High Power CO Overtone Laser Willy L. Bohn*, H.-A. Eckel, W. Riede, and S. Walther DLR Institute of Technical Physics, D-70569 Stuttgart, Germany ABSTRACT A modified electron beam controlled pulsed CO2 laser is used as a multi spectral multi purpose test bed in order to generate high power fundamental and first overtone laser transitions in CO. The revisited concept includes an all solid state power supply which provides a highly reproducible operation at pulse repetition frequencies of up to 100 Hz. The active gas mixture is recirculated in a closed loop and kept at near room temperature using conventional water cooling. Discrimination of the CO fundamental band is obtained by using specially coated dielectric mirrors and introducing additional intracavity diaphragms. Unprecedented laser pulse energies of 25 J are reported in the overtone transitions covering a spectral range between 2 µm and 3.5 µm. Further scaling of pulse energies is expected in the near future using larger diameter resonator mirrors. Keywords: CO overtone, pulsed gas lasers, electron beam controlled lasers, mid-IR lasers.

1. INTRODUCTION Although CO lasers have been widely investigated in the 1970s and 1980s, they disappeared from the mainstream interest of the laser community due to their poor atmospheric transmission properties. Recently, new interest in CO lasers has emerged driven by the capability of generating overtone laser radiation in the mid-IR spectral range centered between 2 µm and 4 µm. This novel approach represents a potential alternative to solid state laser sustained OPO systems and could extend their current performance to considerably higher power levels while maintaining broad frequency coverage. If successful, this might enhance applications in the areas of optical countermeasures, long range LIDAR and chemical agent detection systems. Experimental and theoretical overtone laser has been intensively studied lately, in particular by Russian scientists at the Lebedev and Troitsk Institutes1,2. Those investigations have concentrated on generating overtone radiation in electron beam discharges at liquid nitrogen operating temperatures. We have reported overtone radiation at room temperature3,4 and this article extends our past work to higher power levels. The room temperature (or near room temperature) operation offers substantial advantages for future implementation of a CO overtone laser in any kind of mobile platform. In this article two sets of experimental results determined by the availability of overtone mirrors will be presented and discussed. First, a discharge parameter optimization will be performed with small diameter overtone mirrors. Second, with larger diameter mirrors a greater discharge volume will be accessed leading to unprecedented high overtone output values.

2. THE LASER TEST BED The experimental facility which has been described elsewhere is basically an electron beam controlled non self sustained large volume discharge originally developed for operation of a repetitively pulsed, high average power CO2 laser. The “revisited” concept includes an all solid state advanced pulsed power supply using IGBTs (Integrated Gated Bipolar Transistors) and HCT (High Current Thyristors). A series of capacity charging devices are loading the main discharge up to its designated voltage below the breakdown value. The electron beam fires the discharge and sustains it over the desired period of time which ranges between 2 µs and 15 µs. This leads to a high pulse to pulse reproducibility as shown in Fig.1 by 2 arbitrarily chosen sets of 5 pulses each out of a train of 1000 pulses. The power supply elements are modular and compact and guarantee a reliable laser operation. Also shown in Fig.1 is the schematic of the whole device as well as a photograph of the hardware. The laser is operated in a closed gas loop using three blowers to *

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a b Fig. 1: Multispectral testbed: a) schematics; b) hardware photograph; c) pulse reproducibility

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EL [J/l·bar]

accommodate a homogeneous flow field for the 1.6 m wide transverse discharge with an active volume of 12 l. While running in a CO2 gas mixture the device delivers up to 500 J in single pulse operation and 150 J/pulse at a repetition rate of 100 Hz; thus, totaling an average output power of 15 kW. Since this performance is only limited by the existing power supply, the device is in principle capable of delivering 50 kW of CO2 average power in a compact structure not exceeding 2x2x2 m3. Depending on the gas fill and appropriate adjustment of the discharge parameters the test bed can be operated either as CO2, CO or Ar:Xe laser with 100 wavelengths of 10 µm, 5 µm and 1.7 µm, respectively. CO2 10 % ArXe This is summarized in Fig.2 which exhibits the normalized COfundamental laser pulse energy as a function of the discharge loading. 10 Whereas 6 J/l × bar are achieved in Ar:Xe up to 1% 50 J/l × bar are routinely obtained in CO2. In different series of measurements CO energies of up to 80 J/l × bar have been realized with discharge efficiencies close to 20 1 0.1 % % at room temperature. This is a remarkable result since the only cooling element used in the gas loop is a simple heat exchanger operated with conventional water. 0.1

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Fig. 2: Multispectral specific energy performance

This multi-spectral multi-purpose laser test bed is also used for applications related to paint removal and to laser propulsion. In particular, CO2 laser operation is used in order to propel a laser lightcraft as will be reported by Schall et al. at this conference.

3. GENERATION OF CO OVERTONE RADIATION CO overtone performance was basically limited by the mirror size which has been available to conduct the experiments. In a first step a 1.5 l discharge volume (out of 12 l) has been accessed by small diameter overtone mirrors. In a second step an extended volume of 7 l has been accessed by using larger diameter overtone mirrors. In all cases, however, the rear cavity mirror was a copper mirror since no appropriate dielectric mirror was available during the tests. This has limited the attainable performance in all our investigations. 3.1 Optimized results obtained with a reduced discharged volume The schematics of the first overtone energy level diagram and its corresponding frequency shift as compared to the fundamental transitions is shown in Fig.3. Overtone lasing can only be achieved by partial or total suppression of the fundamental lasing transitions. As

Laser Spectrum

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Fig. 3: Fundamental and overtone spectrum

usual, this is obtained by appropriate choice of the optical cavity mirror coatings. As exhibited in Fig.4 we have used an output coupling mirror with a 98 % transmission for the fundamental and a 2 % transmission for the overtone. Due to the reduced diameter of the available overtone mirror the accessed discharge volume was restricted to 1.5 l. The rear cavity mirror was a copper mirror with no transmission for either wavelength band. Optimization results with respect to CO fraction, output coupling mirror reflectivity and operating gas temperature are shown in Cu T( ) = 98 % Fig.5a, b, and c, respectively. Laser output energies increase by Fig. 4: Overtone optical resonator configuration more than a factor of 2 while decreasing the mirror reflectivity from 99 % to 98 % and 97 %. This is roughly maintained for CO fractions ranging between 12.5 % and 20 %. The most dramatic change in output energy is given as expected by variations in operating temperature. For 5 5 repetitively pulsed operation at 30 Hz without any kind of cooling the gas loop T=3% 4 4 equilibrates at around 45°C. Making use T=2% of conventional water cooling in the 3 3 heat exchanger we are able to decrease 2 2 the gas temperature down to 5°C, and T=1% the laser pulse energy almost increases 1 1 by a factor of 6. This gas temperature will basically be maintained for all 0 0 0 5 10 15 20 25 100 99 98 97 96 further investigations. Experimental CO-fraction (%) R (%) results for fundamental and overtone a b c transitions are summarized in Fig.6 as a Fig. 5: CO-overtone optimization with respect to CO-fraction (a), function of specific loading of the mirror reflectivity (b), and gas temperature (c) discharge. Improved results for the T(1, 2) = 0

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overtone are obtained while reducing the discharge pressure from 400 to 325 mbar. The maximum absolute value corresponds to 8 J at an efficiency slightly exceeding 2 %. The spectral overtone coverage is exhibited in Fig.7 as a function of wavelength and wave number, respectively. The vibrational transitions involved are shown as measured with a corresponding spectrum analyzer without resolving the rotational bands. For further information the 80 21 - 19

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measured transmission profile of the outcoupling mirror has been included into Fig.7. This indicates that the overtone spectrum is probably limited to longer wavelengths by the corresponding mirror transmission data. 3.2 Results obtained with an extended discharge volume Using a second set of mirrors with larger diameters we have been able to access 7 l out of the 12 l available in the main discharge. While the rear cavity mirror was still the same copper mirror as in 3.1, the outcoupling mirror showed a transmission of 2 % and

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Fig. 6: Specific laser performance for fundamental and overtone band

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95 % for the overtone and the fundamental transition, respectively. Two different approaches were chosen to discriminate the overtone and fundamental bands as shown 2 3 4 5 6 in Fig.8. In a first series of experiments a Wavelength (µm)  55 % CO InAs filter was used to suppress the  0 % CO - Overtone overtone as indicated in the transmission Cu T(1) = 95 % profile in the upper right part of Fig.8. In a 1.0 second series of experiments several high 0.8 HR HR 0.6 reflectivity plates have been used with a 0.4 total transmission of 90 % for the 0.2 0.0 fundamental and 2 % for the overtone 2 3 4 5 6 Wavelength (µm) transitions. The corresponding  90 % CO transmission profile is given in the lower  2 % CO - Overtone right part of Fig.8. Both approaches have been cross-checked for every further Fig. 8: Schematic setup of overtone band measurement determination of the CO overtone radiation. An additional discrimination of the CO fundamental band has been achieved by introducing two additional apertures inside the cavity with sandblasted Aperture #1+#2 100 2 1 just in front of the rear and T( ) = 2 % T(1,2) = 0 2 90 the outcoupling mirror, 80 with sandblasted Aperture #1 respectively, as shown in 70 Fig.9a. The corresponding 60 overtone fraction as a 50 without Aperture function of discharge loading 40 is presented in Fig.9b. Cu T(1) = 95 % 1100 1200 1300 1400 1500 1600 Without use of any aperture Accessible Volume: 7 l E0 (J) limit the overtone fraction a b decreases to about 40 % at Fig. 9: Intracavity discrimination of CO fundamental: a) schematics, b) experimental results maximum discharge loading. T(2) = 2 %

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Introducing the sandblasted apertures #1 and #2 increases this figure to about 55 % and 70 %, respectively. For small power loadings the use of both apertures almost completely suppresses the fundamental band. Fig.10 summarizes the overtone laser performance obtained with both apertures for two different pressures and CO fractions. Under current optimum conditions 25 J are obtained in either case with efficiencies of 1.3 % and 1.5 %, respectively. By simple volume scaling of our previous results obtained from a discharge volume of 1.5 l, a laser pulse energy of 40 J should be expected; this value is referenced in Fig.10 by a straight line. This scaling discrepancy is not yet fully understood and needs further investigation. While the difference in laser energy obtained at 400 mbar and 430 mbar is rather substantial at small discharge loadings it completely levels out at higher loading values.

4. CONCLUSIONS AND PERSPECTIVES We have shown CO overtone laser performance at near room temperature operation for two series of measurements determined by the availability of overtone laser mirrors. A first parameter optimization has been undertaken using small diameter mirrors and accessing a discharge volume of only 1.5 l out of 12 l in the main discharge. Using a second set of mirrors an extended discharge of volume of 7 l could be interrogated. Additional discrimination of the CO fundamental band using sand blasted apertures inside the cavity we have obtained unprecedented near room temperature CO overtone energy pulses up to 25 J. From a simple discharge volume scaling overtone energies of at least 40 J are projected. Furthermore, referring to the repetitively pulsed operation capability of the device up to 100 Hz a CO overtone average output power of 4 kW may be expected in the future.

REFERENCES 1. A.A. Ionin, A.A. Kotkov, A.K. Kurnosov, A.P. Napartovich, L.V. Seleznev, N.G. Turkin ”Parametric study of first-overtone CO laser with suppressed fundamental band lasing: experiment and theory” Optics Communications 155 (1998) 197-205 2. N. Basov, G. Hager, A. Ionin, A. Kotkov, A. Kurnosov, J. McCord, A. Narpatovich, L. Seleznev, N.Turkin ”Pulsed first-overtone CO laser with output efficiency higher than 10 %” Optics Communications 171 (1999) 107-112 3. W.L. Bohn ”Pulsed CO2 laser for space propulsion” Proc. High Power Laser Ablation III, Vol. 4065 (2000), paper 4065-58 4. W.L. Bohn ”High Power IR Gas Lasers” Xth Conference on Laser Optics, St. Petersburg, Russia, June 26-30, 2000 (paper Tu A2-2)