We developed a compact Yb:YAG ceramic regenerative amplification system. A rectangular glass block is used to
elongate the cavity. A pulse to be amplified is propagated in a long distance in the glass block by being reflected
repetitively at the end faces of the glass under a condition of total internal reflection. Furthermore, we produced
transmission gratings with a diffraction efficiency of more than 95%. The floor area of the entire amplification system is
reduced to less than 2,000 cm2. In 20-kHz operation, the system generates 1.0-ps compressed pulses of 4.5-W average
power, i.e., 0.225-mJ energy.
In the field of laser processing, it is important to monitor the beam quality such as the spatial- and time-distribution. A novel time-resolved imaging technique named FTOP (Femtosecond Time-resolved Optical Polarigraphy) for visualizing the ultrafast propagation dynamics of intense light pulses in a medium has been proposed and demonstrated. FTOP is used to monitor the 3D intensity distribution of the pump pulse focused in a medium by the probe pulse. Femtosecond snapshot images can be created with a high spatial resolution by imaging only the polarization components of the probe pulse; these polarization components change due to the instantaneous birefringence induced by the pump pulse in the medium. Ultrafast temporal changes in the 2D spatial distribution of the optical pulse intensity were clearly visualized in consecutive images by changing the delay between the pump and probe. We observe that several filaments appear and then come together before the vacuum focus due to nonlinear effects in air. We also prove that filamentation dynamics such as the formation position and the propagation behavior are complex and are strongly affected by the pump energy. The results collected clearly show that this method FTOP succeeds for the first time in directly visualizing the ultrafast dynamics of the self- modulated nonlinear propagation of light.
Direct observation of nonuniform operation in GaAs microwave high-power FETs has been realized by introducing a new electro- optic (E-O) probing system. In the system, ZnTe is used as a longitudinal external E-O crystal in order to make high sensitive measurement. The spatial resolution of the E-O probing system with an electrically synchronized laser diode is as small as 5 micrometers . We apply this system to the measurement of the electric field at the microscopic region of drain electrodes of an X-band 1W FET consisting of 10 FET unit cells (73 micrometers separation). The electric field concentration to the center (1.6 times) and the most outer cells (1.1 times) has directly been measured. The electric fields on the unbalanced FET cells which were damaged artificially by the focused ion beam are also reported.
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