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IEEE Journal of Quantum Electronics

Beam-quality-determining processes in high-energy atomic iodine amplifiers

H. J. Baker, Terence A. King

Published Sep 1, 1981• 12 citations
Publisher

Abstract

The disturbance of the optical homogeneity of high-energy, 1.315 μm atomic iodine amplifier systems by gas density changes, produced by gas-dynamic effects following nonuniform photolysis, has been analyzed for a wide range of line-broadening buffer gases. Mechanisms of photolysis gas heating and the relative merit of different buffer gases for minimizing density changes are discussed. An experimental study of the buffer gas dependence has shown good agreement with the gas-dynamic analysis. The model has been used to determine conditions for diffraction-limited amplification for varying pump times and buffer gas composition for typical large amplifier pumping profiles, showing the possibility for significantly increasing pump pulse duration.

DOI
10.1109/jqe.1981.1071316

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Journal of Applied Physics•Jan 15, 1988•3 citations
The disturbance of the optical homogeneity of high-energy, 1.315 μm atomic iodine amplifier systems by gas density changes, produced by gas-dynamic effects following nonuniform photolysis, has been analyzed for a wide range of line-broadening buffer gases. Mechanisms of photolysis gas heating and the relative merit of different buffer gases for minimizing density changes are discussed. An experimental study of the buffer gas dependence has shown good agreement with the gas-dynamic analysis. The model has been used to determine conditions for diffraction-limited amplification for varying pump times and buffer gas composition for typical large amplifier pumping profiles, showing the possibility for significantly increasing pump pulse duration.
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.
An investigation was made of the dependences of the initial I(5 2 P 1/2 )→I(5 2 P 3/2 )+ hν ( λ = 1315nm) luminescence signal on the pressure and nature of the buffer gases. Atoms of I in the 5 2 P 1/2 state were formed, in initial concentrations of (0.5–3)×10 16 cm −3 , as a result of photolysis of CF 3 I (0.01 atm) by the fourth harmonic of a Q -switched neodymium laser ( E ∼0.6 J, τ ∼2×10 −3 sec). The I(5 2 P 1/2 ) luminescence was recorded by a germanium photodiode whose time constant was (3–4)×10 −7 sec. When CF 3 I was diluted with Ar, Xe, and SF 6 buffer gases, the intensity of the luminescence of the I(5 2 P 1/2 ) atoms increased because of an increase in the probability of the 5 2 P 1/2 →5 2 P 3/2 transition as a result of collisions of the iodine atoms with the atoms or molecules of the buffer gases. The dependence of the probability of the investigated transition on the nature and pressure of the buffer gases was determined in the pressure range ∼0–1 atm. Estimates were obtained of the oscillator strengths corresponding to the matrix elements of the induced electric dipole moment due to the deformation of the electron shells of the iodine atoms in collisions.
The influence of molecular collisions on the production of the degenerate four-wave mixing signal in I(2) is presented. Measurements were performed on gaseous molecular iodine, I(2), contained in a glass cell in which pressure, temperature, and species concentration are easily and independently varied. Frequencydoubled outputs from a seeded Nd:YAG laser and an excimer-pumped dye laser were used as excitation sources. We have studied the dependence of signal strength versus buffer gas pressure, with pump intensity as a third parameter. It is evident from our results that, for pump intensities of less than 1 MW/cm(2), the pressure dependence of the signal follows that given by a simple two-level model in the homogeneously broadened regime. In this regime collisional deexcitation becomes significant, leading to changes in saturation intensity. This is evidenced by a reduction in the signal with an increase in buffer gas pressure. This behavior is similar to that seen in laser-induced fluorescence. At higher pump intensities, the signal is seen to increase with pressure; this behavior cannot be described by the simple two-level model.
Modeling of the efficiency and output beam quality of a Rb flowing-gas diode-pumped alkali amplifier (DPAA) based on a 3D computational fluid dynamic model coupled to a wave optics model for amplified beam propagation is reported. Both end and two-way side pumping schemes were considered and compared for two cases of buffer gas composition where either a He/CH 4 mixture or pure He was used as a buffer gas. Dependencies of the DPAA efficiency and of the output beam quality on different flow and laser parameters were found. For the end pumping scheme and He/CH 4 buffer gas, the amplifier efficiency is almost independent of the gas velocity, whereas beam quality deteriorates at low flow velocities. For side pumping, both efficiency and beam quality are independent of the flow velocity. The effect of amplified spontaneous emission (ASE) on the efficiency of the DPAA is moderate and leads to a decrease in the efficiency of the amplifier by a maximum of ∼10% compared to the case when ASE is not taken into account. The ASE noise to signal ratio in the end-pumped amplifier was found to be negligible, while in the side-pumped amplifier, the ASE noise power at the output was comparable to the power of the coherent amplified output beam. The presented results show that end pumping is preferable to side pumping in terms of efficiency, output beam quality, and ASE noise to signal ratio. The maximum efficiency of the amplifier with end pumping using pure He as a buffer gas is significantly lower than that for the amplifier using He/CH 4 buffer gas. However, despite the low efficiency, an amplifier using pure He has higher output beam quality. The maximum efficiency of a two-way side-pumped amplifier with pure He as a buffer gas does not exceed 0.01. Thus, such an amplifier cannot be used for efficient energy extraction.
An investigation was made of the dependences of the initial I(5 2 P 1/2 )→I(5 2 P 3/2 )+ hν ( λ = 1315nm) luminescence signal on the pressure and nature of the buffer gases. Atoms of I in the 5 2 P 1/2 state were formed, in initial concentrations of (0.5–3) × 10 16 cm −3 , as a result of photolysis of CF 3 I (0.01 atm) by the fourth harmonic of a Q -switched neodymium laser ( E ∼0.6 J, τ ∼2 × 10 −3 sec). The I(5 2 P 1/2 ) luminescence was recorded by a germanium photodiode whose time constant was (3–4) × 10 −7 sec. When CF 3 I was diluted with Ar, Xe, and SF 6 buffer gases, the intensity of the luminescence of the I(5 2 P 1/2 ) atoms increased because of an increase in the probability of the 5 2 P 1/2 →5 2 P 3/2 transition as a result of collisions of the iodine atoms with the atoms or molecules of the buffer gases. The dependence of the probability of the investigated transition on the nature and pressure of the buffer gases was determined in the pressure range ∼0–1 atm. Estimates were obtained of the oscillator strengths corresponding to the matrix elements of the induced electric dipole moment due to the deformation of the electron shells of the iodine atoms in collisions.
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.