We report the first demonstration of a 795-nm Rubidium resonance transition laser using a buffer gas consisting of pure {sup 3}He. This follows our recent demonstration of a hydrocarbon-free 795-nm Rubidium resonance laser which used naturally-occurring He as the buffer gas. Using He gas that is isotopically enriched with {sup 3}He yields enhanced mixing of the Rb fine-structure levels. This enables efficient lasing at reduced He buffer gas pressure, improving thermal management in high average power Rb lasers and enhancing the power scaling potential of such systems

Beach, R J

Kanz, V K

Mitchell, S C

Page, R H

Soules, T F

Wu, S S

UNT Digital Library

UCRL-JRNL-233631Resonance transition 795-nmRubidium laser using 3He buffergasSheldon S.Q. Wu, Thomas F. Soules, Ralph H.Page, Scott C. Mitchell, V. Keith Kanz, RaymondJ. BeachAugust 13, 2007Optics CommunicationsDisclaimer  This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.  Resonance transition 795-nm Rubidium laser using 3He buffer gasSheldon S.Q. Wu,* Thomas F. Soules, Ralph H. Page, Scott C. Mitchell, V. Keith Kanz, Raymond J. BeachLawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551We report the first demonstration of a 795-nm Rubidium resonance transition laser using a buffer gas consisting of pure 3He.  This follows our recent demonstration of a hydrocarbon-free 795-nm Rubidium resonance laser which used naturally-occurring He as the buffer gas. Using He gas that is isotopically enriched with 3He yields enhanced mixing of the Rb fine-structure levels. This enables efficient lasing at reduced He buffer gas pressure, improving thermal management in high average power Rb lasers and enhancing the power scaling potential of such systems.Alkali resonance lasers are being actively investigated as an attractive route for the brightness enhancement, both spatial and spectral, of laser diode arrays.  Since the first demonstration of a rubidium optical resonance transition laser in 2002 [1], multiple demonstrations of alkali resonance transition lasers have been reported in the scientific literature using Rb [2, 3], Cs [4, 5, 6] and K [7] as the gain media.  All these systems have similar lasing schemes, being pumped on their D2 transition (n2S1/2®n2P3/2) and lased on their D1 transition (n2P1/2®n2S1/2). With one exception, all of the published demonstrations have followed Konefal’s original suggestion [8]and used ethane as the buffer gas or a component of the buffer gas to promote rapid F-S (fine-structure) mixing between the terminal pump level (2P3/2) and the initial laser level (2P1/2).  Rapid F-S mixing is a requirement for efficient laser operation.  The problem with this approach, as first reported by Page et al [2], is that it leads to the deposition of carbon on surfaces that simultaneously see high intensity pump light, alkali vapor and ethane. Due to the high optical absorption of carbon compounds to both the pump and laser light, the deposition of carbon quickly leads to thermally-initiated optical damage.  Because we are ultimately interested in using end-pump geometries in which high-intensity pump light is ducted through a cell via reflections with the side walls where pump light, ethane and alkali vapor exist simultaneously, the cell-fouling issue that accompanies the use of ethane requires mitigation, especially for power scaled systems.  Wu et al recently demonstrated one solution to this problem with ahydrocarbon-free resonance transition 795-nm Rubidium laser in which the buffer gas was naturally-occurring He (natural abundance of 3He: 4He is ~1:740000) [3]. In that work, the good agreement between measured laser performance and a laser model supported the clean He-only buffer gas approach as a viable path for power scaling.  Obviated was the issue of carbon formation and degradation of the vapor cell that is observed when ethane is used as a component of the buffer.  One potential disadvantage of the pure He buffer gas approach arises from the smaller F-S mixing cross section of Rb-He, reported to be 4.6x10-17 cm2 based on laser model-fit [3], compared to that of Rb-ethane, reported to be 7.7x10-15 cm2 [9].  The impact of these differing cross section values means that to achieve equivalent F-S mixing rates, higher pressures are required in the pure He systems than would be required in systems using ethane as a buffer gas component.  The primary disadvantage of higher He pressures for power scaled systems is that the refractive index variation with temperature dn/dT in the alkali gain cell is proportional to the He pressure, so higher He pressures will give larger thermal aberrations under equivalent heat loads.  In this letter, we demonstrate one approach to lowering the required He pressure in the alkali vapor gain cell by replacing the He buffer gas having a natural isotopic abundance with isotopically enriched 3He.  The advantage of using isotopically enriched 3He stems from its lower mass and therefore higher thermal speed at a given temperature in comparison with naturally occurring He.  Thehigher thermal speed associated with 3He benefits the F-S mixing rate,2 2 2 23 / 2 1/ 2 3 / 2 1/ 2He rP P P Pn vg s® ®= , where nHe is the number of He atoms per unit volume, 2 23 / 2 1 / 2s®P Pis the Rb-He F-S mixing cross section, and vr is the mean relative speed between He and Rb atoms.  First, at a given temperature vr is higher in 3He than 4He by approximately 4 / 3 1.15» , which not only benefits the F-S mixing rate which depends directly on vr, but also improves thermal management in the cell.  Since the thermal conductivity of an ideal gas is proportional to the mean particle velocity, the thermal conductivity of 3He is larger than that of 4He by the same factor.  Secondly, the F-S mixing cross section itself has a velocity dependence [10] that is expected to give a Rb-3He value larger than the Rb-4He value at a given cell temperature due to the difference in thermal speeds of the two He isotopes.  It can be extrapolated from that velocity dependence that the Rb F-S mixing cross section in 3He is approximately 1.5 times larger than that of 4He at our operating temperatures.  Together, one can expect a Rb F-S mixing rate about 1.7 times larger for the 3He system than the 4He system under same operating conditions. A summary of significant parameters is provided in Table 1.A schematic diagram for the experimental setup used in this demonstration that is nearly identical to that of [3] is shown in Figure 1. The Rb vapor and 3He gas were contained in a 3 cm long cylindrical ceramic cell with misoriented sapphire windows that were anti-reflection coated on their external surfaces, but uncoated on their internal surfaces.  Rb was introduced into the cell as a pure metal.  This was followed by cell evacuation using a laboratory vacuum system followed by the introduction of 40 psi of 3He gas (2.7 atm) at room temperature (~20°C).  The 3He gas is manufactured by Spectra Gases Inc. with a quoted 99.999% chemical purity and 99.9% isotopic enrichment.  The cell was placed in a close-fit copper oven with electric heaters that allowed us to maintain the cell temperature to within 1°C and fix the resulting Rb saturated vapor pressure inside the cell.The pump source used was a Ti:sapphire laser that produced up to 2.4 W of linearly polarized, near-diffraction-limited CW optical radiation.  The Ti:sapphire laser linewidth was ~9 GHz FWHM making the pump laser source narrow compared to the He-broadened D2 pump absorption feature, which we estimate is ~50 GHz wide based on the known Rb-3He collisional broadening rate of 20.8 GHz/amagat [11, 12]. The pump light was coupled into the 41.5 cm physical length laser cavity via a polarizing beam splitter and traversed the vapor cell twice by reflecting off the highly reflecting end mirror.  Stable cavity mode is formed with aid from gain guiding due to the nonuniform transverse gain profile over the length of our cell. The end mirror has a reflectivity of 0.99 at both the pump and lasing wavelengths.  The pump beam was aligned parallel to the laser cavity axis and focused to a 220 μm diameter spot size at the center of the cell, resulting in peak pump irradiance of nearly 5 kW/cm2.  Since the optic axes of the pair of sapphire windows were soldered onto the cell at unknown orientations, a half-wave plate was placed in the cavity to partially compensate for the polarization changes caused by birefringence.  We estimate that even with the half-wave plate, laser light traversing the cell had a 75% transmission efficiency passing the cube polarizer on its return path. Coupled with the uncoated window surfaces, the single-pass passive optical loss is close to 40% in our laser cavity.Laser emission at 795 nm in a TEM00 beam was observed at cell temperatures in the vicinity of 145°C.  Maximum output powers of over 350 mW were measured, corresponding to an optical-optical efficiency greater than 21%.  Large cavity losses along with the high gains that characterize alkali atoms place the optimal output coupler reflectivity below 0.2.  Presented in Figure 2 is the theoretical and measured laser output power versus pump power using different output couplers at a cell temperature of 142°C.  The theoretical curves overlaying the experimental data points were calculated using a laser model previously developed by Beach et al [4].  Because of the larger Rb-3He collision cross section compared to that of 4He, the mixing rate between the F-S levels is high enough to maintain efficient lasing at these pump levels.  The saturation effect observed in [3] for the Rb-4He laser in which the slope efficiency decreased at higher pump powers due to insufficient F-S mixing is significantly diminished here with nearly identical operating conditions, supporting the use of 3He with its larger F-S mixing rate forpower scaled systems.  Treating the Rb-3He F-S mixing rate and the mode overlap efficiency as adjustable parameters, the model curves were generated using an effective Rb-3He F-S mixing cross section value of 7.1×10-17 cm2 and 57% mode overlap efficiency.  Mode overlap is definedin [4] as the fraction of the pump excited volume in the alkali cell extracted by the circulating laser radiation in the resonator.  We note the Rb-3He fine-structure mixing rate that gives the best fit between our model and experimental data is approximately 1.5 times larger than our experimentally fitted value for the Rb-4He cross section reported in [3].In conclusion, we have demonstrated the operation of a 795-nm Rubidium resonance laser system using a buffer gas consisting of pure 3He.  The use of pure He for the buffer gas, either natural abundance or isotopically enriched 3He, as opposed to the more common approach to date of using a hydrocarbon containing buffer gas should enable high reliability Rb based diode-pumped alkali laser (DPAL) systems by eliminating the carbon deposition problem in the laser cells that occurs with the hydrocarbon approach [2].   For our pure He buffer gas approach, the use of isotopically enriched 3He offers multiple advantages over the use of naturally occurring He.  The higher thermal velocity of 3He than that of 4He give a Rb F-S mixing rate about 1.7 times larger for the 3He system than the 4He system under the same operating conditions.  This higher F-S mixing rate, which is critical for efficient laser performance, enables comparable performing 3He systems at only 58% of the He buffer gas pressure required for the 4He systems.  Since thermal aberrations in the laser’s Rb vapor cell are governed by dn/dT which is proportional to the He pressure, the 3He approach will be advantageous for high beam quality lasers in power scaled systems [4]. Another benefit of 3He based systems over 4He based systems is the higher thermal conductivity κ of 3He, 3 4: ~ 1.15 :1He Hek k -- an important consideration in thermal management driven designs of power scaled systems.  In view of these advantages of 3He over 4He based systems, we expect the 3He approach to be the preferred route to power scaling DPAL lasers to efficient, reliable, and good beam quality systems.  Due to their projected efficiency advantages over diode-pumped solid state lasers (DPSSLs), theircompatibility with commercially available laser diode arrays, and now a demonstrated system that promise very high reliability, diode-pumped 3He-only Rb lasers will potentially compete favorably with DPSSLs in many applications that require high beam quality CW or quasi-CW laser operation.We are grateful to Professor Paul Yu of UC San Diego; John O’Pray, Mike Tobin, Kevin Zondervan, Jim Kotora, Denise Podolski and Mark Rotter all of the Missile Defense Agency;and Chris Barty of Lawrence Livermore National Laboratory for their support and many useful discussions.  We also gratefully acknowledge the financial support provided by the Missile Defense Agency.  This work was performed under the auspices of the  U.S. DOE, by UC, LLNLunder Contract W-7405-ENG-48.*Also with the Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, California 92093-0407.Table 1.  Important physical parameters for Rb laser performance.  λMFP denotes the mean free path.  Other symbols have their usual meaning.Physical Parameter Expression Value for 3He / Value for 4HeMean particle velocity 8 8B BrHek T k Tvmp m p= »4 / 3Rb-He fine-structure mixingcross section2 23 / 2 1 / 2s®P P(experimentally determined [10])1.5 at T=142°CFine-structure mixing rate 2 2 2 23 / 2 1/ 2 3 / 2 1/ 2He rP P P Pn vg s® ®= 1.7Thermal conductivity / 2He r MFP Bn v kk l= 4 / 3Collisional broadening rate of Rb D2 line(experimentally determined [12])1.15Figure 1Ti:Sapphirepump laser isolator30 cm FL lens20 cmccO.C.poll/2 waveplate20 cmccH.R.Rb cellFigure 2Figure CaptionsFig. 1.  Schematic diagram of the experimental setup used in our demonstrations. The laser cavity mirrors have ~20 cm radii of curvature and are both concave (cc).  H.R. stands for high reflector, O.C. for output coupler and FL for focal length. The Rb cell is enclosed in a copper oven and heated to the operating temperature of 142°C.Fig. 2.  Rb laser output power for various output couplers plotted against pump power.  Solid curves represent model predictions.  From top to bottom, the reflectivities are: 0.19, 0.32, 0.46,0.48, 0.58, 0.69, 0.74, 0.85, 0.94 and 0.976.  The reflectivities were directly measured using a Ti:sapphire probe beam at 795 nm.References  1. W. F. Krupke, R. J. Beach, V. K. Kanz, and S. A. Payne,‘‘Resonance transition 795-nm rubidium laser,’’ Opt. Lett. 28, 2336–2338 (2003).2. R. H. Page, R. J. Beach, V. K. Kanz, and W. F. Krupke, "Multimode-diode-pumped gas (alkali-vapor) laser," Opt. Lett. 31, 353-355 (2006).3. S. S.Q. Wu, T. F. Soules, R. H. Page, S. C. Mitchell, V. K. Kanz, R. J. Beach, “Hydrocarbon-free resonance transition 795-nm Rubidium laser,”  Accepted for publication in Optics Letters.4. R. J. Beach, W. F. Krupke, V. K. Kanz, S. A. Payne, M. A. Dubinskii, and L. O. Merkle, "End-pumped continuous-wave alkali vapor lasers: experiment, model, and power scaling," J. Opt. Soc. Am. B 21, 2151-2163 (2004).5. T. Ehrenreich, B. Zhdanov, T. Takekoshi, S. P. Phipps, and R. J. Knize, ”Diode Pumped Cesium Laser”, Electronics Lett. 41, 47-48 (2005).6. Y. Wang, T. Kasamatsu, Y. Zheng, H. Miyajima, H. Fukuoka, S. Matsuoka, M.Niigaki, H. Kubomura, T. Hiruma, H. Kan, “ Cesium vapor laser pumped by a volume-Bragg-grating coupled quasi-continuous-wave laser-diode array”, Appl. Phys. Lett. 88, 141112 (2006).7. B. Zhdanov, C. Maes, T. Ehrenreich, A. Havko, N. Koval, T. Meeker, B. Worker, B. Flusche and R. J. Knize, “Optically Pumped Potassium Laser”, Opt. Com. 270, 353-355 (2007).8. Z. Konefal, “Observation of collision induced processes in rubidium-ethane vapour”, Opt. Com. 164, 95-105 (1999).9.  E. S. Hrycyshyn and L. Krause, ‘‘Inelastic collisions between excited alkali atoms and molecules. VII. Sensitized fluorescence and quenching in mixtures of rubidium with H2, HD, D2, N2, CH4 , CD4 , C2H4 , and C2H6,’’ Can. J. Phys. 48, 2761–2768 (1970).10.  A. Gallagher, “Rubidium and Cesium Excitation Transfer in Nearly Adiabatic Collisions with Inert Gases”, Phys. Rev. 172, 88 (1968).11. A. Andalkar and R. B. Warrington, “High-resolution measurement of the pressure broadening and shift of the Cs D1 and D2 lines by N2 and He buffer gases”, Phys. Rev. A 65, 032708 (2002).12. M. V. Romalis, E. Miron, and G. D. Gates, “Pressure broadening of Rb D1 and D2 lines by 3He, 4He, N2, and Xe: Line cores and near wings”, Phys. Rev. A 56, 4569–4578 (1997).

Resonance transition 795-nm Rubidium laser using 3He buffer gas

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Resonance transition 795-nm Rubidium laser using 3He buffer gas

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