Joonhyun Kang

GaInAsP/InP lateral-current-injection distributed feedback laser with a-Si surface grating.

We fabricated a novel lateral-current-injection-type distributed feedback (DFB) laser with amorphous-Si (a-Si) surface grating as a step to realize membrane lasers. This laser consists of a thin GaInAsP core layer grown on a semi-insulating InP substrate and a 30-nm-thick a-Si surface layer for DFB grating. Under a room-temperature continuous-wave condition, a low threshold current of 7.0 mA and high efficiency of 43% from the front facet were obtained for a 2.0-μm stripe width and 300-μm cavity length. A small-signal modulation bandwidth of 4.8 GHz was obtained at a bias current of 30 mA.

Low-Loss Amorphous Silicon Multilayer Waveguides Vertically Stacked on Silicon-on-Insulator Substrate

Low-loss amorphous-silicon (a-Si) waveguides comprising three vertically stacked layers prepared on silicon-on-insulator substrates are demonstrated. We have fabricated multilayer a-Si waveguides and investigated their loss characteristics; this is the first such investigation to our knowledge. All the process temperatures were regulated below 400 °C for the complementary metal oxide semiconductor (CMOS) backend process compatibility. When the surface roughness and sidewall roughness were decreased, the propagation loss decreased to 3.7 dB/cm even in the case of the third layer a-Si waveguide. Such low-loss waveguides can be effectively applied to realize multilayer stacked optical devices.

Lateral-Current-Injection Distributed Feedback Laser With Surface Grating Structure

As a step toward the realization of injection-type membrane distributed feedback (DFB) lasers, which are expected to be important components of optical interconnections, we realized lateral-current-injection DFB (LCI-DFB) lasers with surface grating structures prepared on semiinsulating InP substrates. First, we designed the surface grating structure to have a high index-coupling coefficient together with a high optical confinement in the quantum wells. Then, we investigated the surface grating structure formed on an amorphous-Si (a-Si) layer deposited on the GaInAsP/InP initial wafer containing five quantum wells. A moderately low threshold current of 7.0 mA and a high differential quantum efficiency of 43% from the front facet were obtained under a continuous-wave operating condition at room temperature for a uniform grating LCI-DFB laser with a stripe width of 2.0 μm and a cavity length of 300 μm. A threshold current of 5.8 mA was obtained with a λ/4 phase-shifted LCI-DFB laser with a-Si surface grating. Furthermore, a small-signal modulation bandwidth of 4.8 GHz was obtained at a bias current of 30 mA with a modulation current efficiency factor of 1.0 GHz/mA1/2.

Low Threshold Current Density Operation of a GaInAsP/Si Hybrid Laser Prepared by Low-Temperature N2 Plasma Activated Bonding

The integration of III–V active devices on a Si platform utilizing direct bonding is an attractive way to realize large-scale photonic integrated circuits. Because plasma activated bonding (PAB) is expected to have a higher bonding strength at a lower heating temperature as compared to conventional bonding methods, PAB is attractive for the reduction of non-radiative recombination centers in the III–V active region caused by thermal expansion during the bonding process. A GaInAsP/Si hybrid laser was fabricated with low-temperature (150 °C) N2 PAB, and a low threshold current density of 850 A/cm2 (170 A/cm2 per quantum well) was obtained.

Layer-to-Layer Grating Coupler Based on Hydrogenated Amorphous Silicon for Three-Dimensional Optical Circuits

A high-index-contrast grating coupler based on hydrogenated amorphous silicon (a-Si:H) was demonstrated as a layer-to-layer coupler. a-Si:H was deposited by plasma-enhanced chemical vapor deposition and vertically stacked with SiO2 for multilayer integration. The distance between the multilayer waveguides was fixed to 1 µm for optical isolation. The grating coupler pair was fabricated on the basis of simulation results. From the measurements of a fabricated grating coupler in the C-band, the maximum coupling efficiency was estimated to be 22%.

Amorphous-Silicon Inter-Layer Grating Couplers With Metal Mirrors Toward 3-D Interconnection

Inter-layer grating couplers sandwiched by two metal layers were demonstrated for high coupling efficiency vertical coupling between amorphous-Si:H multi-stacked optical waveguides. A coupling efficiency of 83% was achieved with grating couplers formed on 5 μm wide waveguides separated by 1 μm while theoretical coupling efficiency of 90% was obtained.

Design of apodized hydrogenated amorphous silicon grating couplers with metal mirrors for inter-layer signal coupling: Toward three-dimensional optical interconnection

A pair of grating couplers with apodized structure, sandwiched by metal mirrors, was proposed for inter-layer coupling between multilayer waveguides based on hydrogenated amorphous silicon (a-Si:H). We proved theoretically that the introduction of an apodized grating coupler yields a high coupling efficiency of about 90% even for an inter-layer distance of several micrometers. In addition, this high coupling efficiency was obtained for a large bandwidth by adjusting the grating period.

Multi-stacked silicon wire waveguides and couplers toward 3D optical interconnects

The relationship between the propagation loss and roughness on multi-layered Si waveguides fabricated up to a 3rd layer was investigated. By reducing the surface and sidewall roughness of the waveguides, a low propagation loss of 3.7 dB/cm for the 3 layer a-Si:H waveguides was demonstrated. Furthermore, vertical coupling between multilayer waveguides was demonstrated by use of a grating-type vertical coupler. A coupling efficiency of 22% was obtained for 10 pairs of gratings with a period of 640 nm, even with a layer distance of 1μm.

Analysis of higher-order mode suppressed transmission in low-loss silicon multimode waveguides on silicon-on-insulator substrates

Higher-order mode coupling at bending waveguides between Si multimode waveguides and 500-nm-wide wire waveguides was evaluated for low-loss Si multimode waveguide circuits. By narrowing the multimode waveguides to the wire waveguides at the bending section, the TE fundamental mode propagation was strongly maintained in the entire circuit, and the spectral fluctuation was improved from 10 dB to less than 1 dB for a 4-µm-wide waveguide. Furthermore, propagation losses were reduced to less than 1 dB/cm with waveguide widths greater than 1 µm.

Ultraviolet-induced wavelength trimming of BCB-buried athermal Si slot wavelength filters

Wavelength trimming for athermal Si slot wavelength filter embedded with BCB using DUV exposure was demonstrated. The total shift of 1.21 nm was obtained after exposing 14 J/cm2 of DUV light without any degradation in athermal and propagation characteristics.