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was adjusted to give optimum output power at 100 kHz. The tube pressure here was 1.5 torr. At pulse rates below 100 kHz, the output power consisted (green) wavelength. As the almostentirely of the 5 106 pulse rate was increased, the 5782 (yellow) wavelength became more apparent until, at 150 kHz, the output was almost entirely yellow. At 100 kHz,essentially thesameoutputpower was obtained with both the 50 percent transmitting output mirror and a sapphire etalon of 70 percent effective transmission. Inconclusion,operation of a compactcopper vapor laser achieved, and selfat apulse rate up to 150 kHz has been mode-locked operation has been observed. Efforts are underway to reduce the rise time of the current pulse to increase the efficiency of the device above the value of 0.05 percent achieved to date.

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ACKNOWLEDGMENT The author wishes to thank M. Conroy for his expert technical assistance.

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Fig. 3. Output poweras a function ofbuffergaspressure.Pulse was 100 kHz.

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REFERENCES [ l ] T. S. Fahlen,“Self-heated,multiple-metal vapor laser,” IEEE J. Quantum Electron., vol. QE-12, p. 200, Mar. 1976. [ 21 T. Li, “Diffraction loss and selection of modes in maser resonators with circular mirrors,” Bell Syst. Tech. J . , vol. 45, p. 917, May 1965.

33-W CW Dye Laser P. ANLIKER, H. R. LUTHI, W. SEELIG, J. STEINGER, HEINZ P. WEBER, S. LEUTWYLER, E. SCHUMACHER, AND L. WOSTE Abstract-Acontinuousrhodamine 6G jet stream dye laserwas pumped by a high-power multimode argon laser. Using water with a viscosity-raising additive as a solvent, no thermooptical problems arose of 110 W. Themaximum output up to the availablepumppower power of the CW dye laser was 33 W at an efficiencyof 30 percent.

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Tunable CW lasers with good coherence properties and high power are of great interest for many applications in the fields PULSE REPETITION M T E (KHz) of nonlinear optics and spectroscopy, photochemistry, andisotope separation. The most promising system in the visible for Fig. 4. Output power as a function of pulse rate. Pressure was 1.5 torr. this purpose is at present the jet-stream dye laser. The maximum output power forCW dye lasers reported in the literature is smaller than 10 W [ 1] -[ 31 . This power is usually generated optical pulse rise time was 1 ns, which is equal to the instru- with an overall efficiency of 25 to 35 percent. In this correspondence we report 33-W output power of a rhodamine 6G jet mentation rise time.Theperiod of the mode-lockedpulses stream dye laser pumped by ahigh-power argon laser at an was 3.8 ns, which was equal to the round trip cavity .transit efficiency of 30 percent. time. Thedye laser was pumpedcontinuously by the all-lines After the tube had operated for about an hour and had becomeuniformlyheated,themode lockingdisappeared, the radiation of a high-power Ar-I1 laser [ 41 oscillating in a highly power increased, and the optical pulse duration decreased to 5 transverse multimode. The diameter of the argon laser beam divergence about 2 mrad.This ns. Here, the average output power was 35 mW and the pulse was about 2 mmandthe repetition rate was 100 kHz. The mode-locking phenomenon corresponds to radiation containing up to the30th-order transapparently depends on a nonuniform distribution of copper verse modes. The maximum available output powerwas 110 W. vapor within the tube. It is surmised that mode locking occurs This beam was prefocused bya lens of focal length 50 cm due to saturableadsorption of the beam byheatedcopper vapor lying outside the active discharge volume. Manuscript received March 1, 1977;revised March 29, 1977. A graph of the output power as a function of total pressure P.Anliker, H. R. Luthi, W. Seelig, J. Steinger, and H. P. Weberare is shown in Fig. 3. Here, the pulse rate was 100 kHz and the with the Institute of Applied Physics,University of Berne,Berne, tube was uniformly heated. The power was measured with an Switzerland. Eppley thermopile. A graph of output power as a function of S. Leutwyler, E. Schumacher, and L. Woste are with the Institute for Inorganic Chemistry, Berne, Switzerland. pulse repetition rate is shown in Fig. 4. The heater current 070

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Fig. 1. Dye laser output power versus argon laser power.

to reduce beam diameter, Final focusing with a mirror of 5cm radius of curvature resulted in a spot measured as 60-pm diameter in the jet at 30 W and 90 pm at 110W. Two curved mirrors of5and 7.5-cm radius, respectively, anda plane mirror formed a folded astigmatically compensated cavityas described by Kogelnik [ 51 . The optimized outcoupling transmission was 9 percent. The dye was pumped through a nozzle similar to that described in [ 61, forming a jetof 0.5-mm thickness. Theflow velocity was 10m/s.The solvent emp1,oyed was water with 1.3 percent polyvinyl-alcohol (M.W. 103 000) added to raise the viscosity. Rhodamine 6G chloride, purified by recrystallization, was dissolved at a concentration of 7 X 10-4 M/1 [ 2 ] . Addition of 18 percent AmmonyxLO improved the solubility and prevented dimerisation of the dye. 5 X 10-3 M/1 of COT were added asaotriplet quencher. The temperature of the dye was kept at 35 C by a thermostat. Fig. 1 shows the measured dye laser output power PD versus theargon laser pumppower P A . Thedye laser output increases linearly with the argon laser input up to PD = 33 W at PA = 1 10 W. A further increase of P A , and therefore of P o , is presently limitedby thermaldegradation of theoptical components. Up toanoutput power of 8 W thedye laser oscillates inthefundamental transverse mode, whereas at higher powerthe divergence increasesby afactoroftwo, correspondingtoadditional oscillation inthe second-order transverse modes. The efficiency q = PD/PA is 30 percent. No difference in 77 was measured between a 25-W TEMoo pumping and the 100-W high-order multimode excitation. However, to obtain maximumefficiency it was necessary 1) to avoid thermooptical distortion by using water with a viscosity-raising additive as a solvent and by not focusing the argon laser beam to anintensity greater than3.106 W/cm2 (this thermal limit follows from extrapolation of data out of experiments with ethylene glycol), which was realized by the spot diameter of 60-90 pm, and 2) to completely utilize the invertedregion for laser emissionby adjustingthe beam waist diameter of the dye laser resonatortothepumpedarea, which was fulfilled inour experiment by a TEMoo beam waist diameter of 70 pm in the jet. By observing the output power versus temper:ture for constant flow rate, an optimum temperature of 35 C was found. Thetheoreticallyestimatedoptimum value that minimizes thermal effects is 3OC [ 71 . This indicates that in our laser the chemistry of thedye still leaves openproblems. Presently, possibilities to improvethespatialandtemporalcoherence are under investigation. Note A d d e d in Proof: In a more recent experiment with the same dye and resonator parameters, 52-W dye laser power at 175-W pump power was measured.

[ l ] I. M. Beterov, L. S. Vasilenko, L. A. Kovaleva, A. V. Shishaev, and B. Ya. Yurshin, “Use of a high-power AI+ laser for pumping of a CW dye laser,’’ Sov. J. Quantum Electron., vol. 6, pp. 742-744, June 1976. [2] S. Leutwyler, E. Schumacher, and L. Woste, “Extending the solvent palette for CW jet stream dye lasers,” Opt. Commun., vol. 19, pp. 197-200, NOV. 1976. [ 31 J. M. Yarborough, “10 W CW dye laser‘” Coherent Radiation Co., prospectus. [4] W. H. Seelig and K. V. Banse, “Argon laser emits 150 watts cw,” Laser Focus, pp. 33-37, Aug. 1970. [5] H. W. Kogelnik, E. P. Ippen, A. Dienes, and Ch. V. Shank, “Astigmatically compensated cavities for CW dye lasers,” IEEE J. Quantum Electron., vol. QE-8, pp. 313-379, Mar. 1972. [ 61B. Wellegehausen, H. Welling, and R. Beigang, “A narrow-band jet stream dye laser,” Appl. Phys., vol. 3, pp. 387-391, 1974. [7] B. Wellegehausen, L. Laepple, and H. Welling, “High power CW dye lasers,” Appl. Phys., vol. 6, pp. 335-340, Apr. 1975.

The Infrared N2 Laser as aPump for IR Dyes L. 0. HOCKER Abstract-An infrared molecularnitrogen laser cooled by liquid nitrogen was used to pump a simple grating tuned infrared dye laser in the wavelength range from 0.915 Prn to 1.04 Hm. Broad-bandoperation of thedye lasers produced efficiencies as high as 20 percent,and produced output atwavelengths as longas 1.096 Hm.

Tunable infrared dye lasers are suited for a variety of applications ranging from photochemistry to serving as a source for stimulated Raman scattering. Several different techniques have been used to pump infrared dyes, with direct flashlamp pumping [ 11 and ruby laser pumping [ 21 used with greatest success in pulsed applications. This paper reports the use of the infrared laser lines of molecular nitrogen to pump infrared dyes at moderated high repetition rates producingan output in the range 0.92 pm to 1.09 pm. The infrared lines of Nz were observed t o lase only weakly [ 31, [ 4 ] , producing infrared emission byearlyinvestigators at a small fraction of the efficiency of the well-known ultraviolet lines of N 2 . However, the efficiency of the infrared Nz laser was reported to improve dramatically when the temperature of the working gas in the laser was reduced by cooling the walls of the laser tube to liquid nitrogentemperature [ 5 1 . Using thistechnique, we have built a simplelongitudinally excited molecular nitrogen laser which produced 0.6 mJ pulses with a 150 ns pulsewidth. The laser tube had an active length of 60 cm and a diameter of 19 mm and was equipped with Brewster angle windows, a gold-coated flat mirror, and a 5 0 percent reflecting flat output coupling mirror. Excitation was provided by a 6 nF capacitor discharged into the laser througha self-triggered spark gap. When operatedata charging voltage of 18 kV, the energy stored in the capacitor was -1 J, and the laser output under these circumstances was -0.6mJ, yielding an efficiency of Manuscript received March 31, 1977. Theauthor is withtheDepartment of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.