Takahisa Jitsuno

Laser ablative shaping of plastic optical components for phase control

A new scheme for phase control of optical components with laser ablation has been developed. One can ablate the surface shape of optical plastic material coated on a glass plate by using 193-nm laser light to control the transmission wave front. The surface shape is monitored in situ and corrected to attain the desired aberration level. The irradiation fluence is approximately 40 mJ/cm(2), and the ablation depth/pulse is approximately 0.01 microm/pulse for UV-cured resin. A wave-front aberration of 3.0 lambda is reduced to 0.17 lambda for flat surface shaping. For spherical surface generation, an aberration of 2.5 lambda is reduced to 0.2 lambda. The increase in surface roughness is kept within acceptable levels.

Control of gray-tracking formation for KTP and LBO crystals

Gray-tracking formations for KTP crystals were controlled by thermal annealing process before laser irradiation because hydroxyls in the crystals were controllable. The formation was suppressed with increasing the annealing temperature and humidity. LBO crystals were also examined.

Control of gray-tracking formation in KTiOPO/sub 4/ crystals by thermal annealing preprocess

Summary form only given. The KTiOPO/sub 4/ (KTP) crystal was used for second-harmonic generation (SHG) of high-average-power lasers because it has a large nonlinear optical coefficient and a wide temperature bandwidth. However, the crystal has one disadvantage, i.e., gray-tracking formation. The gray-tracking was investigated by many workers. The gray-tracking susceptibility (GTS) was different for individual crystal. To understand the gray-tracking formation, many crystals from different growth techniques should be compared. Relationship between the absorption coefficient and the GTS or the impurities for different KTP crystals was investigated. Finally, we propose a new approach to improve the gray-tracking.

UV laser ablative figuring of plastic surface for precise optics

We have submitted a paper conceming on a new approach for fabricating a precise optics in a large diameter (up to 1 m) for laser fusion driver using the laser surface ablation by very short wavelength excimer laser in CLEO95. We have made some basic experiments on the measurement of the ablation rate of different kinds of optical plastics and in situ interferometric observation of the surface figure of a PMMA plate. A new approach for mitigating some important disadvantages of optical plastics such as the bulk nonuniformity, the index drift due to the humidity absorption and the strong birefringence, have been proposed using the plastic-glass hybrid component to reduce the thickness of plastic. In our research, we first examined the direct beam ablation of PMMA without a focusing optics. In this case, a very uniform surface ablation have been observed similar to the previous work? The ablation rates for the direct beam irradiation for different values of energy densities are shown in Fig. 1. However, if a focusing lens is introduced to increase the irradiation intensity, some undesired modulation of surface figure have been observed. The source of these modulation may be due to the instantaneous pattem of irradiation beam or the thermal effect on the surface of the plastic. The interaction of the plastic vapor in the optical pass with the laser beam may be an another source of this nonuniformity. We also examined a rod type beam homogenizer for obtaining a uniform beam distribution. However, the ablated surface using the rod homogenizer had some strong surface modulation due to the instantaneous interference of the laser beam in the homogenizer. In the ablation experiment, the surface deterioration has been observed due to the deposition of carbon or particles on the surface of the plastic. These deterioration can be mitigated by the oxygen gas flow on the surface. The environmental condition have also been changed in a chamber with the air or the pure oxygen gas at 1 atm and the vacuum. Although the deposition of carbon or particle have been mitigated in the oxygen or in the vacuum condition,