A 11.5 J at 40 ns output has been obtained from a diode-pumped cryo-cooled Yb:YAG ceramics active-mirror laser amplifier system. The system consists of two amplifier heads which has four Yb:YAG ceramic disks and two pump LD modules. The Yb:YAG ceramics are cooled by conventional cryostat from rear side and are pumped by LD modules from front-side. A pump pulse is delivered to Yb:YAG ceramics coaxially with a seed pulse to reduce damage risk at a dielectric coating of Yb:YAG ceramics due to simplified coating design. To realize this system design, a LD module has been developed to keep a rectangle pattern with side length of around 37 mm among imaging depth of about 10cm at working distance of about 410 mm. As an experimental result of two pass amplification, a 11.5 J pulse energy was obtained with input energy of 1.0 J and total pump energy of 90.2 J. Then, an optical-to-optical conversion efficiency was 11.6% and an extraction efficiency was estimated to be 42%. In our knowledge, this is the highest output energy with nano second pulse duration in cryo-cooled Yb:YAG active-mirror laser amplification scheme. A repetition rate of 0.05 Hz depends on a limitation of a repetition rate of the seed pulse. A dependence of small-signal-gain on pumping repetition rate of the active-mirror laser head was experimentally evaluated. From the experimental result, we have estimated a feasible repetition rate of over 5 Hz. A 10 Hz operation will be demonstrated to reduce a thermal resistance between Yb:YAG ceramics and cryostat. Finally, this laser amplifier system is installed to a 100-J class laser system as preamplifier.
A laser-diode (LD) pumped Nd:YAG compact laser system that is capable of generating 1064-nm, 1-J output pulses in several tens of nanoseconds pulse duration at 300-Hz repetition rate (300-W average power) was developed. A concept of this laser system is based on a ubiquitous machine that is easy to transport and process test rapidly in laboratory. A footprint is 1.2-m in width and 2.4-m in length. This laser system is a master-oscillator power-amplifier (MOPA) architecture that allows for increasing the output energy by adding amplifiers. It consisted of an acousto-optic Qswitched Nd:YAG oscillator, a ϕ3-mm Nd:YAG preamplifier and three ϕ12-mm Nd:YAG main amplifiers. The oscillator generated 6-mJ pulse energy at 37-ns pulse duration that could be adjusted by changing the cavity length. The main amplifier had a small-signal gain (SSG) of 9 by laser diodes (LDs) pumping with maximum 27-kW peak power. The beam size and divergence were adjusted to compensate for thermal lens effect in each amplifier. Single-pass amplification by three main amplifiers increased the pulse energy to 1 J. The pumping repetition rate was fixed to obtain thermally stable condition. However, the output repetition rate is variable from single shot to 300 Hz by controlling the oscillator for the experiment usability.
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