Laser Excitation Dynamics of Argon Metastables Generated in Atmospheric Pressure Flows by Microwave Frequency Microplasma Arrays

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1 Physical Sciences Inc. Laser Excitation Dynamics of Argon Metastables Generated in Atmospheric Pressure Flows by Microwave Frequency Microplasma Arrays W.T. Rawlins, K.L. Galbally-Kinney, S.J. Davis Physical Sciences Inc., Andover, MA A.R. Hoskinson, J.A. Hopwood ECE Department, Tufts University, Medford, MA VG High Energy/Average Power Lasers and Intense Beam Applications VIII SPIE Photonics West 2014 San Francisco CA 2 February 2014 Paper Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

2 Optically Pumped Rare Gas Metastable Laser Physical Sciences Inc. VG Converts high-power diode laser output into high-quality near-infrared output beam Rare gas atomic system offers chemically inert analog to alkali-atom systems (DPAL, XPAL) Similar scaling characteristics Han and Heaven (Emory Univ.) demonstrated lasing on Ar, Kr, Xe using pulsed discharge {J. Han and M.C. Heaven, Opt. Lett. 37, (2012)} Microwave microplasma can be used for CW excitation at atmospheric pressure: scaling gain, output power, efficiency Proprietary Information / SBIR Rights in Data / ITAR or Export Controlled

3 High Power Optically Pumped Gas Lasers Physical Sciences Inc. VG Three level systems Alkali and Rare Gas Lasers Optically efficient (>50%) Excellent beam quality Large range of selectable wavelengths (visible to mid-ir) Compact and take full advantage of high power diode lasers Examples DPAL (Diode-Pumped Alkali Laser ) Optically Pumped Rare Gas Metastables: Microplasma Laser Proprietary Information / SBIR Rights in Data / ITAR or Export Controlled

4 Microwave Micro-Discharge Array Technology J. A. Hopwood and Coworkers, Tufts University VG mm Low-power (<30 W), inexpensive, compact, safe to operate CW operation at atmospheric pressure (~1 GHz) High field strength enables large E/N, [e - ] ~ cm -3 In development: multi-array designs for increased volume

5 Initial Resonator Design for Ar/He Micro-Discharges 915 MHz 15-Resonator Micro-Discharge Array Design: 25 mm gap Computed E-Fields (1 W) Prior to Ignition VG Pre-ignition field strength scales as (power) 1/2 : ~1000 Td at 9 W Estimate [e - ] ~ to cm -3 Visible plasma dimensions 900 mm along flow, 300 mm above board 19 mm array length suitable gain length for initial experiments

6 Ar(I) Energy Levels: 4s and 4p States Paschen (at left) and Racah Notation VG Possible Laser Transitions: Pump s 5 p 9, nm Lase p 10 s 5, nm (illustrated) Pump s 5 p 7, nm Lase p 7 s 4, nm or p 8 s 4, nm or p 9 s 5, nm or p 10 s 5, nm Lower states: s 5, s 3 are metastable s 4, s 2 have strong resonance transitions to ground state: radiatively trapped

7 Schematic of Ar* Optical Pumping Experiment Physical Sciences Inc. VG Diode Laser (Probe) Detector Window Lens Lens Grating Polarizing, BS Cube Window Window Polarizer WM Rotating Mirror Pump Laser Probe Laser Beam Dump PD Experimental setup allows for simultaneous laser-induced fluorescence imaging and gain measurements of the microplasma discharge Infrared camera for spatial distributions InGaAs array monochromator for spectral measurements Easily modified for lasing experiments or gain imaging Optical setup similar to previous experiments with alkali excitation L-0506

8 Photograph of Apparatus: Micro-discharge and Ar* Optical Pumping VG Gas flow Excitation laser beam and cavity are directed along the length of the linear microdischarge array 15 resonator strips, 1.9 cm gain length Ar + He flows, Torr CW Ti:S pump laser, ~1 W

9 Illustration of Microplasma Laser Excitation Physical Sciences Inc. VG Gas Flow Excitation Laser

10 Spectral Measurements of Laser-Induced Fluorescence (1 atm) VG p 10 1s 5 2p 10 1s 4 Scaling of LIF Intensity, 2p 10 1s 5 Observe absolute spectral intensities: photons/cm 2 -s Excellent agreement with NIST branching ratio for 2p 10 1s 5, 1s 4 LIF intensity = {Ti:S on} {Ti:S off, discharge only} Provides means to measure 2p 10 number density Combine with spatially resolved fluorescence images

11 Flow Rate Dependence of 2p 10 (1 atm) Physical Sciences Inc. Discharge Only Laser-Induced Fluorescence VG Laser-pumped 2p 10 has weak dependence on flow rate (velocity) cm/s, t ~ 0.5 to 1 ms Pronounced maximum ~2% Ar in He LIF is clearly separable from discharge variations

12 NIR Images: Discharge and Laser-Induced 2p 10 Fluorescence (1 atm) VG Pump wavelength: nm (1s 5 2p 9 ) NIR images: nm (2p 10 1s 5, 1s 4, 1s 3 ) Microplasma width along gas flow vector mm

13 Images: LIF(2p 10 ) vs. Pump laser Power (1 atm) 25 mm Focal Length Lens VG mw 100 mw 200 mw 300 mw 500 mw 600 mw

14 Profiles of 2p 10 LIF and Discharge Emission (1 atm) 25 mm Focal Length Lens VG Laser-Induced Fluorescence Micro-Discharge Emission Attenuation of 811 nm beam at 50 mw => [1s 5 ] 5/7 [2p 9 ] > 3 x cm -3

15 Spatial and Spectral Scaling with Pump Power (1 atm) 25 mm Lens VG Preliminary Estimate: Maximum 2p 10 intensity => [2p 10 ] ~ 4 x cm -3

16 Effect of Probe Laser on Absorption Measurements (1 atm) VG Initial probe laser power ~1 mw: sufficient to pump the medium (!) Absorption line width consistent with 17 MHz/Torr, 600 K Maximum absorbance => [Ar(1s 5 )] dscg 3 x cm -3 produced in micro-discharge

17 Absorption Line Shape Physical Sciences Inc. VG

18 Optical Gain at nm: 2% Ar/He, 1 atm 9 W Discharge, Ti:S Intensity = 4 kw/cm 2 at nm VG

19 Gain and Absorbance vs. Pump Intensity Physical Sciences Inc. VG Gain = {ln(i/i o )}/(1.9 cm) Absorbance = ln(i o /I)

20 Gain and Absorption Line Widths Physical Sciences Inc. VG Absorption, pump laser off: Positive gain, pump laser on: (Absorption gain) shift: 17.6 ± 0.9 GHz 11.4 ± 2.2 GHz 0.8 ± 0.3 GHz

21 Schematic for Ar* Laser Power Extraction Physical Sciences Inc. VG Lens M1 Window M2 Grating Polarizing, BS Cube Window Window Polarizer WM Rotating Mirror Pump Laser Output Laser M1 = Max-R M2 = Max-R or 15% Trans Beam Dump PD L-0507a

22 Illustration of Laser Cavity Design Physical Sciences Inc. M1 TEMoo mode VG Ti:S Beam Microplasma µm Beam Splitter 300 µm 1.9 cm Output Beam M2 L-0548 Stable resonator: low-order mode with diameter < pump laser diameter

23 Gain Data Prior to Lasing Physical Sciences Inc. VG G o = 1.1 cm -1

24 Optically Pumped Argon Microplasma Laser Atmospheric Pressure Ar/He Micro-Discharge VG CW laser output at nm 22 mw with 15% output coupler 40 mw absorbed at nm Optical efficiency ~ 55% Observed stable output for greater than 30 minutes Excellent beam quality

25 Three-Level Model for Optically Pumped CW Gain Heaven et al. Kinetics VG IB 13 IB 31 A 31 IB 12 k c [M] IB 21 A P (g 3 = 7) 9 2 k Q [M] 2 P (g 2 = 3) 10 n n IB13 k IB 1 c M n 3 31 A31 kc M 2 A k M n 3 n IB A n A n n IB TOT n1 n2 n Q 1 1 S (g 1 = 5) 5 L-0372a Steady-state solution for open-loop gain Zeroth order treatment: no losses outside of 3-level manifold 21 stimulated emission is negligible In saturated ( bleached ) limit: G 21 = (1.76 x )n TOT, cm -1 4-level model: include 1s 4 state 2p 9 1s 4 + hn(966 nm) 1s 4 + M 1s 5 + M cm -1 (slow??) I = (I o /dn) exp(-s 13 N 13 z )dz, N 13 = n 1-5n 3 /3 If n TOT 5.7 x cm -3 G 21 1 cm -1

26 Steady State: Gain vs. Pump Intensity 3-Level Model VG

27 4-Level Model: Bleached Limit Sensitivity to Collisional Coupling Kinetics VG

28 Summary: Initial Ar* Investigations Physical Sciences Inc. VG Microplasma kinetics in dilute Ar/He mixtures support high gain, CW lasing at 1 atm Microwave micro-discharges are viable media for OP-RGL Observe G o ~ 1 cm -1 for [1s 5 ] dscg ~ (3 to 5) x cm -3 Comparable to DPAL gain for similar configuration Consistent with 3-level steady-state model (omit 1s 4 state coupling) 4-level model with Emory kinetics much lower G o /[1s 5 ] dscg Emory data in pulsed discharge show slow 1s 4 1s 5 collisional exchange: CW lasing would require [1s 5 ] dscg > cm -3 PSI data for CW micro-discharge indicate faster 1s 4 1s 5 exchange, possibly due to higher T in discharge-flow (~600 K) Systematic measurements of CW gain kinetics can provide key data for models

29 Acknowledgements Physical Sciences Inc. VG Michael Heaven, Glen Perram, Gordon Hager High Energy Laser Joint Technology Office (Emory University) Air Force Research Laboratory/AFMC (Dr. Steve Adams) Air Force Office of Scientific Research