We are investigating the output and temperature characteristics of Yb:YAG TRAM (Total-Reflection Active Mirror) laser using zero-phonon line excitation (969-nm pumping) and direct water jet cooling for efficient heat removal. The TRAM configuration has an advantage of cooling the surface of the Yb:YAG disk without the high-reflection coating. We have developed an efficient hydrodynamic cooling system, where the disk is directly cooled by impinging water jet with flow rate of up to 52 liter/min., while the water temperature can be controlled from 7 to 80 degrees Celsius. For the estimation of operating temperatures of the Yb:YAG, we measured fluorescence spectra from Yb:YAG using a spectrometer. We tested several types of TRAM with different layer thicknesses and doping concentrations, which were designed to absorb more than 80% of the pump power in a single bounce at room temperature. A fiber-coupled CW laser diode (FCLD) with 600 W output power at 969 nm was used as a pump source. The dependences of oscillator output power and the laser medium temperature on the cooling water temperature and flow rate were investigated. The direct impinging water jet at high flow rate was demonstrated to be effective for cooling the laser medium. It was also confirmed that the zero-phonon line excitation at 969-nm resulted in lower laser medium temperature and hence higher output power compared to the 940-nm pumping. In addition, we demonstrated kW-class laser oscillation using the cooling system and achieved slope efficiency of 63 %.
High-power and high-beam-quality lasers are used in various fields such as high energy physics, laser fusion, aerospace systems and material processing and are expected to have higher output. Power scalability is important for those applications. Solid-state lasers have such a characteristic. One of major problems in solid-state lasers is heat, which may cause damage to the medium due to thermal stress, deterioration of the beam quality, thermal lens effect, etc. Thus an effective cooling method is required. Solid-state lasers include active mirror lasers, slab lasers, thin disc lasers, rod lasers and the like, as is generally known. We have been developing laser systems using active mirror media such as TRAM (Total-Reflection Active Mirror) and ZiZa-AM (Zig-Zag Active-Mirror). In these media, disk like active materials are bonded to the outside of the non-doped material. The seed laser travels in the medium coaxially with the pumping laser while being totally reflected, and is amplified in the active material layer. Since high reflection coating required for a thin disk laser or the like is unnecessary in the case of these media, this is an advantage for removal of heat. We developed a liquid nitrogen jet impingement cooling system. This is an effective system that enables high-level heat removal performance while keeping the medium at an extremely low temperature and jets subcooled liquid nitrogen directly to the active material layer of the medium. In our previous studies, we have achieved development of a kW-class amplifier using a ZiZa-AM. This report presents the various laser characteristics in the case of multi amplifier chain to confirm the scalability of these amplifier systems.