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Electronics Letters

Demonstration of a Highly Efficient Alkali Vapour Amplifier

Huizi Zhao, Zi Ye, Liang Li, Rui Wang, Zining Yang, Weiqiang Yang, Hongyan Wang, Weihong Hua, Xiaojun Xu

Published Jan 1, 2025• 1 citation
Publisher

Abstract

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.

DOI
10.1049/ell2.70402

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jan 13, 2017•1 citation
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.
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.
We report high-efficiency optical amplification with near-extreme-limit gain from a diode-pumped Cs vapor cell. We used wavelength-division multiplexing to couple 852 nm pump and 895 nm seed lasers to achieve nearly overlapping spatial modes in the Cs vapor cell. We investigated the amplification factor as a function of the focal length of the lens focusing on the combined pump and seed signals and determined the optimal focal length under our experimental conditions. The small-signal amplification factor from the Cs vapor cell reached >30 dB at 240 mW pump power, and the optimal optical amplification factor per pump power was 4171/W.
(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.
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.
In the recent years, alkali vapor lasers have become the most promising candidates for realization of a light source with both good beam quality and high output power because of their excellent performances. A rubidium laser is a typical kind of alkali vapor lasers. In this study, we developed a theoretical mode to evaluate the population densities of three levels as well as mode-matching efficiency of a rubidium laser pumped by a narrow-linewidth Ti:Sapphire laser under two different focal conditions. In the evaluation, at least two values of the output power of a rubidium laser must be known, which are usually obtained from the experiment. We performed a series of experiments by using two pump lenses with two focal lengths of 150 and 200 mm, and acquired two corresponding values of the laser output power. Then, we applied the experimental results in the theoretical calculation and then obtained the population densities of three levels and mode-matching efficiency of the rubidium laser. The study demonstrates that the outputted pump power can be used for evaluation of the population densities of three levels and mode-matching efficiency of an alkali laser under the different experimental conditions. The study might be valuable to better understand the physical features of an alkali vapor laser and to optimize the configuration of a high-powered alkali laser in the future.
Alternative wavelengths for optically pumped alkali vapor lasers have been developed using single photon excitation of higher lying P-states, stimulated Raman processes, two-photon excitation of S and D states, and electric quadruple excitation on S-D transitions. Two photon excitation of Cs 72D leads to competing and cascade lasing producing red and infrared lasers operating on the D-P transitions, followed by ultraviolet, blue, the standard near infrared DPAL transitions operating on P-S transitions. The S-D pump transitions are fully bleached at pump intensities exceeding 1 MW/cm2, allowing for lasing transitions that terminate on the ground state. The kinetics of these systems are complex due to competition for population inversion among the many optical transitions. An optically pumped mid-infrared rubidium pulsed, mirrorless laser has also been demonstrated in a heat pipe along both the 62P3/2 - 62S1/2 transition at 2.73 μm and the 62P1/2 - 62S1/2 transition at 2.79 μm with a maximum energy of ~100 nJ. Performance improves dramatically as the rubidium vapor density is increased, in direct contradiction with the prior work. No scaling limitations associated with energy pooling or ionization kinetics have been observed. Practical application for infrared counter measures depends on the further development of blue diode pump sources. Finally, stimulated electronic Raman scattering and hyper-Raman processes in potassium vapor near the D1 and D2 lines have been observed using a stable resonator and pulsed laser excitation. First and second order Stokes and anti-Stokes lines were observed simultaneously and independently for a pump laser tuning range exceeding 70 cm-1. When the pump is tuned between the K D1 and D2 lines, an efficient hyper-Raman process dominates with a slope efficiency that exceeds 10%. Raman shifted laser may be useful as a target illuminator or atmospheric compensation beacon for a high power diode pumped alkali laser.