1006 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. zyxw QE-20, NO. 9, SEPTEMBER 1984 z Repetitively Pulsed Transversely Excited Sr' Recombination Laser M. BRANDT Abstract-Operation of a transversely excitedstrontium-ion recom- bination laser at repetition rates up to zyxwvuts 500 zyxwvuts Hz is reported. Maximum average output power of 90 mW o n the 430.5 nm transition has been obtained from a sealed-off device of active volume - 40 cm3 at a pulse repetition frequency of 200 Hz. This performnace was achieved in the absence of any discharge preionization or forcedrecirculation of the gas mixture. T HE use of transversely excited (TE) discharge tubes for operation of the Sr' and Ca' recombination lasers has resulted in a considerable improvement in extracted energy density compared to longitudinally excited systems [ l ] -[4] . Maximum specific energy densities on the 430.5 nm tran- sition of Sr' and on the 373.7 nm transition of Ca" of approx- imately 50 pJ/cm3 [3] and 30 pJ/cm3 [4], respectively, have been achieved from an active volume of 30 cm3 with no fun- damentallimitations observed. Such energy densities are an order of magnitude greater than have been achieved in longi- tudinally excited systems and compare favorably to the high- est achieved for high-power visible metal-vapor lasers [5]. Tlvs improved performance is attributed to substantially higher operating gas pressures [up to 200 kPa (1500torr)] and higher current densities (up to 1 kA/cm2) achieved in TE devices. Such operating conditions are necessary for effi- cient recombination pumping in these lasers and are more difficult to achieve in longitudinally excited discharges for whichstableoperation is possible onlyfor He gas pressures up to 60 W a (500 torr) and current densities up to approxi- mately 100 A/cm2. Experiments with high-pressure TE recombination lasers have been performed at low repetition frequencies, usually 1-5 Hz, but for a numberofpotentialapplications, devices with high average power outputs are required. In this letter, experiments aimed at increasing the opera- ting frequency of a TE Sr' recombination laser are reported. Operation up to 500 Hz was achieved in the absence of any discharge preionization or forced recirculation of the gas mix- ture. The highest average output power recorded was 90 mW at a repetition frequency of 200 Hz. The corresponding laser pulse energy was 450 pJ. The experiments were performed with a modified version of the single-segment TE discharge tube described previously [l] . The newelectrodestructure was formed by two semi- circular stainless-steel rods of diameter 12 mm with an active length of 200 mm. The gap between the rods was 25 mm. The required vapor pressure of strontium was achieved by externally heating the central region of the tube. Upon vary- ing the pulse repetition frequency the oven temperature was Manuscript received March zyxwvutsrqpo 5, 1984; revised May 11: 1984. The author is with the Department of Defence, Materials Research Laboratories, Melbourne 3032, Australia. manually adjusted to compensate for the discharge heating of the medium. The temperature was monitored with a chromel-alumel thermocouple positioned inside the tube. The excitation circuit was an inductively charged version of the low repetition frequency circuit described previously [l] . The values of the storage and dump capacitances were in the range15-50 and 10-40 nF, respectively, with charging volt- ages up to 15 kV. For each value of the storage capacitance, the dump capacitance was adjusted until minimum oscillations of the voltage and current pulses were obtained following the completion of the electrical avalanche. A pair of 25 mm diameter dielectric mirrors with a 3 m radius of curvature formed a 1.5 m long resonator. One mir- ror was highly reflecting zyxw (R > 0.99) and the other was par- tiallyreflecting (R = 0.6) at 430.5 nm. Laseraverage power was measured by a Scientech model 364 power meter. Laser average power as a function of He gas pressure exhibited similar dependence to that observed in the low-frequency ex- periments [ l ] . The average power increases with He pressure up to -110 kPa (800 torr) and then gradually declines at higher gas pressures until at pressures greater than 133 kPa (1000 torr) the output ceases due to largedischarge instabil- ities. The decline in output power was accompanied by both a gradual decrease in the active volume and shortening of the laser pulse. The decrease in the active volume is due to grad- uallengthwise contraction of the discharge columntowards the middle of the electrodes and is estimated to be -30 per- cent. The shortening of the laser pulse is believed to be due to the disruption of the recombination pumping mechanism as a result of the power being deposited in the discharge dur- ing the current/voltage oscillations following the electrical avalanche [2]. The variation of average power as a function of repetition frequency is shown in Fig. 1 for different values of peak cur- rent. It can be seen that the average power increases with increasingcurrent amplitude and repetition frequency up to 200 Hz. The highest average power obtained was 90 mW at a peak current of 3 kA and a repetition frequency of 200 Hz. Attemptstooperatethe device at currents above 3 kA re- sulted in large discharge instabilities and arcing with conse- quent decrease in laser performance. This behavior was ob- served at all He pressures and for all values of storage and dump capacitances. Gradual increase of the repetition fre- quency,at all values ofpeakcurrent,beyond200Hz was accompanied by a decrease in output power. The laser action ceased at 500 Hz. At high repetition frequencies thetube temperature was adjusted to compensate for the discharge heating. The required temperature was achieved by balancing the heat input due to the discharge with that of the oven. This procedure required significant attention because of the 0018-9197/84/0900-1006$01.00 zyxw 0 1984 IEEE