This paper describes the recent progress made in developing wavelength tunable semiconductor light sources for WDM applications. Wide and quasi-continuous wavelength tunings were investigated for a wavelength-selectable laser and a wavelength tunable distributed Bragg reflector (DBR) laser having a super structure grating (SSG). A wavelength-selectable laser consisting of a DFB laser array, a multi-mode interferometer (MMI), and a semiconductor optical amplifier (SOA) demonstrated a quasi-continuous tuning range of 46.9 nm by using temperature control. A wavelength-tunable DBR laser with SSG exhibited a quasi-continuous tuning range of 62.4 nm by using three tuning current controls. Wavelength stabilization was also demonstrated under the temperature variations of 5.

This paper considers FIR filter design using linear predictive coding technique, for which the coefficients belong to a small set of integers, so that the coefficients have small wordlengths. Previously, integer programming was used to find the coefficients of such filters. However, the design method using integer programming suffers from high computational cost as the filter length increases. The computation can quickly become prohibition. In this paper, we propose two designs of predictive encoded FIR filters based on a modified Karmarkar's linear programming algorithm, which is known to be more suitable for solving large problems. First, we formulate the problem as a weighted minimax error problem and arrange it in a form that the modified Karmarkar algorithm can be applied. The design algorithm has the same (low) complexity as that of the weighted least-square method, but it can solve problems with some constraints, whereas the weighted least-square method cannot. However, the algorithm has a difficulty due to an ill condition caused by matrix inversion when the predictive filter order is high. To avoid this difficulty, we formulate the design as a weighted least absolute error problem. By using this second proposed algorithm, a filter with shorter coefficient wordlength can be found using a higher-order predictor filter at the expense of more computational cost. To further reduce the coefficient wordlength, the filter impulse response is separated into two sections having different ranges of coefficient values. Each section uses a different scaling factor to scale the coefficient values. With small coefficient wordlength, the filter can be realized without hardware multipliers using a low-radix signed-digit number representation. Each coefficient is distributed in space as 2-3 ternary {0,1} or quinary {0,1, 2} coefficients. Ternary coefficients require only add/subtract operation, while quinary coefficients require one-bit shift and add/subtract operations. The shift can be hardwired without any additional hardware.

In the Internet, because of huge scale and distributed administration, it is of practical importance to infer network-internal characteristics that cannot be measured directly. In this paper, based on a general framework we proposed previously, we present a feasible method of inferring packet loss rates of individual links from end-to-end measurement of unicast probe packets. Compared with methods using multicast probes, unicast-based inference methods are more flexible and widely applicable, whereas they have a problem with imperfect correlation in concurrent events on paths. Our method can infer link loss rates under this problem, and is applicable to various path-topologies including trees, inverse trees and their combinations. We also show simulation results which indicate potential of our unicast-based method.

In this paper we discuss the estimation of effort needed to update program codes according to given design specification changes. In the Object-Oriented incremental development (OOID), the requirement changes occur frequently and regularly. When a requirement change occurs, a design specification is changed accordingly. Then a program code is updated for given design specification change. In order to construct the development plan dynamically, a simple and fast estimation method of efforts for code updating is strongly required by both developers and managers. However, existing estimation methods cannot be applied to the OOID. We therefore try to propose a straightforward approach to estimate effort for code updating, which reflects the specific properties of the OOID. We list up following factors of the effort estimation for OOID: (1) updating activities consist of creation, deletion, and modification, (2) the target to be updated has four kinds of types (void type, basic type, library type, and custom type), (3) the degree of information hiding is classified into private, protected and public, and (4) the degree of inheritance affects updating efforts. We then propose a new formula E(P,σ) to calculate the efforts needed to update a program P according to a set of design specification changes σ. The formula E(P,σ) includes weighting parameters: Wupd, Wtype, Winf-h and Winht according to the characteristics (1), (2), (3) and (4), respectively. Finally, we conduct experimental evaluations by applying the formula E(P,σ) to actual project data in a certain company. The evaluation results statistically showed the validity of the proposed approach to some extent.

Optical properties and growth of self-assembled quantum dots (SAQDs) for optoelectronic device applications are discussed. After briefly reviewing the history of research on QD lasers, we discuss growth of InAs SAQDs including the light emission at the wavelength of 1.52-µm with a narrow linewidth (22 meV) and the area-controlled growth which demonstrates formation of SAQDs in selected local areas on a growth plane using a SiO2 mask with MOCVD growth. Then properties of the InGaAs AQDs are investigated by the near-field photoluminescence excitation spectroscopy which reveals gradually increasing continuum absorption connected with the two-dimensional-like (2D-like) wetting layer, resulting in faster relaxation of electrons due to a crossover between 0D and 2D character in the density of states. In the coherent excitation spectroscopy, the decoherence time is determined to be about 15 ps, which is well explained by the phonon induced relaxation mechanism in the SAQDs. Finally, nitride-based SAQDs and perspective of QD optical devices are also discussed.

The differentiated services (Diffserv) architecture is a potential solution for providing quality of service (QoS) on the Internet. Most existing studies focus on providing service differentiation among few service classes. In this paper, we propose an approach which can achieve per-flow weighted fair rate allocation in a differentiated services network. Following the design philosophy of the Diffserv model, in the proposed approach core routers do not need to keep per-flow information. An edge router adjusts the transmission rate of a flow based on the feedback carried on control packets, which are inserted by the ingress edge router and returned by the egress edge router. Core routers periodically estimate the fair share rate of each virtual flow and mark the results in control packets. We use both simulations and analysis to evaluate the performance of the proposed approach. The analytical results show that our approach allows a system to converge to weighted fair rate allocations in limited time. Through the simulation results, we can further validate the analytical results, and demonstrate that better throughput can be achieved.

Several methods have been developed for solving blind deconvolution problem. Recursive inverse filtering method is proposed recently and shown to have good convergence properties. This method requires accurate estimate of the region of support. In this paper, we propose to modify the original method by incorporating split, merge and grouping algorithm to find the region of support automatically.

In Vivo Diffusion Tensor Magnetic Resonance Imaging (DT-MRI) can now be used to elucidate and investigate major nerve pathways in the brain. Nerve pathways are constructed by a) calculating a continuous diffusion tensor field from the discrete, noisy, measured DT-MRI data and then b) solving an equation describing the evolution of a fiber tract, in which the local direction vector of the trajectory is identified with the direction of maximum apparent diffusivity. This approach has been validated previously using synthesized, noisy DT-MRI data. Presently, it is possible to reconstruct large white matter structures in the brain, such as the corpus callosum and the pyramidal tracts. Several problems, however, still affect the method's reliability. Its accuracy degrades where the fiber-tract directional distribution is non-uniform, and background noise in diffusion weighted MRIs can cause computed trajectories to jump to different tracts. Nonetheless, this method can provide quantitative information with which to visualize and study connectivity and continuity of neural pathways in the central and peripheral nervous systems in vivo, and holds promise for elucidating architectural features in other fibrous tissues and ordered media.

We review recent progresses in our studies on the fiber-optic soliton compression and related subjects with special emphasis on dispersion-flattened fibers (DFFs). As for the ultimately short pulse generation, it has been demonstrated to compress 5 ps laser diode pulses down to 20 fs with a 15.1 m-long single-stage step-like dispersion profiled fiber employed. The compression was brought about through a series of the higher order soliton processes in conjunction with a single and ordinary erbium-doped fiber preamplifier, and DFFs contained at its end played a major role. We have performed intensive investigations on the DFF compression mechanisms in the 100-20 fs range. A fairly reliable model was developed for the higher order soliton propagation along a DFF in the temporal range from 100 down to 30 fs by taking into consideration the higher order nonlinear and dispersion effects as well as incident pulse shape dependence. Through the simulation, parametric spectrum generation originating from the modulation instability gain was pointed out at frequencies apart from the pump wave frequency, which agrees with the experimental observation. Its possible application is also discussed.

Recent activities on ultrafast photonic device technology development in the Femtosecond Technology Project sponsored by NEDO are introduced. Topics include management and control of the higher order dispersions of optical fibers, ultrafast mode-locked semiconductor laser, symmetric Mach-Zehnder type all-optical switch, ultrafast serial-to-parallel signal converter and sub-picosecond wavelength switch. Challenges towards novel ultrafast switching material systems are also described.

A GaInNAs alloy on GaAs substrate has been very promising for long-wavelength vertical-cavity surface-emitting lasers (VCSELs) as an active layer. In spite of many groups reported the excellent temperature characteristics of the threshold current of the GaInNAs/GaAs edge-emitting lasers, discussions of the temperature dependence of the lasing characteristics except threshold current is few. In this paper, temperature characteristics of GaInNAs lasers grown by chemical beam epitaxy (CBE) emitting at λ=1.27 µm and λ=1.30 µm were investigated in detail. The characteristic temperature (T0) ranging from 10 to 80 varies from 60 to 130 K and decreased with decreasing cavity length for shorter cavity (< 400 µm) devices. On the other hand, longer cavity (< 400 µm) devices show that the cavity length does not affect so much to T0. The internal losses did not increase with increasing temperature. On the other hand, internal quantum efficiencies decreased with increasing temperature. It is considered that non-radiative recombination center with large temperature dependence may influence the decrease of the internal quantum efficiency due to the insufficient crystal quality of GaInNAs layer. The transparency current densities were unchanged for all temperature range, however, the gain constants decreased with increasing temperature. Thus, the decrease of the gain constant is considered to be due to decreasing of gain. Unchanged both transparency current density and internal loss may also express that these temperature characteristics were not induced by carrier overflow but be done by decreasing of the gain. From the results, it is considered that the temperature dependence of the gain originated from the Fermi-Dirac distribution of carriers was dominant for the temperature characteristics of GaInNAs/GaAs lasers. Due to the temperature dependence on the gain, the T0 decreases with increasing mirror loss.

Although all-optical gate switches based on the intersubband absorption in nitride quantum wells are predicted to operate at 1 Tb/s, realization of strong intersubband absorption at the optical communication wavelength is still difficult. An alternative approach is an interferometer-type gate switch utilizing refractive index change due to the intersubband absorption of a control pulse at a longer wavelength. Feasibility of Mach-Zehnder interferometer (MZI) gate switches, in which 1.55-µm pulses are controlled by 1.85-µm pulses, was theoretically investigated by finite-difference time-domain (FDTD) simulator. Although the effective phase shift does not reach π, 22.5% of the signal pulse energy was predicted to be gated by a 10-pJ control pulse in the MZI switch. Even 1.3-µm pulses can be controlled by 1.85-µm pulses at the expense of the switching energy. This approach provides a way to process signal pulses at the optical communication wavelength utilizing strong intersubband absorption at a longer wavelength.

Breakthroughs in crystal growth and conductivity control of nitride semiconductors during last two decades have led to such developments as high-brightness blue and green light-emitting diodes and long-lived violet laser diodes and so on. All of these nitride-based devices are robust and the most environmentally-friendly ones available. They enable us to save tremendous amount of energy and will be key devices in advanced information technology. Further progress in the area of crystal growth and device engineering will open up new frontier devices based on nitride semiconductors. In this paper, the evolution of nitride-based light-emitting devices is reviewed and the key issues, which must be addressed for nitrides to be fully developed, are discussed.

Several gateway-based congestion control mechanisms have been proposed to support an end-to-end congestion control mechanism of TCP (Transmission Control Protocol). One of promising gateway-based congestion control mechanisms is a RED (Random Early Detection) gateway. Although effectiveness of the RED gateway is fully dependent on a choice of control parameters, it has not been fully investigated how to configure its control parameters. In this paper, we analyze the steady state behavior of the RED gateway by explicitly modeling the congestion control mechanism of TCP. We first derive the equilibrium values of the TCP window size and the buffer occupancy of the RED gateway. Also derived are the stability condition and the transient performance index of the network using a control theoretic approach. Numerical examples as well as simulation results are presented to clearly show relations between control parameters and the steady state behavior.

In the previous work, Lampson et al. proposed an IP lookup algorithm which performs binary search on prefixes (BSP). The algorithm is attractive, even for IPv6, because of its bounded worst-case memory requirement. To achieve fast forwarding, it may need to slow down the insertion speed. Although this can be justified, the routing-table reconstruction in BSP is too time-consuming to handle the frequent route updates. In this work, we propose a fast forwarding-table construction algorithm which can accomplish more than 4,000 route updates per second. Moreover, it is simple enough to fulfill the need of fast packet forwarding. With the enhanced multiway search tree, we further reduced the depth of the tree and eliminated the pointer storage; this reduces the forwarding table size and shortens the lookup time.

785 nm (AlGaAs) laser diode (LD) with a window-mirror structure is demonstrated to be a potential candidate as a highly reliable light source of CD-R. The intermixing of a multi-quantum well structure by silicon implantation is used to form the window-mirror structure. Carbon is adopted as an acceptor because of its low thermal diffusion constant in crystals. As a result, the window-mirror-structure 785 nm AlGaAs LDs with ordinary far field patterns suitable for the actual CD-R drives have shown stable single lateral mode operation up to 250 mW. A mirror degradation level is significantly increased by the window-mirror structure. The pulsed operation current at 160 mW, 70 of the carbon doped LD is reduced by about 15% from that of zinc doped one. Highly reliable 160 mW pulsed operation is also realized at 70. This LD believed to be suited for the next generation high-speed (16-24x) CD-R drives necessitating 160 mW class LD.

In recent years, several inverse solutions of magnetoencephalography (MEG) have been proposed. Among them, the multiple signal classification (MUSIC) method utilizes spatio-temporal information obtained from magnetic fields. The conventional MUSIC method is, however, sensitive to Gaussian noise and a sufficiently large signal-to-noise ratio (SNR) is required to estimate the number of sources and to specify the precise locations of electrical neural activities. In this paper, a new algorithm for solving the inverse problem using the fourth order MUSIC (FO-MUSIC) method is proposed. We apply it to the MEG source estimation problem. Numerical simulations demonstrate that the proposed FO-MUSIC algorithm is more robust against Gaussian noise than the conventional MUSIC algorithm.

In functional brain images obtained by analyzing higher human brain functions using functional magnetic resonance imaging (fMRI), one serious problem is that these images depict false activation areas (artifacts) resulting from image-to-image physiological movements of subject during fMRI data acquisition. In order to truly detect functional activation areas, it is necessary to eliminate the effects of physiological movements of subject (i.e., gross head motion, pulsatile blood and cerebrospinal fluid (CSF) flow) from fMRI time series data. In this paper, we propose a method for eliminating artifacts due to not only rigid-body motion such as gross head motion, but also non-rigid-body motion like the deformation caused by the pulsatile blood and CSF flow. The proposed method estimates subject movements by using gradient methods which can detect subpixel optical flow. Our method estimates the subject movements on a "pixel-by-pixel" basis, and achieves the accurate estimation of both rigid-body and non-rigid-body motion. The artifacts are reduced by correction based on the estimated movements. Therefore, brain activation areas are accurately detected in functional brain images. We demonstrate that our method is valid by applying it to real fMRI data and that it can improve the detection of brain activation areas.

We have demonstrated active mode hopping suppression in external-cavity semiconductor lasers including a diffraction grating as a wavelength-selecting device. The feedback control nullifies the difference between the oscillation wavelength and selected wavelength based on observed changes in diffraction angle. The control has suppressed mode hopping over a 7.5 times wider span than without control. And when combined with conventional mode hopping suppression techniques, mode-hop-free oscillation is achieved over 130 nm. Our approach can be used for most Littman-type external-cavity semiconductor lasers with simple attachments; it will be useful for continuous wavelength sweeping and for long-term wavelength stabilization.

A tunable external-cavity InGaAs/AlGaAs quantum-well laser using a grating coupler monolithically integrated in a selectively disordered waveguide is demonstrated. The laser consists of an amplifier with a narrow channel for lateral single-mode guiding and a tapered section, a grating coupler for output beam collimation and wavelength dispersion, and an external half mirror. Selective quantum-well disordering technique using SiO2 caps of different thicknesses and rapid thermal annealing was employed to reduce the passive waveguide loss in the grating coupler region. Loss reduction from 40 cm-1 to 3 cm-1 was accomplished. Resultant increase of the grating coupler efficiency and expansion of the effective aperture length led to significant improvement of the laser performances. The maximum output power of 105 mW and wide tuning range of 21.1 nm centered at 997 nm were obtained. The well collimated output beam of full diffraction angles at half maximum of 0.16 0.18 was obtained.