Published papers

PIC multi-band WSS is evaluated with 33.3 GBd 64-QAM data. BER values below the FEC-limit for the S, C and L-bands are obtained. The 3dB bandwidth scaling of cascading WSSs is also investigated for a metro-access network.

A multi-stage low-crosstalk Bragg grating filter (BGF) for an arrayed-waveguide grating (AWG) with 100 GHz spacing is designed. The wavelength demultiplexer consisting of the AWG and the BGFs has a crosstalk of -42 dB.

For the next generation network, ultra-low latency network must be use for smart society vehicle. For this purpose, we have built a campus network using hollow-core fiber, aiming for a breakthrough from devices that will meet the ultra-low delay requirements of Beyond 5G. Hollow-core fibers have a air hole in the core and have succeeded in trapping light in the core. Using the characteristics of this fiber, we will report on the current experimental results regarding the possibilities of multi-branch access, ultra-multi-wavelength networks, radio over fiber (RoF), and power over fiber (PWoF). We also opened the Keio Future Optical Network Open Research Center to make these available for open use.

A new type of coupling structure with an angled cut fiber and a tapered silicon waveguide is proposed and the characteristics of this structure are measured. The coupling loss is 6.09 dB for TE wave and 6.53 dB for TM wave with 5-degree cut fiber.

The objective of our research is to develop optical switches utilizing the phase transition of manganese telluride (MnTe). Initially, we investigated the fundamental optical properties of MnTe by measuring the complex refractive index spectra of α- and β-MnTe thin films. Notably, the imaginary component of the refractive index exhibited a shift of approximately 1 eV toward higher energy due to the phase transition from the α-phase to the β-phase. First-principles calculations indicated that this shift results from an increase in the band gap, mainly attributed to a reduction in the interatomic distance between manganese (Mn) and tellurium (Te). Furthermore, we conducted physical simulations of light propagation in two optical switch models—the Mach–Zehnder type and the ring resonator type—incorporating MnTe thin films. Both models demonstrated effective ON–OFF operation with extinction ratios ranging from 16 to 17 dB, facilitated by the phase transition of MnTe.

Optical signal delay processor for a beamforming antenna in a radio over fiber system is fabricated. The relative delay between four optical signals is controlled from 0 to 31.2 ps, corresponding to the phase control from 0 to 7p/4 for 28-GHz radio wave.

We present a high-power photodiode (HPPD)-driven antenna system using hollow-core fibers (HCFs). This system employs a configuration that does not use an electrical power amplifier (PA), with the aim of overcoming the degradation of PA performance associated with the increasingly high frequencies of radio-frequency (RF) signals. To enhance the RF output power of the HPPD, we introduce an optimized modulation scheme for analog radio-over-fiber (A-RoF) signals based on a comparison of various modulation schemes. After demonstrating the effectiveness of the high-power A-RoF transmission using HCFs, we conducted detailed performance evaluations of a conventional downlink antenna system and an HPPD-driven antenna system using HCFs. The HPPD-driven antenna system achieved a 24.6 dB improvement in the 5 G new radio (NR) output power with a carrier frequency of 28 GHz compared with the conventional system, with an error-vector magnitude of less than 8%. Furthermore, we simultaneously transmitted the power for driving an HPPD-driven antenna over a 1 km HCF, and experimentally demonstrated the ability to drive the HPPD-driven antenna via power-over-fiber without requiring an external power supply. To the best of our knowledge, this is the first demonstration of an optically powered HPPD-driven antenna with a 5 G NR output power of approximately 10 dBm based on the practical 28 GHz band wireless standard.

We propose and demonstrate an optical coupling structure using an angled cut fiber and an island-shaped silicon spot size converter that has low coupling loss, weak wavelength and polarization dependence, and is suitable for high port count surface mount applications. The coupling structure was designed and optimized via simulation using the 3D-finite domain time domain (FDTD) method. The angled cut fiber and the test silicon chip were fabricated, and the coupling structure was set up. The simulation results showed that the lowest loss was 0.98 dB for a TE wave and 1.09 dB for a TM wave, and the measurement results showed the lowest loss was 5.68 dB for a TE wave and 5.95 dB for a TM wave. The discrepancy between the simulation and experimental losses may be due to the thicker matching liquid layer between the angled-cut fiber facet and silicon waveguide, which differs from the simulation assumption. The remaining difference may be the result of an unparalleled alignment of the facet and the waveguide substrate. The wavelength dependence of the coupling loss, including polarization difference in the wavelength range from 1500 to 1600 nm, was 0.78 dB for the simulation and 1.08 dB for the experiment.

The coupling structure using an angled cut fiber and a forked waveguide was proposed. A coupling loss of 1.5 dB was achieved for both TE and TM waves. The polarization dependence was only 0.2 dB for the wavelength from 1.54 μm to 1.58 μm.

Newly structured fiber called Hollow-Core Fiber (HCF) had been equipped into the campus for power over fiber (PWoF) access network. In Keio university's open lab, we constructed HCF network among four building. HCF transfers the high power optical signal through the air core not glass core materials. Therefore, ultra high-power transmission having maximum 1,000 times larger which can apply to energy transmission (i.e. PWoF). And also high-linearity can realize optical analog signal transmission without degradation called analog radio over fiber. This paper also describes an automatic power off Internet of Things optical network unit for remote cameras application in the Keio university future optical network open research center. These technologies will be key to a reliable and smart beyond 5G society.

A highly reliable, broadband, in-vehicle optical network is proposed. The fragile laser diodes are placed only in the master and optical packets are transmitted between the master and slave nodes in a ring topology network.

We propose a novel low-loss phase-change optical switch using MnTe thin film, which has a smaller absorption coefficient than conventional phase-change materials. The designed phase-change optical switch has a loss of 0.388 dB, a crosstalk of -37.7 dB, and an extinction ratio of 37.3 dB.

A monolithic wavelength selective switch (WSS) using a silicon waveguide was fabricated and its characteristics were measured. The crosstalk of the silicon arrayed-waveguide grating (AWG) is reduced by introducing a Bragg grating filter (BGF), which can realize a waveguide type WSS with both fast switching and low crosstalk performance. The extinction ratio of the BGF was approximately 20 dB with a 3-dB bandwidth of 0.6 nm. The AWG had an average loss of 9.5 dB with a large second peak crosstalk. Switching experiments showed that the total loss averaged 20.5 dB, the crosstalk averaged −13.9 dB, and that switching responses were 13 µs for both the rise and fall.

A hollow core fiber is used to transmit ultra-high power optical signals to deploy a large number of optical network units in the access network. The optically powered optical amplifier is placed at the splitter to compensate for the loss of upstream transmission lines.

We demonstrate the ultralow nonlinearity of HCF for high-speed THz data transport and stable LO distribution, successfully delivering 100-Gb/s data and stable LO signal in the 300-GHz band at launch powers up to 29 dBm over a 3.9-km PBG-HCF link.

Silica frustules of most planktonic diatoms have many shallow holes in which the length (L) is smaller than the width (W). The present study focuses on a silica ultrastructure of setae of a planktonic diatom having deep (L/W > 1) holes. Here, we characterized microscopically patterned nanoholes on the silica walls of thick, robust, and hollow setae of a colony of Chaetoceros coarctatus. Basically, tetragonal poroid arrangements with and without a costa pattern are observed on the inner and outer surfaces, respectively, for three kinds of curving hollow setae attached to the anterior, intercalary, and posterior parts of the colony. The seta structures including specific poroid arrangements and continuity of deep nanoholes depend on the location. The deep nanoholes ∼90 nm wide are elongated from 150 to 1500 nm (L/W ∼17) with an increase in the wall thickness of the polygonal tubes of the setae. The inside poroid array, with a period of 190 nm in the extension direction of setae, is lined by parallel plates of the costae. However, the poroid arrangement on the outer surface is disordered, with several holes obstructed with increasing wall thickness of the posterior terminal setae. According to the movement of a colony in a fluid microchannel, the thick curving terminal setae is suggested to involve attitude control and mechanical protection. Using an optical simulation, the patterned deep through-holes on the intercalary setae were suggested to contribute anti-reflection of blue light in the wavelength range of 400 to 500 nm for the promotion of photosynthesis in seawater.

Newly structured fiber called Hollow-Core Fiber (HCF) had been equipped into the campus for ultra-low latency network. In Keio university's open lab, we constructed HCF network among four building. HCF transfers the optical signal through the air core not glass core materials. Therefore, three main futures are achieved as follows: 1st is Ultra high-power transmission having 1,000 times larger which can apply to energy transmission like power over fiver (PWoF). 2nd is very-low delay because of small refractive index of about 1. 3rd is optical transmission without degradation due to optical non-linearity which can apply to analog signal transmission for example radio over fiber (RoF). So that we propose massively-wavelength parallel optical paths with HCF to realize the ultra-low latency path. We will use multi-band including S, C, L-Band etc. And using less-nonlinearity, denser wavelength can be transmitted. Because we can use huge number of wavelengths, so one user uses one wavelength without packet or flame multiplexing. So multiplexed delay can me neglected. This paper also describes other new futures such as analogue radio over fiber (ARoF) for 5G signal extension tested in the Keio university future optical network open research center. These technologies will be key to a reliable and smart Beyond 5G society.

Over +30 dBm, penalty-free, 32 Gbit/s intensity-modulation direct-detection transmission using 4-km photonic bandgap fiber (PBGF) has been successfully demonstrated. The results show that PBGF can be useful for application within a high split ratio access network, which connects the optical line terminal to many users and IoT devices. In addition, we show that PBGF can be applied to the wireless network consisting of very small cells, configured with a large number of optically powered remote radio heads.

+36 dBm, four-channel analog radio over fiber signals for the beam forming was successfully transmitted through a hollow core fiber. The relative delay between channels can be controlled by the optical signal delay processor.

We have proposed a novel in-vehicle optical network, called SiPhON (Si-Photonics-Based In-Vehicle Optical Network). The main features of SiPhON are: (1) High reliability through the use of temperature-controlled laser diodes (LDs) and a redundant configuration. (2) A large capacity of up to 100 Gbps. (3) Variable communication bandwidth between the master and gateways. (4) Small size and low power consumption through the use of silicon photonics chips. (5) Very low latency. A principle of concept system incorporating 4 K cameras, light detection and rangings (LiDARs), and CAN Ethernet interfaces has been constructed and successfully demonstrated.