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Journal of physics. A, Proceedings of the Physical Society. General

Amplified spontaneous emission. IV. Beam divergence and spatial coherence

G I Peters, L. Allen

Published Apr 1, 1972• 30 citations
Publisher

Abstract

For pt. III see abstr. A54492 of 1971. The origins of beam divergence in amplified spontaneous emission are discussed and theoretical models are developed to predict what their effect would be in real systems. The divergence of the beam is experimentally investigated for the 3.39 mu m He-Ne and the 0.614 mu m Ne systems and compared with the theories. Good agreement is achieved in pure neon for a theory which depends upon the parametric relationship between intensity and length of active medium. In the case of He-Ne although the theory gives the correct qualitative prediction, the absolute magnitude of the effect is wrong by a factor of about 1.7. Measurements of the degree of spatial coherence as a function of length and inversion density of the medium and for the variation of spatial coherence across the tube for a given length and inversion density of medium, are presented for the neon system.

DOI
10.1088/0305-4470/5/4/011

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Amplified spontaneous emission III. Intensity and saturation

L. Allen, G I Peters

For pt. II see abstr. A35654 of 1971. A theory is developed to account for the intensity of ASE as a function of the length and diameter of the amplifying medium and of its inversion density. Detailed results are presented from a study of the 3.39 mu m transition in He-Ne which verify the theory. The theory is also shown to fit the experimental data obtained from the 0.337 mu m transition in N 2 .

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Amplified Spontaneous Emission and External Signal Amplification in an Inverted Medium

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An improved version of the theory of amplified spontaneous emission (ASE) is presented and shown to be satisfied by experimental data obtained using the 3.39-\ensuremath{\mu}m transition of He-Ne. The new approach allows the effect of an external signal injected at one end of the system to be investigated and its influence on both the positive-going and negative-going waves to be calculated for He-Ne. A special case occurs when the injected signal is replaced by a mirror which reflects the negative-going wave back into the medium and the theory of this commonly occurring laboratory system is experimentally verified at 3.39 \ensuremath{\mu}m. The treatment allows the importance of the interaction between spontaneous and stimulated emission to be considered. The beam divergence predicted by the theory is evaluated and compared with experiment. The spectral distribution for a pure ASE system is considered, as is the distribution of inversion which results from the positive- and negative-going waves and which is shown to have minima at the medium extremities. Finally the implications of this work for the problem of interstellar OH emission is considered.

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Spectral distribution of amplified spontaneous emission

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Multibeam gain competition deduced from intensity statistics of amplified spontaneous emission

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The univariate and bivariate intensity statistics of amplified spontaneous emission (ASE) from continuously excited HeNe and HeXe gas discharges have been measured for the purpose of characterizing the dependence of ASE fluctuations on gain length and degree of homogeneous broadening. Comparison with theoretical results from a model for intensity fluctuations in heavily saturated multimode systems confirms that increasing the homogeneous broadening lowers the number of competing "modes" in the system. Intrinsic to this simple coupled-intensity model are unique combinations of cumulants that are independent of the number of modes and the strength of their coupling. These combinations have been measured, and they are independent of the degree of homogeneous broadening and are constant as the discharge length is increased. The normalized cumulants and cross-correlations for coprapogating and counterpropagating orthogonal, circularly polarized beams in a HeXe homogeneously broadened medium are consistently smaller in magnitude than those for orthogonal, linearly polarized beams. Comparison of results for neon and xenon discharges will distinguish whether this is the result of back-reflections or is a property of the counterpropagating beams quasiresonant with these noble-gas transitions, which differ in their upper- and lowerstate angular momenta.

Annual Meeting Optical Society of America•Jan 1, 1990•0 citations

Flashlamp Pumped Dye Amplifiers

Thomas Efthimiopoulos

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.

•Oct 1, 1978•0 citations

Amplified spontaneous emission I. The threshold condition

G I Peters, L. Allen

The threshold condition for the onset of amplified spontaneous emission is derived and experimentally verified in the 3.39 mu m He-Ne and the 0.614 mu m Ne systems. A short discussion is given concerning the confusion between this type of radiation and superradiance.

Journal of physics. A, Proceedings of the Physical Society. General•Mar 1, 1971•108 citations
For pt. III see abstr. A54492 of 1971. The origins of beam divergence in amplified spontaneous emission are discussed and theoretical models are developed to predict what their effect would be in real systems. The divergence of the beam is experimentally investigated for the 3.39 mu m He-Ne and the 0.614 mu m Ne systems and compared with the theories. Good agreement is achieved in pure neon for a theory which depends upon the parametric relationship between intensity and length of active medium. In the case of He-Ne although the theory gives the correct qualitative prediction, the absolute magnitude of the effect is wrong by a factor of about 1.7. Measurements of the degree of spatial coherence as a function of length and inversion density of the medium and for the variation of spatial coherence across the tube for a given length and inversion density of medium, are presented for the neon system.
An improved version of the theory of amplified spontaneous emission (ASE) is presented and shown to be satisfied by experimental data obtained using the 3.39-\ensuremath{\mu}m transition of He-Ne. The new approach allows the effect of an external signal injected at one end of the system to be investigated and its influence on both the positive-going and negative-going waves to be calculated for He-Ne. A special case occurs when the injected signal is replaced by a mirror which reflects the negative-going wave back into the medium and the theory of this commonly occurring laboratory system is experimentally verified at 3.39 \ensuremath{\mu}m. The treatment allows the importance of the interaction between spontaneous and stimulated emission to be considered. The beam divergence predicted by the theory is evaluated and compared with experiment. The spectral distribution for a pure ASE system is considered, as is the distribution of inversion which results from the positive- and negative-going waves and which is shown to have minima at the medium extremities. Finally the implications of this work for the problem of interstellar OH emission is considered.
A theory is described which accounts for the dependence of the spectral distribution of amplified spontaneous emission on the length and inversion density of the active medium. For an inhomogeneously broadened line an unanticipated dip occurs in the plot of linewidth against length at lengths slightly greater than the critical length, but is explained by a simple physical analysis. Agreement occurs between the theory and that of Yariv and Leite (1963) in the regime over which their theory is expected to work. Observations by other workers on the spectral distribution of amplified spontaneous emission systems are discussed.
The univariate and bivariate intensity statistics of amplified spontaneous emission (ASE) from continuously excited HeNe and HeXe gas discharges have been measured for the purpose of characterizing the dependence of ASE fluctuations on gain length and degree of homogeneous broadening. Comparison with theoretical results from a model for intensity fluctuations in heavily saturated multimode systems confirms that increasing the homogeneous broadening lowers the number of competing "modes" in the system. Intrinsic to this simple coupled-intensity model are unique combinations of cumulants that are independent of the number of modes and the strength of their coupling. These combinations have been measured, and they are independent of the degree of homogeneous broadening and are constant as the discharge length is increased. The normalized cumulants and cross-correlations for coprapogating and counterpropagating orthogonal, circularly polarized beams in a HeXe homogeneously broadened medium are consistently smaller in magnitude than those for orthogonal, linearly polarized beams. Comparison of results for neon and xenon discharges will distinguish whether this is the result of back-reflections or is a property of the counterpropagating beams quasiresonant with these noble-gas transitions, which differ in their upper- and lowerstate angular momenta.
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.