Published papers

A low crosstalk optical wavelength filter consisting of a Si arrayed-waveguide grating and Bragg grating tunable filters was designed and fabricated. The simulation results indicated the crosstalk of the optical wavelength filter was dramatically improved.

A low crosstalk optical wavelength filter consisting of a Si arrayed-waveguide grating and Bragg grating tunable filters was designed and fabricated. The simulation results indicated the crosstalk of the optical wavelength filter was dramatically improved.

An optical wavelength filter consisting of a silicon arrayed-waveguide grating and Bragg tunable grating filters was designed and fabricated. The characteristics of the optical wavelength filter were experimentally measured.

We propose a temperature-insensitive silicon waveguide using a polymer on the top surface of the cladding. The temperature insensitivity of the waveguide is achieved by combining materials with positive and negative thermo-optic coefficients. The material used for the athermal silicon optical circuit proposed in this study is silicone resin. The length of the temperature compensation waveguide was determined based on the phase shifts in the light. This is because, if TO effect is nullified, there should be no phase variation. We input light into the waveguide and simulated the phase after passing through the temperature compensation structure. The amount of phase shift varied with the length of the waveguide when silicone resin was used for the top surface of the clad. The phase shift of the athermal waveguide was approximately 1% to 12% compared to the phase shift of the conventional waveguide.

Vivid-colored ammonite fossils that are mined in Alberta, Canada, are known as gemstones called “Ammolite.” The specific colors having narrow reflectance spectra are ascribed to an interference phenomenon on the preserved nacre of fossils. However, the essence of the vivid tones that are different from the structural coloration of other kinds of shells has not been revealed. In this work, we clarified that specific reflection by interlaminar nanogaps (~ 4 nm) of aragonite plates with the homogeneous lamination period in the fossilized nacreous layers is essential for the emergence of vivid structural colors depending on the thickness of the layer.

Optical packet generation by multiplexing low- and high-frequency signals is investigated. The cut-off frequency of the bias tee is optimized in terms of the eye-opening ratio for optical packet trains with different euard ×and intervals.

This paper presents a broadband optical packet generation method by multiplexing low- and high-frequency signals using a bias tee. The cut-off frequency of the bias tee, fc is optimized in terms of the eyeopening ratio for optical packet trains with different guard times and intervals. As a result, the optimal condition is observed to be fGT < fc < f1, and fc should be closer to fGT. (where fGT is the guard frequency and f1 is the high-pass cut-off frequency of a low-frequency amplifier). We implemented optical packet generation using a bias tee based on the simulation results and confirmed the effectiveness of this method.

10 Gbps packet transmission in Si-Photonics-Based In-Vehicle Optical Network (SiPhON) with one master and 3 zonal gateways was successfully demonstrated. In addition, bandwidth variable feature of SiPhON is also confirmed.

We demonstrate a high-power 1:256 PON with optical powering using a hollow-core fiber. We have succeeded in transmitting A-RoF signals with input power exceeding 36 dBm and feeding optical power of 40 dBm for powering.

We demonstrate a high-power 1:256 PON with optical powering using a hollow-core fiber. We have succeeded in transmitting A-RoF signals with input power exceeding 36 dBm and feeding optical power of 40 dBm for powering.

We designed and fabricated a 200-GHz-spacing, 16-channel, 1 × 4 silicon monolithic wavelength selective switch (WSS), adopting a wavefront control configuration with no waveguide crossings. Its basic switching operations were demonstrated with four wavelength channels.

We designed and fabricated a 200-GHz-spacing, 16-channel, 1 × 4 silicon monolithic wavelength selective switch (WSS), adopting a wavefront control configuration with no waveguide crossings. Its basic switching operations were demonstrated with four wavelength channels.

The wavelength selective crossconnect (WXC) is a key component of the reconfigurable optical add/drop multiplexer (ROADM). Waveguide type WXC is difficult to increase the number of ports and channels, and free-space type WXC has a low switching speed of milliseconds. To solve these problems, we have proposed a hybrid type WXC. It has microsecond switching speed, where switching is performed by silicon optical circuit and wavelength division (de)multiplexing is performed by free-space optical system. In this paper, we designed the free-space optics and simulated the transmission spectra of a 16-channel 2×2 hybrid-type WXC using CodeV optical simulator. The thickness and the position of the microlens array to be attached to the silicon optical circuit has been designed. The angle of incidence on the grating coupler was 9 degrees, and the thickness of the lens was 0.53 mm. The center of the microlens array was offset by 60.1 μm from the center of the grating coupler. The distance between the lenses in the free-space optics was optimized for the x-z and y-z planes, respectively. The loss spectra with the light emitted from the grating coupler were simulated for each of the 16 channels. The loss at the center frequency of each channel varies from -0.89 dB to -2.87 dB. The loss can be reduced by optimizing the grating coupler design to be -0.89 dB to -0.91 dB.

A hybrid wavelength selective crossconnect (WXC) with microsecond switching speed, in which wavelength division multiplexing is performed by a free-space optical system and switching is performed by a silicon optical circuit, is proposed and designed.

An optical triode switch using phase change material (PCM) operating as a Mach-Zehnder interferometer (MZI) or as a directional coupler (DC) is proposed. The characteristics of optical triode switches are calculated by using finite-difference time-domain (FDTD) method. Ge2Sb2Te5 (GST) and Ge2Sb2Se4Te1 (GSST) are used as the PCM. In our calculation models, the signal light has a wavelength of 1550 nm, and the phase of the PCM is controlled by light with a wavelength of 1300 nm input into additional waveguides. In the MZI-type triode switch, PCM patches are laid on several crossings between the signal waveguides and the control waveguides. The DC-type triode switch consists of three parallel waveguides, and the control light is input to the center waveguide which has the PCM layer on it. The structures are optimized to have low loss and low crosstalk. In addition, the energy and pulse power required to change the phase are estimated for both types of switches.

The Artificial Intelligence (AI) platform is needed to control the dynamic Internet of Things (IoT)-network with many IoT devices, such as self-driving vehicles, robots, etc., whose movements are changing. We describe the architecture of an access network that should serve as an AI platform for this purpose, connecting an area of about 10 km with low latency and tightly integrating edge computers, etc. We propose a time-space-synchronous digital twin, part of which is being developed as a proof of concept (PoC). For AI applications, information from sensors and other IoT devices will be sent to the network (digital twin) in real time, where AI processing will be performed by linking edge devices and the center cloud. For this purpose, we have created an IoT Box for AI that can be powered from the network, and can link AI networks with low power consumption.For this comprehensive research, Keio University created the "Keio Future Optical Network Open Research Center"with the support of the Ministry of Internal Affairs and Communications (MIC), and began research on a testbed.

Feasibility evaluation of hollow-core photonic bandgap fiber cables which are installed under the Keio University canpus was conducted with actual applications. No degradation of cable attenuation and propagation latency were observed over 1 year.

This study presents high-power passive optical network (PON) and time- and wavelength-division multiplexed PON (TWDM-PON) architectures that use hollow-core fibers (HCFs) for mobile fronthaul networks. To demonstrate the advantages of HCFs, we experimentally evaluate and compare the transmission characteristics of analog radio-over-fiber (A-RoF) signals over a PON or TWDM-PON link between an optical line terminal and a splitter using a standard single-mode fiber and an HCF. In PONs, we simultaneously transmit high-power A-RoF signals and feed light for optically powering remote radio heads in mobile fronthaul networks. In TWDM-PONs, we simultaneously transmitted four-channel A-RoF signals modulated by a 5G new radio signal with a carrier frequency of 28 GHz. Furthermore, we analyzed the PON and TWDM-PON architectures to clarify feasible configurations using actual parameters consistent with the experimental setups.

The C- and L-bands (1530 nm ∼ 1625 nm) are used for long-haul optical communication systems. The operating wavelength range needs to be extended to realize much broader transmission capacity with a low-cost modulation format. In this paper, we propose a large-scale wavelength multiplexer/demultiplexer using a conventional spatial grating in combination with arrayed-waveguide gratings (AWGs) to cover the whole fiber transmission wavelength range. The diffracted light from the spatial grating is spectrally decomposed and coupled to several AWGs for further demultiplexing. In this study, we designed a multi/demultiplexer which covers the O∼L-bands (186.2 THz ∼ 243.7 THz) with a channel spacing of 100 GHz. This has 9 sub-bands and 576 wavelength channels. The groove density of the spatial grating and the beam diameter on it are assumed to be 300 lines/mm and 1 mm x 1 mm, respectively. The diffracted light is coupled to 9 tapered waveguides on the edge of a planar lightwave circuit (PLC). The shape of the beam is changed by a cylindrical lens, to enhance the coupling efficiency. Each tapered waveguide is connected to an AWG with a center wavelength corresponding to the particular sub-band. AWGs with 64 output channels have a frequency spacing of 100 GHz. The transmission loss in the AWGs was calculated to be between -14.9 dB and -6.6 dB. The relatively large transmission loss was due to misalignment between the beam and the tapered waveguides.

As fiber-optic systems turn toward multi-band transmission (MBT), exploiting the complete low loss window of optical fibers, novel optical components, able to operate in bands other than the conventional C-band, become necessary. In light of this, we report on a multi-band photonic integrated 1 × 2 wavelength selective switch (WSS) operating in the O, S, C and L-bands. The photonic integrated WSS presented in this work uses a folded arrayed-waveguide grating (AWG) as the filtering element, while the wideband operation of the thermo-optic switches allows the routing of the individual channels from those bands to any of the device output ports. The operation of the WSS is experimentally validated for different bands and modulation formats. Results show error-free operation with limited penalty with intensity-modulation direct-detection (IM/DD) non-return-to-zero on-off keying (NRZ-OOK) up to 35 Gb/s in O, S, C and L-bands and up to 169.83 Gb/s with coherent 64-quadrature amplitude modulation (QAM) data transmission in the S, C and L-bands.