The initial phase alternation of RZ pulses having duty cycle beyond 50% in dispersion-managed-link is found to help stabilize DM solitons transmissions. The stable soliton propagation of such wide RZ pulses should ease the difficulties designing soliton-based DWDM systems due to less spectral occupancy/channel. For the proof of concept, 40 Gbit/s WDM transmissions are numerically investigated and the initial phase alternation improved the transmission distance by the factor of 2 in the soliton-soliton interaction limited regime. The advantage of this concept has also been verified by conducting 40 Gbit/s single and 8 channels WDM transmission experiments using OTDM techniques with initial phase alternation.

Phase-inversion between neighboring pulses appearing in carrier-suppressed return-to-zero pulses is effective in reducing the signal distortion due to chromatic dispersion and nonlinear effects. A generation method of the anti-phase pulses at 40 GHz is demonstrated by using semiconductor mode-locked lasers integrated with chirped gratings. Operation principle and pulse characteristics are described. Suppression of pulse distortion due to fiber dispersion is confirmed for generated anti-phase pulses. Repeaterless 150-km dispersion-shifted-fiber L-band transmission at 42.7 Gbit/s is demonstrated by using the pulse source.

Today, an ultra-high capacity transmission system based on N40 Gb/s channel rate is the most promising approach to achieve multi-terabit/s of capacity over a single fiber. We have demonstrated 5.12 Tbit/s transmission of 128 channels at 40 Gbit/s over 3100 km and 10.24 Tbit/s transmission of 256 channels at 42.6 Gbit/s (using FEC) over 100 km, based on four main technologies: 40 Gbit/s electrical time-division multiplexing (ETDM), vestigial sideband demultiplexing (VSB), advanced amplifier technology including Raman amplification and TeraLightTM fiber. A record spectral efficiency of 1.28 bit/s/Hz is applied to achieve 10.24 Tbit/s transmission within the C- and L-band.

The mobile computing system is a set of functions on a distributed environment organized to support mobile hosts. In this environment, mobile hosts should be able to move without any constraints and should remain connected to the network even while moving. Also, they should be able to get necessary information regardless of their current location and time. Distributed mutual exclusion methods for supporting distributed algorithms have hitherto been designed for networks only with static hosts. However, with the emergence of mobile computing environments, a new distributed mutual exclusion method needs to be developed for integrating mobile hosts with underlying distributed systems. In the sense, many issues that should be considered stem from three essential properties of mobile computing system such as wireless communication, portability, and mobility. Thus far, distributed mutual exclusion methods for mobile computing environments were designed based on a token ring structure, which has the drawback of requiring high costs in order to locate mobile hosts. In this paper, we propose not only a distributed mutual exclusion method that can reduce such costs by structuring the entire system as a tree-based logical structure but also recovery schemes that can be applied when a node failure occurs. Finally, we evaluate the operation costs for the mutual exclusion scheme and the recovery scheme.

Watermarking techniques are widely used to protect the secret document. In some valuable literatures, most of them concentrate on the binary data watermarking by using comparisons of an original image and a watermarked image to extract the watermark. In this paper, an efficient watermarking algorithm is presented with two-layer hidden for gray-level image watermarking. In the first layer, the key information is found based on the codebook concept. Then the secret key is further hidden to the watermarked image adopting the encryption consisting of spatial distribution in the second layer. The simulations demonstrate that the watermarking information is perceptually invisible in the watermarked image. Moreover, the gray-level watermark can be extracted by referring key parameters rather than the original image, and the extracting quality is very good.

A new dimension-reduced interference suppression scheme is proposed for DS-CDMA systems over multipath channels. The proposed receiver resolves the problems of interference and multipath effects without needing to estimate the channel and training sequences. The minimum mean squared error (MMSE) criterion is used to obtain an algorithm to cancel the interference of each path. The MMSE filter is composed of two stages based on multipath effects. The proposed receiver has low complexity without great degradation of performance compared with the full dimension MMSE receiver with known channel information. Simulation results show that the proposed receiver converges to the optimal value rapidly because of its reduced dimension.

We achieved a dispersion tolerance of 25-ps/nm at 80-Gbit/s using novel carrier-suppressed return-to-zero (CS-RZ) coding realized by duty ratio and optical multiplexing phase control. We also show that the dispersion tolerance strongly depends on the relative optical phase difference between adjacent time slots, and demonstrate 80-Gbit/s 60-km DSF transmission without dispersion compensation by using a newly-fabricated stable 80-Gbit/s OTDM transmitter.

A 1.6-Tb/s dense WDM signal was successfully transmitted over 480 km using the carrier-suppressed return-to-zero (CS-RZ) modulation format. The CS-RZ format was chosen because it exhibited better transmission performance over a wide fiber-input power window than the NRZ and RZ formats in a 40-Gb/s-based WDM transmission experiment with 100-GHz channel spacing, confirming its nonlinearity-insensitive nature in dense WDM systems. With the wide power window of CS-RZ, we achieved stable transmission of 4040-Gb/s WDM signals over a 480-km (680 km) standard SMF line with only the C-band, in which a spectral ripple remained during transmission. Distributed Raman amplification and forward error correction were not used, providing a margin for already installed transmission lines.

We demonstrated variable dispersion compensation using the Virtually Imaged Phased Array (VIPA) for a 40-Gbit/s dense-WDM transmission system. The large tunable range from -800 to +800 ps/nm in the entire C-band wavelength range and the high tunable resolution of 1 ps/nm was achieved by using a 3-dimensional mirror equipped with a stepping motor that we developed. We adopted the dispersion monitor of 40-GHz intensity extracted from the received 40-Gbit/s baseband signals, and verified that this dispersion monitoring method is applicable to nonlinear transmission by detecting the monitor peak. Using the VIPA variable dispersion compensator and the dispersion monitor, we demonstrated 1.28-Tbit/s (40-Gbit/s 32 ch) automatic dispersion compensation. As a result, we confirmed that only two VIPAs and one fixed dispersion compensating fiber (DCF) are needed to make our method applicable to the entire C-band for dense WDM 40-Gbit/s systems having a large transmission range of 80 km.

The effectiveness of Aeff enlarged positive dispersion fiber (EE-PDF) and hybrid amplification configuration with erbium-doped fiber amplifier (EDFA) and fiber Raman amplifier for reducing the fiber nonlinearity and improving the transmission performance in long distance 40 Gbit/s-based WDM transmission was investigated. We have confirmed that the use of EE-PDF in modified dispersion map for 40 Gbit/s transmission is quite effective to increase the transmissible distance and have successfully demonstrated 16 40 Gbit/s WDM transmission over 2000 km with proper dispersion management. We have also confirmed that the use of distributed Raman amplification is quite effective to extend the repeater spacing. By adding the optimum Raman amplification, almost the same transmission performance was obtained with a doubled repeater spacing in long distance 40 Gbit/s-based WDM transmission.

Y. S. Han et al. have proposed an efficient maximum likelihood decoding (MLD) algorithm using A* algorithm which is the graph search method. In this paper, we propose a new MLD algorithm for linear block codes. The MLD algorithm proposed in this paper improves that given by Han et al. utilizing codewords of dual codes. This scheme reduces the number of generated codewords in the MLD algorithm. We show that the complexity of the proposed decoding algorithm is reduced compared to that given by Han et al. without increasing the probability of decoding error.

Quantization is an essential step which leads to compression in discrete cosine transform (DCT) domain. In this paper, we show how a statistically non-optimal uniform quantizer can be improved by employing an efficient reconstruction method. For this purpose, we estimate the probability distribution function (PDF) of original DCT coefficients in a decoder. By applying the estimated PDF into the reconstruction process, the dequantization distortion can be reduced. The proposed method can be used practically in any applications where uniform quantizers are used. In particular, it can be used for the quantization scheme of the JPEG and MPEG coding standards.

Methodologies for more efficient Raman amplification and a more suitable modulation format for 40 Gbit/s WDM unrepeatered transmission are investigated. Management of the fiber effective area is proposed to realize low noise distributed Raman amplification. An Aeff management technique in which low-Aeff fiber is located in a median section instead of the last section, was confirmed numerically and experimentally to improve the OSNR and Q-factor. Carrier-suppressed-return-to-zero (CS-RZ) modulation has the advantage of reducing fiber-nonlinearity effects and permitting denser multiplexing of the wavelengths. 40 Gbit/s 32-channel unrepeatered WDM transmission over 202 km was demonstrated employing the proposed methodologies.

Two planar asymmetric coupled waveguides were fabricated by using different materials (InGaAsP and TiO2/Si) and tested as dispersion compensators (or pulse compressors). Compression of a more-than-10-ps chirped pulse is experimentally demonstrated by using an InGaAsP planar asymmetric coupled waveguide whose group velocity dispersion (GVD) is enhanced by structural optimization and is spectrally tuned to an input pulse as precisely as possible. A large polarization dependence of the pulse compression was also observed and indicates that the observed pulse compression results from dispersion compensation due to the GVD associated with supermodes. A new planar, asymmetric coupled waveguide with a large difference in refractive indices of the two waveguides was fabricated by using a combination of dielectric (TiO2) and semiconductor (Si) materials in order to obtain better GVD characteristics than semiconductor (for example, InGaAsP) asymmetric coupled waveguides. A preliminary experiment on pulse compression using the TiO2/Si planar asymmetric coupled waveguide was conducted. A 2.8-ps blue chirped pulse was compressed down to about 1 ps by a 1-mm-long waveguide (compression ratio: 0.375, which is better than those of the previous InGaAsP planar asymmetric coupled waveguides). This compression ratio agrees well with a theoretical result obtained by a numerical model based on a supermode's GVD.

Space-variant approaches subject to local image characteristics are useful in practical image restoration because many natural images are nonstationary. Motivated by the success of denoising approaches in the wavelet domain, we propose a region-adaptive restoration approach which adopts a wavelet denoising technique in flat regions after an under-regularized constrained least squares restoration. Experimental results verify that this approach not only improves image quality in mean square error but also contributes to ringing reduction.

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.

Progress on a single wavelength channel OTDM terabit/s transmission is described. In particular, we focus on 1.28 Tbit/s OTDM transmission over 70 km which we realized recently. A pre-chirping technique using a high speed phase modulator is emphasized to simultaneously compensate for third- and fourth-order dispersion. The input pulse width was 380 fs, and the pulse broadening after a 70 km transmission was as small as 20 fs. All 128 channels time-division-demultiplexed to 10 Gbit/s had a bit error rate of less than 110-9, in which we employed a lot of new technique for pulse generation, dispersion compensation and demultiplexing. These techniques help pave the path for OTDM technology of the 21 century.

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.

In this paper, we review recent developments in the field of optical regeneration for both ultra long-haul transmission and terrestrial networking applications. Different techniques (2R/3R) using nonlinear properties of materials and/or devices are proposed such as saturable absorber or InP based interferometer structures showing regenerative capabilities. Principles of operation as well as system experiments are described.

The purpose of our study is to estimate the imaging of ischemic myocardial muscles in rats. The magnetocardiograms (MCG) of rats were measured by a 12-channel high resolution gradiometer, which consisted of 5 mm diameter pick-up coils with a 7.5 mm distance between each coil. MCGs of seven male rats were measured in a magnetically shielded room pre and post coronary artery occlusion. The source imaging was estimated by minimum norm estimation (MNE). Changes of the current source imaging pre- and post coronary artery occlusion were clarified. As a result, in the ST segment, the current distribution significantly increased at the ischemic area. In the T wave, the direction of the current distribution clearly shifted to the left thorax. We proved that the increased area of the current distribution in the ST segment was related to the ischemic area of the ventricular muscles.