We demonstrate phase demodulation of 10-Gbps DPSK signals using a silicon micro-ring resonator with a radius of 10 µm and with various coupling gaps for light of ∼1550 nm in wavelength. Influence of the Q factors and transmissions of the resonators on the response speed and power balance of the two output ports is discussed. Furthermore, temperature sensitivity on resonance peak was measured and we discuss its effect on practical demodulation application.

We studied various types of 2D and 3D Si-based photonic crystal structures that are promising for future photonic integrated circuit application. With regard to 2D SOI photonic crystal slabs, we confirmed the formation of a wide photonic bandgap at optical communication wavelengths, and used structural tuning to realize efficient single-mode line-defect waveguides operating within the bandgap. As regards 3D photonic crystals, we used a combination of lithography and the autocloning deposition method to realize complicated 3D structures. We used this strategy to fabricate 3D full-gap photonic crystals and 3D/2D hybrid photonic crystals.

An electroabsorption (EA) modulator array using a double optical-pass (DP) configuration has been developed to obtain high-speed modulation in parallel. Feeding electrical signals from the highly reflective side of the modulator eliminated component assembly problems with lenses and microwave feeder lines. Passive waveguide integration enabled wafers to be cleaved with very short absorbers. The degradation in frequency response was theoretically calculated to be <0. 2 dB compared to that of EA modulators without a passive waveguide. A common upper doping layer in the absorber and passive waveguide regions was introduced to attain high product throughput due to good epitaxial flatness and processing. The integrated 4-channels multiquantum well DP EA modulator array demonstrated high overall performance for a wavelength range from 1545 to 1558 nm. It features a drive voltage of 2 V for 10 dB attenuation, an insertion loss of 12 dB, and 4 channels17 GHz bandwidths for each channel, with low -20 dB crosstalk between adjacent waveguides.

In this paper, we introduce and describe the computational environment that we have developed for cellular automata (CA). CA are powerful methods to understand and simulate the behavior of complex systems such as traffic jams, fluid crosscurrents, and natural disasters. In CA method, modeling of such a system or a phenomenon is to define a transition function, which determines local interactions, so-called "CA rules." However, no systematic method for design of CA rules has been established. We require a CA simulator for "trial and error" in study of modeling based on CA. Furthermore, the CA simulation environment that does not require special knowledge of a user for parallel processing is desired. The purpose of this study is to develop a comprehensive system that enables us to expedite the design of local rules and to accelerate simulations. We have implemented two kinds of simulators differing in their characteristics to improve both design efficiency and execution speed. The major difference between the two simulators is whether a source code is compiled or not. The source code is described in DORA language the authors have designed for the system. DORA language is designed for describing CA rules simply.

The design, fabrication, and measurement of photonic-band-gap (PBG) waveguides and resonators in two-dimensional photonic crystal slabs have been investigated. Although photonic crystal slabs have only partial gaps, efficient waveguides and resonators can be realized by appropriate design. As regards PBG waveguides, we show various designs for efficient single-mode waveguides in PhC slabs with SiO2 cladding, we report group dispersion measurements of PBG waveguides in PhC slabs, and describe the successful fabrication of PBG waveguides with adiabatic connectors that enable us to couple the light from single-mode fibers efficiently to PBG waveguides. As regards PBG resonators, we show how to realize very high-Q and small volume resonators in hexagonal PhC slabs, and report the fabrication of resonant tunneling filters that consist of PBG resonators coupled with PBG waveguides. We also describe the successful fabrication of resonant tunneling mode-gap filters with adiabatic mode connectors.

Polarization insensitive discrete electroabsorption modulators have been designed as an optical gating device. It reveals the first finding, to our knowledge, that the ratio of the optical confinement factor (Γ) to the differential of the values (ΔΓ) between TE and TM polarized lights decides polarization dependence of attenuation. The ratio ΔΓ/Γ is significantly reduced by increasing core thickness. Large optical confinement structures combining a thick InGaAsP bulk absorption layer and polyimide-buried mesa-ridge waveguide have fabricated. The ratio ΔΓ/Γ of the high-mesa structure was estimated to be less than 0.05 in the gain-region of an erbium-doped fiber amplifier (EDFA), which enable us extremely low polarization sensitivity less than 1 dB up to 20 dB extinction. Proper waveguide length of the structure allowed low insertion loss ( 9.3 dB), small loss-change ( 1.8 dB) and sufficient modulation depth ( 30 dB) simultaneously in the EDFA's gain region. The low-mesa structure provided low insertion loss around 7 dB with small deviation in the wavelength region. High modulation band-width and a polarization-insensitive optical gating waveform have also demonstrated.

A wavelength tunable DBR laser monolithically integrated with an EA-modulator as a WDM system light source was fabricated by selective area MOVPE growth. The lasing wavelength and band-gap energy were simultaneously controlled on the same epitaxial wafer by using a modulated grown thickness of InGaAsP/InGaAsP MQW layers. A wavelength tuning range of 3.5 nm, an output power of 3 mW, and an extinction ratio of 14 dB for 3 V were achieved. The measured 3 dB frequency bandwidth was 2 GHz. No significant change in modulation characteristics were observed when wavelength tuning by injecting the current into the DBR.

Effect of the post-growth annealing on the morphology of a Ge mesa selectively grown on Si was studied from the viewpoint of near-infrared photodiode applications. By ultrahigh-vacuum chemical vapor deposition, Ge mesas were selectively grown at 600 on Si (001) substrates partially covered with SiO2 masks. The as-grown Ge mesas showed trapezoidal cross-sections having a top (001) surface and {311} sidewall facets, as similar to previous reports. However, after the subsequent post-growth annealing at ~800 in the ultrahigh-vacuum chamber, the mesas were deformed into rounded shapes having a depression at the center and mounds near the edges. Such a deformation cannot be observed for the samples annealed once after cooled and exposed to the air. The residual hydrogen atoms on the Ge surface from the germane (GeH4) decomposition is regarded as a trigger to the observed morphological instability, while the final mesa shape is determined in order to minimize a sum of the surface and/or strain energies.

A silicon (Si) wire waveguiding system fabricated on silicon-on-insulator (SOI) substrates is one of the most promising platforms for highly-integrated, ultra-small optical circuits, or microphotonics devices. The cross-section of the waveguide's core is about 300-nm-square, and the minimum bending radius are a few micrometers. Recently, crucial problems involving propagation losses and in coupling with external circuits have been resolved. Functional devices using silicon wire waveguides are now being tested. In this paper, we describe our recent progress and future prospects on the microphotonics devices based on the silicon-wire waveguiding system.

One of the most serious challenges facing the exponential performance growth in the information industry is a bandwidth bottleneck in inter-chip interconnects. We therefore propose a photonics-electronics convergence system with a silicon optical interposer. We examined integration between photonics and electronics and integration between light sources and silicon substrates, and we fabricated a conceptual model of the proposed system based on the results of those examinations. We also investigated the configurations and characteristics of optical components for the silicon optical interposer: silicon optical waveguides, silicon optical splitters, silicon optical modulators, germanium photodetectors, arrayed laser diodes, and spot-size converters. We then demonstrated the feasibility of the system by fabricating a high-density optical interposer by using silicon photonics integrated with these optical components on a single silicon substrate. As a result, we achieved error-free data transmission at 12.5 Gbps and a high bandwidth density of 6.6 Tbps/cm2 with the optical interposer. We think that this technology will solve the bandwidth bottleneck problem.

LSI on-chip optical interconnections are discussed from the viewpoint of a comparison between optical and electrical interconnections. Based on a practical prediction of our optical device development, optical interconnects will have an advantage over electrical interconnects within a chip that has an interconnect length less than about 10 mm at the hp32-22 nm technology node. Fundamental optical devices and components used in interconnections have also been introduced that are small enough to be placed on top of a Si LSI and that can be fabricated using methods compatible with CMOS processes. A SiON waveguide showed a low propagation loss around 0.3 dB/cm at a wavelength of 850 nm, and excellent branching characteristics were achieved for MMI (multimode interference) branch structures. A Si nano-photodiode showed highly enhanced speed and efficiency with a surface plasmon antenna. By combining our Si nano-photonic devices with the advanced TIA-less optical clock distribution circuits, clock distribution above 10 GHz can be achieved with a small footprint on an LSI chip.

Graphene is attracting attention in electrical and optical research fields recently. We measured the optical absorption characteristics and polarization dependence of single-layer graphene (SLG) on sub-micrometer Si waveguide. The results for graphene lengths ranging from 2.5 to 200 $mu$ m reveal that the optical absorption by graphene is 0.09 dB/$mu$ m with the TE mode and 0.05 dB/$mu$ m with the TM mode. The absorption in the TE mode is 1.8 times higher than that in the TM mode. An optical spectrum, theoretical analysis and Raman spectrum indicate that surface-plasmon polaritons in graphene support TM mode light propagation.

Monolithic integration of various kinds of optical components on a silicon wafer is the key to making silicon (Si) photonics practical technology. Applying silicon photonics to telecommunications further requires low insertion loss and polarization independence. We propose an integration concept for telecommunications based on Si and related materials and demonstrate monolithic integration of passive and dynamic functional components. This article shows the great potential of Si photonics technology for telecommunications.

Wide deployment of artificial intelligence (AI) is inducing exponentially growing energy consumption. Traditional digital platforms are becoming difficult to fulfill such ever-growing demands on energy efficiency as well as computing latency, which necessitates the development of high efficiency analog hardware platforms for AI. Recently, optical and electrooptic hybrid computing is reactivated as a promising analog hardware alternative because it can accelerate the information processing in an energy-efficient way. Integrated photonic circuits offer such an analog hardware solution for implementing photonic AI and machine learning. For this purpose, we proposed a photonic analog of support vector machine and experimentally demonstrated low-latency and low-energy classification computing, which evidences the latency and energy advantages of optical analog computing over traditional digital computing. We also proposed an electrooptic Hopfield network for classifying and recognizing time-series data. This paper will review our work on implementing classification computing and Hopfield network by leveraging silicon photonic circuits.