Multipass pumping scheme produces 13 W in Nd:YLF
Kristin Lewotsk
A high-power diode-pumped, neodymium-doped yttrium lithium fluoride (Nd:YLF) laser has been demonstrated by engineers at Schwart¥Electro-Optics Research Division (SEO, Concord, MA). The system operates at 1047 nm in continuous-wave or pulsed mode. Reported results include 13 W of output with 33% optical efficiency and a beam quality M2 of less than 1.2.
Segmented endface coatings are used to force multiple passes through the bar-shaped gain region, in contrast to the zig-zag gain path of conventional total-internal-reflection slab lasers. A high-reflection coating covers the majority of each endface, while the remaining area is an anti-reflection-coated coupling window (see figure). The 3-cm-long Nd:YLF bar is side-pumped at 808 nm by a pair of 20-W diode-laser arrays. The arrays are laterally offset and the crystal sideface coatings segmented such that pum¥power is reflected from the crystal face opposite each diode bar, resulting in double-pass pum¥absorption.
The three-mirror cavity configuration consists of a cylindrical mirror, a spherical mirror, and a flat output coupler. This arrangement produces a diffraction-limited beam at 1047 nm while minimizing thermal lensing in the Nd:YLF crystal. To remove excess heat, the crystal is mounted on a brass water-cooled heat sink. The laser can be Q-switched by an intracavity acousto-optic modulator at repetition rates ranging from 5 to 120 kHz.
Researcher James Harrison described achieving better than 13 W of output power with the laser in a postdeadline paper at CLEO `95 (CPD20). He also summarized results of second-harmonic generation experiments in which 6.8 W of 523.5-nm output was produced with a noncritically phase-matched lithium borate crystal.
There is widespread interest in the development of diode-pumped 1-µm lasers for many applications. Among the possibilities suggested by Harrison is a dry-ink printing process in which the laser fires on ink-impregnated plastic to release the pigment onto the paper. Such a process eliminates messy and volatile inks. A commercial Nd:YLF system rated at more than 10-W output was introduced by SEO at InterOpto (July 11-14, Chiba, Japan).