The proton-exchanged waveguide formed on MgO-doped lithium niobate crystals is resistant to the optical damage or the photorefractive effect. Therefore, this waveguide is believed to be a promising device for optical information and processing. However, the optical damage can also be an important problem for this waveguide in the communication wavelength since the high-power optical source is used. In this report, a brief general review on the optical properties and its practical application of the lithium niobate crystal as the optical waveguide are given. Then the experimental research work aimed to clarify the properties and its mechanism of the electrooptic effect and the optical damage or photorefractivity of the lithium niobate optical waveguide is described. In this work, the optical damage in this proton-exchanged waveguide is measured quantitatively at various optical wavelengths including blue and red light by using the holographic grating method and the infrared communication wavelength (1550 nm) by using the prism coupler method. The optical damage is significant not only in blue wavelength but also in the red, and even at 1550 nm with high power (100 mW) laser diode for communication. So the optical damage cannot be negligible also in the communication wavelengths. The effect of annealing temperature is also discussed. At the relatively high temperatures, the optical damages are founde to be annealed out. The effect of the applied electric field to the optical damage is experimentally discussed and its enhancement is observed to the applied d.c. and a.c. fields. In conclusion, the optical properties as the electrooptic constant and the optical damage are experimentally measured and the many fundamental data are obtained to realize the useful and practical optical devices.

Autocloned photonic crystals have corrugated multilayer structure. By related process technology, we can easily fabricate an array of polarizers or waveplates. Patterned photonic crystals are versatile component of many optical systems. This paper focuses on their use in optical microscopy. The main topics are: 1. Use of polarization imaging in microscopy, 2. generation of radial/circular polarization by a polarizer having concentric corrugations, 3. a "longitudinal polarization slit" (a new component) and its function in confocal microscopy, and 4. a polarization converter for generating "z-polarized" light at the focal point. In every application above, autocloned photonic crystals play a central role.

This study proposes a novel structure of index-guiding square photonic crystal fibers (SPCF) having simultaneously ultra-flattened chromatic dispersion characteristics and low confinement losses in a wide wavelength range. The finite difference method (FDM) with anisotropic perfectly matched layers (PMLs) is used to analyze the various properties of square PCF. The findings reveal that it is possible to design five-ring PCFs with a flattened negative chromatic dispersion of 0-1.5 ps/(nm.km) in a wavelength range of 1.27 µm to 1.7 µm and a flattened chromatic dispersion of 01.15 ps/(nm.km) in a wavelength range of 1.25 µm to 1.61 µm. Simultaneously it also exhibited that the confinement losses are less than 10-9 dB/m and 10-10 dB/m in the wavelength range of 1.25 µm to 1.7 µm.

A novel procedure for an efficient and rigorous solution of electromagnetic scattering problems is presented. It is based on the use of universal bases that are obtained by applying the SVD procedure to PO-derived basis functions. These bases, constructed by totally bypassing any matrix-type approach, can be used for all angles of incidence and their use leads to a matrix with relatively small dimensions. The method enables us to solve 2D scattering problems in a computationally efficient and numerically rigorous manner.

This paper presents the Physical Optics field calculation in terms of only line integrations by using the Modified Edge Representation technique (MER), the alternative way of the surface integration. Not only the diffracted fields as in the conventional method of equivalent edge currents (EEC) but also the scattering geometrical optics fields are expressed in terms of the MER line integrals. The far field patterns of parabolic reflector antennas with the defocused dipole feed are discussed and the satisfactory agreement with those obtained by the Physical Optics surface integration is demonstrated.

We propose a simple technique to form miniature optical circuits using microfibers embedded into a low refractive index matrix. As an example we demonstrate a silica microfiber knot resonator embedded in a fluoroacrylate polymer. Fabrication issues and initial experimental results are reported. We also present simulations aimed at understanding the current limitations to the Q-factor and the role of the embedding polymer refractive index on the Q-factor of future resonators. It is anticipated that using commercially available polymers high Q-factor resonators with radii as small as 100 micrometers can be made by this technique.

We fabricated various microscopic optical devices with Si photonic wire waveguides and demonstrated their fundamental characteristics. The bending loss of the waveguide was practically negligible when the bending radius of the waveguide exceeded 5 µm. Therefore, we can fabricate very compact optical devices with the waveguide. We demonstrated an optical directional coupler with the waveguide. The coupling length of the directional coupler was extremely small, several micrometers, because of strong optical coupling between the waveguide cores. We also demonstrated ultrasmall optical add/drop multiplexers (OADMs) with Bragg grating reflectors constructed from the waveguides. The dropping wavelength bandwidth of the OADM device was less than 2 nm and the dropping center wavelength could be tuned using thermooptic control with a microheater formed on the Bragg reflector. Using the Si photonic wire waveguide, we also demonstrated thermooptic switches by forming a microheater on a branch of a Mach-Zehnder interferometer made up of the waveguides. In this switching operation, we observed an extinction ratio exceeding 30 dB, a switching power less than 90 mW, and a switching response speed less than 100 µs using a 12 optical switch with an 8530 µm2 footprint.

The Modified edge representation (MER) is the concept to be used in the line integral approximation for computing the surface radiation integrals of diffraction. The MER as applied to the physical optics (PO-MER), has remarkable accuracy in the surface-to-line integral reduction even for the curved surfaces and for sources very close to the scatterer. In the discussion of the mathematical foundation for this accuracy, the evaluation of the singularities in the integrand of the PO-MER line integration was left for further study.

Locality in high frequency diffraction is embodied in the Method of Moments (MoM) in view of the method of stationary phase. Local-domain basis functions accompanied with the phase detour, which are not entire domain but are much larger than the segment length in the usual MoM, are newly introduced to enhance the cancellation of mutual coupling over the local-domain; the off-diagonal elements in resultant reaction matrix evanesce rapidly. The Fresnel zone threshold is proposed for simple and effective truncation of the matrix into the sparse band matrix. Numerical examples for the 2-D strip and the 2-D corner reflector demonstrate the feasibility as well as difficulties of the concept; the way mitigating computational load of the MoM in high frequency problems is suggested.

A lasing charactrization of a Brillouin/erbium optical fiber laser (BEFL) is experimentally discussed. In the BEFL, an erbium-doped fiber amplifier (EDFA) is incorporated into the Brillouin laser resonator to enhance small Brillouin gain, which makes the configuration of the Brillouin laser resonator easy and flexible. The experimental results show that the output power of the BEFL has a threshold against the Brillouin pump power, and above the Brillouin threshold, the output power increases linearly with the EDFA pump power. The BEFL threshold decreases with increasing the length of the optical fiber in the laser resonator used as a Brillouin gain medium. The BEFL oscillates in a stable single longitudinal mode because the bandwidth of the Brillouin gain profile is very narrow ( 30 MHz). The relative intensity noise (RIN) and the spectral lineshape were measured. The noise floor level decreases with increasing the EDFA pump power, and the full-width at half maximum of the BEFL was measured to be about 8 kHz.

In order to achieve omni-directional coverages on base station antennas for fixed wireless access systems (FWA), a TM01 mode conical horn with 4.6λ aperture size was employed as a feed horn for an axisymmetrical reflector antenna. Here, a shaped dielectric lens was inserted in the conical horn so as to achieve low sidelobe radiation characteristics. However, it was pointed out that radiation pattern shaping ability was degraded in this small lens antenna. In this paper, deteriorations of aperture distributions in a shaped lens are clarified through FDTD calculations. Severe phase delays are shown in the aperture phase distributions. A novel lens shaping method of compensating the phase delays is developed. Aperture distributions and radiation patterns of the corrected lens are estimated through FDTD calculations. Satisfactory uniform phase distributions in aperture distributions and low sidelobe radiation patterns are ensured.

This paper reviews the approximation principle of Physical Optics in view of diffraction theory. Two key error factors are identified for PO, that is, 1) errors in edge diffraction coefficients and 2) fictitious penetrating rays. Improved methods named PO-AF and PTD-AF are proposed as the methods which suppress the fictitious penetrating rays from PO and PTD respectively. In deep shadow regions of the reflector antennas, PO-AF and PDT-AF approach to PO-EEC and UTD respectively, while the continuity is assured. The effectiveness is numerically demonstrated for two dimensional scatterers.

The recent progress in the field of Ti:Er:LiNbO3 waveguide lasers with emission wavelengths in the range 1530 nm < λ < 1603 nm is reviewed. After a short discussion of the relevant fabrication methods concepts and properties of different types of lasers with grating resonator, acoustooptically tunable Fabry Perot type lasers and new ring laser structures are presented.

Holey Fiber (HF) technology has progressed rapidly in recent years and has resulted in the development of a wide range of optical fibers with unique and highly useful optical properties including endlessly single-mode guidance, and high optical nonlinearity. In this paper the state-of-the-art HF technology for all-optical signal processing devices is reviewed from a perspective of possible application for telecommunications.

The add/drop wavelength filter is an essential component in the new-generation photonic network. Microring resonator filters using high index contrast (HIC) optical waveguides are recently attracting attention as add/drop filters owing to their compactness, functionality such as dispersion compensation, and ease of filter synthesis. In particular, the vertically coupled microring resonator (VCMRR) filter is highly suited for the dense, large-scale integration of filter circuits. In this review, the fundamental characteristics advantageous to the add/drop filter nodes are introduced, and the recent progress in the development of vertically coupled microring resonator filters achieved mainly by the author's group is described.

High frequency (HF) diffraction is known as local phenomena, and only parts of the scatterer contribute to the field such as the edge, corner and specular reflection point etc. Many HF diffraction techniques such as Geometrical Theory of Diffraction (GTD), Uniform Theory of Diffraction (UTD) and Physical Theory of Diffraction (PTD) utilize these assumptions explicitly. Physical Optics (PO), on the other hand, expresses the diffraction in terms of radiation integral or the sum total of contributions from all the illuminated parts of scatterers, while the PO currents are locally defined at the point of integration. This paper presents PO-based visualization of the scattering and diffraction phenomena and tries to provide the intuitive understanding of local property of HF diffraction as well as the relations between PO and the ray techniques such as GTD, UTD etc. A weighting named "eye function" is introduced in PO radiation integrals to take into account of local cancellation between rapidly oscillating contributions from adjacent currents; this extracts important areas of current distribution, whose location moves not only with the source but also with the observation point. PO visualization illustrates both local property of HF scattering and defects associated with ray techniques. Furthermore, careful examination of visualized image reminds us of the error factor in PO as applied for curved surfaces, named fictitious penetrating rays. They have been scarcely recognized if not for visualization, though they disturb the geometrical shadow behind the opaque scatterer and can be the leading error factors of PO in shadow regions. Finally, visualization is extended to slot antennas with finite ground planes by hybrid use of modified edge representation (MER) to assess the significance of edge diffraction.

Point-to-point optical and millimeter wave communication has recently been of interest, especially in urban areas. Its benefits include simpler and easier installation compared with a land-based line. However, this technology suffers when adverse weather conditions are present, such as rain, fog and clouds, which induce scattering and absorption of the optical wave. The effects of scattering and absorption degrade the quality of the communication link resulting in increase of bit-error-rate. Therefore, there exists a need for accurate channel characterization in order to understand and mitigate the problem. In this paper, radiative transfer theory is employed to study the behavior of amplitude modulated signal propagating through a random medium. We show the effect of the medium to a modulated signal and relate the outcome on the quality of the communication link.

Symmetric Mach-Zehnder (SMZ) type all-optical swit-ches are discussed. The SMZ type all-optical switches feature the so-called differential phase modulation scheme to achieve a speed unrestricted by efficient, thus usually slow nonlinearities. In these switches, semiconductor optical amplifiers (SOAs) are often used to realize low optical power switching. We discussed SOAs from a view point of all-optical switch applications, rather than amplifier applications. Finally, all-optical signal processing experiments are discussed with the SMZ type all-optical switches. These include ultrafast demultiplexing of 336 Gb/s signal pulses and random operations at 42 Gb/s for all-optical logic operation and wavelength conversion.

In this paper, the absorbing property of the discrete Green's function ABC, which was based on a powerful concept of the TLM method, has been improved by relocating loss process from the time domain to the space domain. The proposed scheme simply adds a loss matrix to the connection matrix in the basic TLM algorithm to make the formulation of the ABC more efficient. Various lengths of absorbing layers discretized for a WR-90 empty waveguide have been tested in terms of reflection property. An expression for an optimum absorbing property has been also derived with respect to the length of the layer. Comparison of the layer with the discrete Green's function ABC shows that the layer in this study has improved reflection property better than approximately 3 and 6 dB, respectively, when 50Δ and 60Δ absorbing layers have been adopted for the WR-90 waveguide. Finally, the layer has been applied to a WR-75 metal insert filter as an example.

We developed a process to fabricate optical functions such as, lens, prism, or diffuser directly on to a glass substrate. Processes include precision mastering by diamond cutting, and multi-layer photopolymer (2P) molding process to realize flat surface and integration of multiple functions with a good alignment within few micrometers.