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Scaling of an Optically Pumped Mid-Infrared Rubidium Laser

Paul J. Moran

Published Mar 26, 2015• 0 citations
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Abstract

Abstract : An optically pumped mid-infrared rubidium (Rb) pulsed laser has been demonstrated in a heat pipe along the 62P3/2 - 62S1/2 transition at 2.730 micrometer and the 62P1/2 - 62S1/2 transition at 2.790 micrometer. The bleached limit, slope efficiency, and maximum laser output energy of the mid-IR Rb laser have been shown to scale linearly with increasing Rb density, contrary to prior laser demonstrations. A maximum output energy of 5 nJ per pulse had previously been observed before a rollover occurred in the scaling of output energy with Rb concentration. In this experiment, the maximum laser output energy observed was 100 nJ, with linear scaling to a Rb concentration of 7 x 10 to the 15th power cm-3. A maximum slope efficiency of 1.7 x 0.0001 was observed. Evidence that only a small percentage (2%) of the available pump photons were absorbed at a concentration of 2 x 10 to the 15th power cm-3 and a pump energy of 10 (mu)J. This indicated that there were 20,000 pump photons per Rb atom at a pump energy of 10 mJ, and that the effective laser efficiency was 0.65 percent or higher. The hyperfine structure and absorption spectral profile of the 52S1/2 - 62P1/2 and 52S1/2 - 62P3/2 (blue) pump transitions were studied using a continuous wave (cw) pump source.

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Mirrorless Lasing in Optically Pumped Rubidium Vapor

Ryan M. Richards

Abstract : Mirrorless lasing has been successfully demonstrated in a Rubidium heat pipe. Lasing was observed on the 6(2)P(sub 3/2)-6(2)S(sub 1/2) energy transition at 2.73 microns and on the 6(2)P(sub 1/2)-6(2)S(sub 1/2) transition at 2.79 microns. The transitions were optically pumped from 5(2)S(sub 1/2)-6(2)P(sub 3/2) at 420.2 nm and from 5(2)S(sub 1/2)-6(2)P(sub 1/2) at 421.7 nm, respectively. The 5(2)S(sub 1/2)-6(2)P(sub 3/2) transition was excited over a 36 GHz band, while the 5(2)S(sub 1/2)-6(2)P(sub 1/2) transition was excited over a 24 GHz band. Both pump transitions showed a high degree of saturation and only a small fraction (1%) of the incident pump energy was absorbed. Output energies of up to 5 nJ were obtained at 2.73 microns and of up to 5.4 nJ at 2.79 microns when pumped at pulse energies of 4 mJ. Both transitions experienced bleaching at this 4 mJ pump energy, limiting further IR output energy. Increasing rubidium concentration at bleached pump energies showed no increase in laser energy after 170 deg. C, likely due to second order processes. Slope efficiency for both transitions, however, increased linearly with rubidium concentration up to 11 x .000001 The addition of small amounts of argon, helium, and ethane (5-45 Torr) were found to fully quench lasing action on both transitions.

•Mar 1, 2013•0 citations

Optically Pumped Atomic Rubidium Lasers: Two-Photon and Exciplex Excitation Mechanisms

Jeffrey Gallagher

Abstract : The Doppler-broadened two-photon absorption (TPA) cross-section for the 52S1/2- 52D5/2 transition in Rb is measured using direct absorption methods. A detailed model of the intensity profile was developed to account for a focused Gaussian beam propagating through a two-photon absorption medium. A peak absorbance of 24% was observed for an intensity of 6.28 kW/cm2 at the focus, a Rb density of 4.6 x 1015 cm-3 concentrations from 1.61-8.52 x 10(15) cm-3 line. Extracting the hyperfine-broadened TPA cross-section from 87 test configurations, while varying the pump power, alkali concentration and focal length, yielded an error weighted average of 6.75x10-21 cm4/W. This cross-section is sufficient for a pulsed dye laser to bleach the pump transition in the Two-Photon Pumped Alkali Laser (TPAL) that lases at 420 nm and 5.2 mum. Additionally, optically pumped atomic rubidium lasers pumped in the blue satellite of the D2 line from the ground Rb-Ar or Rb-Kr collision pair to the dissociative B2 sum 1/2 produce laser emission at 780.2 nm. Lasing is achieved for the Rb-Ar system and 757.1 - 760.4 nm for the Rb-Kr system. Slope efficiencies exceed 0.25% using a heat pipe configuration. The gain is very high with photon build-up times of 1-3.7 ns.

•Jun 1, 2013•2 citations

Wavelength diversity in optically pumped alkali vapor lasers

Glen P. Perram

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.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jan 13, 2017•1 citation

IR laser oscillation on caesium and rubidium atomic transitions upon pumping to high-lying energy levels

A. A. Babin, Michail V. Volkov, Sergey G Garanin +5 more

Abstract Laser oscillation is obtained in caesium and rubidium atoms with wavelengths in the range of 2 – 5.5 μm under pumping to high-lying energy levels. Longitudinal resonant pumping is implemented using the second harmonic of radiation from an optical parametric oscillator. The pump wavelength is tuned over discrete levels from 8P to 10P in caesium atoms and from 6P to 8P in rubidium atoms. The width of the pump radiation spectrum is 12 cm−1. When caesium atoms are pumped, the pump pulse energy is no more than 10 mJ; when rubidium atoms are pumped, it does not exceed 3 mJ. The pulse repetition rate is 10 Hz. The maximum output energy of IR laser radiation upon pumping the 9P3/2 level of caesium atoms is about 100 μJ at a cell temperature of ∼170°C, while the efficiency of pump conversion into radiation energy with a wavelength λ ∼ 3.1 μm turns out to be ∼1 %. For rubidium atoms, an estimate of the output energy of IR radiation gives a value of ∼80 μJ at a cell temperature of 180 °C, which corresponds to an energy efficiency of ∼2.7 %.

Quantum Electronics•Mar 30, 2022•0 citations

Experimental investigation of a pulsed Rb–Ar excimer-pumped alkali laser

Hongling Cheng, Zhimin Wang, Fengfeng Zhang +5 more

Abstract We present experimental results of an exciplex-pumped alkali laser (XPAL) at 780 nm based on the 5 2 P 3/2 → 5 2 S 1/2 transition of the Rb atom in mixtures of Rb vapor and Ar. A laboratory-built Ti:sapphire laser with a pulse repetition rate of 3 kHz and a pulse width of 100 ns is used as the pump source. The maximum laser pulse energy of 0.26 µJ at 780 nm is obtained under an absorbed pump pulse energy of 42 µJ at 755 nm in mixtures of Rb vapor and Ar at a temperature of 423 K, corresponding to an optical conversion efficiency of 0.62%. Further experiments show that the output laser at 780 nm can always be detected for pump wavelengths ranging from 754 to 759 nm, indicating that Rb–Ar mixtures can be effectively pumped by commercial laser diodes (LDs) with a bandwidth of 5 nm.

Japanese Journal of Applied Physics•Feb 3, 2017•4 citations

Determining the Two-Photon Absorption Cross-Section for the 52S1/2 --> 52D5/2 Transition in Naturally Occurring Rubidium

Jeffrey Gallagher, Glen P. Perram

Recently we demonstrated lasing at 420 nm and 455 nm inside of rubidium and cesium vapor cells respectively, by non-resonant, two photon absorption at near infrared pump wavelengths. Two photon absorption from the ground S1/2 state to excited D3/2,5/2 or higher lying S1/2 state via virtual states and subsequent infrared stimulated emission pumps the upper laser levels, the second excited P1/2,3/2 states. To better understand the mechanism and phenomenology of this type of laser, this paper presents a method for determining the two-photon absorption (TPA) cross-section for the 5S1/2 −→ 5D5/2 transition in naturally occurring rubidium. For an incident pump intensity of 392 W cm2 and a rubidium concentration of 3.34 × 10 atoms cm3 , a peak absorbance of 0.10 is observed. The concentration of the Rb in the test cell is determined by one photon absorption spectra of the D2, indicating a vapor curve shifted by 14 C from the exterior wall temperature. The incident laser beam is focused to a waist of 75 μm. The beam quality is consistent with an M = 1.05 − 1.09. A model for the intensity as a function of propagation distance is developed to interpret the observed absorption spectrum and determine a cross-section for the two photon absorption.

AIAA Plasmadynamics and Lasers Conference•Jun 1, 2011•1 citation
Abstract : An optically pumped mid-infrared rubidium (Rb) pulsed laser has been demonstrated in a heat pipe along the 62P3/2 - 62S1/2 transition at 2.730 micrometer and the 62P1/2 - 62S1/2 transition at 2.790 micrometer. The bleached limit, slope efficiency, and maximum laser output energy of the mid-IR Rb laser have been shown to scale linearly with increasing Rb density, contrary to prior laser demonstrations. A maximum output energy of 5 nJ per pulse had previously been observed before a rollover occurred in the scaling of output energy with Rb concentration. In this experiment, the maximum laser output energy observed was 100 nJ, with linear scaling to a Rb concentration of 7 x 10 to the 15th power cm-3. A maximum slope efficiency of 1.7 x 0.0001 was observed. Evidence that only a small percentage (2%) of the available pump photons were absorbed at a concentration of 2 x 10 to the 15th power cm-3 and a pump energy of 10 (mu)J. This indicated that there were 20,000 pump photons per Rb atom at a pump energy of 10 mJ, and that the effective laser efficiency was 0.65 percent or higher. The hyperfine structure and absorption spectral profile of the 52S1/2 - 62P1/2 and 52S1/2 - 62P3/2 (blue) pump transitions were studied using a continuous wave (cw) pump source.
Abstract : Mirrorless lasing has been successfully demonstrated in a Rubidium heat pipe. Lasing was observed on the 6(2)P(sub 3/2)-6(2)S(sub 1/2) energy transition at 2.73 microns and on the 6(2)P(sub 1/2)-6(2)S(sub 1/2) transition at 2.79 microns. The transitions were optically pumped from 5(2)S(sub 1/2)-6(2)P(sub 3/2) at 420.2 nm and from 5(2)S(sub 1/2)-6(2)P(sub 1/2) at 421.7 nm, respectively. The 5(2)S(sub 1/2)-6(2)P(sub 3/2) transition was excited over a 36 GHz band, while the 5(2)S(sub 1/2)-6(2)P(sub 1/2) transition was excited over a 24 GHz band. Both pump transitions showed a high degree of saturation and only a small fraction (1%) of the incident pump energy was absorbed. Output energies of up to 5 nJ were obtained at 2.73 microns and of up to 5.4 nJ at 2.79 microns when pumped at pulse energies of 4 mJ. Both transitions experienced bleaching at this 4 mJ pump energy, limiting further IR output energy. Increasing rubidium concentration at bleached pump energies showed no increase in laser energy after 170 deg. C, likely due to second order processes. Slope efficiency for both transitions, however, increased linearly with rubidium concentration up to 11 x .000001 The addition of small amounts of argon, helium, and ethane (5-45 Torr) were found to fully quench lasing action on both transitions.
Abstract : The Doppler-broadened two-photon absorption (TPA) cross-section for the 52S1/2- 52D5/2 transition in Rb is measured using direct absorption methods. A detailed model of the intensity profile was developed to account for a focused Gaussian beam propagating through a two-photon absorption medium. A peak absorbance of 24% was observed for an intensity of 6.28 kW/cm2 at the focus, a Rb density of 4.6 x 1015 cm-3 concentrations from 1.61-8.52 x 10(15) cm-3 line. Extracting the hyperfine-broadened TPA cross-section from 87 test configurations, while varying the pump power, alkali concentration and focal length, yielded an error weighted average of 6.75x10-21 cm4/W. This cross-section is sufficient for a pulsed dye laser to bleach the pump transition in the Two-Photon Pumped Alkali Laser (TPAL) that lases at 420 nm and 5.2 mum. Additionally, optically pumped atomic rubidium lasers pumped in the blue satellite of the D2 line from the ground Rb-Ar or Rb-Kr collision pair to the dissociative B2 sum 1/2 produce laser emission at 780.2 nm. Lasing is achieved for the Rb-Ar system and 757.1 - 760.4 nm for the Rb-Kr system. Slope efficiencies exceed 0.25% using a heat pipe configuration. The gain is very high with photon build-up times of 1-3.7 ns.
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.
Abstract Laser oscillation is obtained in caesium and rubidium atoms with wavelengths in the range of 2 – 5.5 μm under pumping to high-lying energy levels. Longitudinal resonant pumping is implemented using the second harmonic of radiation from an optical parametric oscillator. The pump wavelength is tuned over discrete levels from 8P to 10P in caesium atoms and from 6P to 8P in rubidium atoms. The width of the pump radiation spectrum is 12 cm−1. When caesium atoms are pumped, the pump pulse energy is no more than 10 mJ; when rubidium atoms are pumped, it does not exceed 3 mJ. The pulse repetition rate is 10 Hz. The maximum output energy of IR laser radiation upon pumping the 9P3/2 level of caesium atoms is about 100 μJ at a cell temperature of ∼170°C, while the efficiency of pump conversion into radiation energy with a wavelength λ ∼ 3.1 μm turns out to be ∼1 %. For rubidium atoms, an estimate of the output energy of IR radiation gives a value of ∼80 μJ at a cell temperature of 180 °C, which corresponds to an energy efficiency of ∼2.7 %.
Abstract We present experimental results of an exciplex-pumped alkali laser (XPAL) at 780 nm based on the 5 2 P 3/2 → 5 2 S 1/2 transition of the Rb atom in mixtures of Rb vapor and Ar. A laboratory-built Ti:sapphire laser with a pulse repetition rate of 3 kHz and a pulse width of 100 ns is used as the pump source. The maximum laser pulse energy of 0.26 µJ at 780 nm is obtained under an absorbed pump pulse energy of 42 µJ at 755 nm in mixtures of Rb vapor and Ar at a temperature of 423 K, corresponding to an optical conversion efficiency of 0.62%. Further experiments show that the output laser at 780 nm can always be detected for pump wavelengths ranging from 754 to 759 nm, indicating that Rb–Ar mixtures can be effectively pumped by commercial laser diodes (LDs) with a bandwidth of 5 nm.
Recently we demonstrated lasing at 420 nm and 455 nm inside of rubidium and cesium vapor cells respectively, by non-resonant, two photon absorption at near infrared pump wavelengths. Two photon absorption from the ground S1/2 state to excited D3/2,5/2 or higher lying S1/2 state via virtual states and subsequent infrared stimulated emission pumps the upper laser levels, the second excited P1/2,3/2 states. To better understand the mechanism and phenomenology of this type of laser, this paper presents a method for determining the two-photon absorption (TPA) cross-section for the 5S1/2 −→ 5D5/2 transition in naturally occurring rubidium. For an incident pump intensity of 392 W cm2 and a rubidium concentration of 3.34 × 10 atoms cm3 , a peak absorbance of 0.10 is observed. The concentration of the Rb in the test cell is determined by one photon absorption spectra of the D2, indicating a vapor curve shifted by 14 C from the exterior wall temperature. The incident laser beam is focused to a waist of 75 μm. The beam quality is consistent with an M = 1.05 − 1.09. A model for the intensity as a function of propagation distance is developed to interpret the observed absorption spectrum and determine a cross-section for the two photon absorption.