Narrow-beam and low threshold current characteristics have been realized for a 1.3 µm FS-BH (Facet Selective growth Buried Heterostructure) laser diode monolithically integrated with a tapered waveguide lens by a selective area epitaxial growth technique. The beam divergences in the perpendicular and horizontal directions have been reduced down to about 12. By the introduction of the strained quantum well structure and the optimized cavity structure, the threshold current has been kept as low as 6 mA which is comparable to the conventional Fabry-Perot laser diodes. Even at high temperature as high as 85, the threshold current and the operation current (P=10 mW) have been suppressed to as low as 23 mA and 63 mA, respectively. Furthermore error-floor-free characteristics for 622 Mbps-50 km transmission have been confirmed under severe optical feedback condition.

A low dark current CCD linear image sensor with pixels consisting of a photodiode and a storage area has been developed. In order to suppress the dark current, the wafer process has been improved. An impurity profile of a photodiode was modified to minimize depletion width, which was monitored by the photodiode potential. Surface states under the storage gate were decreased by hydrogen annealing with plasma-deposited silicon nitride as an inter metal dielectric film. As the isolation dose decreased, the dark current both in the photodiode and in the storage region were effectively suppressed. Finally, low dark currents of 5 pA/cm2 at photodiode and 120 pA/cm2 at storage area were obtained.

This paper is concerned with a point-oriented finite volume time domain (FVTD) method in the Cartesian coordinate system for analyzing electromagnetic wave scattering by arbitrary shaped metallic gratings. The perfectly matched layer (PML) is used for the absorbing boundary conditions (ABC's) in the directions corresponding to transmitted and reflected wave regions. An FVTD version of the Floquet's theorm is described to impose the periodic condition in the direction where conducting rods are located periodically. The boundary conditions for a conductor rod which is not well suited to the Cartesian coordinate system are satisfied in an average fashion by introducing image fields at image points. It is shown that the present method gives accurate numerical results. Numerical calculations are also carried out for thick conducting rods which seem difficult to deal with in an analytical way.

Current-voltage characteristics of triple-barrier resonant tunneling diodes are theoretically analyzed taking phase breaking into account. The peak current in predicted using conventional theories is much smaller, typically by a factor of 1/3000 for a coherent length of 100 nm, than that measured because the incoherent tunneling process is neglected. We take both the coherent and the incoherent tunneling processes into account in the analysis and show that the product of the peak current and the voltage width at half maximum of the peak current is almost constant even when the phase coherent length varies between 50 and 1000 nm. The peak current density increases by two orders of magnitude in the model developed here.

The resistive-fuse network for early vision was studied using circuit simulation to clarify the potential of implementation with resonant tunneling diodes (RTDs). To over-come the fundamental problem of the RTD network, i.e., the RTDs cannot perform simulated annealing (SA), pseudo SAs were proposed. These methods are based on the time-variation of the input signal strength, and are found to be effective in restoring images. A resistive-fuse network is shown to be one of the most promising applications of RTDs.

The multichannel distortions of direct modulated laser diode were studied from the view point of rate equations. A novel technique for compensating the composite second order distortion (CSO) was proposed. Meanwhile, the related calibration procedures were indicated. After the compensation, 10 dB improvement in CSO was obtained

From a GF(q) sequence {ai}i0 with period qn - 1 we can obtain new periodic sequences {ai}i0 with period qn by inserting one symbol b GF(q) at the end of each period. Let b0 = Σqn-2 i=0 ai. It Is first shown that the linear complexity of {ai}i0, denoted as LC({ai}) satisfies LC({ai}) = qn if b -b0 and LC({ai}) qn - 1 if b = -b0 Most of known sequences are shown to satisfy the zero sum property, i.e., b0 = 0. For such sequences satisfying b0 = 0 it is shown that qn - LC({ai}) LC({ai}) qn - 1 if b = 0.

This paper presents the scattering characteristics of a TE electromagnetic plane wave by metallic strip gratings on an optically plasma-induced silicon slab at millimeter wave frequencies. The characteristics were analyzed by using the spectral domain Galerkin method and estimated numerically. We examined to control the resonance anomaly by changing the optically induced plasma density, and the metallic strip grating structures were fabricated on highly resistive silicon. The optical control characteristics of the reflection, and the forward scattering pattern by the grating structures, were measured at Q band and are discussed briefly with theory.

A quasi-optical Superconductor-Insulator-Superconductor (SIS) mixer has been designed and tested in the 270-GHz band. The mixer used a substrate-lens-coupled log-periodic antenna and a tuning circuit for RF matching. The antenna is planar and self-complementary, and has a frequency-independent impedance of around 114 Ω over several octaves. The tuning circuit consists of two Nb/AIOx/Nb tunnel junctions separated by inductance for tuning out the junction capacitances and a λ/4 impedance transformer for matching the resistance of the two-junction circuit to the antenna impedance. The IF output from the mixer is brought out in a balanced method at each edge of the antenna, and is coupled to a low noise amplifier through a balun transformer using a 180-degree hybrid coupler for broadband IF matching. Double sideband receiver noise temperatures, determined from experimental Y-factor measurements, are about 150 K across the majority of the desired operating frequency band. The minimum receiver noise temperature of 120 K was measured at 263 GHz, which is as low as that of waveguide receivers. At this frequency, measurement of the noise contribution to the receiver results in input losses of 90 K, mixer noise of 17 K, and multiplied IF noise of 13 K. We found that the major sources of noise in our quasi-optical receiver were the optical losses.

Orthogonal frequency division multiplexing (OFDM) has been receiving a lot of attention in the field of broadcasting because of its ruggedness under multipath environments. One of important issues to realize high quality reception of OFDM signals is to correct frequency and timing offsets between the transmitter and receiver so that orthogonality of the carriers can be maintained. This paper discusses a frequency and timing period acquisition technique for OFDM systems. A new offset estimation technique is introduced that detects both the frequency and timing peirod offsets at the same time by using only one pilot symbol with its suitable frequency assignment. A pseudo noise (PN) sequence is also introduced to assign these frequencies of the pilot symbol so that the frequency acquisition range can be widened. Numerical examples are given to show the estimate variances of the proposed frequency and timing period estimator over both additive white Gaussian noise (AWGN) and multipath fading channels. Also the bit error rate (BER) performance for an open loop acquisition system is examined.

The use of high-performance portable computers has become widespread. It is expected that many people will carry large amounts of multimedia information in these portable computers. In face-to-face communication, however, few systems are capable of exchanging multimedia information. Previously, we developed the Nomadic Collaboration Support System, which supports face-to-face communication through conversation and the distribution of documents. The system makes it possible for each participant in face-to-face communication to distribute electronic documents to other participants and edit them synchronously. However, it is often impossible for participants to obtain suitable amounts of information during face-to-face communication because of the difficulty in tailoring the documents for each participant. In this paper, we propose a technique to exchange hypertext documents on the Nomadic Collaboration Support System, which will allow each participant to obtain the most suitable amount of information possible from the distributor without his tailoring documents for each participant.

Precise direct mounting of laser diode (LD) and photodiode (PD) chips on silica planar lightwave circuits (PLCs) has been investigated for application to transceiver modules. To achieve submicron optical alignment, self-aligned index marks on the PLCs and LDs were directly detected by transmission infrared light. The repeatability of the positioning was measured to be within 0.125 µm. The output power of the resultant module was 0.2 mW at 80 mA. A waveguide-type PD was also mounted in the same way, and module sensitivity of 0.25 A/W was demonstrated.

We proposed a new marker design for passive alignment of a laser to a fiber on a silicon waferboard. Our fiducial marker is simple form and easy to fabricate. With a unique marker design, high accurate positioning of the laser chip is easily achieved using a conventional flip-chip bonder. We have successfully fabricated laser modules with uniform coupling, within 1 dB for a flat end single-mode fiber and within 2 dB for a hemispherical end fiber. This assembly method offers the potential for low-cost optical module packaging.

There are some results indicating that a quantum computer seems to be more powerful than ordinary computers. In fact, P.W. Shor showed that a quantum computer can find discrete logarithms and factor integers in polynomial time with bounded error probability. No polynomial time algorithms to find them using ordinary computers are known. In this paper, we show that the cycles in some kinds of periodic functions, e.g., functions proposed as pseudo-random generators, can be found in polynomial time with bounded error probability on a quantum Turing machine. In general, it is known that ordinary computers take exponential time to find the cycles in periodic functions.

We have proposed a multiplexed fiber-optic sensor system using an optical loop with a frequency shifter. The measured output power spectrum of the system has shown that the multiprexed signals superimpose upon a noise pedestal which is like a series of hill peaks. In this paper, the output power spectrum is theoretically analyzed from the output intensity autocor-relation function. It displays that the noise pedestal originates from the laser phase-induced intensity noise. The noise level depends on the coherence time of the laser source. The positions of peaks are decided by the working frequency of the frequency shifter in the optical loop. The sensitivity of the system are related to the bandwidth B, the coherence time Tc, the sensor number n to be multiplexed, the loop loss α, and the fiber coupler parameters. Properly choosing these parameters is beneficial to improve the sensitivity of system.

This letter proposes a pulse width modulated (PWM) control method which can stabilize chaotic orbits onto unstable fixed points and unstable periodic orbits. Some numerical experiments using the Lorenz equation show that chaotic orbits can be stabilized by the PWM control method. Furthermore, we investigate the stability in the neighborhood of an unstable fixed point and discuss the stability condition of the PWM control method.

This paper describes the performance of a predistorter implementation to a superluminescent diode (SLD) in fiber-optic wireless systems under the optical reflection. SLD intensity noise and 3rd-order intermodulation distortion (IM3) are experimentally compared with those of DFB-and FP-LD. It is observed that the IM3 of SLD has ideal 3rd characteristics and output noise remains unchanged against the number of optical connectors. It is also found that the predistorter reduces IM3 by 8 dB. Receiver sensitivity of the system is discussed from the view point of overall design. the BER performance of an SLD with predistorter using a π/4-QPSK signal as a subcarrier is also described theoretically and experimentally.

Ultrasonic diffraction image of specimen informs its acoustic structure as X ray diffraction method for analysis of the crystal structure. This ultrasonic diffraction method has a feature that focused ultrasound beam is used and diffraction image is observed on focal plane. When the structure of specimen is perfectly periodic, its diffraction image produces symmetrical respect to origin, but the diffraction image of weak periodic structure such as living tissue has some asymmetricity. In this paper, the principle of ultrasonic diffraction method, and data processing for asymmetrical and scattered diffraction image caused by weak periodic structure are described. The results of diffraction image of plant tissue and animal tissue, and its discussion are also described. This method is expected to be useful in evaluation of acoustic structure such as living tissue and internal tissue of bone.

In this paper we study the bifurcations of the periodic solutions induced by the symmetrical properties of a system of hybridly coupled oscillators of the Rayleigh type. By analogy with the results concerning with the equilibria, we classify the periodic solutions according to their spatial and temporal symmetries. We discuss the possible bifurcations of each type of periodic solution. Finally we analyze the phase portraits of the system when the parameters vary.

In this paper, electromagnetic scattering by infinite double two-dimensional periodic array of resistive upper and lower elements is considered. The electric field equations are solved by using the moment method in the spectral domain. Some numerical results are shown and frequency selective properties are discussed.