This paper proposes a voice conversion (VC) method based on a model that links linguistic and acoustic representations via latent phonological distinctive features. Our method, called speech chain VC, is inspired by the concept of the speech chain, where speech communication consists of a chain of events linking the speaker's brain with the listener's brain. We assume that speaker identity information, which appears in the acoustic level, is embedded in two steps — where phonological information is encoded into articulatory movements (linguistic to physiological) and where articulatory movements generate sound waves (physiological to acoustic). Speech chain VC represents these event links by using an adaptive restricted Boltzmann machine (ARBM) introducing phoneme labels and acoustic features as two classes of visible units and latent phonological distinctive features associated with articulatory movements as hidden units. Subjective evaluation experiments showed that intelligibility of the converted speech significantly improved compared with the conventional ARBM-based method. The speaker-identity conversion quality of the proposed method was comparable to that of a Gaussian mixture model (GMM)-based method. Analyses on the representations of the hidden layer of the speech chain VC model supported that some of the hidden units actually correspond to phonological distinctive features. Final part of this paper proposes approaches to achieve one-shot VC by using the speech chain VC model. Subjective evaluation experiments showed that when a target speaker is the same gender as a source speaker, the proposed methods can achieve one-shot VC based on each single source and target speaker's utterance.

For low-density parity-check (LDPC) codes, the penalized decoding method based on the alternating direction method of multipliers (ADMM) can improve the decoding performance at low signal-to-noise ratios and also has low decoding complexity. There are three effective methods that could increase the ADMM penalized decoding speed, which are reducing the number of Euclidean projections in ADMM penalized decoding, designing an effective penalty function and selecting an appropriate layered scheduling strategy for message transmission. In order to further increase the ADMM penalized decoding speed, through reducing the number of Euclidean projections and using the vertical layered scheduling strategy, this paper designs a fast converging ADMM penalized decoding method based on the improved penalty function. Simulation results show that the proposed method not only improves the decoding performance but also reduces the average number of iterations and the average decoding time.

This paper proposes a new and effective convolutional neural network model termed OFR-Net for optical flow refinement. The OFR-Net exploits the spatial correlation between images and optical flow fields. It adopts a pyramidal codec structure with residual connections, dense connections and skip connections within and between the encoder and decoder, to comprehensively fuse features of different scales, locally and globally. We also introduce a warp loss to restrict large displacement refinement errors. A series of experiments on the FlyingChairs and MPI Sintel datasets show that the OFR-Net can effectively refine the optical flow predicted by various methods.

A long-reach coexisting PON system (1G/10G-EPON, video, and TWDM-PON) that uses the Wavelength Selective-Asymmetrical optical SPlitter (WS-ASP) without any active devices like optical amplifiers is proposed. The proposal can take into account the subscriber distribution in an access network and provide specific services in specific areas by varying the splitting ratios and the branch structure in the optical splitter. Simulations confirm the key features of WS-ASP, its novel process for deriving the splitting-ratios and greater transmission distance than possible with symmetrical splitters. Experiments on a prototype system demonstrate how wavelengths can be assigned to specific areas and optical link budget enhancement. For 1G-EPON systems, the prototype system with splitting-ratio of 60% attains the optical link budget enhancement of 4.2dB compared with conventional symmetrical optical splitters. The same prototype offers the optical link budget enhancement of 4.0dB at the bit rate of 10G-EPON systems. The values measured in the experiment agree well with the simulation results with respect to the transmission distance.

We evaluated the noise properties of a periodically poled lithium niobite phase-sensitive amplifier (PSA) using a phase-locked local oscillator as a pump generated by an optical phase-locked loop (OPLL-LO). To examine whether or not the LO pump generated by an OPLL degrades the noise figure (NF) of the PSA, we compared the noise levels of a PSA using an OPLL-LO with that of one using a master local oscillator (M-LO) that utilizes the master light itself as a pump in the electrical domain. With the OPLL, the phase-locked local light had almost the same frequency noise components as the master light. We observed almost the same output noise spectra for the OPLL-LO PSA and M-LO PSA and confirmed the absence of excess noise components in the OPLL-LO PSA in the 0.1 to 20-GHz range. The OPLL-LO PSA exhibited low-noise amplification with an average NF of 1.7-dB at a 23.2-dB gain within an input power range of -31 to -21dBm, which is a feasible input power for repeater amplifiers used in the optical signal transmission systems. We also investigated the influence of the noisy master light, which emulates the accumulation of optical noise from the amplifiers in the transmission system. The OPLL-LO PSA was highly tolerant to the optical noise because the difference in the NF was negligibly small within a master light OSNR range of 5 to 55dB. These results indicate that the OPLL-LO PSA will be useful as a low-noise repeater amplifier for the spectrally efficient large-capacity photonic networks of the future.

We propose a novel non-orthogonal multiple access (NOMA)-based optimal multiplexing method for multiple downlink service channels to maximize the integrated system throughput. In the fifth generation (5G) mobile communication system, the support of various wireless communication services such as massive machine-type communications (mMTC), ultra-reliable low latency communications (URLLC), and enhanced mobile broadband (eMBB) is expected. These services will serve different numbers of terminals and have different requirements regarding the spectrum efficiency and fairness among terminals. Furthermore, different operators may have different policies regarding the overall spectrum efficiency and fairness among services. Therefore, efficient radio resource allocation is essential during the multiplexing of multiple downlink service channels considering these requirements. The proposed method achieves better system performance than the conventional orthogonal multiple access (OMA)-based multiplexing method thanks to the wider transmission bandwidth per terminal and inter-terminal interference cancellation using a successive interference canceller (SIC). Computer simulation results reveal that the effectiveness of the proposed method is especially significant when the system prioritizes the fairness among terminals (including fairness among services).

In this paper, we reformulate a sensitivity analysis method for function-expansion-based topology optimization method without using gray area. In the conventional approach based on function expansion method, permittivity distribution contains gray materials, which are intermediate materials between core and cladding ones, so as to let the permittivity differentiable with respect to design variables. Since this approach using gray area dose not express material boundary exactly, it is not desirable to apply this approach to design problems of strongly guiding waveguide devices, especially for plasmonic waveguides. In this study, we present function-expansion-method-based topology optimization without gray area. In this approach, use of gray area can be avoided by replacing the area integral of the derivative of the matrix with the line integral taking into acount the rate of boundary deviation with respect to design variables. We verify the validity of our approach through applying it to design problems of a T-branching power splitter and a mode order converter.

A grating consisting of a periodic array of InSb-coated dielectric cylinders on a substrate is analyzed at THz frequencies using the frequency-dependent finite-difference time-domain method based on the trapezoidal recursive convolution technique. The transmission characteristics of an infinite periodic array are investigated not only at normal incidence but also at oblique incidence. The incident field is shown to be coupled to the substrate due to the guided-mode resonance (GMR), indicating the practical application of a grating coupler. For the sensor application, the frequency shift of the transmission dip is investigated with attention to the variation of the background refractive index. It is found that the shift of the dip involving the surface plasmon resonance is almost ten times as large as that of the dip only from the GMR. We finally analyze a finite periodic array of the cylinders. The field radiation from the array is discussed, when the field propagates through the substrate. It is shown that the radiation direction can be controlled with the frequency of the propagating field.

This paper describes the effectiveness of the geometric multi-grid method in a current density analysis using a numerical human body model. The scalar potential finite difference (SPFD) method is used as a numerical method for analyzing the current density inside a human body due to contact with charged objects in a low-frequency band, and research related to methods to solve faster large-scale simultaneous equations based on the SPFD method has been conducted. In previous research, the block incomplete Cholesky conjugate gradients (ICCG) method is proposed as an effective method to solve the simultaneous equations faster. However, even though the block ICCG method is used, many iterations are still needed. Therefore, in this study, we focus on the geometric multi-grid method as a method to solve the problem. We develop the geometric-multi-grid method and evaluate performances by comparing it with the block ICCG method in terms of computation time and the number of iterations. The results show that the number of iterations needed for the geometric multi-grid method is much less than that for the block ICCG method. In addition, the computation time is much shorter, depending on the number of threads and the number of coarse grids. Also, by using multi-color ordering, the parallel performance of the geometric multi-grid method can be greatly improved.

In this paper, we analyzed the pulse responses of dispersion medium with periodically conducting strips by using a fast inversion Laplace transform (FILT) method combined with point matching method (PMM) for both the TM and TE cases. Specifically, we investigated the influence of the width and number of the conducting strips on the pulse response and distribution of the electric field.

Silicon photonics technology is a promising candidate for small form factor transceivers that can be used in data-center applications. This technology has a small footprint, a low fabrication cost, and good temperature immunity. However, its main challenge is due to the high baud rate operation for optical modulators with a low power consumption. This paper investigates an all-Silicon Mach-Zehnder modulator based on the lumped-electrode optical phase shifters. These phase shifters are driven by a complementary metal oxide semiconductor (CMOS) inverter driver to achieve a low power optical transmitter. This architecture improves the power efficiency because an electrical digital-to-analog converter (DAC) and a linear driver are not required. In addition, the current only flows at the time of data transition. For this purpose, we use a PIN-diode phase shifter. These phase shifters have a large capacitance so the driving voltage can be reduced while maintaining an optical phase shift. On the other hand, this study integrates a passive resistance-capacitance (RC) equalizer with a PIN-phase shifter to expand the electro-optic (EO) bandwidth of a modulator. Therefore, the modulation efficiency and the EO bandwidth can be optimized by designing the capacitor of the RC equalizer. This paper reviews the recent progress for the high-speed operation of an all-Si PIN-RC modulator. This study introduces a metal-insulator-metal (MIM) structure for a capacitor with a passive RC equalizer to obtain a wider EO bandwidth. As a result, this investigation achieves an EO bandwidth of 35.7-37 GHz and a 70 Gbaud NRZ operation is confirmed.

We review our research progress of multi-port optical switches based on the silicon photonics platform. Up to now, the maximum port-count is 32 input ports×32 output ports, in which transmissions of all paths were demonstrated. The switch topology is path-independent insertion-loss (PILOSS) which consists of an array of 2×2 element switches and intersections. The switch presented an average fiber-to-fiber insertion loss of 10.8 dB. Moreover, -20-dB crosstalk bandwidth of 14.2 nm was achieved with output-port-exchanged element switches, and an average polarization-dependent loss (PDL) of 3.2 dB was achieved with a non-duplicated polarization-diversity structure enabled by SiN overpass waveguides. In the 8×8 switch, we demonstrated wider than 100-nm bandwidth for less than -30-dB crosstalk with double Mach-Zehnder element switches, and less than 0.5 dB PDL with polarization diversity scheme which consisted of two switch matrices and fiber-type polarization beam splitters. Based on the switch performances described above, we discuss further improvement of switching performances.

A compact silicon photonic crystal waveguide (PCW) slow-light modulator is presented. The proposed modulator is capable of achieving a 64 Gbps bit-rate in a wide operating spectrum. The slow-light enhances the modulation efficiency in proportion to its group index ng. Two types of 200-µm-long PCW modulators are presented. These are low- and high-dispersion devices, which are implemented using a complementary metal-oxide-insulator process. The lattice-shifted PCW achieved low-dispersion slow-light and exhibited ng ≈ 20 with an operating spectrum Δλ ≈ 20 nm, in which the fluctuation of the extinction ratio is ±0.5 dB. The PCW device without the lattice shift exhibited high-dispersion, for which a large or small value of ng can be set on demand by changing the wavelength. It was found that for a large ng, the frequency response was degraded due to the electro-optic phase mismatch between the RF signals and slow-light even for such small-size modulators. Meander-line electrodes, which bypass and delay the RF signals to compensate for the phase mismatch, are proposed. A high cutoff frequency of 55 GHz was theoretically predicted, whereas the experimentally measured value was 38 GHz. A high-quality open eye pattern for a drive voltage of 1 V at 32 Gbps was observed. The clear eye pattern was maintained for 50-64 Gbps, although the drive voltage increased to 3.5-5.3 V. A preliminary operation of a 2-bits pulse amplitude modulation up to 100 Gbps was also attempted.

Horizontal slot waveguides enable light to be strongly confined in thin regions. The strong confinement of light in the slot region offers the advantages of enhancing the interaction of light with matter and providing highly sensitive sensing devices. We theoretically investigated fundamental characteristics of horizontal slot waveguides using Nb2O5. The coupling coefficient between SiO2 slot and air slot waveguides was calculated. Characteristics of bending loss in slot waveguide were also analyzed. The etching conditions in reactive ion etching needed to obtain a sidewall with high verticality were studied. We propose a process for fabricating horizontal slot waveguides using Nb2O5 thin film deposition and selective etching of SiO2. Horizontal slot waveguides were fabricated that had an SiO2 slot of less than 30 nm SiO2. The propagated light passing through the slot waveguides was also obtained.

The injection locking properties of rotary dissipative solitons developed in a closed traveling-wave field-effect transistor (TWFET) are examined. A TWFET can support the waveform-invariant propagation of solitary pulses called dissipative solitons (DS) by balancing dispersion, nonlinearity, dissipation, and field-effect transistor gain. Applying sinusoidal signals to the closed TWFET assumes the injection-locked behavior of the rotary DS; the solitons' velocity is autonomously tuned to match the rotation and external frequencies. This study clarifies the qualitative properties of injection-locked DS using numerical and experimental approaches.

This paper discusses a static analysis problem, called absolute consistency problem, for relational schema mappings. A given schema mapping is said to be absolutely consistent if every source instance has a corresponding target instance. Absolute consistency is an important property because it guarantees that data exchange never fails for any source instance. Originally, for XML schema mappings, the absolute consistency problem was defined and its complexity was investigated by Amano et al. However, as far as the authors know, there are no known results for relational schema mappings. In this paper, we focus on relational schema mappings such that both the source and the target schemas have functional dependencies, under the assumption that mapping rules are defined by constant-free tuple-generating dependencies. In this setting, we show that the absolute consistency problem is in coNP. We also show that it is solvable in polynomial time if the tuple-generating dependencies are full and the size of the left-hand side of each functional dependency is bounded by some constant. Finally, we show that the absolute consistency problem is coNP-hard even if the source schema has no functional dependency and the target schema has only one; or each of the source and the target schemas has only one functional dependency such that the size of the left-hand side of the functional dependency is at most two.

We propose an ultra-low inter-core crosstalk (XT) multi-core fiber (MCF) with standard 125-μm cladding. We show the fiber design and fabrication results of an MCF housing four cores with W-shaped index profile; it offers XT of less than -67dB/km over the whole C+L band. This enables us to realize 10,000-km transmission with negligible XT penalty. We also observe a low-loss of 0.17dB/km (average) at a wavelength of 1.55μm and other optical properties compatible with ITU-T G.654.B fiber. We also elucidate its good micro-bend resistance in terms of both the loss and XT to confirm its applicability to high-density optical fiber cables. Finally, we show that the fabricated MCF is feasible along with long-distance transmission by confirming that the XT noise performance corresponds to transmission distances of 10,000km or more.

We demonstrated 1-Tb/s-class transmissions of field-deployed large-core low-loss fiber links, which is compliant with ITU-T G.654.E, using our newly developed real-time transponder consisting of a state-of-the-art 16-nm complementary metal-oxide-semiconductor (CMOS) based digital signal processing application-specific integrated circuit (DSP-ASIC) and an indium phosphide (InP) based high-bandwidth coherent driver modulator (HB-CDM). In this field experiment, we have achieved record transmission distances of 1122km for net data-rate 1-Tb/s transmission with dual polarization-division multiplexed (PDM) 32 quadrature amplitude modulation (QAM) signals, and of 336.6 km for net data-rate 1.2-Tb/s transmission with dual PDM-64QAM signals. This is the first demonstration of applying hybrid erbium-doped fiber amplifier (EDFA) and backward-distributed Raman amplifier were applied to terrestrial G.654.E fiber links. We also confirmed the stability of signal performance over field fiber transmission in wavelength division multiplexed (WDM) condition. The Q-factor fluctuations respectively were only less than or equal to 0.052dB and 0.07dB for PDM-32QAM and PDM-64QAM signals within continuous measurements for 60 minutes.

We fabricated an InP-based dual-polarization In-phase and Quadrature (DP-IQ) modulator consisting of a Mach-Zehnder (MZ) modulator array integrated with RF termination resistors and backside via holes for high-bandwidth coherent driver modulators and revealed its high reliability. These integrations allowed the chip size (Chip size: 4.4mm×3mm) to be reduced by 59% compared with the previous chip without these integrations, that is, the previous chip needed 8 chip-resistors for terminating RF signals and 12 RF electrode pads for the electrical connection with these resistors in a Signal-Ground-Signal configuration. This MZ modulator exhibited a 3-dB bandwidth of around 40 GHz as its electrical/optical response, which is sufficient for over 400 Gbit/s coherent transmission systems using 16-ary quadrature amplitude modulation (QAM) and 64QAM signals. Also, we investigated a rapid degradation which affects the reliability of InP-based DP-IQ modulators. This rapid degradation we called optical damage is caused by strong incident light power and a high reverse bias voltage condition at the entrance of an electrode in each arm of the MZ modulators. This rapid degradation makes it difficult to estimate the lifetime of the chip using an accelerated aging test, because the value of the breakdown voltage which induces optical damage varies considerably depending on conditions, such as light power, operation wavelength, and chip temperature. Therefore, we opted for the step stress test method to investigate the lifetime of the chip. As a result, we confirmed that optical damage occurred when photo-current density at the entrance of an electrode exceeded threshold current density and demonstrated that InP-based modulators did not degrade unless operation conditions reached threshold current density. This threshold current density was independent of incident light power, operation wavelength and chip temperature.

Effective user accommodation will be more and more important in passive optical networks (PONs) in the next decade since the number of subscribers has been leveling off as well and it is becoming more difficult for network operators to keep sufficient numbers of maintenance workers. Drastically reducing the number of small-scale communication buildings while keeping the number of accommodated users is one of the most attractive solutions to meet this situation. To achieve this, we propose two types of long-reach repeater-free upstream transmission configurations for PON systems; (i) one utilizes a semiconductor optical amplifier (SOA) as a pre-amplifier and (ii) the other utilizes distributed Raman amplification (DRA) in addition to the SOA. Our simulations assuming 10G-EPON specifications and transmission experiments on a 10G-EPON prototype confirm that configuration (i) can add a 17km trunk fiber to a normal PON system with 10km access reach and 1 : 64 split (total 27km reach), while configuration (ii) can further expand the trunk fiber distance to 37km (total 47km reach). Network operators can select these configurations depending on their service areas.