High-average-power, high-repetition-rates picosecond-pulsed regenerative ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser amplifiers were developed. The architecture used in the amplifiers, which are named as thin-rod, has a unique cooling scheme like slab lasers and also has a unique pumping scheme like photonic crystal fiber lasers, is suitable for high-average power Ytterbium lasers. This architecture also has high gain characteristics which is appropriate for the regenerative spectral and pulse shaping on high-repetition-rate, ultrashort-pulse amplifications.
High-efficiency cavity-dumped ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser was developed. Although the
high quantum efficiency of ytterbium-doped laser materials is appropriate for high-efficiency laser oscillation, the
efficiency is decreased by their quasi-three/four laser natures. High gain operation by high intensity pumping is suitable
for high efficiency oscillation on the quasi-three/four lasers without extremely low temperature cooling. In our group,
highest efficiency oscillations for continuous wave, nanosecond to picosecond pulse lasers were achieved at room
temperature by the high gain operation in which pump intensities were beyond 100 kW/cm2.
Yb:YAG is an attractive material for high-power and ultra-short pulse laser because of its high quantum efficiency exceeding 90% and wide gain spectrum larger than about 10 nm. Our research is development of high-average power Kerr-lens mode-locked Yb:YAG laser. A Kerr-lens mode-locked laser has been designed with thin-rod Yb:YAG for high average power oscillator. Thin and long Yb:YAG crystal is used for gain medium and it was pumped directly by stacked LD bar. The Z-folded cavity was designed by simple ABCD matrix law. An additive Kerr-medium is used for stable mode-locked operation instead of semiconductor saturable absorber mirrors (SESAM), because Kerr-effect of the Yb:YAG crystal is weak. SF57 glass is used to stabilize the mode-locked oscillation to decrease the intra-cavity loss to increase the oscillation bandwidth for transform limited pulse generation. The Group Delay Dispersion (GDD) of the optical element in the laser cavity was compensated by SF10 prism pair. The output is expected to be the transform limited ultrashort pulses of sub 200 fs width with a high average output power around 60 W with 200 W pump power.
The Yb (ytterbium) laser material is promising for high-power all-solid-state ultrashort pulse lasers because of its high quantum efficiency and wide gain spectrum. A didoe end-pumped technology is developed for high-average-power and efficient all-solid state ultrashort pulse lasers with high beam quality output. Two 100 W CW-LDs are used for pumping and the beams are focused on the end surfaces of a thin and long Yb:YAG rod. A pair of parallel side surfaces of the rod is placed in contact with cupper heat sinks to remove heat. A one-dimensional distribution of thermal stress induced birefringence was observed in the rod by this simple heat flow. The loss due to thermal birefringence was measured to be smaller than 3%, when the probe beam was linearly polarized in parallel or perpendicular to the direction of cooling. It can be expected theoretically that a moderate single-path gain of 4 for efficient amplification and the optical conversion efficiency of 50% are realized for multimode operation with a pump power of 200 W. In the experiment, output power was measured to be 55 W with beam quality factor of Mx2 x My2 = 5.5 x 6.0 and 27 W with Mx2 x My2 = 1.3 x 1.4 for simple plane-plane linear cavity configuration with no thermal lens compensations.
In this paper, the performance characteristics of diode-pumped miniature CW Yb:YAG laser such as the temperature distribution and thermal lensing effect inside laser rod, and cw laser output are investigated. The laser output power of 2.5 W with the slope efficiency of 48.6% and opt-optical efficiency of 19.8% at the absorbed pump power of 12.6 W are obtained in experiment.
A compact direct-detection Doppler lidar system has been developed for tropospheric wind measurements. ALD-pumped Nd:YAG Q-switched laser with frequency stabilized and two-frequency oscillation was realized for the differential Doppler wind measurements. Basic principle of system performance and system parameters are shown as well as the experimental results.
C60 vapor was irradiated with an intense femtosecond laser pulse. Multiply charged carbon ions, up to C4+, were detected in the time-of-flight spectra. We have measured both energy and angular distributions with respect to the laser polarization direction for ions with different charge number. The kinetic energy of ions are distributed above 1 keV. The angular distribution measurement shows that the highly charged ions of C3+ and C4+ are mostly distributed in the direction parallel to the laser polarization, and the C+ ions are slightly distributed in the direction perpendicular to it. These observations clearly indicate that an anisotropic explosion takes place. The average energy of ions with the different charge number is found to be proportional to the square of the charge number, while the maximum energy is proportional to the charge number. Classical molecular dynamics simulations have been successfully carried out reproducing not only the energy spectra but also the angular distributions of ions. The simulations suggest that the most crucial process for the anisotropic Coulomb explosion of C60 is not the electron impact ionization, but the cascade hopping of electrons.
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