Koichi Yamakawa

30-mJ, diode-pumped, chirped-pulse Yb:YLF regenerative amplifier

A diode-pumped chirped-pulse regenerative amplifier with a cooled Yb:YLF crystal has been developed. The output pulse energy is 30 mJ at 20-Hz repetition rate. A high effective extraction efficiency of 68% is obtained, which is attributed to reduced saturation fluence at low temperature and to a high effective pulse energy fluence during regenerative amplification. After pulse compression by use of a parallel grating pair, 18-mJ pulse energy and 795-fs pulse duration are obtained.

Two-color chirped-pulse amplification in an ultrabroadband Ti:sapphire ring regenerative amplifier.

We have developed a high-energy, ultrabroadband Ti:sapphire ring regenerative amplifier capable of producing in excess of 20-mJ output at a 10-Hz repetition rate. The technique of chirped-pulse amplification is used to generate two-color, time-synchronized pulses with central wavelength separations of up to approximately 120 nm and with a total energy of 10 mJ by use of a regenerative pulse-shaping technique. Mid-infrared pulses tunable from 6 to 11 microm are generated by difference frequency mixing the two-color outputs.

360-W average power operation with a single-stage diode-pumped Nd:YAG amplifier at a 1- kHz repetition rate

We report a high-average-power laser-diode-pumped Nd:YAG master oscillator power amplifier system that has a minimum number of elements in the single multipass zigzag-slab amplifier stage and is used to pump a high-peak-power and high-average-power Ti:sapphire laser system. This phase-conjugated system produces an average power of 362 W at 1 kHz in a 30-ns pulse with an optical-to-optical conversion efficiency of 14%. With an external KTP doubler this system generates 132 W of green average output power at 1 kHz with a conversion efficiency of 60% when pumped at a power level of 222 W. To the best of our knowledge these results represent the highest average output power at both infrared and green wavelengths achieved in a single amplifier stage.

Improved high-field laser characteristics of a diode-pumped Yb:LiYF4 crystal at low temperature.

We have demonstrated a diode-pumped Yb:LiYF4 laser oscillator at liquid nitrogen temperature in free-running mode. The obtained laser gain was 21 cm-1, which was 15 times as high as that at room temperature. The effective tuning range was broadened to 35 nm due to absorption spectral narrowing.

Characterization of wave-front corrected 100 TW, 10 Hz laser pulses with peak intensities greater than 1020W∕cm2

An improvement of laser-focused peak intensity has been achieved in a JAERI 100 TW Ti:sapphire chirped-pulse amplifier chain with a feedback controlled adaptive optics system operating at a 10 Hz repetition rate. The Strehl ratio was enhanced to 0.8 by means of a Bimorph deformable mirror with a Shack-Hartmann wave-front sensor. Measurements of optical parameters of the laser pulse and an experimental tunneling ionization yield of helium have practically confirmed focusing to ultrarelativistic intensities of over 1020W∕cm2 within 16% accuracy.

Wave-front measurements of terawatt-class ultrashort laser pulses by the Fresnel phase-retrieval method

We demonstrate single-shot wave-front measurements of high-peak-power 100-fs laser pulses using the Fresnel phase-retrieval method. The wave fronts are reconstructed from only two intensity distributions at two planes along the optical axis and are measured by means of simple charge-coupled-device cameras. Wave fronts of a terawatt-class laser pulse are measured to be better than 0.36λ, peak to valley, when compared with the measured far-field intensity distributions of the beam.

Noise characterization of an all-solid-state mirror-dispersion-controlled 10-fs Ti:sapphire laser

Abstract We characterized the phase and amplitude noise of a mirror-dispersion-controlled 10-fs Ti:sapphire laser pumped by a frequency-doubled cw diode-pumped Nd:YVO4 laser and compared with these of the Ti:sapphire laser pumped by an Ar-ion laser. The rms timing jitters and rms amplitude noise for the all-solid-state and Ar-ion laser pumped Ti:sapphire lasers are calculated to be 0.31 ps rms and 0.71 ps rms and 0.15% rms and 0.32% rms, in the frequency range from 20 kHz to 400 kHz, respectively. The phase and amplitude noise characteristics of the Ti:sapphire laser were greatly improved by using the diode-pumped solid state laser as a pump source.

Second - harmonic generation of ultra-high intensity femtosecond pulses with a KDP crystal.

We investigated second harmonic generation with ultrahigh intensity femtosecond laser pulses from a terawatt Ti: sapphire laser system. Energy conversion efficiency of about 80 % for a type I potassium dideuterium phosphate crystal was obtained with 130 fs laser pulses at an intensity as high as 192 GW/cm 2.

Proposal for Ultrabroadband Phase-Matching Optical Parametric Chirped Pulse Amplification with a Diverged Pump Beam

We propose a novel scheme for optical parametric chirped pulse amplification with a diverged pump beam to achieve an ultrabroadband phase-matching range, in which different signal wavelengths correspond to different inclination angles of the diverged pump beam. Simulation results show that the proposed scheme amplifies a signal pulse shorter than 10 fs at a wavelength of approximately 800 nm.

Relativistic laser plasma from micron-sized argon clusters as a debris-free x-ray source for pulse x-ray diffraction

We have demonstrated diffraction from Si(111) crystal using x rays from highly ionized Ar ions produced by laser irradiation with an intensity of 6×1018W∕cm2 and a pulse duration of 30 fs acting upon micron-sized Ar clusters. The measured total photon flux and linewidth in the Heα1 line (3.14 keV) were 4×107photons∕shot∕4πsr and 3.7 eV (full width at half maximum), respectively, which is sufficient to utilize as a debris-free light source for time-resolved x-ray diffraction studies.