Shingo Oishi

Measurement of tiny droplets using a newly developed optical fibre probe micro-fabricated by a femtosecond pulse laser

Optical fibre probing is widely applied to measurements in gas–liquid two-phase flows. We have already developed and reported a four-tip optical fibre probe (F-TOP) for millimetre-size bubbles/droplets, and a single-tip optical fibre probe (S-TOP equipped with a wedge-shaped tip) for sub-millimetre-size bubbles/droplets. However, it is difficult to measure micrometre-size bubbles/droplets by S-TOP. The main purpose of the present study was to develop a new type of optical fibre probe micro-fabricated by a femtosecond pulse laser (Fs-TOP). First, we confirmed the performance of the new probe by examining millimetre- and sub-millimetre-size droplets; the results from the Fs-TOP were compared with those obtained from the visualization of the droplets by high-speed video camera, and showed satisfactory agreement. In addition, we demonstrated the measurement of velocities and chord lengths of micrometre-size droplets (about 50 µm in diameter) using the Fs-TOP and compared them with those from the S-TOP to clarify the limits and strengths of each type of probe.

Compact, Rigid, and High-Power Ultrafast Laser System Applying a Glass-Block Cavity

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.