Yuki Atsumi

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.

Low-Loss GaInAsP Wire Waveguide on Si Substrate with Benzocyclobutene Adhesive Wafer Bonding for Membrane Photonic Circuits

Low-power and compact optical interconnects can be realized using III–V membrane photonic integrated circuits fabricated from thin InP-based films bonded to a Si wafer using benzocyclobutene (BCB) adhesive wafer bonding method. However, the conditions of the bonding and fabrication process strongly affect the propagation loss of waveguides. This study investigated the process conditions during bonding and waveguide fabrication with a view to reducing the propagation loss. It was found that clean, large GaInAsP thin films without air voids or fractures could be obtained by using an appropriate solvent volatilization time of 20–30 min at 200 °C for BCB. Furthermore, by using a resist with high dry etching resistance and an SiO2 mask for the etching process, the sidewall roughness of the waveguide was significantly reduced, which finally afforded a waveguide propagation loss as low as 4 dB/cm.

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.

Athermal Wavelength Characteristics of Si Slot Ring Resonator Embedded with Benzocyclobutene for Optoelectronic Integrated Circuits

An athermal Si-slot-waveguide ring resonator embedded with benzocyclobutene (BCB), a low-k material used in electronics, was proposed. By controlling the width of the BCB-filled gap to 90 nm, the temperature coefficient of equivalent index of a Si slot waveguide could be reduced to 3.70×10-6, which is two orders of magnitude smaller than that of a conventional Si waveguide. The dependences of peak wavelength shift on temperature in the fabricated devices of a conventional Si wire waveguide with SiO2 cladding, Si wire waveguide with BCB cladding, and Si slot waveguide with BCB cladding were 26, 17, and -0.6 pm/K, respectively.

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.

Simultaneous retrieval of fluidic refractive index and surface adsorbed molecular film thickness using silicon wire waveguide biosensors

For silicon wire based ring resonator biosensors, we investigate the simultaneous retrieval of changes in the fluidic refractive index ∆n(c) and surface adsorbed molecular film thickness ∆d(F). This can be achieved by monitoring the resonance shifts of the sensors operating in the TE and TM polarizations at the same time. Although this procedure is straightforward in principle, significant retrieval errors can be introduced due to deviations in the sensor waveguide cross-sections from their nominal values in the range commonly encountered for silicon photonic wire devices. We propose a method of determining the fabricated waveguide size using the group indices derived from measured free spectral range (FSR) of the resonators. We further demonstrate that using experimentally measured group index values, the waveguide size can be determined to accuracies of ± 2 nm in width and ± 1 nm in height. By using this procedure, ∆n(c) and ∆d(F) can be obtained to a precision of within 10% of the true values using optically measurable parameters, improving the retrieval accuracy by more than 3 times.

50 Gbps data transmission through amorphous silicon interlayer grating couplers with metal mirrors

The fabrication of highly efficient interlayer grating couplers for multilayered a-Si:H waveguides by introducing two metal mirrors to reflect back the diffracted light was demonstrated. The coupling efficiency from one layer to the next was 83% (loss of 0.8 dB). The 50 Gbps signals were successfully transmitted from one layer to the next without any eye degradation.