of inhomogeneous broadening, because different groups of atoms have different transition frequencies and therefore interact with different modes thanks to the spectral hole-burning (see Section 15.2 and Fig. 15.3). Again, this weakens the mode–mode competition and allows multimode operation. However, these are qualitative considerations, and the only reliable and quantitative method to determine the possibility of multimodal operation is the linear-stability analysis of the single-mode stationary solutions.
A theory is presented for the effect of cross relaxation (CR) on the saturation with driving-signal intensity of the steady-state gain of a laser amplifier with an inhomogeneously broadened linewidth. The condition of steady-state gain results when the amplifier excitation level W and the driving-signal intensity are constant for about 3τ or longer, where τ is the fluorescence lifetime of the upper laser level. The driving signal is derived from an oscillator operated in the normal mode and which does not saturate the amplifier inversion density over times short compared with τ. We assume that CR, at a rate F, restores to thermal equilibrium any distortion of the spectral inversion density caused by the driving signal. Formulas are derived for steady-state gain saturation with arbitrary F for two cases: the spectral width of the driving signal is (1) narrow and (2) broad relative to the ionic or homogeneous line. The ionic linewidth Δvi is assumed to be much less than the inhomogeneous linewidth Δva and refers to the minimum frequency interval for which infinitely fast CR is operative. The theory also yields the CR coefficients f=(Δvi/Δva) [F/(τ−1+W)] for the narrow-band case and f*=(Δvs/Δva) [F/(τ−1+W)] for the broadband case, where Δvs is the driving-signal bandwidth. These coefficients significantly parametrize the CR effect: for f, f*>1, hole-burning is not severe, whereas for f, f*<1, considerable hole-burning takes place and the gain saturates faster. Finally, the theory is applied to experimental results obtained for Nd3+:glass broadband and narrow-band amplification; an approximate lower bound of 4×105 sec−1 is determined for F.
High inhomogeneous broadening of impurity spectra in amorphous media reflects one type of dispersion, characteristic for disordered systems: dispersion of electronic transition frequency. In principle, there can exist a pronounced dispersion of other parameters: homogeneous line width (Γ), Debay-Waller factor (α), hole burning efficiency (ϕ), etc. The problem of the burning efficiency dispersion (BED) was extensively discussed in the recent years ([1-6] and references therein). The nonexponentiality of the burning kinetics was treated in some publications [1-3,6] as an evidence of such dispersion. But burning kinetics is always nonexponential in strongly inhomogeneously broadened systems, if a burning laser line width Δνl is much less then Γ. That was found experimentally and treated theoretically in one of the first publications on a hole burning [7] (see, also [8]). All other sources of the nonexponentiality are, in fact, additional to this one. So, it’s not easy to attain experimentally the BED parameters on such background. The procedure of a numerical fit ting of the burning kinetic with BED, developed in [4,5], is correct, but it looks unreliable due to many fitting parameters.
One-step (0 to 1) spectral hole burning (SHB) is widely used as a method of eliminating inhomogeneous broadening of spectra: the minimal width of the hole in the inhomogeneous distribution function (IDF) of the transition frequency Omega 01 is obtained by SHB with monochromatic light and is determined by the homogeneous linewidth sigma 0 = gamma 1 + Gamma (where gamma 1 and Gamma are the constants of the energetic and the phase relaxation of the (first) excited level respectively). Two-step (0 to 1 to 2) SHB is considered where the selective excitation of the first step is fixed by a photochemical transformation through the second excitation step. If the selective pulse is a coherent and single-sided exponential then, in the case of an extremely short fixing pulse ( delta pulse), on the increase of the time delay T between the selective and fixing pulses, a monotonic narrowing of the spectral hole in the IDF of the frequency Omega 01 takes place up to the limit width Gamma + mod gamma 1 - Delta mod (where Delta is the spectral width of the selective pulse).
We have studied microscopic effects leading to mode competition in an inhomogeneously broadened medium of a gas laser with a weakly anisotropic cavity. The polarization bistability observed in some lasers, which is believed to be caused by hole burning, we attribute to microscopic coherent effects induced by two light fields. Due to these effects one field can be coupled with another. Hole burning is found to be important, but not crucial. The exact solution of the equation of motion for field amplitudes is compared with the third order approximation of perturbation theory. The role of four wave mixing, important for bistability of axial modes in a homogeneously broadened medium, is discussed.