I. INTRODUCTION The diode-pumped laser has been under development for more than 20 years [1]. For most of the development, continuous wave (cw) diode lasers have been used as a pumping source [2]. The employed gain medium was usually a solid state crystal, such as Nd:YAG or Nd:YVO4. Due to their great advantages of high efficiency and low thermal load, cw diode-pumped lasers are replacing most of the low- or medium-power flashlamp-pumped cw solidstate lasers.

The average power of single cw diode bar can reach several tens of Watts. These cw diode-pumped lasers can emit pulses when acousto-optic Q-switches are used inside the cavity. The average power of a cw diodepumped, Q-switched laser can reach the kilowatt level; however, the pulse energy is limited to less than several millijoules and the pulse duration is usually longer than several tens of nanoseconds [3]. Nonetheless, in many cases, the peak power of pulse is too small for practical applications in LIDAR (light detection and ranging). For pulsed excitation, flashlamps have been the most frequently used pumping source. Although the overall efficiency obtained in flashlamp pumping is typically limited to 1 ∼ 3%, peak pumping power of several kilowatts can be easily achieved even with low-price flashlamps. To get higher pulse energies, pumping by using intense pulsed-diode lasers is required.

Quasi-cw (QCW) diode lasers have been used for this application. Still, pulsed diode lasers with peak powers of several kilowatts are rather expensive and rarely used. As diode lasers can have wavelengths very close to the absorption maximum of the gain medium, the absorption efficiency can be very large. Usually, due to the high absorption coefficient, the profile of the absorbed pump laser inside the gain medium becomes non-uniform when single-side pumping is used. Therefore, multi-side pumping is usually used to achieve uniform absorption of the pump laser’s energy [4]. Moon et al. used three-side pumping [5]. Le Garrec et al. used five-side pumping [6]. Brioschi et al. used twelve-side pumping [7].

With the increase in the number of pumping directions, the mounting mechanism becomes more complicated, and a laser with a complicated mounting mechanism is more vulnerable to vibration. For applications requiring a robust mounting and high-power pumping, single-side pumping is the most suitable, considering its simple structure and maximum obtainable pumping power. However, achievement of uniform and efficient pumping in single-side pumping is still a challenge, so various designs of the reflector shape and the reflector material have been investigated [8–11]. Brand used a cusp-shaped reflector for uniform pumping [8]. Koshel and Walmsley used nonimaging optics [9,10]. In this work, we report on a QCW diode-laser-pumped Nd:YAG laser. The laser used a single-side pumping geometry. The pumping uni- -355- -356- Journal of the Korean Physical Society, Vol. 57, No. 2, August 2010

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