Papers - Tanabe, Takasumi

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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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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Demonstration of Nighttime Thermoelectric Power Generation with Radiative Cooling Utilizing Low-Cost Commercial Materials

Tanto Y., Yomoda A., Kokubu J., Sugano R., Tanabe T.

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

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We conducted controlled experiments on nighttime thermoelectric generation using cost-effective, commercially available materials under various weather conditions, both with and without a blackbody filter for radiative cooling, achieving efficiencies of up to 53.3 mW/m2,.

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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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Vicinity of exceptional point-induced mode-locking in coupled microresonators

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

CLEO: Science and Innovations, CLEO: S and I 2024 in Proceedings CLEO 2024, Part of Conference on Lasers and Electro-Optics  2024

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A coupled-cavity system with gain and loss enables mode-locking without natural saturable absorbers. Our theoretical and numerical findings demonstrate significant modulation in system Q near the exceptional point (EP), even with minor refractive index changes and minimal Kerr effect involvement.

Transmission in the 300 Ghz Band Using a Soliton Comb

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

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

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We generated terahertz waves at 300 GHz using a soliton comb, focusing on characteristics such as low phase noise, and successfully demonstrated an error-free 10 Gbit/s transmission with a bit-error rate below 10-9.

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Towards Deterministic Perfect Soliton Crystals in Microtoroids with Saturable Absorption

Matsuyama H., Takano A., Imamura R., Wang H., Fujii S., Tanabe T.

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

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We utilized silica microtoroid resonators to achieve high repetition rate mode-locking and perfect soliton crystals. We then transfer graphene onto microtoroids to enable saturable absorption and study the impacts on its properties, including the Q value.

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Study on Chaos Synchronization of Cascaded Microresonator Optical Frequency Combs

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

CLEO: Science and Innovations, CLEO: S and I 2024 in Proceedings CLEO 2024, Part of Conference on Lasers and Electro-Optics  2024

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By numerically investigating the synchronization of cascaded microresonator frequency combs, we find the optimal transmittance parameters and discover that partial injection from the leader is sufficient. We also present preliminary experimental results.

Soliton Comb Generation Using a Small Prism Coupling Module

Mototani Y., Fujii S., Kimura S., Hashimoto Y., Yamakawa T., Imamura R., Tanabe T.

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

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We developed a compact prism coupling module conforming to the 1U CubeSat standard and demonstrated soliton microcomb generation with an ultrahigh-Q MgF resonator to explore the robustness of the miniaturized multi-wavelength light source.

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Small-error transmission and filter operation at 300 GHz using dissipative Kerr soliton frequency comb

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

CLEO: Science and Innovations, CLEO: S and I 2024 in Proceedings CLEO 2024, Part of Conference on Lasers and Electro-Optics  2024

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Using a soliton microcomb, we generated 300 GHz waves and demonstrated a transmission experiment at 10 Gbit/s with an ultra-low bit-error rate of 10-9. We also demonstrated a photonic filter in the 300 GHz region, which adds another functionality in the terahertz regime.

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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Versatile tuning of Kerr soliton microcombs in crystalline microresonators

Fujii S., Wada K., Sugano R., Kumazaki H., Kogure S., Kato Y.K., Tanabe T.

Communications Physics 6 ( 1 )  2023.12

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Microresonator-based optical frequency combs emitted from high-quality-factor microresonators, also known as microcombs, have opened up new horizons to areas of optical frequency comb technology including frequency metrology, precision sensing, and optical communication. To extend the capability of microcombs for such applications, large and reliable tunability is of critical importance. Here, we show broad spectral tuning of Kerr soliton microcombs in a thermally controlled crystalline microresonator with pump-detuning stabilization. The fundamental elements composing frequency combs, namely the center frequency, repetition frequency, and carrier-envelope offset frequency, are spectrally tuned by up to −48.8 GHz, −5.85 MHz, and −386 MHz, respectively, leveraging thermal effects in ultrahigh-Q crystalline magnesium fluoride resonators. We further demonstrate a 3.4-fold enhancement of soliton comb power resulting from thermal expansion with a temperature change of only 28 K by employing quantitative analyses of the fiber-to-resonator coupling efficiency.

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Stability and mutual coherence of Raman combs in high-Q silica microresonators

Sugawara S., Fujii S., Kawanishi S., Tanabe T.

Optics Continuum 2 ( 7 ) 1588 - 1596 2023.06

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We investigated the stability and mutual coherence of a Raman microcomb in a silica microrod resonator by monitoring the output power and longitudinal mode spacings. The results indicate that we can obtain a stable Raman comb formation without the need for four-wave mixing processes. The use of a Raman comb will open the possibility of simplifying the setup because it will relax the phase matching condition usually required for microresonator frequency comb generation. Although there are some restrictions in regard to using a Raman comb for applications due to the coexistence of the comb components in different mode families, a proof-of-concept demonstration shows that it is sufficiently stable and robust for applications such as optical communications.

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Assessment of the Feasibility of Employing Chaos Synchronization in Two Cascaded Microresonators for Secure Data Transmission

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

2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023  2023

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As the limits of computational power continue to increase, concern has arisen regarding insufficient security along with a demand for improvements in encryption methods for data transmission, which might be achieved by chaos synchronization [1, 2]. Using modified Lugiato-Levefer equations (LLEs), we numerically explore two cascaded microresonator frequency combs [3]. To obtain a chaotic state, both cavities are pumped by continuous-wave lasers until modulation instability (MI) combs are generated. By injecting the output of the first resonator into the second resonator, both chaotic fields in the cavities are synchronized. In this work, we investigate to what extent the chaos is synchronized and how this phenomenon may be implemented in existing data transmission systems over a chaotic channel to enable secure communication.

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A small spectrometer with extremely high resolution (0.07 nm) realized with an improved reconstruction algorithm

Tanabe T., Kokubu J., Sugano R.

International Conference on Metamaterials, Photonic Crystals and Plasmonics    291 - 292 2023

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We successfully reconstruct the spectrum of a multi-wavelength input by using the inverse matrix method for data obtained with a random chirped photonic crystal waveguide.

Field Demonstration of Low-Latency Massively Parallel Communication with Microresonator Frequency Comb

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

2023 Conference on Lasers and Electro-Optics, CLEO 2023  2023

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We demonstrated low-latency optical transmission with a microresonator frequency comb using a 9-km optical fiber in a metropolitan area. The delay time of the entire system, excluding the transmission fiber, was only 3.1 us.

Synchronization of microresonator frequency combs in chaotic regime

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

Proceedings of SPIE - The International Society for Optical Engineering 12407 2023

ISSN  0277786X

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We numerically study the chaotic synchronization of microresonator frequency combs. The chaotic state of microresonators could be a key factor in optical communications because the modulation instability state (chaotic) has a larger output than the soliton state (stable), which may enable us to realize a higher signal-to-noise ratio. In addition, it will allow secure communication. We show that two microresonator frequency combs in a leader-follower configuration can be synchronized in chaotic regimes. Interestingly, the follower comb synchronizes even when some longitudinal modes of the leader comb are absent. We also show that the Turing pattern comb in the follower ring becomes chaotic and synchronizes when we inject the leader's modulation instability comb.

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