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Soviet Journal of Quantum Electronics

Investigation of the physical parameters of an iodine amplifier pumped by open high-current discharge radiation

Nikolai G. Basov, V. S. Zuev, V A Katulin, A Yu Lyubchenko, V Yu Nosach, Alexei L. Petrov

Published Feb 28, 1979• 9 citations
Publisher

Abstract

Investigations were made of the magnitude and distribution of the population inversion, homogeneity of the medium, and influence of an amplifier on the master-oscillator pulse contrast. It was found that the population inversion distribution was strongly inhomogeneous. The active medium in the amplifier was very homogeneous and ensured a divergence of 10 –4 rad. The iodine amplifier had a negligible (for the contrast) luminescence noise level.

DOI
10.1070/qe1979v009n02abeh008727

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Investigations were made of the magnitude and distribution of the population inversion, homogeneity of the medium, and influence of an amplifier on the master-oscillator pulse contrast. It was found that the population inversion distribution was strongly inhomogeneous. The active medium in the amplifier was very homogeneous and ensured a divergence of 10 –4 rad. The iodine amplifier had a negligible (for the contrast) luminescence noise level.
Pulse propagations in an amplifier in the case of the photodissociation iodine laser are investigated with the use of semiclassical treatments, especially when the input pulse fed into the amplifier has a width comparable to or shorter than the transverse relaxation time of the amplifier medium. In our semiclassical equations for pulse propagation in an iodine amplifier, the populations of all hyperfine levels [2P1/2(F= 3, 2),2P3/2(F = 4, 3, 2, 1)], the polarizations of optically allowed transitions (\DeltaF = 0, \pm 1), and the Doppler detuning effects were taken into consideration. Based on the numerical integrations of the semiclassical equations, the propagated pulse shape and the population distributions of the hyperfine levels are discussed. From our calculations, it has been made clear that bandwidth limitations of pulses in the range of pulsewidths of 0.1 ns and below can be avoided by making use of the nonlinear amplification that results from the coherent interaction between the propagated pulse field and the amplifier medium.
A superradiant and amplified spontaneous transition pulse is obtained from an iodine photodissociation laser amplifier which uses i-C3F7I as an active molecule and is pumped with a flashlamp (full width at half maximum=3 μs). We have observed a sharp leading pulse of superradiance whose rise time is less than 50 ns, and the amplified spontaneous emission follows the superradiance during the pumping flash. By measuring the population inversion density in the active medium of the amplifier, we have obtained the threshold condition for the superradiance to take place. Also, the buildup time and peak intensity of the pulse are measured as a function of the flashlamp input energy. By placing a reflecting mirror at a varying distance (from 173 to 673 cm) behind the amplifier, we have found a periodic intensity fluctuation in the superradiance; the period (Δt) depends linearly on the distance (d), i.e., Δt=2d/c, where c is the light velocity.
A calculation is made of the steady-state population inversion distribution in homogeneously pumped active media of spherical or cylindrical form. It is assumed that reflections from the boundaries of the active medium have been completely eliminated and it is shown that the inversion decrease, caused by the amplification of spontaneously emitted radiation, is mainly a function of the overall volume of the active medium. For active media of equal volume, the stored inversion is minimal for a sphere.
Introduction We were able to show in Chapter 3 that a medium in which we can obtain a population inversion (i.e. a situation in which the population density in the excited state is greater than that in the fundamental level) allows for optical gain of an electromagnetic wave having a frequency near to the resonant frequency of the system. By introducing feedback of the amplified signal into the medium, the system can be made to oscillate naturally, resulting in laser oscillations . To obtain this population inversion, we must introduce at least a third (and perhaps even a fourth) energy level into the system. (We saw how a two-level system under the influence of an intense pump beam will saturate with no resulting population inversion.) The aim then of this chapter is to introduce the concepts necessary to extend our two-level system into a working model capable of illustrating the phenomenon of laser oscillation. We will not spend too much time discussing atomic transition lasers as they do not figure readily in our treatment of quantum electronic properties of semiconductors. An exception will be made, however; we brush upon the particular topics of a diode pumped laser in Complement 4.E and a quantum cascade laser in Complement 13.H. Population inversion and optical amplification Population inversion We will show how population inversion can be achieved by carrier transfer from higher lying levels to the upper level of a two-level subsystem of interest.
An analysis is made of the kinetics of physical processes in a hydrogen-iodine active medium. It is shown that, in principle, it is possible to achieve a population inversion due to the I * ( 2 P 1/2 )–I( 2 P 3/2 ) spin–orbit transition in atomic iodine as a result of near-resonant transfer of hydrogen vibrational energy to I * ( 2 P 1/2 ) ( VE process). A necessary condition for inversion is that at T = 300 K the mixture should contain more than 10% of the vibrationally excited hydrogen H 2 ( ν = 2) relative to the ground-state H 2 ( ν = 0). Some typical features of a hydrogen–iodine system are noted and its potential usefulness is demonstrated.
Equations appropriate for amplification of short pulses in the multilevel atomic iodine amplifier are developed from the Maxwell-Bloch equations with homogeneous line broadening. The equations are implemented in the form of a two-dimensional time-dependent finite-difference computer code with six population densities, six medium polarization densities, and adaptive propagation capabilities. The Sandia atomic iodine laser was used to examine the adequacy of the theoretical description for amplification conditions covering the full range of gain saturation. Whole-beam self-focusing is observed with a strongly saturating small-waist beam in agreement with predictions. Calculations predict that this self-focusing instability may be controlled by tuning the multiline amplifier bandwidth.