Small Signal Gain in DPAL Systems

Transcription

1 Physical Sciences Inc. VG Small Signal Gain in DPAL Systems Kristin L. Galbally-Kinney, Daniel L. Maser, William J. Kessler, Wilson T. Rawlins, and Steven J. Davis 20 New England Business Center Andover, MA SPIE Conference 7915, High Energy/Average Power Lasers and Intense Beam Applications VI Paper , Time: 11:10 AM - 11:30 AM January 2011 Acknowledgement of Support and Disclaimer This material is based upon work supported by Air Force Office of Scientific Research under Contract Number FA Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of Air Force Office of Scientific Research. Distribution Statement A: Approved for Public Release; Distribution is Unlimited 20 New England Business Center Andover, MA 01810

2 Outline VG Alkali atom-rare gas molecules Description of PSI apparatus Absorption spectroscopy of exciplex species Multi-photon excitation Alkali atom absorption/gain spectroscopy Summary

3 Exciplex Effect: Alkali-Rare Gas Collision Pairs Cs-Ar Potential Energy Diagram 1.2 Cs-Ar Absorption Spectra Cs Torr Ar, 448 K VG Transmission Cs-Ar Cs-Ar + 75 Torr Ethane Wavelength, nm Van der Waals collision pair provides continuum molecular absorption over several nm spectral range B-state dissociates directly to 2 P 3/2, can lase on either transition Allows efficient coupling of spectrally broad excitation sources to alkali atoms NO LINE NARROWING REQUIRED

4 Apparatus for Alkali-Rare Gas Spectroscopy VG Longitudinal pump: 0.5 W Ti:S laser Co-linear TDL beam for gain measurements Side view for fluorescence spectrometer

5 Exciplex Absorption and Fluorescence Spectroscopy Cs, Rb + He, Ar, Kr, Xe

6 Laser Excitation of Cs(6 2 P) Fluorescence via CsKr Exciplex Absorption Direct pumping of D 1, D 2 lines throughout exciplex band VG Fluorescence Intensity CsKr(B X) (within Exciplex Band)

7 Absorption Spectroscopy: General Observations VG Rare gas effects for Ar, Kr, Xe are similar EXCEPT: Bandhead moves to shorter wavelengths for smaller rare gas atoms Broadening effect Ar > Kr > Xe UIUC: combine rare gases to fill in exciplex absorption spectrum between bandhead and D 2 line C 2 H 6 effect enhances exciplex absorption Mechanism is unknown Are there other collision partners that do this? Emory Univ.: potential energy calculations for Rb + CH 4 in progress Exciplex effect is small for He: not significant in conventional DPAL

8 Emission Spectra Produced by Multiphoton Absorption

9 Multiphoton Excitation of Cs(I) Fluorescence Excitation Near 852 nm VG Cs Torr Kr, 473 K

10 Blue Cs Doublet Emission at λ excitation = 852 nm Cs Torr Kr, 473 K VG (a) (b)

11 Multiphoton Excitation of Infrared Alkali Transitions by Atom and Exciplex Absorption VG P 3/2 1/2 7 2 P 3/2 1/2 6S 1/2 7 2 S 1/2 3/2 4D 5/2 3/2 52 D 5/2 Cs 3/2 1/2 62 P Rb 3/2 1/2 5P 6 2 S 1/2 5S 1/2 From: Sharma et al., APL 38, 209 (1981) K-2120 Infrared lines can be lased via multiphoton excitation of higher states

12 Infrared Cs(I) Fluorescence: InGaAs Array Spectrometer Spectral Resolution = 0.3 nm Excitation of CsXe at 852 nm VG FTIR Spectrometer Spectral Resolution = 2 cm -1 (0.002 nm) E-03 Signal Intensity, counts/s P J' D J" 3/2 3/2 1/2 1/2 3/2 5/2 1/2 5/2 7 2 S J' 6 2 P J" 1/2 3/2 Signal Intensity 1.6E E E E E E E E-04 3/2 1/2 Quartz Transmission 3/2 1/2 1/2 1/2 7 2 P J' 7 2 S J" 5 2 D J' 6 2 P J" 5/2 3/2 3/2 3/ E Wavelength, μm -2.0E Wavelength, μm Excitation observed at very low pump power (~100 mw) Implies 2-photon pumping via intermediate exciplex state Will be prominent process at high pump power

13 Cs-Ar Energy Level Diagrams: Pathways for Resonant 2-Photon Excitation Current XPAL Stable and Repulsive Upper Levels Cs(7 2 S)+Ar( 1 S) VG B 2 Σ + 1/2 A 2 Π 3/2 A 2 Π 1/ nm 837 nm 6 2 P 3/2 6 2 P 1/ nm Excitation (1)X 2 Σ + (AQCC5-AB1) (2)B 2 Σ + (AQCC5-AB1) (3)C 2 Σ + (AQCC5-AB1) (4)E 2 Σ + (AQCC5-AB1) (1) 2 Δ (AQCC5-AB1) (1)A 2 Π (AQCC5-AB1) (2)D 2 Π (AQCC5-AB1) Cs(5 2 D)+Ar( 1 S) 0 X 2 Σ + 1/2 6 2 S 1/ Cs(6 2 P)+Ar( 1 S) Interatomic Distance (Å) Potential energy curves for Cs-Ar suggest possibility of efficient two-photon pumping path R(A) from Prof. M. Heaven K-2131

14 Gain Spectroscopy and Imaging

15 DPAL/XPAL Gain Measurement Test Bed (Diode laser scanning D 1 line) VG Direct probe of population inversion dynamics Aids in design of optical resonators Portable: take to other facilities Can extend to spatial imaging of gain Expect significant spatial effects in power scaling Valuable tool for scaling DPAL to high powers

16 Optical Layout for DPAL/XPAL Gain Measurements VG

17 Relative Absorbance Computed D 1 Absorption Spectra: Cs Low Pressure, Doppler broadening F" = 4 F" = 3 F' = 3 4 F' = 3 4 Collisional Broadening Effect Cs 2 S 1/2 2 P 1/2, 894 nm Relative Absorbance VG High Pressure, collisional broadening Computed Spectrum Cs D 1 Multiplet 500 Torr Kr + 75 Torr C 2 H K Relative Frequency, MHz Relative Frequency, MHz Collisional broadening greatly expands required scan range High optical thickness at elevated temperatures

18 Absorption/Gain Spectra: Cs( 2 S 1/2,F =4 2 P 1/2,F ), 894 nm 500 Torr Kr + 75 Torr C 2 H 6, 338 K VG Pump Laser: 2 S 1/2 2 P 3/2, 852 nm -2 Net Absorbance, ln(i o /I) GAIN W/cm ABSORPTION Relative Frequency, MHz Continuing work: investigate absorption and gain dynamics for DPAL, XPAL configurations: Cs, Rb, K

19 Dependence of Gain on Pump Power VG Integrated D 1 Gain, G(ν)dν Rare Gas = 500 Torr C 2 H 6 = 75 Torr T = K Pump D 2 Absorption Cs-Xe-C 2 H 6 Cs-Kr-C 2 H 6 Rb-He-C2H6 Cs-Ar Cs-Xe Ti:S Pump Power, mw

20 State-Selected Absorption and Saturation VG Neat Cs: D 1 Absorption, Pump D 2 (F") F" = 4 F" = 3 Absorbance, %/cm F' = 3 4 F' = No Pump F" = 4 pump Relative Probe Laser Frequency, GHz D 1 (F") Peak Absorbance, %/cm Neat Cs T = 350 K Pump D 2 (F") F" = 4 F" = Ti:S Power, mw

21 Imaging Gain Diagnostic: A Critical Tool for Power Scaling of Alkali Lasers VG Concept: Pump Lasers Pump Lasers Gain Profile Alkali Gain Cell K-4802 Experimental Verification Single pass unsaturated gain Provides estimate for available output power Spatially resolved gain Probe Beam Can measure both longitudinal and transverse gain Predicts output beam quality Applicable to static and flowing systems Effects of heat deposition, fluid dynamics, radiation trapping, particles Key data to optimize resonator design, output power, and beam quality Alkali Cell Ti: S Pump Beam K-5034

22 3-D Image of D 1 Gain, Absorption Cs Torr Kr + 75 Torr C 2 H 6 Probe beam diameter > pump beam diameter Gain profile follows Gaussian profile of pump beam Elevated base absorbance: effects of radiation trapping VG

23 Exciplex effect: Ar, Kr, Xe Summary VG Broadens absorption to several nm, direct inversion of D 2 line Ethane enhances exciplex absorption: mechanism? other collision partners? Laser-induced fluorescence: tracks molecular potentials Evidence for direct excitation of D 1, longer B-state lifetime Multiphoton excitation of infrared atomic transitions (1 to 4 μm): possible laser candidates Observe exciplex-assisted pumping of higher Cs* states Excitation mechanism may involve real molecular states rather than virtual intermediates more efficient process Likely significant phenomenon at moderate to high pump power Optical diagnostic tools required for gain, key species concentrations Fiber-coupled gain diagnostics in progress for Rb, Cs Emphasize high-resolution spectral, spatial information imaging

24 Acknowledgements M. C. Heaven, J. Merritt, J. Han Emory University J. G. Eden, J. D. Readle, C. J. Wagner, J. J. Coleman, N. L. Dias, V. B. Verma University of Illinois at Urbana-Champaign J. T. Verdeyen, D. L. Carroll CU Aerospace T. Madden, D. Hostutler AFRL HEL-JTO and AFOSR