Yasuki Takeuchi

Highly efficient cryogenically-cooled Yb:YAG laser

A MOPA laser system for high pulse energy and high average power has been developed by using a cryogenic Yb:YAG. In the regenerative amplifier with our original TRAM architecture, the high pulse energies of 6.5 and 1.5 mJ were obtained at the repetition rate of 200 Hz and 1 kHz, respectively. An optical efficiency was as high as ηo-o = 9.3% with an excellent beam quality of M2 < 1.1, which ensured that a cryogenic Yb:YAG TRAM had a high thermal strength. The following four pass power amplifier with a cryogenic Yb:YAG rod showed 140 mJ at 100 Hz. Both a high optical efficiency of ηo-o = 30% and a high slope efficiency of ηs = 44% showed that an efficient laser operation could be realized for a power amplification with both a high pulse energy and a high average power by using a cryogenic Yb:YAG.

Spectral properties of second-harmonic generation at 800 nm in a BiB(3)O(6) crystal.

We have investigated spectral distribution and walk-off effect of second-harmonic generation in a 3-mm-long type I BiB(3)O(6) crystal. Linearly turning ability of the BiB(3)O(6) crystal is confirmed for wavelengths around 800 nm. In addition, the walk-off effect of fundamental beams is quantitatively measured by introducing a little vertical polarization component into pumping fundamental pulses. A conversion efficiency of 28% from fundamental to second harmonic is achieved at a 3.2-GW/cm(2) fundamental intensity.

Nonlinear refractive index of a YAG crystal at low temperature

Recently, pulse lasers with both of high pulse energy and high average power are strongly desired for various industrial and scientific applications. In such laser systems, the nonlinear refractive index is a critical factor for spatial phase distortion and the B-integral has been used to evaluate it.

Electro-Optic Characteristics of a Cooled Deuterated Potassium Dihydrogen Phosphate Crystal

The half-wave voltage of a deuterated potassium dihydrogen phosphate (DKDP) crystal was measured between 231 K and room temperature. It was found to decrease linearly with crystal temperature at a rate of 55.1 V/K. The lowest voltage of 511 V was observed at 231 K, which is one eighth of that at room temperature. The cooled DKDP crystal was discussed for use as an electro-optic modulator as it provides various advantages such as the reduced voltage of the power supply needed and the lower B-integral.

New design of the temperature controlled pockels cell for IFE driver

We have observed the temperature dependence of the electro-optic coefficients of the DKDP crystal from 298K to the 231K. The half-wave retardation voltage was decreased proportionally with the crystal temperature at the rate of 55.1 V/K. The lowest half-wave voltage was 551 V at the crystal temperature of 231K, which corresponded to 1/8 of that at the room temperature. Note that the low operation voltage of 511 V may permit the use of semiconductor electrical switches instead of the thyratron.

Curve-fitting method for pure spectrum measurement of femtosecond laser pulses by a pinhole pair.

We present a curve fitting method for measuring the spectral distribution of femtosecond laser pulses with Youngs double-pair interference. The method is applicable to cancel the influence of the mutual coherent portion in the spectrum measurement.

Cryogenic Yb:YAG total-reflection active-mirror lasers

Both high pulse energy and repeatable operation are simultaneously desired for high-power lasers in the next generation. These lasers are expected to be useful tools for various kinds of advanced fields such like inertial fusion energy, neutron sources, debris remove, PET and so on. The target of 1-kJ, 100Hz in pico seconds is set for novel laser technology developments. The most significant problem in the developments is laser materials which require high thermal strength, material size scalability and suitable emission cross section (saturation fluence). Cryogenic Yb:YAG ceramic has shown good performances via our previous basic experiments.[1,2] In addition, a total-reflection active-mirror (TRAM) amplifier has been proposed for high efficiency, compactness and high optical damage threshold. So-called active-mirror amplifiers show high efficiency due to a good spatial overlap between a pump and a laser beam. The optical damage threshold reduced due to interference on the input/output surface is, however, significant for pulse duration. The TRAM recovers it. A few temperature rise in cw operation showed considerably high thermal strength.[3] In pulse operation as following, a regenerative amplifier is developed and an additional joule-class amplifier system at 100 Hz is under construction.

Chirped-Pulsed Yb 3+:YAG Regenerative Amplifier using a Total-Reflection Active-Mirror

The first chirped-pulse regenerative amplifier using a total-reflection active-mirror with a cryogenic Yb 3+:YAG/ YAG monolithic composite ceramic was demonstrated. 3.6 mJ of output pulse energy was obtained at 100 Hz repetition rate.

Milli-Joules, Kilo-Hertz Regenerative Amplifier Using Total-Reflection Active-Mirror with Cryogenic Yb:YAG

A regenerative amplifier by using total-reflection active-mirror with a cryogenic YAG/Yb:YAG composite ceramics has been demonstrated up to kHz repetition rate. A 6.5-mJ pulse energy and a 9.3% optical efficiency were obtained at 200-Hz.

Development of high power Yb:YAG pump source for few-cycle pulse amplification

Recently few cycle pulses with an ultrahigh peak power and a high average power have been actively developed by using a conventional Ti:sapphire system and an optical parametric chirped-pulse amplification (OPCPA) system. A powerful pump source is enthusiasmly desired in nano- and pico-seconds for Ti:sapphire and OPCPA, respectively. A “GENBU (Generation of ENergetic Beam Ultimate)”-laser has been conceptually designed for peta-watt peak power at 100 Hz by using the broadband OPCPA technique.[1] A 100 J pulse energy is necessary as a pump source and a diode-pumped solid-state laser is one of the most promising system, which uses novel technologies of a cryogenic Yb:YAG [2] and an active mirror amplification scheme. A feasibility of the pump source has been studied at sub-joule pulse energy to show its viability of this new laser system. A master-oscillator power-amplifier (MOPA) system has been developed at 150 mJ at 100 Hz in 10 ns.