Takashi Sawada

Elimination of polarization sensitivity in silica-based wavelength division multiplexer using a polyimide half waveplate

This paper proposes a low-loss technique for eliminating polarization sensitivity in a silica-based planar lightwave circuit (PLC) which uses a polarization mode converter formed at the center of the circuit. This converter consists of a waveguide gap housing a polyimide half waveplate. The excess loss of the converter was drastically reduced to 0.26 dB with a /spl Delta/=0.75% waveguide by employing an 18 /spl mu/m-wide waveguide gap and a 14.5 /spl mu/m-thick polyimide half waveplate. A polarization mode conversion crosstalk of -37 dB was achieved at 1.55 /spl mu/m. Using this converter, we successfully eliminated the polarization sensitivity in some silica-based PLC-type wavelength division multiplexers. The converter is also insensitive to temperature and offers long term stability.

Deposition Pressure Effects on the Laser Plume of YBa2Cu3O7-δ

The control of the preferential orientation in oxide superconducting films, which is generally done by deposition temperatures, was achieved by choosing the oxygen pressures during the deposition. Time-resolved optical observations of the laser plume were carried out at the same time with the film deposition. Upon increasing the oxygen pressure, the velocities of the emissive species ablated from the target were decreased. The kinetic energy effects of the ablated species on the film orientation are discussed, and the orientation change can be qualitatively understood by a surface migration model as well as the case of the deposition temperature dependence.

Origin of Intrinsic Second-Harmonic Generation in Crystallized GeO2–SiO2 Glass Films

The origin and properties of second-harmonic generation (SHG) from crystalline particles and their crystalline phases in GeO2–SiO2 glass films were investigated. GeO2–SiO2 glass films with a thickness of approximately 5 µm were fabricated by the chemical vapor-phase deposition (CVD) method. Heat treatments in the temperature range of 1100–1260°C for durations of 2–8 h have been performed to crystallize the glass films. X-ray diffraction (XRD) peaks at around 2θ=22° in the crystallized GeO2–SiO2 films were observed, and the obtained XRD patterns in the glass films are exactly the same as those in ultraviolet (UV) poled GeO2–SiO2 glasses with a large second-order optical nonlinearity comparable to that in LiNbO3 crystals. Using a SHG microscopic technique, it has been found that the intensity of SHG emission from crystalline particles is clearly dependent on stress-induced optical retardation which is accompanied by the formation of crystalline particles of the pseudo-β-cristobalite containing GeO2. In addition, a new technique for enhancing the intrinsic SHG by means of modification of defect states is demonstrated.

Direct observation of second-harmonic generation from crystalline particles in Ge-doped SiO2 glass films.

The authors report what is, to our best knowledge, the first observation of second-harmonic generation (SHG) directly from the crystalline particles in Ge-doped SiO2 (Ge:SiO2) glass films. Ge:SiO2 glass films with approximately 5 microm thickness were fabricated by chemical vapor-phase deposition. X-ray diffraction (XRD) peaks at around 2theta=22 degrees in thermally crystallized Ge:SiO2 films were observed, and obtained XRD patterns are exactly the same as those in ultraviolet-laser-poled Ge:SiO2 glasses. Using SHG microscopic technique with a Nd:YAG laser, it has been successfully found that SH emitting with 532 nm wavelength is observed directly from the crystalline particles induced in the crystallized films.

Ultrathin (5 µm) Flexible Reflective Waveplate of Fluorinated Polyimide and Elimination of Polarization Sensitivity in Titanium-Diffused Lithium Niobate Waveguide Circuits

An ultrathin, flexible reflective half-waveplate, consisting of a 5-µm-thick polyimide quarter-waveplate sputtered with a thin gold film, was fabricated, and its property of eliminating polarization sensitivity in titanium-diffused lithium niobate waveguide circuits was investigated. Transverse electric/transverse magnetic mode-conversion ratios of 15–20 dB at 1.55 µm were obtained, and a polarization-insensitive intensity modulator with a low half-wave voltage of 4.5 V was successfully fabricated using a titanium-diffused lithium niobate waveguide. The excess loss of the polyimide reflective waveplate (1 dB) was much smaller than that using a conventional quartz waveplate (6 dB).

YBa2Cu3O7−δ - PrBa2Cu2TaO8 heterostructures made by pulsed laser deposition

Abstract The YBa 2 Cu 3 O 7−δ (YBCO)/PrBa 2 Cu 2 TaO 8 (PBCT) heterostructure was formed by pulsed laser deposition. The great possibility of PBCT for the barrier of junctions using YBCO has been confirmed. The lattice flexibility of PBCT is poorer than that of YBCO and YBCO can be used as a buffer layer between PBCT and substrates being lattice mismatch.

In-plane birefringence of polyimide films prepared on substrates having thermal expansion anisotropy

Polyimide films whose refractive indices in the film plane show anisotropy (in-plane birefringence, Δn) were prepared by a novel method based on the anisotropy of thermal expansion coefficients (Δα) of single crystal substrates. Polyimide molecular chains were oriented along the crystal axis having a larger thermal expansion coefficient. The Δn of polyimide films increases as the Δα and the larger thermal expansion coefficient of the substrates increase.

Preparation of YBCO Thin Films by Plasma Assisted Pulsed Laser Deposition

YBCO films were deposited in a RF discharge oxygen plasma by pulsed laser deposition. Critical temperatures (Tc) decreases with an increasing RF power. Since Tc increases up to almost the same value by annealing at 500 °C in oxygen followed by slow cooling, films deposited in an oxygen plasma were deficient in oxygen. Scanning electron microscopic observations revealed that the crystal grain size in films deposited in an oxygen plasma was larger than that of the film without plasma, which corresponds to a smaller oxygen diffusivity into films prepared in oxygen plasma