Simulation of nonlinear propagation of femtosecond laser pulses in air for quantitative prediction of the ablation crater shape.

The utilization of sub-100 fs pulses has attracted attention as an approach to further improve the quality and precision of femtosecond laser microfabrication. However, when using such lasers at pulse energies typical for laser processing, nonlinear propagation effects in air are known to distort the beam's temporal and spatial intensity profile. Due to this distortion, it has been difficult to quantitatively predict the final processed crater shape of materials ablated by such lasers.

Influence of thermal stress on continuous-wave second-harmonic generation in periodically poled LiTaO crystals.

Thermal stress effects on continuous-wave second-harmonic generation in periodically poled LiTaO crystals are evaluated via a numerical simulation that is based on nonlinear propagation equations and a thermal conduction equation. The thermal performance and absorption coefficients used in the numerical simulation were determined by comparison with experimental results. The results show that the thermal stress caused by a small residual absorption would restrict the maximum output power of a second-harmonic-continuous-wave laser.

Simplified system for relative phase control between two input beams for coherent polarization beam combination.

We devised a simplified system for coherent polarization beam combination (CPBC), in which two beams with orthogonal polarizations are combined with a polarizing beam splitter (PBS). In a CPBC system, control of the relative phase between two beams is important to obtain an output beam with stable polarization. Herein, the beam leaked from PBS is used to control the relative phase, realizing a robust system.

Thermal performance in high power SHG characterized by phase-matched calorimetry.

We proposed a method to determine device quality in heat removal. Temperature change depending on SH power was analyzed by fitting with a new model to characterize heat removal performance of SHG modules, named as phase-matched calorimetry (PMC). The thermal disposal performance of SHG devices was improved by combination of metal housing and reduced crystal aperture.

High-efficiency electro-optic amplitude modulation with delayed coherent addition.

Amplitude modulation of laser light is required for resonant sideband extraction employed in gravitational-wave detectors. Amplitude modulation with electro-optic phase modulators is realized by interferometric phase-to-amplitude conversion. Although two outputs modulated at opposite phases to each other are obtained, usually only one of them is utilized and the other is abandoned.

Wideband and high-gain frequency stabilization of a 100-W injection-locked Nd:YAG laser for second-generation gravitational wave detectors.

Second-generation gravitational wave detectors require a highly stable laser with an output power greater than 100 W to attain their target sensitivity. We have developed a frequency stabilization system for a 100-W injection-locked Nd:YAG (yttrium aluminum garnet) laser. By placing an external wideband electro-optic modulator used as a fast-frequency actuator in the optical path of the slave output, we can circumvent a phase delay in the frequency control loop originating from the pole of an injection-locked slave cavity.

Formulation of frequency stability limited by laser intrinsic noise in feedback systems.

We investigated the influence of amplitude modulation (AM) noise and phase modulation (PM) noise of a laser source on the frequency stability in frequency stabilization systems. We estimated the frequency stability and evaluated the efficacy of a noise reduction technique (the Doppler-trend subtraction method) of a laser diode frequency stabilization system, where enhanced intensity noise arising from PM-to-AM noise conversion through a reference gas cell is reduced using the technique employed in modulation transfer spectroscopy. To evaluate the relationship between the laser's intrinsic noise and its frequency stability, we performed noise spectrum measurements and formulated frequency stability in addition to measuring Allan standard deviation.

Thermal effects in high-power CW second harmonic generation in Mg-doped stoichiometric lithium tantalate.

We investigated thermal behaviors of single-pass second-harmonic generation of continuous wave green radiation with high efficiency by quasi-phase matching in periodically poled Mg-doped stoichiometric lithium tantalate (PPMgSLT). Heat generation turned out to be directly related to the green light absorption in the material. Strong relation between an upper limit of the second harmonic power and confocal parameter was found.

Development of a wavelength-stabilized distributed bragg reflector laser diode to the Cs-D2 line for field use in accurate geophysical measurements.

We have developed a wavelength-stabilized laser diode (LD) for geophysical measurement devices, which benefit from the uniformity of laser light. Regarding this purpose, a system that has such characteristics as low power consumption, sturdiness against mechanical disturbances, and a long life with long-term frequency stability is especially required. Therefore, we adopt as the light source a distributed Bragg reflector (DBR) LD because it has various advantages concerning such properties.

100 W, single-frequency operation of an injection-locked Nd:YAG laser.

We have built a single-frequency Nd:YAG laser capable of producing an output power of 101 W by injection locking a slave laser that can emit an output power of 121 W in the free-running state to a 2-W master laser. We confirmed that the output mode was diffraction limited and linearly polarized.

Development of a frequency-detuned interferometer as a prototype experiment for next-generation gravitational-wave detectors.

We report on our prototype experiment that uses a 4-m detuned resonant sideband extraction interferometer with suspended mirrors, which has almost the same configuration as the next-generation, gravitational-wave detectors. We have developed a new control scheme and have succeeded in the operation of such an interferometer with suspended mirrors for the first time ever as far as we know. We believe that this is the first such instrument that can see the radiation pressure signal enhancement, which can improve the sensitivity of next-generation gravitational-wave detectors.