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Applied Optics

Ē-Level Population of Ruby vs Pumping

V. Daneu, C. A. Sacchi, O. Svelto

Published Jul 1, 1965• 14 citations
Publisher

Abstract

The Ē-level population of a cylindrical ruby rod is experimentally measured as a function of the pump energy at two points of the cross section of the rod, namely, the center and the periphery. The measurement is performed by measuring the fluorescent energy emitted after pumping by the points considered. The rod is pumped by a spiral flashtube. It is found that the experimental data of population vs pumping follow exponential curves as expected theoretically. From these curves the pump energy necessary for equalizing the populations between ground state and laser level is obtained in a straightforward way. The knowledge of this quantity leads to the measure of the losses (scattering plus diffraction) per pass in the rod. A value of ∼45% of losses is obtained for our rod.

DOI
10.1364/ao.4.000863

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In this study, we take the pump rate into consideration for the first time to give a theoretical description of radiation trapping in three-level systems. We numerically verify that under strong pumping, the population of the ground state is depleted, which leads to saturation of the radiation trapping within the pumped region. This saturation inevitably clamps the lifetime lengthening that is experimentally verified on a 0.05 at% thin ruby crystal based on the axial pinhole method. Our model is confirmed to be valid in lifetime measurement when the ruby fluorescence is collected from both the pumped and the unpumped regions.

Optics Express•Oct 26, 2012•10 citations
The Ē-level population of a cylindrical ruby rod is experimentally measured as a function of the pump energy at two points of the cross section of the rod, namely, the center and the periphery. The measurement is performed by measuring the fluorescent energy emitted after pumping by the points considered. The rod is pumped by a spiral flashtube. It is found that the experimental data of population vs pumping follow exponential curves as expected theoretically. From these curves the pump energy necessary for equalizing the populations between ground state and laser level is obtained in a straightforward way. The knowledge of this quantity leads to the measure of the losses (scattering plus diffraction) per pass in the rod. A value of ∼45% of losses is obtained for our rod.
The radial distribution of the pump energy absorbed within a ruby rod when pumped by a helical flash-tube is experimentally measured. The measurement is performed by taking near-field pictures of the emission pattern of the red fluorescence of the ruby. The emission pattern in this case is a visual reproduction of the energy absorbed at each point of the rod. The following three cases are considered: (a) ruby rod in air, (b) ruby rod covered by a cylindrical sheath of water, and (c) ruby rod covered by a cylindrical sheath of benzyl benzoate. The influence of the ground-state depopulation due to the pumping on the radial distribution is also studied. Experimental curves for the energy density are compared with theoretical curves obtained from the theory developed in a previous paper. The agreement is fairly good, the theoretical curves not differing from the experimental ones by more than 20%.
It was found that by observing the fluorescent intensity of ruby along its longitudinal axis during pumping, an appreciable amount of information may be obtained. It was observed that the time at which population inversion occurs during a pump cycle is directly observable from the time-resolved fluorescent intensity. A method for establishing the actual population in the metastable state from the fluorescent intensity is demonstrated and cross correlated with two other methods for measuring population inversion. The rate equation describing population inversion has been solved including the nonlinear stimulated emission loss term, and complete agreement with experiment is demonstrated. For the particular pumping geometry used, maximum energy available, and ruby quality and size, maximum metastable population of 1.21×1019 ions/cm3 was achieved, representing 75% inversion.
Depopulation of ground-state Cr ions in ruby (DGS) under optical pumping with a Xe flash light has been measured as a function of time. Experiments have been made by means of measuring the decrease in absorbing power for a probe light (5790 Å or 5461 Å) from a Hg lamp. DGS reaches its maximum value after some delay from the peak of Xe light, and decreases after discontinuation of optical pumping with a time constant of 3.4 ms, which is the lifetime of metastable-state 2E. During laser oscillation mean value of DGS is forced to take a constant value, which varies with the condition of laser oscillation. So long as the input energy to the Xe lamp is not altered, the shorter the duration of Xe flash light is, the higher becomes the maximum value of DGS. On the basis of the rate equation, these experimental results have been explained.
The real and imaginary parts of the complex index of refraction have been measured in the R1 line of optically pumped ruby. The experiments, which employed a temperature-controlled, single-mode ruby laser as a variable-frequency monochromatic source, were performed near liquid-nitrogen temperature, where the R1 doublet structure is well resolved. R1 absorption profiles were measured for several optical-pumping conditions, ranging from zero up to a pump power that excited a population in the 2E state equal to about 60% of the population of the 4A2 ground state. For each pumping condition, the anomalous-dispersion curve was measured interferometrically. The spectral profiles of the absorption coefficient and the anomalous dispersion display a dependence on the optical-pumping conditions that is more complicated than simple saturation of the R1 transitions, but for given pumping conditions the two quantities are correlated as predicted by the Kramers–Kronig relations.
A spatially periodic distribution of excited electronic states is produced in ruby which is placed into the interference region of two light beams derived from a pulsed laser (wavelength 530 nm). The spatially modulated population density acts like an optical grating and is detected by diffraction of a quasi-cw laser beam (488 nm). The lifetimes of the population density gratings are measured for ruby crystals with 0.05 to 5% Cr-doping and compared with the fluorescence lifetimes of the R-lines (694 nm). For the 0.05% ruby both lifetimes are nearly equal. At higher Cr-concentrations the grating decay is considerably faster than the fluorescent decay. The grating lifetimes are independent of the grating period A = 1 to 100 μm. From this observation an upper limit of the diffusion constant for energy transfer between excited electronic states in ruby is estimated.
In this study, we take the pump rate into consideration for the first time to give a theoretical description of radiation trapping in three-level systems. We numerically verify that under strong pumping, the population of the ground state is depleted, which leads to saturation of the radiation trapping within the pumped region. This saturation inevitably clamps the lifetime lengthening that is experimentally verified on a 0.05 at% thin ruby crystal based on the axial pinhole method. Our model is confirmed to be valid in lifetime measurement when the ruby fluorescence is collected from both the pumped and the unpumped regions.