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 high gain cryogenic Yb:YAG ceramics laser amplifier for a high energy laser amplification system has been developed. The laser system consists of a fiber oscillator and two stage LD pumped cryogenic Yb:YAG ceramic amplifiers. The preamplifier stage has a 5-pass laser amplifier head and the main amplifier stage has a 2-pass laser amplifier head, respectively. The preamplifier obtained an average stored energy density of 0.836 J/cc and small-signal gain (SSG) of 60 with 33 J of stored energy. Then about 1 μJ of input energy from the oscillator was amplified to 3.6 J. The main amplifier head had four pumping LD modules which irradiated the Yb:YAG ceramics directly. This original angular pumping scheme ideally increases irradiation intensity and homogenizes irradiation pattern on the Yb:YAG ceramics due to superposition effect of all of the LD modules. A maximum peak power of over 100 kW was generated by one LD module. When the output energy of the LD modules was 450 J, a 20 of SSG at single pass was obtained. Stored energy density was evaluated to 0.429 J/cc when 148 J energy was stored in 346 cc of Yb:YAG ceramics. As a result, a 55-J output energy with 10 ns pulse duration was demonstrated at a pumping energy of 450 J. The optical-tooptical conversion efficiency which includes transmissivity of the LD modules was 12 %. The extraction efficiency was estimated to 37%.