Masahide Itoh

Photorefractive effect in GaP

The photorefractive effect in undoped semi-insulating GaP was observed. GaP has a photorefractive sensitivity in the spectral region of 0.6 to 0.9 microm. The photorefractive center is the deep donor resulting from the P antisite defect. We measured the two-beam coupling gain coefficient and its time constant as functions of the grating period and the pumping intensity, using a 633-nm He-Ne laser. The maximum gain coefficient of 0.33 cm(-1) was obtained at a grating period of 1.1 microm. The time constant, which depends on the grating period, was 3-20 msec at a pumping intensity of 30 mW/cm(2).

Measurement of spatial coherence of a copper vapour laser beam using a reversal shear interferometer

We measured the spatial coherernce of a copper vapour laser (CVL) beam with an unstable resonator by the reversal shear interferometer. By this method, we can evaluate the spatial coherence function from a single-shot measurement. The spatial coherence width was 5 mm when an unstable resonator with a magnification factor of 60 was used. Moreover we verified the result by the theoretical calculation on the basis of the passive resonator model.

Non-collinear interaction model of the second harmonic generation of a copper vapor laser

Abstract We estimated theoretically the conversion efficiency of the second harmonic generation of a copper vapor laser (CVL) beam. Since the CVL beam is only partially coherent, the theory for the coherent beam is useless for the second harmonic generation of the CVL beam. The focused CVL beam was considered as a set of bundles of rays with propagated in different directions through a crystal and non-collinear interaction between these bundles of rays was investigated. On the basis of this model, we found that the conversion efficiency depends on the product of the beam divergence and the beam diameter, and the length of a nonlinear crystal.

Laser-induced damage threshold and absorption measurements in rare-gas-halide excimer laser components

Abstract We have developed a new system to measure the damage threshold with high reproducibility. We controlled the total energy on the sample by an optical system, without changing a spot-size on the sample surface. We made high-reflection coatings with and without over-coat, and anti-reflection coatings with and without under-coat for 248 nm KrF excimer laser light. In high-reflection coating, non-quarter-wavelength design was also tested. Damage threshold of coatings for 193 nm ArF excimer laser were also tested. We measured the absorption of coatings by the interferometric laser calorimetry. Revealing a cause of damage, we made a qualitative and quantitative analysis, so that the main cause of the damageof the optical coatings for 248 nm light was the residual absorption of the coatings.

Time-resolved measurement of beam divergence of a copper vapor laser using a saturable absorber

Abstract We measured the temporal change of the beam divergence of a copper vapor laser using the saturable absorber DASBTI. The recovery time of the absorption of the DASBTI (∽ 50 ps) is much shorter than the temporal change of the intensity of the copper vapor laser (∽ 20 ns). In this case, the transmissivity is a linear function of the instantaneous intensity. The beam divergence can be derived from the input and output power.

Absorption measurement of the optical materials by real time holographic interferometry

Abstract A new calorimetric technique is described for measuring a small absorption coefficient using real time holographic interferometry. The wavefront reconstructed by the hologram interferes with the wavefront which is deformed by the change of the optical path length caused by the temperature rise of the sample under radiation of an argon laser beam (1W, λ = 488 nm). From the movement of the fringes temporal and spatial temperature variation is measured, and fitting these values to the theoretical ones we obtain an absorption coefficient β = 0.14 (cm -1 ) of Corning glass filter No. 3–80 at λ = 488 nm.

Vibration analysis using photorefractive two-wave mixing

Abstract Vibration analysis by two-wave mixing in photorefractive BaTiO 3 crystals is made. The theoretical solution when the signal is exposed to a vibration with a period much smaller than the time constant of the crystal is deduced. The beam coupling and the gain saturation effects are considered. Experimental result is given to support the analysis. The application of photorefractive two-wave mixing in mode pattern visualization of vibrating structures is proven feasible.

Intensity stabilization in photorefractive two-wave mixing by controlling the phase shift of the fringe

Abstract The amplified signal beam of photorefractive two-wave mixing was stabilized by feedback system. The amplification factor of the two-wave mixing was controlled by changing the phase difference between the interference fringe and the index grating in BaTiO 3 . The interference fringe was shifted by frequency tuning of a laser diode. The short-term fluctuation of the output beam was reduced to 0.8%. We controlled the output intensity within the range from 10 to 4 mW although the change of the input power was 5%.

Preparation of YBa2Cu3O7−x crystals by verneuil process with laser-heating

Abstract Premelted YBa 2 Cu 3 O 7− x was mixed with a BaCuO 2 −CuO flux and formed into a boule melting the fed powder under laser-heating. Holding the boule at 910°C, the surplus flux was removed by melting and YBa 2 Cu 3 O 7− x crystals were extracted. The size of the crystals was from 0.01×0.01 to 0.5×0.5 mm with 0.02V0.04mm in thickness. After annealing at 600°C the crystals became superconducting with the transition temperature at 92 K. Interference contrast microscopy revealed microstep line structures on the as-grown crystal surfaces as well as domain structures in the orthorhombic phase. It was found that the crystallographic orientation of the orthorhombic lattice could be determined from the contrast of the domains in the microscope.