Kazuya Takayama

Fabrication of DAST Channel Optical Waveguides

In order to use the large, electro-optic coefficient of a nonlinear optical ionic crystal, 4-(p-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST), a channel optical waveguide structure is needed. We successfully fabricated a waveguide using two methods: by a dry-etching technique and by photo-bleaching. Because DAST has a large optical loss, parts of the waveguide should be composed of a transparent polymer. We used photolithography and a reactive ion etching method to fabricate a serially grafted (conjunct) waveguide of DAST with a transparent polymer waveguide. The waveguide was also fabricated by photobleaching, whereby the refractive indices of the crystal’s a- and b-axes were decreased by degrading the crystal. The cladding part of the DAST waveguide was photobleached by irradiating with UV light. The under- and over-cladding layers of these channel waveguides were composed of a UV-cured resin that did not dissolve the DAST crystal. The loss of the crystal waveguide for the crystal b-direction was around 10 dB/cm, due to the scattering loss of the DAST single crystal.

Biological effect of shock waves on rat brain: pathological evaluation by compact Ho:YAG-laser-induced cavitational shock wave generator

To introduce shock wave as a new treatment modality for the lesions in the vicinity of brain and skull, pressure-dependent brain damages after exposure of shock wave were investigated. A novel compact Ho:YAG laser-induced cavitational shock wave generator (diameter: 15 mm, weight: 20g) was used intstead of clinical lithotriptors due to their wide distribution of shock waves. In the first part, we have developed and investigated characteristics of present generator by means of high-speed photography, shadowgraphy, and pressure measurement. Generation of localized shock wave without harmful effect of laser was observed after irradiation of Ho:YAG laser in the brass tube with internal water supply. Mechanical effect of accompanying laser-induced liquid jet was mitigated after placement of latex diaphragm with acrylic water reservoir. Maximum overpressure of generated shock wave was 15 MPa before placement of diaphragm, and 5 MPa after placement of diaphragm. In the second part, shock wave-induced brain damages were investigated in 5 male Sprague-Dawley rats. While subarachnoid hemorrhage could be observed between 1 and 5 MPa, intracerebral hemorrhage, and laceration of tissue were also observed above 5 MPa. We therefore conclude that overpressure of exposing shock wave over brain surface should be managed under 1 MPa.

Serially Grafted Waveguide Fabrication of Organic Crystal and Transparent Polymer

We have successfully fabricated a serially grafted waveguide combining a type of organic nonlinear optical (NLO) crystal, 4-(p-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST), and a transparent polymer. To utilize the large electrooptic coefficient of the DAST effectively, a waveguide structure is desirable. Since organic crystals with large NLO effects show large optical losses, a major part of this serially grafted waveguide was composed of the transparent polymer. The input/output optical beam has been effectively coupled to the DAST crystal waveguide through the transparent polymer waveguide. This serially grafted structure is a potential candidate for fabricating many types of practical devices with optical nonlinearity such as the Mach-Zehnder interferometer(MZI).

Polymer Optical Waveguide Chip: New Tool for Simple Evaluation of Optical Parameters of Waveguide Elements

A polymer optical chip consisting of 45°-angled cut straight waveguides, Y-splitters, and S-bend waveguides was designed and fabricated to examine a simple evaluation of multimode waveguides. Appropriate input condition was investigated for standardization of an evaluation method of optical characteristics of multimode waveguides.

Fabrication of nonlinear optical waveguide

Abstract A ridge waveguide of an organic salt, 4-(4-dimethylaminostyryl)-l-methylpyridinium tosylate, DAST, which has the largest second-order optical nonlinearities, was fabricated for the first time to our knowledge. The DAST crystal was covered by poly (methylmethacrylate) as a protective layer for a photo-resist solution, then standard photolithography and oxygen reactive ion etching (RIE) processes were applied. The DAST ridge waveguide, 34 × 20 (μm and 6×6 μm channels with 2 mm in length were successfully fabricated by this process. For the DAST crystal, photo bleaching process was effectively applied to reduce the refractive index of the crystal that can be applied to fabricate a cladding layer for the ridge waveguide. Fabrication of serially grafted waveguide of the DAST and UV cured resin was also tried that is important technology for actual application of the crystal waveguide.

Fabrication and optical properties of an organic crystal waveguide

A ridge waveguide of an organic salt, 4-N,N-methyl- stilbazorium tosylate, DAST, which has the largest second- order optical nonlinearities among organic materials, was fabricated for the first time to our knowledge. DAST crystal was covered by poly (methyl methacrylate) as a protect layer for a photo-resist solution, then standard photolithography and oxygen reactive ion etching (RIE) processes were applied. DAST ridge waveguide, 34x20 micrometers and 6x6 micrometers channel s with 2mm in length were successfully fabricated by this process. To reduce the refractive index of the crystal, photo bleaching was effectively applied for DAST crystal. So, photobleaching can be applied to fabricate a cladding layer for the ridge waveguide.

Simple evaluation method of multimode polymer optical waveguides for next generation FTTH application

Polymer optical waveguide devices are getting popular for next generation FTTH application. In order to accelerate the development of polymer optical devices, evaluation of waveguide characteristics should be speeded up. Polymer optical chip containing a combination of 45°-angled cut waveguide, Y-splitter and S-bend structures was designed and fabricated for simple evaluation of multimode waveguides. Input launching such as light source, mode scrambler was investigated for reliable measurement.

A relationship between the optical anisotropy and the optical nonlinearity of a poled polymer film

A uniaxial refractive index anisotropy was introduced by a corona poling. And, and biaxial refractive index anisotropy was introduced by a physical film stretching with the presence of the nonlinear optical chromophore. The refractive indices of the film before poling and after the poling were nx=ny=nz=1.541 and nx=ny=1.539, nz=1.552, respectively. The mechanical stress was then applied to the film to introduce another anisotropy. The refractive indices after the stretching were nx=1.540, ny=1.538, and nz=1.551. We also proposed an in-situ film elongation technique with poling. Above the grass transition temperature, the polymer film is plasticized and can be stretched longer so that the chromophore is reoriented effectively. As a result, the decreasing of film transparency due to the stretching can be prevented by this temperature.

Visualization of interfacial instability induced by initially disturbed converging shock waves

The paper reports an experimental study of the interaction of initially disturbed converging and diverging cylindrical shock waves with cylindrical interfaces and the resulting Richtmyer- Meshkov instability. Experiments were conducted in an annular vertical diaphragmless shock tube, in which cylindrical soap bubbles filled with He and SF6 were placed co-axially in its test section. In order to the clarify effects of initial disturbances, 10 mm dia. pins were installed in the annular section, 92 mm from the shock tubes test section and before the 90 degree smooth bend at the top of the shock tube. The inner ends of the pins were rounded to ensure completel sealing with the inner wall of the low pressure channel. Weak disturbances in the converging cylindrical shock waves were produced by inserting the pins. Double exposure holographic interferometry was used for quantitative visualization of the interfacial instability and the subsequent mixing between the interface gases. It is known that the lower mode numbers of disturbances are dominant near the converging center, therefore experiments with mode eight and mode four disturbed converging shock waves were performed and the effects of weak waves behind the shock wave on the interfacial growth rates were clarified.