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Journal of the Optical Society of America B

Three-dimensional wave optics and fluid dynamics modeling of a Rb flowing-gas diode-pumped alkali amplifier

Karol Waichman, Boris D. Barmashenko, Salman Rosenwaks

Published Mar 28, 2023• 8 citations

Abstract

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.

DOI: 10.1364/josab.488095

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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.

Comprehensive analysis of kinetic and fluid dynamic processes in flowing-gas diode-pumped alkali vapor amplifiers is reported. Taking into account effects of the temperature, the amplified spontaneous emission, the saturation power, the excitation of the alkali atoms to high electronic levels and the ionization, a detailed physical model is established to simulate the output performance of flowing-gas diode-pumped alkali vapor amplifiers. Influences of the flow velocity and the pump power on the amplified power are calculated and analyzed. Comparisons between single and double amplifier, longitudinal and transverse flow are made. Results show that end-pumped cascaded amplifier can provide higher output power under the same total pump power and the cell length, while output powers achieved by single- and double-end pumped, double-side pumped amplifiers with longitudinal or transverse flow have a complicated but valuable relation. Thus the model is extremely helpful for designing high-power flowing-gas diode-pumped alkali vapor amplifiers.

ABSTRACT We demonstrate a high‐efficiency diode‐pumped alkali vapor amplifier with high performance. The output power reaches 30 W with 25 dB magnification, a spectral linewidth of 3.7 MHz, and a coherence length of 80 m. An factor of 1.6 is achieved, and higher beam quality can be further improved by flow field. This advanced laser source exhibits significant potential for a wide range of applications, including precision laser spectroscopy, lidar systems, quantum optics research, and related fields.

(F = 4) to obtain an efficient diode-pumped alkali amplifier (DPAA). To realize highly efficient optical gain conditions, both the spatial modes of the pump and signal lasers are made to overlap in the Cs vapor cell with the use of a single-mode optical fiber. An amplification factor of 430 ± 15 is achieved under the following conditions: cell temperature of 90 °C, signal power of 0.1 mW, and pump power of 200 mW. We believe that our results can aid in the development of highly efficient diode-pumped alkali-vapor lasers and amplifiers.

The use of high-power ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$

{\mathrm{Al}}_{\mathit{x}}

As diode laser arrays for optical pumping of Rb vapor has been examined both theoretically and experimentally. Our studies show that large atomic polarizations can be obtained using the broad spectral output of a diode laser array if the Rb resonance line is collisionally broadened by several atmospheres of a buffer gas. The objective was to study the effectiveness of diode laser arrays as light sources for optically pumped spin-exchange polarized

^{3}\mathrm{He}

targets. The average Rb polarization was deduced at three different Rb vapor pressures using the polarization induced in the

^{3}\mathrm{He}

buffer gas. The largest

^{3}\mathrm{He}

polarization observed was 53% using a 20-W diode array with 1.9-nm full width at half maximum spectral width in 10.5 amagat of

^{3}\mathrm{He}

. This result compares favorably with other light sources for this target. The measured average Rb polarizations are compared to theoretical calculations.

The parametric four-wave mixing (4WM) process responsible for the generation of coherent blue light in alkali vapors is a self-seeded process which starts by population inversion and amplified spontaneous emission (ASE). Lately, attention has been turned toward frequency up- and down-conversion in alkali vapors, using relatively low pump powers with CW diode lasers. In this Letter, we investigate the interplay between ASE and 4WM in rubidium (Rb) vapors by studying the mid-infrared (mid-IR) radiation emitted in the forward direction at 5.23 μm. We show that the ASE can be suppressed by 4WM in the CW regime. Thus, we demonstrate the generation of coherent mid-IR light at 5.23 μm in hot Rb vapors via parametric 4WM.

Paraxial Maxwell-Bloch theory is used to describe amplified spontaneous emission (ASE) in strongly inverted media. Counterpropagation of waves coupled through gain saturation and diffraction, and pumping by a laser beam which propagates transverse to the amplifier are included in the treatment. The reactive part of the medium polarization is calculated by population-rate equations in the limit of rapid dephasing of the atomic dipoles. The spontaneous emission noise source needed for the numerical simulation of Maxwell's equations is derived from microscopic theory by coarse-graining, thereby establishing the compatibility of the spherically symmetric microscopic source with paraxial propagation theory. The three coupled Maxwell equations for the counterpropagating ASE waves and the transversely propagating pump wave are solved numerically in time and two spatial dimensions. This simulation is used to calculate the longitudinal profile of the transverse fluorescence, which measures the saturation of the gain, the longitudinal profile of the transmitted pump wave, which measures the saturation of pump absorption, and the transverse coherence function of the radiation emitted from the ends of the amplifier.