Papers - Tanabe, Takasumi

Broadbandwidth Edge Coupler Structure for SiN-Si Heterogeneous Integration

Sugano R., Kokubu J., Otake R., Fujii S., Tanabe T.

IEICE Transactions on Electronics E108C ( 11 ) 572 - 578 2025.11

ISSN  09168524

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SUMMARY We study how to optimize the coupling efficiency between silicon nitride and silicon waveguides using an edge coupler with an inverse-tapered structure, specifically targeting broad bandwidth applications. Initially, we hypothesized that mode matching alone would suffice for effective coupling. However, our results indicate that maximizing the coupling efficiency requires the simultaneous optimization of mode coupling and effective refractive-index matching. Theoretical simulations predicted a maximum coupling efficiency of 0.963, while experimental measurements yielded an efficiency of 0.588. We analyzed the wavelength dependence of our coupler, and found that the edge coupler exhibited broader bandwidth properties than a standard multilayer coupler, making it more suitable for applications across a wide wavelength range. These results indicate the potential for integrating such couplers with microresonator frequency comb systems, offering the possibility of a wide range of photonic applications.

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Record-high-Q AMTIR-1 microresonators for mid- to long-wave infrared nonlinear photonics

Yang L., Sugano R., Takabayashi R., Kanzawa H., Kumazaki H., Zhuang Y., Wei X., Tanabe T., Fujii S.

Optics Letters 50 ( 21 ) 6554 - 6557 2025.11

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AMTIR-1 chalcogenide glass has shown its potential for use in thermal imaging systems owing to its low refractive index, thermal resistance, and high transparency across the infrared wavelength regime. Here, we report a millimeter-scale high-Q whispering gallery mode microresonator made of AMTIR-1. The recorded Q-factor has reached 1.2×107 at 1550 nm, which is almost two orders of magnitude higher than previously reported values. We characterize the thermal properties, where low thermal conductivity plays an important role in thermal resonance tuning. We further show that AMTIR-1 resonators support anomalous dispersion as well as a low absorption coefficient near the 7 µm wavelength band, thus offering the possibility of providing suitable platforms for mid-infrared, long-wave infrared nonlinear optics, including microresonator frequency comb generation.

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Soliton-comb-based photonic microwave filter operating in the 300 GHz band

Tanikawa K., Imamura M., Yomoda A., Kogure S., Sugano R., Kawanishi S., Fujii S., Tanabe T.

Photonics Research 13 ( 5 ) 1282 - 1289 2025.05

ISSN  23279125

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We present a 300 GHz photonic filter based on a soliton microcomb, addressing the increasing demand for high-frequency, low-phase-noise signals in THz-band wireless communications. Utilizing a uni-traveling-carrier photodiode (UTC-PD), our configuration enables the generation of 300 GHz waves through beat signals from adjacent longitudinal modes of the microcomb. By combining the UTC-PD with a photonic filter approach, we demonstrate that RF photonic filters with high suppression ratios are achieved in the 300 GHz band. Additionally, by mitigating the effects of higher-order dispersion, we achieve a stopband rejection exceeding 30 dB. These results underscore the potential of our photonic filter for advanced THz applications and provide a promising pathway for low-noise, high-bandwidth wireless communication systems.

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Scalable fabrication of erbium-doped high-Q silica microtoroid resonators via sol-gel coating

Imamura R., Fujii S., Nagashima K., Tanabe T.

Optics Continuum 4 ( 3 ) 512 - 521 2025.03

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This study explores sol-gel methods for fabricating erbium-doped silica microtoroid resonators, addressing the limitations of conventional doping techniques and enhancing device scalability. We develop a reproducible sol-gel process that yields defect-free films for photonic applications and detail common defects and troubleshooting strategies. Two fabrication methods are compared: traditional film deposition on substrates and the direct coating of prefabricated resonators. The latter enables the fabrication of larger resonator diameters (up to 450 pm) without buckling while achieving a high-g factor and a low lasing threshold of 350 pW. These erbium-doped resonators exhibit multi-mode laser oscillations at 1550 nm, revealing the sol-gel method’s potential for realizing scalable, gain-doped photonic devices.

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Field Trial of Optical Transmission Experiment Employing a Microresonator Frequency Comb Light Source for Low-Latency, Short-Reach Optical Communication

Tanikawa K., Fujii S., Kogure S., Tanaka S., Tasaka S., Wada K., Kawanishi S., Tanabe T.

IEICE Transactions on Electronics E108.C ( 3 ) 146 - 152 2025.03

ISSN  09168524

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We present first-time demonstration of short-reach and low-latency optical communication within a real network, employing a microresonator frequency comb as a light source. The modulated signal is transmitted through a 9-km single-mode optical fiber installed in a metropolitan network. This demonstration paves the way for realizing low-latency massively parallel optical communication, which is the key to beyond-5G and 6G network. For a proof-of-concept experiment, we employ an MgF<inf>2</inf> crystalline microresonator with a 20-GHz free-spectral range that could be used for dense wavelength division multiplexing communication. We generated a stable soliton comb and modulated it with simple 10-Gbps intensity modulation and direct detection to achieve a small excess delay of 3.1 µs.

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Architecture for coherent dual-comb spectroscopy and low-noise photonic microwave generation using mechanically actuated soliton microcombs

Murakami T., Wada K., Kogure S., Takabayashi R., Yang L., Shibata R., Kumazaki H., Watanabe S., Ishizawa A., Tanabe T., Fujii S.

Optics Letters 50 ( 4 ) 1417 - 1420 2025.02

ISSN  01469592

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Dissipative Kerr soliton microcombs have inspired various intriguing applications such as spectroscopy, ranging, telecommunication, and high purity microwave generation. Mechanically actuated soliton microcombs provide enhanced controllability and flexibility for Kerr solitons, thus enabling technological progress to be made on such practical applications. Here, we present architectures for coherent dual-comb techniques and ultralow-noise microwave generation by exploiting the mechanical actuation of ultrahigh-Q crystalline microresonators. By unifying a pump laser, we demonstrate highly coherent dual-soliton combs using distinct resonators with slightly different repetition rates. We also report significant phase-noise reduction achieved by directly generating Kerr solitons from a sub-Hz linewidth ultrastable laser. This study paves the way for further advancements in a wide variety of applications based on Kerr soliton microcombs.

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Transition between low- and high-noise modulation instability microcombs in a silicon nitride microresonator

Kogure S., Sugano R., Fujii S., Tanabe T.

Japanese Journal of Applied Physics Part 1 Regular Papers and Short Notes and Review Papers 64 ( 1 )  2025.01

ISSN  00214922

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We developed a systematic measurement method that simultaneously captures pump detuning, the optical spectrum, RF beat noise, and the linewidth of a microresonator frequency comb. This comprehensive measurement approach enables us to investigate and visualize the phase transition between two distinct states within the modulation instability (MI) comb: a low-noise state (MI comb phase 1) and a high-noise state (MI comb phase 2). While phase 2 is typically recognized as the MI comb, our findings suggest that phase 1 represents a transition from the Turing pattern to the MI comb, maintaining a low-noise profile despite exhibiting the spectral characteristics of an MI comb.

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Coherent dual soliton combs with mechanically actuated Kerr microcombs

Murakami T., Wada K., Kogure S., Takabayashi R., Yang L., Shibata R., Kumazaki H., Watanabe S., Ishizawa A., Tanabe T., Fujii S.

2025 Conference on Lasers and Electro Optics Europe and European Quantum Electronics Conference CLEO Europe Eqec 2025  2025

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Dissipative Kerr soliton (DKS) microcombs have various intriguing applications thanks to their features including compactness, high-repetition rate, low-power operation, and a broadband spectrum covering up to one-octave. Such unique features of DKSs have been utilized in dual-comb technologies to achieve accuracy, a high acquisition rate, and a small footprint simultaneously. However, conventional dual-comb systems using microcombs face challenges in controlling the difference in repetition rates (Δfrep) between two soliton combs while maintaining mutual coherence. When independent excitation sources and distinct microresonators are used for dual comb generation [1], it is usually difficult to maintain mutual coherence. On the other hand, the dual-combs with a single excitation source in a counter-propagating method [2] can achieve mutual coherence, but the controllability of Δfrep is limited. In this presentation, we report on a coherent dual-comb system with a shared excitation source using mechanically actuated soliton microcombs [3]. Soliton generation enabled by rapid resonance control allows us to obtain mutually coherent dual-combs with a fixed-frequency excitation source in two individual microresonators, balancing the flexibility of Δfrep and mutual coherence [4]. Figure 1 shows the experimental setup. Magnesium fluoride microresonators with FSRs of ~9.4 GHz are used as platforms. Figure 2(a) shows the combined optical spectrum of the dual soliton combs. The electrical spectrum of the dual-comb is shown in Figs. 2(b, c), where peaks are observed at intervals of Δfrep. Furthermore, we confirmed that the RF comb lines have very narrow linewidth compared to the linewidth when distinct pump lasers and resonators are used to generate dual soliton combs [Fig. 2(d,e)]. This study envisions practical dual comb applications and deployment that fully exploit the features of Kerr microcombs.

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Stokes Solitons and PT-Symmetric Coupled Cavities: Two Paths to Mode-Locking Without Saturable Absorbers

Tanabe T., Otake R., Imamura R., Wang H.

International Conference on Metamaterials Photonic Crystals and Plasmonics    1021 - 1022 2025

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Through numerical analyses, we demonstrate that robust mode-locking can be achieved without the use of saturable absorbers by either exploiting stimulated Raman scattering in dispersion-engineered microresonators or by operating near exceptional points in PT-symmetric coupled cavities.

Seamless Conversion of a Photonic Convolution Output to a 300-GHz Wireless Transmitter

Sato K., Kokubu J., Yomoda A., Kawanishi S., Sugano R., Imamura M., Uemura H., Fujii S., Tanabe T.

2025 Conference on Lasers and Electro Optics CLEO 2025  2025

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We performed optical convolution operations using a soliton comb and reduced the computational complexity by 23.1% compared to the electronic digital computer. We successfully modulated and transmitted by radio waves at 300-GHz.

Investigating the Mechanism Behind the Formation of a Stable sech2-shaped Raman Comb in a Silica Microtoroid Driven by Continuous-Wave with Specific Dispersion

Wang H., Otake R., Imamura R., Yang L., Matsuyama H., Fujii S., Tanabe T.

2025 Conference on Lasers and Electro Optics Europe and European Quantum Electronics Conference CLEO Europe Eqec 2025  2025

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Recent experiments have shown that when a microresonator is pumped with coherent optical pulses under appropriate conditions, optical pulses with durations shorter than 50 fs can be generated [1]. These Raman solitons arise from the complex interplay between nonlinearity and dispersion, and their ultrashort duration makes them highly promising for applications that require high temporal resolution. Such solitons require precise control over the repetition rate of the pump pulse train and the round-trip time of the microresonator. However, theoretical analyses suggest that ultrashort Raman optical pulses will also exist under continuous-wave (CW) pumping conditions. We aim to explore the Raman soliton dynamics and the formation conditions using a microtoroid driven by CW.

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Implementation of a Photonic Filter Using a Soliton Comb for 300 GHz Band Communication

Imamura M., Tanikawa K., Yomoda A., Sugano R., Kawanishi S., Fujii S., Tanabe T.

2025 Conference on Lasers and Electro Optics CLEO 2025  2025

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We propose and demonstrate a photonic filter operating in the 300 -GHz band utilizing a soliton comb. Through transmission experiments, we show its ability to suppress jamming signals, paving the way for advances in terahertz communication.

High-Speed Photonic Convolution with Direct 300 GHz Wireless Output

Tanabe T., Sato K., Kokubu J., Yomoda A., Imamura M., Fujii S.

LEOS Summer Topical Meeting  2025

ISSN  10994742

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We demonstrate a soliton microcomb-based photonic convolution with direct 300 GHz wireless output. By encoding kernel weights onto comb lines and summing them in a uni-traveling-carrier photodiode, we achieve a 23.1% reduction in computational cost. This approach links photonic computation directly to 300 GHz transmission.

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Mechanically Actuated Kerr Soliton Microcombs

Fujii S., Wada K., Kogure S., Kumazaki H., Tanabe T.

Laser and Photonics Reviews 18 ( 9 )  2024.09

ISSN  18638880

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Mode-locked ultrashort pulse sources with a repetition rate of up to several tens of gigahertz greatly facilitate versatile photonic applications such as frequency synthesis, metrology, radar, and optical communications. Dissipative Kerr soliton microcombs provide an attractive solution as a broadband, high-repetition-rate compact laser system in this context. However, its operation usually requires sophisticated pump laser control to initiate and stabilize the soliton microcombs, particularly in millimeter-sized ultrahigh-Q whispering-gallery resonators. Here, a mechanically actuated soliton microcomb oscillator is realized with a microwave repetition rate of 15 GHz. This enables direct initiation, subsequent long-term stabilization, and fine tuning of soliton combs without pump laser tunability that is generally required for soliton microcomb operation. The prospects for using this method with a wide range of applications are revealed that will benefit from mechanical soliton actuation such as optical clocks, spectral extension, and dual-comb spectroscopy.

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Exceptional point proximity-driven mode-locking in coupled microresonators

Imamura R., Fujii S., Nakashima A., Tanabe T.

Optics Express 32 ( 13 ) 22280 - 22290 2024.06

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We show theoretically and numerically that mode-locking is feasible with a coupled-cavity system with gain and loss, notably, without any natural saturable absorber. We highlight that in the vicinity of the exceptional point, system Q exhibits substantial modulation even with minor refractive index changes and a minimal Kerr effect contribution. Leveraging this unique behavior, we propose an unprecedented approach wherein the lossy auxiliary cavity functions as an efficient artificial saturable absorber, thus facilitating mode-locking. This approach is not only novel, but also presents considerable advantages over conventional systems where both gain and saturable absorption are contained within a single microcavity. These benefits include reduced operational power and ease of post-adjustment, achievable through the manipulation of the coupling strength between the two microcavities.

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Compact photonic crystal spectrometer with resolution beyond the fabrication precision

Sugano R., Kokubu J., Jin S., Kodama T., Hofs J.J., Zhang J., Okano M., Tanabe T.

Optics Express 32 ( 12 ) 21563 - 21576 2024.06

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We present a compact silicon photonic crystal spectrometer with a footprint of 740 × 9 μm2 and excellent wavelength resolution (∼0.01 nm at single and <0.03 nm at multiple wavelength operation) across a telecom bandwidth of 10 nm. Although our design targets a wavelength resolution of 1.6 nm, within the current state-of-the-art fabrication precision of 2 nm, we achieve a resolution that exceeds these limits. This enhanced resolution is made possible by leveraging the random localization of light within the device.

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Synchronization of two chaotic microresonator frequency combs

Moreno D., Fujii S., Nakashima A., Lemcke D., Uchida A., Sanchis P., Tanabe T.

Optics Express 32 ( 2 ) 2460 - 2472 2024.01

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We explore the synchronization of chaotic microresonator frequency combs, emphasizing the modulation instability state, which is known for its inherent chaotic behaviors. Our study confirms that the synchronization of two such combs is feasible by injecting the output from the lead microresonator into the next microresonator’s input. We also identify the optimal parameters for this synchronization. Remarkably, even partial injection from the leader is sufficient for synchronization, paving the way for versatile future system configurations. Such systems could simultaneously utilize distinct spectral components for synchronization and transmission. This work advances our understanding of chaotic microresonator combs, showing them to be pivotal elements in next-generation optical communication systems.

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Mechanical Actuation of Kerr Soliton Microcombs in Ultrahigh-Q Crystalline Microresonators

Fujii S., Wada K., Kogure S., Kumazaki H., Tanabe T.

16th Pacific Rim Conference on Lasers and Electro-Optics, CLEO-PR 2024  2024

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We demonstrate a mechanically actuated soliton microcomb with a microwave repetition rate of 15 GHz in ultrahigh-Q crystalline microresonators. This enables direct initiation, subsequent long-term stabilization, and precise tuning of soliton combs.

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Fabrication of silicon nitride based high-Q microring resonators prepared by the hot-wire CVD method and their applications to frequency comb generation

Sota S., Handa K., Fujii S., Tanabe T., Uzawa Y., Furusawa K., Sekine N.

Optical Materials Express 14 ( 5 ) 1128 - 1138 2024

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Fabrication of silicon nitride (SiN) based high-Q microring resonators prepared by the hot-wire chemical vapor deposition (HWCVD) method is presented. By the virtue of low-stress HWCVD films, no special precautions against crack propagation were required for high confinement waveguide device fabrication. By using an additional annealing process, the intrinsic Q factor in excess of 5 × 105 was obtained in the telecommunication C band, and which allowed us to observe frequency comb generation. We also investigated into the anneal temperature dependence of the residual hydrogen concentration in the film as well as the optical properties of the microring resonators.

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Dispersion Engineering of Crystalline Microresonators for Optical Frequency Comb Generation beyond C-Band

Takabayashi R., Kodama H., Tanikawa K., Kakinuma Y., Tanabe T., Fujii S.

16th Pacific Rim Conference on Lasers and Electro-Optics, CLEO-PR 2024  2024

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We detail a dispersion engineering strategy for microresonators allowing us to generate microcombs in the near-infrared and mid-infrared regimes, and identifying optimal shapes for achieving anomalous dispersion suited to different wavelength regimes.

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