In order to realize receiver (Rx) photonic integrated circuits (PICs) applicable to 400 Gb/s-class coherent passive optical network (PON) systems, we integrated tunable wavelength filters (TWFs), PIN photodiodes (PIN-PDs), and optical 90-degree hybrids (90HBs) into Rx PICs fabricated on a commercial standard silicon photonics (SiPh) platform. As a result of device evaluation, we observed characteristics indicating the potential for application in 400 Gb/s-class coherent PON systems. Subsequently, we prototyped Rx assemblies in which the Rx PIC and a transimpedance amplifier (TIA) chip are mounted. When the Rx assembly received a 60-Gbaud dual-polarization (DP) quadrature phase shift keying (QPSK) 200 Gb/s C-band optical signal after 2 km loopback transmission, we achieved an optical received power of $-$24 dBm at a bit error ratio (BER) of 2$ imes 10<^>{-2}$, which corresponds to the forward error correction (FEC) limit. Based on this result, the Rx assembly is suitable for a PON system with a maximum transmission distance of 20 km and up to 16 terminations. Similarly, when the Rx assembly received a DP-16 quadrature amplitude modulation (QAM) 400 Gb/s optical signal, we achieved an optical received power of $-$6 dBm at the FEC limit. Although some issues remain, these results suggest the Rx assembly can be applied to 400 Gb/s-class coherent PON systems. In the future, we will improve the Rx characteristics by reducing insertion loss of the Rx PICs and will prototype and evaluate transmitter (Tx) assemblies.

We propose an optical biosensor using Si waveguides to detect a bio target with magnet beads as a transducer. A gap is placed between input and output waveguides. The antibody of the target antigen is coated on the floor of the gap. When the antigen arrived at the gap, it binds with the antibody. The magnet beads are coated with 2nd antibody of the target. By dropping fluid containing the magnet beads to the gap, it binds with the captured antigen and shuts down the light transmission. The lowered light transmission indicates that the antibody is detected.

In this paper, we report a silicon photonics-based laser Doppler vibrometer (LDV) aiming for a small-footprint LDV system. The LDV system was based on the arctangent approach of phase-generated carrier (PGC-Arctan) demodulation in a phase-modulating interferometer using an integrated silicon optical phase modulator (SiPh-PM). The LDV performances were experimentally evaluated using a discrete SiPh-PM, and an integrated core-chip module in which the main parts of the LDV system were monolithically integrated in a single silicon-photonics chip. They were compared with the results of a conventional heterodyned-based LDV configuration. The results demonstrated sufficient performance for LDV uses, indicating that the phase modulator-based, integrated silicon photonics LDV is usable in practical cases.

We report a structure for bonding III-V devices on Si waveguide with improved mis-alignment tolerance. The multilevel crossing and tapered waveguide structures are proposed. In the multilevel crossing waveguide structure, the light couples from a III-V waveguide to a Si waveguide whenever there is a crossing point between them. The misalignment tolerance becomes very large when these two waveguides have sufficient length, and the value can be as high as > 3 μm Various types of III-V waveguide can be employed including a tapered structure. The tapered structure consists of a tapered III-V waveguide and a thin and wide Si waveguide beneath it The propagating light in misaligned tapered III-V waveguide can efficiently couple to the wide Si waveguide if this is wide enough. The Si should be thin enough to ensure only the fundamental mode is excited in this waveguide. The structure can tolerate a misalignment of 3 μm.

The characteristics of Moiré one-dimensional photonic crystal (1DPhC) based on topological structure is described. The two 1DPhC lattices with different hole sizes and periods are overlapped to form a 1D Moiré lattice. The filter response of the device changes with the relative position and period difference of two lattices. The resonance mode is excited in two unique positions in the1D Moiré lattice. Higher Q value is attained by smaller period difference and optimized hole size ratio. The Q value can be almost an order higher than the conventional topological 1DPhC resonator. The device shows high sensitivity of 270 nm/RIU against clad refractive index change which may be beneficial to be used as an optical sensor. Sensor sensitivities of 450 to 660 nm/RIU are obtained by using slot waveguide structures.

We studied a high-speed Ge/Si electro-absorption optical modulator (EAM) evanescently coupled with a Si waveguide of a lateral p-n junction for a high-bandwidth optical interconnect over a wide range of temperatures from 25 °C to 85 °C. We demonstrated 56 Gbps high-speed operation at temperatures up to 85 °C. From the photoluminescence spectra, we confirmed that the bandgap energy dependence on temperature is relatively small, which is consistent with the shift in the operation wavelengths with increasing temperature for a Ge/Si EAM. We also demonstrated that the same device operates as a high-speed and high-efficiency Ge photodetector with the Franz-Keldysh (F-K) and avalanche-multiplication effects. These results demonstrate that the Ge/Si stacked structure is promising for both high-performance optical modulators and photodetectors integrated on Si platforms.

A technology of co-packaged optics, which is mounting photonics integrated circuits and electronic integrated circuits on the same board, is essential to meet the demands of high-capacity transmission in data centers. In this respect, polymer optical waveguides have attracted much attention because of mechanical stability, excellent processability, and compatibility with electronic circuits. To achieve high-capacity optical transmission, we are developing a new package substrate, which we call active optical package (AOP) substrate, as a solution of co-packaged optics. The AOP substrate consists of a conventional organic package substrate such as glass-epoxy substrates, on which silicon photonics dies are embedded and SMF connectors are mounted. In the AOP, the silicon photonics inputs/outputs (I/Os) and the optical connectors are connected by 3D optical wiring technology that has a function of converting the pitch size. The 3D optical wiring is realized with a pair of micro-mirrors and a single-mode polymer waveguide. First, we have evaluated the high-speed optical transmission performance of a single-mode polymer optical waveguide for LAN-WDM network. We observed there was no noticeable penalty for optical transmission for all LAN-WDM channels. And then, we demonstrate transmission of an AOP substrate comprising of silicon waveguide, two micro-mirrors and polymer waveguide. Transmission of 112-Gb/s PAM4 optical signal was performed without noticeable penalty up to 85 °C.

We propose silicon (Si)-photonics-embedded interposers as a novel packaging platform to achieve co-packaged optics. An interposer is an organic substrate that has Si-photonics transceiver dies buried in it and polymer optical waveguides connecting the embedded Si chips and optical connectors. We also developed a Si-photonics-device-embedding process and investigated the properties and operations of embedded Si-photonics devices as a feasibility study. The embedded arrayed waveguide grating and reflective optical filter showed a wavelength shift on the order of 0.1 nm with our embedding process. The shifts seem to be due to the difference in ambient temperature during the measurements and induced strain. Though this embedding process is presumed to affect the spectrum for Si-photonics devices, the difference is small enough to be controlled. Embedded Si-photonics transmitter- and receiver-integrated circuits successfully demonstrated 25-Gb/s operations. The proposed Si-photonics-embedded interposer is a promising candidate for a co-packaged optics platform to eliminate the interconnect bandwidth bottleneck for high-performance computing systems.

The receiving characteristics of the silicon photonic-integrated chip assembly consisting of transimpedance amplifiers and butt-joint waveguide germanium avalanche photodiodes with lateral separated absorption and multiplication (SAM) structures were demonstrated based on conventional complementary metal-oxide-semiconductor processes. We experimentally verified a clear open eye diagram at 10 Gbps with a reverse bias of 17 V. Receiving sensitivities of -22.8 dBm and -22.0 dBm were obtained for the best and worst polarizations, respectively, with optical signals that had wavelengths longer than those of the absorption edges of Ge at a bit rate of 10 Gbps and a wavelength of 1600 nm. The proposed waveguide butt-joint germanium avalanche photodiodes with lateral SAM structures can be used to receive long wavelengths of downstream signals with low polarization dependence in next-generation optical access network systems.

Multimode interference (MMI) coupler for multimode system is proposed based on Si waveguide platform. Each mode is separated and injected into the mNxmN MMI coupler with m being the number of modes and N is the multiplication number of input image.

We previously proposed a new package substrate called active optical package (AOP) substrate to realize co-packaged optics. An optical redistribution technology on silicon photonics dies was developed to fabricate the AOP substrate. It is composed of a polymer waveguide, with a mirror-based optical coupling between the polymer and silicon waveguides. The fabricated optical redistribution loss was characterized in this study. An average loss of approximately 4 dB and wavelength dependent loss of ±1 dB were observed for the wavelength range of 1460–1600 nm. It was shown that the low wavelength-dependent optical redistribution was available owing to the broadband characteristics of mirror-based optical coupling and polymer waveguide.

Polarization rotating (PR) Mach Zehnder Interferometer (MZI) device is proposed based on Si waveguide platform using multimode concept and experimentally demonstrated. Its concept is to use the mode conversion between TE first order and TM fundamental modes. The device is composed of mode converters, multimode directional couplers and PR rib waveguides inserted to the center of each arm waveguide. Fabricated PR MZI device shows good matching in the spectra between TE and TM polarizations in terms of the operation wavelength and the device loss.

To realize a new package substrate for co-packaged optics, a silicon-photonics hybrid glass-epoxy substrate was demonstrated. In the substrate, silicon photonics dies working as optical/electrical conversion engines are embedded. Additionally, it includes optical redistribution composed of polymer waveguides and mirror-based optical coupling structures between the polymer and silicon waveguides. A demonstration sample was designed for a total bandwidth of 10 Tbps using silicon photonics dies with arrayed waveguide gratings, wavelength splitters, and polarization splitters/rotators for 16-ch wavelength division multiplexing (WDM). It was fabricated using unique key technologies, such as silicon photonics embedding, micromirror fabrication, and single-mode polymer waveguide fabrication. Its wavelength multiplexing operation and signal transmission characteristics were evaluated. As a result, the hybrid substrate was discovered to be capable of 112 Gbps pulse amplitude modulation 4 (PAM-4) transmission with a 16-ch WDM function because the transmitter dispersion and eye closure quaternary (TDECQ) values of less than 3.4 dB were obtained and 16-ch WDM spectrum were clearly visible. To the best of our knowledge, the working of such a hybrid substrate was demonstrated for the first time. This demonstration implies that the hybrid substrate is feasible, and the above-mentioned novel technologies are crucial to its development.

In high performance computer systems and large-scale data centers, data movement becomes a critical problem. To overcome this problem, co-packaged optics are attracted much attention for high performance LSIs like upcoming high-capacity switch ASICs. We proposed a new package substrate called active optical package (AOP) substrate. The AOP substrate is an organic package substrate where Si photonics dies are embedded. On the surface of the substrate, fan-out polymer waveguides, connecting high density Si-photonics I/O and low density single-mode fiber (SMF) array, are integrated. the AOP substrate will be used as same as conventional standard package substrates. Only a different point is to connect a pluggable optical fiber. Interlayer optical coupling between the Si and polymer waveguides was achieved with two micro mirrors. For example, light output from the Si waveguide end facet was reflected to upper side by the 1st micro mirror. The light was reflected again to the polymer waveguide end facet by the 2nd mirror fabricated above the 1st mirror. Compared to popular grating couplers, the mirror is suitable for low wavelength and polarization dependent optical coupling. Error free transmission of 28-GBaud NRZ optical signal through the Si waveguide, mirror coupling, and polymer waveguide was achieved without significant penalties. The optical coupling of the polymer waveguides to SMFs was also achieved by passive alignment using a low-cost MT-ferrule-compatible optical connector. The loss penalty of the passive alignment was about 3 dB.

We demonstrate temperature insensitive operation of an active optical package substrate comprising of silicon waveguide, two micro-mirrors and polymer waveguide. Transmission of 112-Gb/s PAM4 optical signal was performed without noticeable penalty at 85 °C.

We propose silicon (Si)-photonics-embedded interposers to minimize the Si photonics chip size and achieve highly precise optical fiber alignment. The interposers are organic interposer substrates that have Si photonics transceiver dies buried in the substrate and polymer optical waveguides connecting the Si chips and optical connectors. We investigated the effect of the embedding process on the operation of half-buried Si-photonics transceiver chips in an organic substrate and successfully observed 25-Gb/s operations. The Si-photonics-embedded interposer is expected to be a co-packaged optics platform to eliminate the interconnect bandwidth bottleneck for high-performance computing systems.

A device performing both the polarization-alignment and the wavelength selection is demonstrated by fabricated device. Waveguides incorporating Bragg gratings for polarization rotation and mode conversion having different widths are placed near to each other. For the polarization rotation, experimental results show that the field strength ratio in the two waveguides defines the diffraction coupling coefficient and excitation strength of unwanted modes. The excitation of unwanted mode is reduced, when a large radius short curved waveguide is used for input/output waveguides as shown in the experiment. Polarization aligned wavelength peaks were observed at the backward drop port in the experiment.

We propose a next generation co-packaged substrate using Si photonics dies, a polymer optical waveguide, and a optical connector to achieve beyond 10 Tb/s and WDM optical links. The two micro-mirrors and polymer waveguides were integrated, and their optical characteristics were evaluated.

Novel optical circuit with a microring resonator and polarization rotators was proposed for process control monitoring. The extraction method by TE and TM spectral analysis using the circuit showed sensitivity to sub-nm order fabrication deviations as well as robustness to measurement errors.