Yasuo Ohtera

Photonic crystals for the visible range fabricated by autocloning technique and their application

We fabricate photonic crystals for the visible range by the ‘autocloning’ technique, in which multilayers are stacked by an appropriate combination of sputter deposition and sputter etching. TiO2/SiO2 and Ta2O5/SiO2 are chosen as materials since they are transparent in the range and give a high contrast of refractive indices. The fabrication technique has flexibility regarding materials and size and is very reliable and reproducible even if the pitch is less than 0.2 μm. We also study the application of photonic crystals to birefringent elements such as waveplates and polarization selective gratings and experimentally verify that they are useful for optical pick-up systems.

Polarization stabilizer using liquid crystal rotatable waveplates

In standard single-mode fiber (SMF) transmission systems, a polarization stabilizer is useful for optimizing the performance of polarization dependent systems or devices. We have proposed and developed a polarization stabilizer which employs liquid crystal (LC) rotatable waveplates. It has a simple configuration, no moving parts, and endless-control characteristic. High polarization stability was confirmed under various state of polarization (SOP) fluctuations. The maximum response speed of the polarization stabilizer is about 900/spl deg//s, but this can be increased by improving the controller. We applied the polarization stabilizer to a semiconductor optical amplifier (SOA) having polarization dependence, and achieved good gain stabilization.

Numerical analysis of eigenmodes and splice losses of thermally diffused expanded core fibers

Field distributions and far-field patterns (FFPs) of the eigenmodes of thermally diffused expanded core (TEC) fibers are clarified by numerical analysis. The spot size of the TEC fiber is estimated by using the FFP that can be measured more easily than the near-field pattern (NFP). Splice losses of two TEC fibers are analyzed numerically considering a single or composite effect of gap, offset, and tilt. The results are useful for estimating isolations of fiber-integrated optical isolators and insertion losses of other fiber-integrated type optical devices. Splice losses considering multiple reflection between two endfaces of the TEC fibers with gaps are also analyzed numerically. This is useful for the design of fiber-integrated tunable wavelength filters.

Cylindrical Vector Laser Beam Generated by the Use of a Photonic Crystal Mirror

A concentrically patterned photonic crystal mirror with polarization selectivity fabricated by the autocloning technique was used for the generation of radially and azimuthally polarized beams from an Nd:YAG laser cavity. By adopting the photonic crystal mirror as the output coupler of the cavity, both radially and azimuthally polarized beams were obtained with similar output power as that of an unpolarized beam generated by a conventional, non-polarization-selective output coupler.

Propagation of light beams along line defects formed in a-Si/SiO2 three-dimensional photonic crystals: Fabrication and observation

We have fabricated optical waveguides in three-dimensional (3D) photonic crystals and observed propagation of light beams. Light beams with wavelengths of 1.15 μm propagate along the line defects formed in the 3D photonic crystals. The 3D photonic crystals consist of a-Si/SiO2 multilayers laminated alternately by rf bias sputtering on a periodically hollowed silica substrate with a triangular lattice. The pit diameter is 0.2 μm and the pitch of the lattice is 0.5 μm. The thickness of each laminated layer is 0.2 μm. Line defects are formed normal to the surface by laminating a-Si/SiO2 multilayers with ten periods on the substrate in which the corrugation patterns have been omitted in a certain area corresponding to the core. The measurements of transmittance normal to the surface show that the wavelength of 1.15 μm used in observation of propagation is in the passband for the one-dimensional periodic region corresponding to the core and in the stop band for the 3D periodic region corresponding to the cladd...

Multichannel Photonic Crystal Wavelength Filter Array for Near-Infrared Wavelengths

Multichannel wavelength filters consisting of 2-D photonic crystal (PhC) for the near-infrared wavelength region (~800 nm) are demonstrated. The filter is a thin-film wavy multilayer structure and is fabricated by the autocloning method, which is based on a radio frequency bias sputtering process. Twelve long-pass-type filter regions are integrated on a common silica substrate. Sharp cutoff characteristics and almost equal channel spacing are experimentally verified. To precisely control the effective lattice constant of PhC, a modulated lattice structure on the substrate is utilized

Liquid crystal rotatable waveplates

We propose and demonstrate rotatable waveplates using liquid crystal for the first time which can be used for polarization control. To realize a waveplate with an arbitrary direction of birefringence, we use a vertically-aligned nematic liquid crystal cell with a lateral electric field. The orientation of the waveplate can be rotated endlessly. It was confirmed that the direction of the waveplate could be rotated endlessly at a rate of 7200/spl deg//s, which is sufficient to construct polarization controlling devices for most SMF transmission systems.

Reduction of propagation and bending losses of heterostructured photonic crystal waveguides by use of a high-Δ structure

We describe the fabrication and evaluation of a low-loss, high-delta optical waveguide consisting of heterostructured photonic crystals. The waveguide is composed of a multilayer stack of Ta2O5/SiO2 and is prepared by use of the autocloning technique. Light is guided in the waveguide by the difference in the effective refractive indices of the constituent photonic crystals. By improving the design of the core region so that is has a flat multilayer structure, we achieve delta = 3.09% for the in-plane direction and net propagation loss of 0.56 dB/mm at lambda = 1.6 microm. Experiments also suggest that the bending loss of the waveguide can be reduced to less than 0.1 dB if the curvature radius is larger than 700 microm.

In-plane light propagation in Ta/sub 2/O/sub 5//SiO/sub 2/ autocloned photonic crystals

We investigate the characteristics of in-plane light propagation in 2-D photonic crystals fabricated by the autocloning method, by which multiple layers are stacked while preserving periodic surface corrugation. A numerical analysis shows that Ta/sub 2/O/sub 5//SiO/sub 2/ autocloned photonic crystals are applicable to waveguides and lenses since a sufficient amount of change of the effective refractive index is obtained by adjusting the aspect ratio of the unit structure. It is also shown that the crystals have a wide bandgap applicable to Bragg reflectors and resonators. Experimental results reveal that in-plane propagation losses for the two directions are less than 1.4 dB/mm at /spl lambda/=1.55 /spl mu/m, showing the crystals are useful for low loss in-plane devices.

Photonic crystals for the application to spectrometers and wavelength filters

This paper reviews the application of photonic crystals (PhCs) and related periodic structures to spectrometers and wavelength filters. In the first half of the paper in-plane type spectrometers utilizing superprism effect are reviewed. In the latter half out-of-plane and surface-normal type devices, where guided mode resonance (GMR) plays a center role, are described. Finally, Bragg reflection type wavelength filters are dealt with.