Kazuto Itoh

Crystalline/Amorphous Si Integrated Optical Couplers for 2D/3D Interconnection

The in-plane and interlayer waveguide-type couplers between crystalline Si and amorphous-Si:H wire waveguides, for 2D/3D hybrid-material integration are presented in this paper. The in-plane-type coupler achieves stable coupling between two waveguides by using tapers located at the tips of the waveguides. The interlayer-type coupler can connect two waveguides, despite an interlayer distance of 1 μm, with a simple process flow, by introducing a trident structure. An experiment was conducted in which the in-plane and interlayer-type couplers realized low coupling losses (coupling efficiencies) of 0.16 dB (96%) and 0.49 dB (89%) per coupler, respectively.

GaInAsP/silicon-on-insulator hybrid laser with ring-resonator-type reflector fabricated by N2 plasma-activated bonding

III–V/Si hybrid integration with direct bonding is an attractive method of realizing an electrophotonic convergence router with a small size and a low power consumption. Plasma-activated bonding (PAB) is an effective approach for reducing thermal stress during the bonding process because PAB achieves a high bonding strength with low-temperature annealing. This time, the fabrication of a GaInAsP/silicon-on-insulator (SOI) hybrid laser with Si ring-resonator-type reflectors was demonstrated by N2 PAB. By measuring the lasing spectra, we confirmed the reflective characteristics resulting from the cascaded Si ring resonators. We also investigated kink characteristics, which occur around the threshold current, of the current–light output (I–L) characteristics, and successfully approximated the kink characteristics by considering saturable absorption occurring at the III–V/Si taper tip. The taper structure was investigated in terms of a passive device as well as an active device, and a structure for eliminating saturable absorption was proposed.

Vertical trident coupler for 3D optical interconnection

A vertical trident coupler for 3D optical circuit was demonstrated. This is novel interlayer directional-type coupler between crystalline Si and amorphous-Si:H wire waveguides. The trident structure can manage both easiness of fabrication and high coupling even with relatively thick interlayer distances. In experiment, this trident coupler realized low coupling loss (coupling efficiency) of 0.58dB (87%) at 1550 nm wavelength and almost constant coupling loss of around 0.5dB within the wavelength range from 1530 nm to 1610 nm.

High-efficiency silicon Mach-Zehnder modulator with vertical PN junction based on fabrication-friendly strip-loaded waveguide

We demonstrate a vertical p-n junction silicon Mach-Zehnder modulator constructed with hydrogenated amorphous silicon strip-loaded waveguides on a flat SOI platform. A 3-mm-long phase shifter shows 0.80- to 1.86-Vcm modulation efficiency, 7.3- to 16.9-dBV loss-efficiency product, 3-dB bandwidth of 17 GHz, and 25-Gb/s operation.

High-efficiency strip-loaded waveguide based silicon Mach-Zehnder modulator with vertical p-n junction phase shifter

We demonstrate a silicon Mach-Zehnder modulator (MZM) based on hydrogenated amorphous silicon (a-Si:H) strip-loaded waveguides on a silicon on insulator (SOI) platform, which can be fabricated by using a complementary metal-oxide semiconductor (CMOS) compatible process without half etching of the SOI layer. Constructing a vertical p-n junction in a flat etchless SOI layer provides superior controllability and uniformity of carrier profiles. Moreover, the waveguide structure based on a thin a-Si:H strip line can be fabricated easily and precisely. Thanks to a large overlap between the depletion region and optical field in the SOI layer with a vertical p-n junction, the MZM provides 0.80- to 1.86-Vcm modulation efficiency and a 12.1- to 16.9-dBV loss-efficiency product, besides guaranteeing a 3-dB bandwidth of about 17 GHz and 28-Gbps high-speed operation. The αVπL is considerably lower than that of conventional high-speed modulators.

Analysis of plasmonic phase modulator with furan–thiophene chromatophore electro-optic polymer

We analyzed two types of Mach-Zehnder plasmonic modulators on a silicon-on-insulator platform with a different furan-thiophene chromophore electro-optic polymer to compare to other reports. The metal-taper coupling structure and the metal-insulator-metal cross section in our design have been optimized based on the new material parameters. According to the simulation result, a modulator with a slot width of 50 nm and an on-off voltage of Vπ=20  V can be 21 μm long, leading to a total modulator loss of 15 dB, which is comparable to previously reported devices.

Double taper-type mode convertor for direct bonded III-V/SOI hybrid photonic devices

The double taper-type mode convertor for direct bonded GaInAsP/SOI hybrid photonic devices was proposed and the characteristics of the fabricated structure using plasma activated bonding were measured. The calculated and measured results showed the coupling efficiencies of 90% and 75%, respectively.

Apodized amorphous silicon grating coupler with metal mirrors for 3D optical interconnection

Inter-layer coupling between multilayer waveguides was demonstrated using hydrogenated amorphous silicon (a- Si:H) grating couplers with metal mirrors. The fabricated device which has the inter-layer distance of 2 2 μm successfully showed wider bandwidth compared with uniform grating structure.