Jun Ushida

On-Chip Optical Interconnect

We describe a cost-effective and low-power-consumption approach for on-chip optical interconnection. This approach includes an investigation into architectures, devices, and materials. We have proposed and fabricated a bonded structure of an Si-based optical layer on a large-scale integration (LSI) chip. The fabricated optical layer contains Si nanophotodiodes for optical detectors, which are coupled with SiON waveguides using surface-plasmon antennas. Optical signals were introduced to the optical layer and distributed to the Si nanophotodiodes. The output signals from the photodiodes were sent electrically to the transimpedance-amplifier circuitries in the LSI. The signals from the photodiodes triggered of the circuitries at 5 GHz. Since electrooptical modulators consume the most power in on-chip optical interconnect systems and require a large footprint, they are critical to establish on-chip optical interconnection. Two approaches are investigated: 1) an architecture using a fewer number of modulators and 2) high electrooptical coefficient materials.

Systematic design of antireflection coating for semi-infinite one-dimensional photonic crystals using Bloch wave expansion

We present a systematic method for designing a perfect antireflection coating (ARC) for a semi-infinite one-dimensional (1D) photonic crystal (PC) with an arbitrary unit cell. We use Bloch wave expansion and time reversal symmetry, which leads exactly to analytic formulas of structural parameters for the ARC and renormalized Fresnel coefficients of the PC. Surface immittance (admittance and impedance) matching plays an essential role in designing the ARCs of 1D PCs, which is shown together with a practical example.

Mode identification of high-quality-factor single-defect nanocavities in quantum dot-embedded photonic crystals

We investigate the quality (Q) factor and the mode dispersion of single-defect nanocavities based on a triangular-lattice GaAs photonic-crystal (PC) membrane, which contain InAs quantum dots (QDs) as a broadband emitter. To obtain a high Q factor for the dipole mode, we modulate the radii and positions of the air holes surrounding the nanocavity while keeping sixfold symmetry. A maximum Q of 17 000 is experimentally demonstrated with a mode volume of V=0.39(λ∕n)3. We obtain a Q∕V of 44 000(n∕λ)3, one of the highest values ever reported with QD-embedded PC nanocavities. We also observe ten cavity modes within the first photonic band gap for the modulated structure. Their dispersion and polarization properties agree well with the numerical results.

25 GHz operation of silicon optical modulator with projection MOS structure

We report a high-speed and compact silicon optical modulator based on the free carrier plasma dispersion in a silicon rib waveguide with a MOS (metal-oxide-semiconductor) junction structure. To achieve high-speed and high-efficiency performance, an improved structure of a very small rib waveguide including a projection MOS junction was studied. We demonstrated high speed of 25 GHz operation in case of 120-200 μm phase-shift length and high optical modulation efficiency of 0.5-0.67 Vcm for VπL by using a projection MOS junction structure. According to the carrier-density simulation, higher operation bandwidth up to 40 GHz can be realized.

Waveguide-integrated Si nano-photodiode with surface-plasmon antenna and its application to on-chip optical clock distribution

We developed a waveguide-integrated Si nano-photodiode (PD) with a surface plasmon (SP) antenna for on-chip optical clock distribution. The interfacial periodic nano-scale metal-semiconductor-metal Schottky electrodes were shown to function as an SP optical antenna and also as an optical coupler between a SiON waveguide and a very thin Si-absorption layer. Furthermore, a very high speed response of 17 ps as well as enhanced photoresponsivity was achieved for a 10-mum coupling length. By using this technology, we fabricated a prototype of a large-scale-integration (LSI) on-chip optical clock system and demonstrated 5 GHz of optical clock circuit operation connected with a 4-branching H-tree structure.

Immittance matching for multidimensional open-system photonic crystals

An electromagnetic (EM) Bloch wave propagating in a photonic crystal (PC) is characterized by the immittance (impedance and admittance) of the wave. The immittance is used to investigate transmission and reflection at a surface or an interface of the PC. In particular, the general properties of immittance are useful for clarifying the wave propagation characteristics. We give a general proof that the immittance of EM Bloch waves on a plane in infinite one- and two-dimensional (2D) PCs is real when the plane is a reflection plane of the PC and the Bloch wave vector is perpendicular to the plane. We also show that the pure-real feature of immittance on a reflection plane for an infinite three-dimensional PC is good approximation based on the numerical calculations. The analytical proof indicates that the method used for immittance matching is extremely simplified since only the real part of the immittance function is needed for analysis without numerical verification. As an application of the proof, we describe a method based on immittance matching for qualitatively evaluating the reflection at the surface of a semi-infinite 2D PC, at the interface between a semi-infinite slab waveguide (WG) and a semi-infinite 2D PC line-defect WG, and at the interface between a semi-infinite channel WG and a semi-infinite 2D PC slab line-defect WG.

Tunable Optical Notch Filter Realized by Shifting the Photonic Bandgap in a Silicon Photonic Crystal Line-Defect Waveguide

A tunable optical notch filter was realized by thermally shifting the TM-like (the lights electric field perpendicular to the substrate) bandgap of a silicon photonic crystal slab W1 line-defect waveguide with silica cladding. This device is compact-its footprint is 340times16 mum2, excluding the electrode pads. The 3-dB bandwidth of the device was about 5 nm, and the extinction ratio at the center wavelength was as high as 40 dB. A maximum center wavelength shift of 17.9 nm was attained at a heating power of 0.7W, with a tuning efficiency of 25.5 nm/W. The tuning response time was less than 100 mus

Low-loss Silicon Oxynitride Waveguides and Branches for the 850-nm-Wavelength Region

We developed silicon oxynitride (SiON) waveguides and branches working at around 850 nm wavelength for on-chip optical interconnection. SiON films were deposited by plasma-enhanced chemical vapor deposition (PECVD) and were very transmissive in this wavelength region. The propagation losses of fabricated waveguides were as low as 0.2–0.3 dB/cm. The branches are based on multimode interference (MMI) and exhibited excellent 3 dB characteristics. We also integrated a SiON waveguide with a Si nano-photodiode (PD) with a surface plasmon antenna. A large photocurrent of about 0.1 mA at a coupling length of only 10 µm and a high-speed response of 17 ps were demonstrated for the waveguide-integrated Si nano-PD.

125-µm-pitch × 12-channel “optical pin” array as I/O structure for novel miniaturized optical transceiver chips

We have developed an optical I/O structure using an array of optical pins for a chip-scale parallel optical module named an “optical I/O core.” The optical pin is a kind of vertical polymer waveguide, which is made from UV curable resins. The optimum shape and combination of resins for the optical pins were determined by ray-trace simulation. The numerical aperture (NA) of the developed optical pins is more than 0.4. A photolithographic technique was used to produce a 125-μm-pitch × 12-channel optical pin array. The coupling losses between a GI-50 multi-mode optical fiber (MMF) and the optical pins for a receiver (RX) and transmitter (TX) were 0.41 dB and 2.3 dB, respectively. Wide coupling tolerance of more than 25 μm was also obtained when the allowable excess loss was 0.5 dB. Furthermore, clear eye diagrams were obtained for 25-Gbps back-to-back transmission by using the optical I/O cores with the optical pins and GI-50 MMF.

Efficient transmission mechanisms for waveguides with 90° bends in pillar photonic crystals

Transmission mechanisms for simple 90° bends in waveguides in photonic crystals consisting of a square lattice of pillars (pillar PCs) were investigated. We utilized the fact that the electromagnetic field of a guided wave is characterized by two predominant plane-wave components. It was revealed by theoretical analyses that, if kd is the wave vector of those plane-wave components, a guided wave with a Bloch wave vector k can efficiently propagate through a 90° waveguide bend when ∣k∕kd∣=1/√¯2. Under this wave-vector condition, a single plane-wave component of a guided wave carries nearly all the electromagnetic power through the waveguide bend. Impedance matching at the waveguide bend is also established by that power-carrying plane-wave component. It was demonstrated, using three-dimensional finite-difference time-domain simulation, that a simple 90° bend of a waveguide in a pillar-PC slab efficiently transmitted a guided wave under that wave-vector condition.