Double-pulsed output with a Xe iv laser excited by long current pulse discharges.

Laser action in a highly ionized xenon gas has been re­ ported by a number of authors. Above all, there has been considerable interest in pulsed Xe IV lasers for their possibility as a simple, efficient, and coherent source from the UV to visible region of the spectrum. Hansch et al., Schearer, and Fahey and Schearer reported the use of a pulsed xenon laser for pumping dye. When some dispersive elements such as an etalon or a diffraction grating are employed in a dye laser cavity, long pulse excitation has the advantage that the nar­ rowband operation can be achieved by multiple round trips of light in the cavity. However, little attention has been paid to the operation of the Xe IV laser by a long current pulse of the order of several tens of microseconds. Double-pulsed output for Ar and Kr ion lasers has been observed by Cheo and Cooper, Hattori and Goto, and Sakuma and Tako. This phenomenon is attributed to quenching and saturation processes. In this Letter we dem­ onstrate that the Xe IV laser can be operated in the long-pulse mode and can have a double-pulsed output at low filling pressures, which seems to be different from those found be­ fore. Excitation processes of Xe IV lasers are discussed on the basis of observed output waveforms, and some new excitation mechanisms are also proposed for this double-pulsed output. The laser tube used in this experiment had a 3-mm i.d. and was made of Pyrex glass terminated at both ends with fusedquartz Brewster windows. The over-all tube length between the windows was 120 cm, and the discharge was turned on between an aluminum cold cathode and a tungsten anode separated by 90 cm. The optical resonator consisted of two dielectric-coated mirrors with 2-m radius of curvature. These mirrors were total reflectors (99.7% reflectivity) over a spectral range from 480 nm to 640 nm, and hence the optimum output coupling was not made. To obtain a long discharge current pulse (~100 μsec) the power supply used in this experiment consisted of a ten-stage pulse-forming network (PFN), which was charged up to 14 kV. Each stage of the PFN consisted of a 50-μH inductor and a 0.5-μF capacitor. The character­ istic impedance of the PFN was thus 10 Ω. As shown in os­ cilloscope traces of the discharge current (Fig. 1), there was a slight impedance missmatch between the PFN circuit and the discharge tube. The discharge impedance of our Xe IV laser tube was considered to be <10 Ω. Initiation of dis­ charges was switched by triggering a spark gap, which was connected to the laser tube in series. The laser output was detected with a Si photodiode (EG&G SD-100). The oscilloscope traces of discharge current (upper traces) and total laser output (lower traces) at a gas pressure of 0.02 mm Hg are shown in Fig. 1. The output power is distributed principally over six strong lines at 496.4, 500.8, 526.0, 535.3, 539.5, and 595.6 nm. A sharp pulse appears at the rising edge of the discharge with a duration of ~ 5 μsec, and it increases in peak power with discharge current. After a few tens of microseconds, a second broad pulse is turned on and oscillates for about 50 μsec, when the peak value of discharge current exceeds 300 A. The peak power of the second pulse also in­ creases with discharge current. There is a time interval beFig. 1. Temporal display of laser output and excitation current pulse, time base 20 μsec/div.