This paper newly proposes and theoretically analyzes the performance of multi-hop free-space optical (FSO) systems employing optical amplify-and-forward (OAF) relaying technique and wavelength division multiplexing (WDM). The proposed system can provide a low cost, low latency, high flexibility, and large bandwidth access network for multiple users in areas where installation of optical fiber is unfavorable. In WDM/FSO systems, WDM channels suffer from the interchannel crosstalk while FSO channels can be severely affected by the atmospheric turbulence. These impairments together with the accumulation of background and amplifying noises over multiple relays significantly degrade the overall system performance. To deal with this problem, the use of the M-ary pulse position modulation (M-PPM) together with the OAF relaying technique is advocated as a powerful remedy to mitigate the effects of atmospheric turbulence. For the performance analysis, we use a realistic model of Gaussian pulse propagation to investigate major atmospheric effects, including signal turbulence and pulse broadening. We qualitatively discuss the impact of various system parameters, including the required average transmitted powers per information bit corresponding to specific values of bit error rate (BER), transmission distance, number of relays, and turbulence strength. Our numerical results are also thoroughly validated by Monte-Carlo (M-C) simulations.

We numerically and experimentally demonstrate for the first time a novel all-optical quantization technique using dense spectral slicing with a specially designed arrayed waveguide grating for orthogonal frequency division multiplexed signals. By using a mode-locked laser diode with low jitter, the quantization technique can be achieved a high-speed and low-jitter operation. Both numerical and experimental results confirm the feasibility of 10 GSample/s, completely linear 3-bit step quantization for photonic analog to digital conversion. This optical quantization technique will be beneficial for ultra-high-speed optical communication using digital signal processing.

We experimentally investigate the performance of a distributed Raman amplifier (DRA)-based pulse compressor for a phase modulated signal. A 10 Gb/s return-to-zero (RZ)-differential phase shift keying (DPSK) signal is compressed to picosecond range after transmission. Pulsewidth is continuously compressed in a wide range from 20 to 3.2 ps by changing the pump power of the DRA while the compressed waveforms are well-matched with sech2 function. Error-free operations at bit-error-rate (BER) of 10-9 are achieved for the compressed signals of various pulsewidths with low power penalties within 2.3 dB compared to the back-to-back. After the compression, the 10 Gb/s signal is used to generate a 40 Gb/s RZ-DPSK optical time division multiplexing (OTDM) signal. This 40 Gb/s OTDM signal is then successfully demultiplexed to 10 Gb/s DPSK signal by using an optical gate based on four-wave mixing (FWM) in a highly nonlinear fiber (HNLF).

In all-optical switches using the cascade of second harmonic generation and difference frequency mixing in periodically poled lithium niobate (PPLN) waveguide devices, walk-off between the fundamental and second harmonic pulses causes crosstalk between neighboring symbols, and limits the switching performance. In this paper, we numerically study retiming characteristics of all-optical switches that employ the PPLN waveguide devices with consideration for the effects of the crosstalk and for the input timing of the data and clock pulses. We find that the time offset between the data and clock pulses can control the timing jitter of the switched output; an appropriate offset can reduce the jitter while improving the switching efficiency.

All-optical 1-to-6 wavelength multicasting of a 10-Gb/s picosecond-tunable-width converted return-to-zero (RZ)-on-off-keying (OOK) data signal using a wideband-parametric pulse source from a distributed Raman amplifier (DRA) is experimentally demonstrated. Width-tunable wavelength multicasting within the C-band with approximately 40.6-nm of separation with various compressed RZ data signal inputs have been proposed and demonstrated. The converted multicast pulse widths can be flexibly controlled down to 2.67 ps by tuning the Raman pump powers of the DRA. Nearly equal pulse widths at all multicast wavelengths are obtained. Furthermore, wide open eye patterns and penalties less than 1.2 dB at the 10-9 bit-error-rate (BER) level are found.

A combination of nonreturn-to-zero (NRZ)-to-return-to-zero (RZ) waveform conversion and wavelength multicasting with pulsewidth tunability is experimentally demonstrated. A NRZ data signal is injected into a highly nonlinear fiber (HNLF)-based four-wave mixing (FWM) switch with four RZ clocks compressed by a Raman amplification-based multiwavelength pulse compressor (RA-MPC). The NRZ signal is multicast and converted to RZ signals in a continuously wide pulsewidth tuning range between around 12.17 and 4.68 ps by changing the Raman pump power of the RA-MPC. Error-free operations of the converted RZ signals with different pulsewidths are achieved with negative power penalties compared with the back-to-back NRZ signal and the small variation among received powers of RZ output channels at a bit-error-rate (BER) of 10-9. The NRZ-to-RZ waveform conversion and wavelength multicasting without using the RA-MPC are also successfully implemented.

The physical optics (PO) approximation is one of the widely-used techniques to calculate scattering fields with a reasonable accuracy in the high frequency region. The computational load of PO radiation integral dramatically increases at higher frequencies since it is proportional to the electrical size of scatterer. In order to suppress this load, a variety of techniques, such as the asymptotic evaluation by the stationary phase method (SP), the equivalent edge currents (EECs), the low-order polynomial expansion method and the fast physical optics (FPO), have been proposed and developed. The adaptive sampling method (ASM) proposed by Burkholder is also one of the techniques where the sampling points in radiation integral should be adaptively determined based upon the phase change of integrand. We proposed a quite different approach named ``Localization of the radiation integrals.'' This localization method suggests that only the small portions of the integration with a slow phase change contribute to the scattering field. In this paper, we newly introduce the ASM in the localization method and applied the proposed method into the radar cross section (RCS) analysis of 2-dimensional strip and cylinder. We have confirmed that the proposed method provides the frequency-independent number of division in the radiation integrals and computational time and accuracy. As the starting point for extension to 3-D case, the application of the proposed method for a reflection from an infinite PEC plane and a part of sphere was also examined.

Recent progress in research on the finite element method (FEM) for optical waveguide design and analysis is reviewed, focusing on the author's works. After briefly reviewing fundamentals of FEM such as a theoretical framework, a conventional nodal element, a newly developed edge element to eliminate nonphysical, spurious solutions, and a perfectly matched layer to avoid undesirable reflections from computational window edges, various FEM techniques for a guided-mode analysis, a beam propagation analysis, and a waveguide discontinuity analysis are described. Some design examples are introduced, including current research activities on multi-core fibers.

For next generation planar lightwave circuit (PLC) devices, high function and high-density integration are required as well as downsizing and cost reduction. To realize these needs, high refractive index difference between a core and a clad $(Delta)$ is required. To use PLC for practical applications, silica-based PLC is one of the most attractive candidate. However, degradation of the optical properties and productivity occur when $Delta$ of the core becomes high. Thus, $Delta$ of most of the conventional PLC with GeO$_2$-SiO$_2$ core is designed less than 2.5%. In this paper, we report a silica-based ultra-high $Delta $ PLC with ZrO$_2$-SiO$_2$ core. 5.5%-$Delta$ ZrO$_2$-SiO$_2$ PLC has been realized with low propagation loss and basic characteristics has been confirmed. Potential of chip size reduction of the ZrO$_2$-SiO$_2$ PLC is shown.

We propose an accurate modeling of the wavelength conversion process by dynamic tuning of a dielectric cavity. Since the process involves the long-distance propagation of light, the finite-difference time-domain (FDTD) method is not suitable for modeling of the wavelength conversion process owing to the numerical dispersion error of the FDTD method. The proposed modeling is based on the constrained interpolation profile (CIP) method, which was developed in the field of computational fluid dynamics for the purpose of reducing considerably the numerical dispersion error, and is formulated for a one-dimensional problem using an interpolation function of a higher order than that used in the original CIP method. Numerical experiments reveal that the proposed method can achieve accurate prediction of the wavelength conversion process even with a coarse grid model and is superior to both the original CIP method and the FDTD method.

In this paper, a novel hybrid technique for analyzing complex antennas around the coated object is proposed, which is termed as “iterative vector fields with Physical Optics (PO)”. A closed box is used to enclose the antennas and the complex field vectors on the box' surfaces can then be obtained using Huygens principle. The equivalent electromagnetic currents on Huygens surfaces are computed by Higher-order Method of Moments (HOB-MoM) and the fields scattered from the coated object are calculated by PO method. In addition, the parallel technique based on Message Passing Interface (MPI) and Scalable Linear Algebra Package (ScaLAPACK) is employed so as to accelerate the computation. Numerical examples are presented to validate and to show the effectiveness of the proposed method on solving the practical engineering problem.

A new wavelength-selective optical modulator was proposed and discussed. The modulator consists of three kinds of distributed Bragg reflectors (DBRs) integrated in a single straight waveguide. The waveguide can guide TE$_0$ and TE$_1$ modes, and an in-line Michelson interferometer is constructed by the three DBRs. An operation-wavelength wave among incident wavelength-division-multiplexed TE$_1$ guided waves is split into TE$_0$ and TE$_1$ guided waves by one of DBRs, and combined by the same DBR to be TE$_0$ output wave with interference after one of waves is phase-modulated. A modulator using an electro-optic (EO) polymer is designed, and the static performance was predicted theoretically. An operation principle was confirmed experimentally by a prototype device utilizing a thermo-optic effect instead of the EO effect.

In the Knill-Laflamme-Milburn (KLM) scheme, quantum teleportation is nearly deterministically carried out with linear optics. To reconstruct an original quantum state, however, a phase shift is required for an output state. We exhibit a proper phase shift to complete quantum teleportation.

We have succeeded in demonstrating high-performance four-channel 25 Gb/s integrated receiver for 100 Gb/s Ethernet with a built-in spatial Demux optics and an integrated PD array. All components which configure to the Demux optics adhered to a prism. Because of the shaping accuracy for prism, the insertion loss was able to suppress to 0.8 dB with small size. The connection point of the package for high speed electrical signals was improved to decrease the transmission loss. The small size of 12 mm 17 mm 7 mm compact package with a side-wall electrical connector has been achieved, which is compatible with the assembly in CFP2 form-factor. We observed the sensitivity at average power of -12.1 dBm and the power penalty of sensitivity due to the crosstalk of less than 0.1 dB.

One of the most serious challenges facing the exponential performance growth in the information industry is a bandwidth bottleneck in inter-chip interconnects. We therefore propose a photonics-electronics convergence system with a silicon optical interposer. We examined integration between photonics and electronics and integration between light sources and silicon substrates, and we fabricated a conceptual model of the proposed system based on the results of those examinations. We also investigated the configurations and characteristics of optical components for the silicon optical interposer: silicon optical waveguides, silicon optical splitters, silicon optical modulators, germanium photodetectors, arrayed laser diodes, and spot-size converters. We then demonstrated the feasibility of the system by fabricating a high-density optical interposer by using silicon photonics integrated with these optical components on a single silicon substrate. As a result, we achieved error-free data transmission at 12.5 Gbps and a high bandwidth density of 6.6 Tbps/cm2 with the optical interposer. We think that this technology will solve the bandwidth bottleneck problem.

We report a 40-Gb/s and highly accurate intensity limiter with a single Erbium-Doped Fiber Amplifier (EDFA) for low-power-consumption driving intensity limiting. The intensity limiter based on self-phase modulation with an appropriate pre-chirping procedure makes it possible, which provides a highly accurate limiting of less than 0.01 dB. We fed 40-Gb/s signals with 2.69 dB intensity fluctuation and 4.7 dB improvement on the receiver sensitivity was obtained for a bit error rate of 10-9 by using a numerical simulation.

For the modeling of multipath propagation in every wireless systems, the ray tracing method has been widely studied. However, large errors may result due to the approximation of geometrical optics in curved surfaces. This paper therefore focused on the curved surfaces and edges, which are difficult to handle in ray tracing. Examples of curved surfaces can be found in arched cross-section tunnels which are common in highway networks of mountainous areas. The traditional ray tracing method of dividing the curved surface into smaller flat plates is not so accurate as the size of smaller plates may not satisfy the geometrical optics assumption, and the reflection point which satisfies Fermat's principle may not exist. In this work, a new ray tracing method is proposed with 2 contributions. The first one is the implementation of the reflection coefficient for curved surfaces in ray tracing. The second is applying the physical optics method on the caustics region. To evaluate these methods, path gain simulation results for an arched cross-section model are compared with measurements made inside an arched tunnel. To further improve the simulation results, the effect of rough surface is introduced, and the results are again compared with measurement.

We deal with the scattering of a scalar plane wave by a half plane with a sinusoidally deformed edge from a straight edge by a physical optics approximation. The normal incidence of a plane wave to an edge is assumed. A contribution of an edge to the field integral is asymptotically evaluated and the basic properties of the scattering caused by the edge deformation is clarified. The scattering pattern has peaks at specific scattering angles, which agree with diffraction angles calculated by the well-known grating formula for normal incidence. Some numerical examples are shown and it is shown that the results are in good agreement with the results obtained by the GTD method for low angle incidence.

This paper presents a compact metal plate lens antenna for evaluating a wave absorber placed on ceiling of the ETC gate. The focal distance of the lens was derived to be 129 cm by the geometrical optics procedure. By arranging the lens in front of a horn antenna, the gain and beamwidth characteristics were improved from 18 dBi to 26 dBi and from 22 degrees to 7 degrees, respectively. Then the antenna characteristics were evaluated when the distance between the antenna and the lens was changed in order to miniaturize the lens antenna. As the result, the changes in beamwidth were held to within 1 dB when the lens came close to the horn antenna. Scattering, phase and electric field intensity of electromagnetic wave were evaluated to clarify the foundation of the given characteristics. It was found that the field intensity for the miniaturized lens antenna is stronger than that for GO designed one though the phase uniformity is worse. The distance between the horn antenna and lens can be reduced to 80 cm. The absorption characteristics for the arranged absorbers which have different absorptions were measured, and it was shown that the proposed method was suitable for specifying the deteriorated absorber in the ETC system.

Electromagnetic scattering at high-frequencies is computationally heavy. Radar cross section (RCS) of electrically large concave and convex objects are solved by using the hybrid method. For convex and concave surfaces, Modified-Vector Physical Optics (MPO) with enhanced accuracy and Iterative Physical Optics (IPO) taking multiple-reflections into account, are selectively and independently applied for convex and concave parts of the scatterer. The accuracy of RCS by this hybrid method is tested with the MoM based simulator Wipl-D as the reference. The RCS from relatively small scatterers with the dimension of the order of a few wavelengths can be successfully predicted.