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Applied Optics

Novel gain measurement technique for high gain amplifiers

Brian K. Garside, T. Efthymiopoulos

Published Sep 1, 1979• 2 citations
Publisher

Abstract

A new method for determining the gain in high gain amplifiers is reported. The technique only requires measurements of the relative intensity and the output spectral width of the amplified spontaneous emission from the amplifier as a function of input pumping. The results of an experimental test of this technique using a flashlamp-pumped rhodamine 6G dye amplifier are described. Simple approximate analytical models are used to compare with experiment thereby determining the gain. The results are found to be in good agreement with both more accurate gain measurements and an alternative technique which can be applied over a restricted range of operational parameters.

DOI
10.1364/ao.18.003053

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Amplification of spontaneous emission and gain measurement in dye amplifiers

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A comparison is reported of the observed spectral narrowing of amplified spontaneous emission from dye amplifiers with predictions from a dynamical model based on rate equations. This is employed to develop a generally applicable method of determining the unsaturated gain within the dye medium. Comparison is made with another technique for determining gain over the restricted pumping range for which it is applicable. The matching of theory to experiment is sufficiently sensitive that good values can also be determined for the wavelength dependence of the excited singlet state absorption cross section.

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The major purpose of the work described in this thesis is the experimental and theoretical study of the amplification of the spontaneous emission in dye amplifiers. This work falls into three distinct sections: (a) the study of high-current short-pulse flashtubes operating in (or near) the ablation regime; (b) the study of the amplification process of the spontaneous emission in a homogeneously broadened medium, such as a dye material, in the thin cell case, where there is no variation of the pumping across the cell; and, (c) the study of the amplification of the spontaneous emission in the thick cell case, where gain and index of refraction inhomogeneities exist across the cell thickness. In each case, we have developed theoretical models and compared their predictions with the experimental results. In particular, for the ablation-type flashlamps, it is shown that their behaviour is well represented by an analysis in which the discharge plasma is supposed to radiate as a blackbody at the temperature of the plasma. This model accounts for the fact that the light output from a flashlamp saturates whereas the current does not, provides a more accurate value for the plasma resistivity as a function of temperatures and gives a useful design criterion for determining the physical dimensions of an ablation-type flashlamp. With respect to the amplification of the spontaneous emission in a thin cell, it is mainly shown that (i) the spontaneous emission propagating along the gain medium is subject to narrowing along with shifting of its peak wavelength; (ii) even at moderate pumping the population of the excited singlet state, which determines the gain of the system, is non-uniform along the cell; (iii) it is possible to calculate the unsaturated gain by calculating the intensity of a particular wave-length and its spectral narrowing both as a function of the exciation and compare them with the same quantities measured experimentally; (iv) it is also possible to develop a semianalytic system independent method to measure the gain approximately. The theory developed to account for the propagation of the amplified spontaneous emission (ASE) has been partly extended to cover the case of the propagation and spectral narrowing of the ASE in a thick cell, where non-uniformities of the index of refraction and the gain are important along with the diffraction losses. In this particular case, it is shown that a model, which assumes an index step variation across the cell, and solves the Maxwell's equation for the field propagation along the cell, can account for the spatial distribution of the field across the cell and its manifestation at the far field. Most of the predictions of the model are verified experimentally. In particular, it is shown that (i) in a thick cell, part of the ASE propagates outside the gain medium, because of the lensing effect caused by heating of the lasing material; (ii) the field across the cell can exhibit stable spatial modes, which can produce a far field far from the amplifying cell axis; (iii) it is possible to improve the spectral narrowing of the ASE since saturation can be delayed to higher pumping than before and most of the narrowing occurs below saturation of the gain; (iv) it is also possible to measure the temperature and the index of refraction change due to heating by measuring the far field angle, and estimate the diffraction losses involved in the propagation of the ASE in a thick cell. Finally, a number of suggestions on further research are proposed.

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•Jan 1, 2015•0 citations
A new method for determining the gain in high gain amplifiers is reported. The technique only requires measurements of the relative intensity and the output spectral width of the amplified spontaneous emission from the amplifier as a function of input pumping. The results of an experimental test of this technique using a flashlamp-pumped rhodamine 6G dye amplifier are described. Simple approximate analytical models are used to compare with experiment thereby determining the gain. The results are found to be in good agreement with both more accurate gain measurements and an alternative technique which can be applied over a restricted range of operational parameters.
A comparison is reported of the observed spectral narrowing of amplified spontaneous emission from dye amplifiers with predictions from a dynamical model based on rate equations. This is employed to develop a generally applicable method of determining the unsaturated gain within the dye medium. Comparison is made with another technique for determining gain over the restricted pumping range for which it is applicable. The matching of theory to experiment is sufficiently sensitive that good values can also be determined for the wavelength dependence of the excited singlet state absorption cross section.
The major purpose of the work described in this thesis is the experimental and theoretical study of the amplification of the spontaneous emission in dye amplifiers. This work falls into three distinct sections: (a) the study of high-current short-pulse flashtubes operating in (or near) the ablation regime; (b) the study of the amplification process of the spontaneous emission in a homogeneously broadened medium, such as a dye material, in the thin cell case, where there is no variation of the pumping across the cell; and, (c) the study of the amplification of the spontaneous emission in the thick cell case, where gain and index of refraction inhomogeneities exist across the cell thickness. In each case, we have developed theoretical models and compared their predictions with the experimental results. In particular, for the ablation-type flashlamps, it is shown that their behaviour is well represented by an analysis in which the discharge plasma is supposed to radiate as a blackbody at the temperature of the plasma. This model accounts for the fact that the light output from a flashlamp saturates whereas the current does not, provides a more accurate value for the plasma resistivity as a function of temperatures and gives a useful design criterion for determining the physical dimensions of an ablation-type flashlamp. With respect to the amplification of the spontaneous emission in a thin cell, it is mainly shown that (i) the spontaneous emission propagating along the gain medium is subject to narrowing along with shifting of its peak wavelength; (ii) even at moderate pumping the population of the excited singlet state, which determines the gain of the system, is non-uniform along the cell; (iii) it is possible to calculate the unsaturated gain by calculating the intensity of a particular wave-length and its spectral narrowing both as a function of the exciation and compare them with the same quantities measured experimentally; (iv) it is also possible to develop a semianalytic system independent method to measure the gain approximately. The theory developed to account for the propagation of the amplified spontaneous emission (ASE) has been partly extended to cover the case of the propagation and spectral narrowing of the ASE in a thick cell, where non-uniformities of the index of refraction and the gain are important along with the diffraction losses. In this particular case, it is shown that a model, which assumes an index step variation across the cell, and solves the Maxwell's equation for the field propagation along the cell, can account for the spatial distribution of the field across the cell and its manifestation at the far field. Most of the predictions of the model are verified experimentally. In particular, it is shown that (i) in a thick cell, part of the ASE propagates outside the gain medium, because of the lensing effect caused by heating of the lasing material; (ii) the field across the cell can exhibit stable spatial modes, which can produce a far field far from the amplifying cell axis; (iii) it is possible to improve the spectral narrowing of the ASE since saturation can be delayed to higher pumping than before and most of the narrowing occurs below saturation of the gain; (iv) it is also possible to measure the temperature and the index of refraction change due to heating by measuring the far field angle, and estimate the diffraction losses involved in the propagation of the ASE in a thick cell. Finally, a number of suggestions on further research are proposed.
The widely used technique for determining optical gain and absorption from single-pass, amplified spontaneous emission (ASE) is analyzed with particular attention given to active-volume saturation and three-dimensional effects. The results for homogeneously broadened media show that the population inversion and hence the gain in the active region is diminished by high levels of spontaneous and ASE. The externally measured gain is further reduced for large output collection apertures that admit incompletely amplified off-axis rays. Criteria are given for the excitation and detection geometries required to ensure accurate (∼10-percent) gain measurement. The simultaneous measurement of the equilibrium absorption coefficient can be made under less restrictive conditions.
A theory is developed to describe the way in which modulations in the pump intensity produce modulations in the gain of a four-level, homogeneously broadened laser amplifier. The theory is tested by carrying out an experiment using an alexandrite crystal pumped by a cw dye laser. A second dye laser is used to measure the gain in the inverted laser transition. The dependence of the magnitude and the bandwidth of the gain on the pumping rate is determined. Agreement between theory and experiment is good.