Excitonic optical properties of GaN homoepitaxial layers have been studied by means of magneto-luminescence and time-resolved luminescence spectroscopy. The luminescence lines due to the radiative recombination of excitons bound to neutral donors and acceptors have been measured under magnetic field up to 8 T, which was aligned perpendicular and parallel to the hexagonal c-axis. Under the magnetic field aligned perpendicular to the hexagonal c-axis, both the donor- and acceptor-bound-exciton lines clearly split into two components, which originated from the Zeeman splitting. The effective g-factors for both the donor- and acceptor-bound excitons were estimated to be 2.02 and 2.47, respectively. Under the magnetic field aligned parallel to the hexagonal c-axis, slight broadening of the bound-exciton lines was observed and the Zeeman splitting was too small to be detected. On the other hand, the diamagnetic shift for both the donor- and acceptor-bound-exciton luminescence lines was observed under the magnetic field aligned both perpendicular and parallel to the hexagonal c-axis. It was found that the diamagnetic shift of the donor-bound exciton was smaller than that of the acceptor-bound exciton. Furthermore, recombination dynamics of excitonic transitions was measured under high-density excitation. An excitation-density-dependent transition of the dominant radiative recombination process from donor-bound excitons to biexcitons was clearly observed in the temporal behavior. In addition, double-exponential decay of biexciton luminescence was observed, which is one of the characteristics of biexciton luminescence at high excitation densities.

Bit error rates (BER) for playback of (1,7) code employed in optical disc recording were simulated using an ideal (Gaussian) playback waveform, with playback being performed by PRML (Partial Response Maximum-Likelihood) combining a partial response equalizer and a double clock weighted Viterbi decoder. It was found that best BER occurs for PR(2,3,3,2) +7/10 level Viterbi decoding at a weighted value of w = 0.5 for data consisting of 107 symbols. For a minimum bit length of 0.28 µm, BER of 10-4 and less than 10-6 was obtained for SN ratios of 15.6 dB and 17.7 dB, respectively. And for a minimum bit length of 0.26 µm, BER of 10-4 and less than 10-6 was obtained for SN ratios of 16.7 dB and 18.8 dB, respectively. These results demonstrate the feasibility of a minimum bit length of 0.26 µm in current optical disc recorders.

Several methods have be proposed or used to optimize the phase distribution of a kinoform. In this paper, we proposed a fast algorithm for optimization of the kinoform based on simulated annealing to reduce the large computation cost. This method uses a simplified equation to calculate the energy function after perturbation.

II-VI laser diode was fabricated using a ZnCdSe/ZnS0.06Se0.94/ZnMgSSe SCH structure on GaAs, in which ZnMgSSe was originally proposed by our group. ZnMgSSe is lattice-matched to GaAs and the bandgap energy of ZnMgSSe is larger than that of ZnSe and ZnS0.06Se0.94 lattice-matched to GaAs. As for the crystal growth mechanism, the composition of ZnMgSSe is not changed and the RHEED pattern becomes spotty in group II-rich growth conditions and S incorporation is difficult in group VI-rich growth conditions. From these results, we consider that the optimized growth condition of ZnMgSSe is in the stoichiometric region (both (21) and c(22) were observed). As for the device quality, although the current density of this device is minimized to 500 A/cm2, it was difficult to improve the reliability of the electrode and the active layer ZnCdSe. We found that the thin ZnTe and thick ZnSSe based electrode is necessary for reliability of the electrode, and that an optimized VI/II ratio is necessary for the reliability of the active region. To fabricate a relatively low-operating-voltage device, the stripe width is also an important parameter. In spite of relatively weak covalent bond of II-VI compounds, we can produce a device lifetime as long as 400 h.

PID control schemes have been widely used in most of process control systems. Most of these processes are often treated as first-order systems with dead times. And also, in many cases, PID parameters are usually tuned based on the process parameters, i. e. , the time constant, the dead time and the process gain. However, since these process parameters can not be obtained exactly, it is well known that it is difficult to find the suitable PID parameters in practice. In this paper, we propose a genetic tuning scheme of PID parameters for first-order systems with large dead times. The authors have already proposed a tuning method of PID parameters using a genetic algorithm (GA), which was based on the relationship between PID control and generalized minimum variance control(GMVC) laws. In practice, for large dead time systems, first-order low pass pre-filters are often used. The proposed method is an extended version of the previously proposed method mentioned above to the system with a pre-filter due to the large dead time, i. e. , a tuning method of both PID parameters and the pre-filter using a GA. The proposed control scheme is numerically evaluated on some simulation examples.

In a phase-locked-loop (PLL) frequency synthesizer with binary phase comparison, jitter is hard to suppress. In this paper, we propose a PLL frequency synthesizer with an improved binary phase comparison which can solve the above problem. The effectiveness of the proposed method is confirmed by PSpice simulation results.

A new manufacturing process for advanced Fiber Bragg Gratings which uses phase plates is described. Its high versatility allows to achieve many type of filters in optical fibers (phase-shifted, apodised, Fabry-Perot).

This paper reports the performance of an AC-voltage sensor with a LiNbO3 integrated retroreflective structure based on the Y-junction Mach-Zehnder interferometer. This structure is capable of realizing a low-cost sensor chip because of the small chip size and single optical-fiber connection. In the sensitivity and frequency response evaluation, detection sensitivities of 6.3 µ V / Hz have been measured with a frequency response from 6 Hz to 2 GHz. These measurement limitations were also analyzed theoretically and compared with the experimental results. This unique sensor enables precise voltage measurement in an EMI environment, even inside a computer.

For measuring high frequency ultrasonic fields which are often spatially distributed and transient, an array probe with small element sensors is highly required. In this paper, we propose a fiber-optic micro-probe array which is based on wavelength-division-multiplexing technique. The element sensor consists of a micro optical cavity of 100 µm long made at the end of optical fiber. Optical path length of the cavity is changed by the applied acoustic field, and the modulation of output light intensity is monitored at another end of the fiber for the information of the acoustic field. Array of sensor elements and a light source as well as a photo detector are connected together by an optical star coupler. The Fabry-Perot resonance wavelength of each sensor element is designed different one another, and the outputs from the sensors are discriminated by sweeping the wavelength of light source with the use of a tunable semiconductor laser. In this paper, the performance of the micro-probe array is discussed experimentally.

We study the problem of stabilizing a general nonlinear control system globally based on direct gradient descent control which is a dynamic feedback control law. The direct gradient descent control and the general nonlinear control system (original system) form a new system (extended system). Under an appropriate assumption we can make the extended system become globally asymptotically stable if its unforced system is stable in the sense of Lyapunov.

In this paper, a puzzle called Cell-Maze is analyzed. In this puzzle, cells are arranged in checker board squares. Each cell is rotated when a player arrives at the cell. Cell-Maze asks whether or not a player started from a start cell can reach a goal cell. The reachability problem for ordinary graphs can be easily solved in linear time, however a reachability problem for the network such as Cell-Maze may be extremely difficult. In this paper, NP-hardness of this puzzle is proved. It is proved by reducing Hamiltonian Circuit Problem of directed planar graph G such that each vertex involved in just three arcs. Furthermore, we consider subproblems, which can be solved in polynomial time.

Given a combinatorial problem on a set of weighted elements, if we change the weight using a parameter, we obtain a parametric version of the problem, which is often used as a tool for solving mathematical programming problems. One interesting question is how to describe and analyze the trajectory of the solution. If we consider the trajectory of each weight function as a curve in a plane, we have a set of curves from the problem instance. The curves induces a cell complex called an arrangement, which is a popular research target in computational geometry. Especially, for the parametric version of the problem of computing the minimum weight base of a matroid or polymatroid, the trajectory of the solution becomes a subcomplex in an arrangement. We introduce the interaction between the two research areas, combinatorial optimization and computational geometry, through this bridge.

This paper surveys two methods for designing numerically robust geometric algorithms. The first method is the exact-arithmetic method, in which numerical computations are done in sufficiently high precision so that all the topological judgements can be done correctly. This method is usually accompanied with lazy evaluation and symbolic perturbation in order to reduce the computational cost and the implementation cost. The second method is the topology-oriented method, in which the consistency of the topological structure is considered as higher-priority information than numerical computation, and thus inconsistency is avoided. Both of the methods are described with the implementation examples.

We survey recent developments in the study of approximation algorithms for NP-hard geometric optimization problems. We focus on those problems which, given a set of points, ask for a graph of a specified type on those points with the minimum total edge length, such as the traveling salesman problem, the Steiner minimum tree problem, and the k-minimum spanning tree problem. In a recent few years, several polynomial time approximation schemes are discovered for these problems. All of them are dynamic programming algorithms based on some geometric theorems that assert the existence of a good approximate solution with a simple recursive decomposition structure. Our emphasis is on these geometric theorems, which have potential uses in the design and analysis of heuristic algorithms.

The vehicle routing and facility location fields are well-developed areas in management science and operations research application. There is an increasing recognition that effective decision-making in these fields requires the adoption of optimization software that can be embedded into a decision support system. In this paper, we describe the implementation details of our software components for solving the vehicle routing and facility location problems.

A convex hull is one of the most fundamental and interesting geometric constructs in computational geometry. Considerable research effort has focused on developing algorithms, both in serial and in parallel, for computing convex hulls. In particular, there are few problems whose parallel algorithms are so thoroughly studied as convex hull problems. In this paper, we review the convex hull parallel algorithms and their paradigm. We provide a summary of results and introduce several interesting topics including typical techniques, output-size sensitive methods, randomized approaches, and robust algorithms for convex hull problems, with which we may see the highlights of the whole research for parallel algorithms. Most of our discussion uses the PRAM (Parallel Random Access Machine) computational model, but still we give a glance at the results of the other parallel computational models such as mesh, mesh-of-trees, hypercube, recofigurable array, and models of coarse grained multicomputers like BSP and LogP.

We describe FMCW reflectometry for characterization of long optical fibers by using an external-cavity laser diode as a light source. Since the optical path difference between the reference beam and the reflected beam from the optical fiber under test is much longer than the coherence length of the light source, the reference and the reflected beams are phase-decorrelated. As a result, the beat spectrum between the reference and the reflected beams is measured. In the phase-decorrelated FMCW reflectomety, the spatial resolution is enhanced by narrowing the spectral linewidth of the light source and increasing the repetition frequency of the optical frequency sweep as well as increasing the chirping range of the optical frequency sweep. In the experiments, an external-cavity DFB laser is used as a narrow linewidth light source, and the optical frequency is swept by minute modulation of the external cavity length. Long single mode optical fibers are characterized, and the maximum measurement range of 80 km is achieved, and the spatial resolutions of 46 m, 100 m and 2 km are achieved at 5 km, 11 km and 80 km distant, respectively. The Rayleigh backscattering is clearly measured and the propagation loss of optical fiber is also measured. The optical gain of an erbium-doped optical fiber amplifier (EDFA) is also estimated from the change in the Rayleigh backscattering level in the optical fiber followed after the EDFA.

A general analytical expression for describing the growth of the resonant peak wavelengths of long period gratings is derived. The theoretical calculations explain the shift of peak loss wavelengths in the direction of either shorter or longer wavelengths as the induced index change of grating increases. We have calculated and experimentally verified the sensitivity of the resonant peak wavelengths with respect to an overlay index for various grating periods. It is shown that the center wavelength shift of the claddding modes depends strongly on the grating period and the claddding mode order.

A number of Tonpilz piezoelectric transducers generally form a large matrix array. They may be required to be only light in weight, or both miniaturized and light in weight as well as to have high-power capability, according to their use. This paper describes the results obtained in an investigation of miniaturized, light-weight Tonpilz piezoelectric transducers with high-power capability. An electromechanical energy conversion system and mechanical vibration system for the transducer are theoretically investigated on transducer configuration, using a Martin equivalent circuit and distributed-constant equivalent mechanical circuits. For simplification of the theoretical analysis, the transducer was considered as being divided into a front-half section, from the head mass end to the vibration nodal section, and a rear-half section, from the vibration nodal section to the tail mass end. Figures of merits, FMm for the front-half section and FM'm for the rear-half section, are calculated to achieve lightening as well as high-power capability. In addition, figures of merits FMml for the front-half section and FM'ml for the rear-half section are calculated to achieve miniaturization and lightening as well as high-power capability. To corroborate the theoretical results obtained, three kinds of Tonpilz transducers having the same 30 kHz resonant frequency but different configurations were built. Then, three kinds of 33 matrix arrays composed of nine Tonpilz transducers with the same configuration were fabricated. To evaluate the high-power characteristics for the arrays, a water tank to which high hydraulic pressure can be applied was adopted and a high-power test was carried out. As a result, it was found that transducers with high FMm and FM'm figures of merits are light in weight and exhibit high performance in high-power handling capability. Also, it was found that transducers with high FMml and FM'ml figures of merits are miniaturized and light in weight and exhibit high performance in high-power handling capability.

In this study, a simple chaos communication system including modulation-demodulation circuits is studied. The influence of modulation-demodulation circuits to chaos synchronization is investigated. For the estimation of communication quality, bit error rate (BER) is calculated by computer simulation when a sequential random pulse information signal is transmitted via this proposed system.