Koji Tsubakimoto

Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator

The optical properties, Faraday effect and Verdet constant of ceramic terbium gallium garnet (TGG) have been measured at 1064 nm, and were found to be similar to those of single crystal TGG at room temperature. Observed optical characteristics, laser induced bulk-damage threshold and optical scattering properties of ceramic TGG were compared with those of single crystal TGG. Ceramic TGG is a promising Faraday material for high-average-power YAG lasers, Yb fiber lasers and high-peak power glass lasers for inertial fusion energy drivers.

Intense longitudinal electric fields generated from transverse electromagnetic waves

We present a simple method for generating an intense longitudinal electric field from transverse electromagnetic waves (laser pulses) with radially symmetric polarization, to which a liquid crystal device converts linear polarization with energy efficiency of ∼99%. The laser-generated longitudinal electric field was observed in two dimensions and distinguished from the transverse component using the optical Kerr shutter method. The measured amplitude was 1.1 GV/m at the focus of a pulsed Nd:YAG laser beam of 0.25-MW peak-power.

Experimental demonstration of spatially coherent beam combining using optical parametric amplification

We experimentally demonstrated coherent beam combining using optical parametric amplification with a nonlinear crystal pumped by random-phased multiple-beam array of the second harmonic of a Nd:YAG laser at 10-Hz repetition rate. In the proof-of-principle experiment, the phase jump between two pump beams was precisely controlled by a motorized actuator. For the demonstration of multiple-beam combining a random phase plate was used to create random-phased beamlets as a pump pulse. Far-field patterns of the pump, the signal, and the idler indicated that the spatially coherent signal beams were obtained on both cases. This approach allows scaling of the intensity of optical parametric chirped pulse amplification up to the exa-watt level while maintaining diffraction-limited beam quality.

Thermally Induced Birefringence Compensation in High Average Power Nd:YAG Laser

Efficient compensation of thermally induced birefringence with thermal lensing in a rod geometry Nd:YAG laser amplifier was achieved by an in-cavity 90° polarization rotator with three sets of image relaying optics between the laser rods. The measured birefringence loss per transit was 2.8% for pumping power of 580 W per rod. A nearly diffraction limited output beam of 38 W was observed.

84 dB amplification, 0.46 J in a 10 Hz output diode-pumped Nd:YLF ring amplifier with phase-conjugated wavefront corrector

A diode-pumped joule class in a 10 Hz output Nd:YLF ring amplifier has been developed. A phase conjugate plate was developed as a wavefront corrector for the residual wavefront distortion of an Nd:YLF rod. We have demonstrated a 0.46 J output of 10 ns pulse duration at 10 Hz repetition rate with 1.5 nJ of input energy. The effective gain of the ring amplifier system was 84.8 dB. To our knowledge, this is the highest magnification with joule-level output energy in a single-stage amplifier system that has ever been built. As a preamplifier system, this system contributed a demonstration of 21.3 J in a 10 Hz output diode-pumped Nd:glass zigzag slab laser system with a stimulated Brillouin scattering- phase conjugation mirror. We describe a robust and effective method of wavefront correction for high-energy laser systems.

Simultaneous compensation of birefringence and thermal lensing effect in a high average power Nd:YAG amplifier

Thermally induced birefringence and thermal lensing in Nd:YAG rod amplifier was compensated by a 90deg rotator with three sets of imaging optics. Experimentally, 2.8% single pass birefringence loss was achieved and a nearly diffraction limited amplified output was observed.

Two-stage optical parametric chirped-pulse amplifier using sub-nanosecond pump pulse generated by stimulated Brillouin scattering compression

We demonstrate optical parametric chirped-pulse amplification (OPCPA) based on two-beam pumping, using sub-nanosecond pulses generated by stimulated Brillouin scattering compression. Seed pulse energy, duration, and center wavelength were 5 nJ, 220 ps, and ~1065 nm, respectively. The 532 nm pulse from a Q-switched Nd:YAG laser was compressed to ~400 ps in heavy fluorocarbon FC-40 liquid. Stacking of two time-delayed pump pulses reduced the amplifier gain fluctuation. Using a walk-off-compensated two-stage OPCPA at a pump energy of 34 mJ, a total gain of 1.6 × 105 was obtained, yielding an output energy of 0.8 mJ. The amplified chirped pulse was compressed to 97 fs.

Spatial asymmetry of optical parametric fluorescence with a divergent pump beam and potential applications

The spatial distribution of the optical parametric fluorescence generated in a negative uniaxial nonlinear crystal is asymmetric with respect to the pump when the pump beam has a slight divergence angle. The formation mechanism of this phenomenon and the influence of parameters were analyzed and discussed from a theoretical standpoint. Moreover, two potential applications of this phenomenon were experimentally demonstrated, showing the temporal contrast improvement of ultra-intense lasers and the intensity enhancement of special light sources induced by the optical parametric generation.

Degradation of femtosecond petawatt laser beams: Spatio-temporal/spectral coupling induced by wavefront errors of compression gratings

Recently, several petawatt (PW, 1015 W) lasers with pulse duration of ~20–30 fs have been introduced throughout the world, pushing the upper limit on laser peak power. However, besides well-known spatio-temporal coupling effects, such as residual spatial/angular chirps and pulsefront tilt/curvature, the spatio-temporal/spectral coupling in compressors induced by wavefront errors of gratings, which could dramatically distort ultra-intense pulses, has been neglected. In this work, for the first time we analyzed this phenomenon and the peak power/intensity degradation induced by it. Our results suggest that the actual performance of femtosecond PW lasers may be worse than previously estimated.

Generation of High Efficiency 2 μm Laser Pulse from a Periodically Poled 5 mol % MgO-Doped LiNbO3 Optical Parametric Oscillator

We report on an optical parametric oscillator (OPO) based on a large aperture periodically poled 5 mol % MgO-doped LiNbO3 (PPMgLN). A high average power Q-switched Nd:YAG laser operating at 1.064 µm producing 10 ns pulses at a repetition rate of 10 Hz was used to pump the OPO. Total output pulse energy of 126 mJ at 2 µm with 62% slope efficiency was achieved at relatively low pump power intensity. Temperature tuning behavior of the quasi-phase matched OPO was also observed.