Details of a Researcher

Fujii, Shun

写真a

Affiliation: Faculty of Science and Technology, Department of Physics （Yagami）

Position: Research Associate/Assistant Professor/Instructor

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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 （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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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 （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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Quantization of Mode Shifts in Nanocavities Integrated with Atomically Thin Sheets

Fang N., Yamashita D., Fujii S., Otsuka K., Taniguchi T., Watanabe K., Nagashio K., Kato Y.K.

Advanced Optical Materials （Advanced Optical Materials) 10 （ 19 ） 2022.10

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The unique optical properties of 2D layered materials are attractive for achieving increased functionality in integrated photonics. Owing to the van der Waals nature, these materials are ideal for integrating with nanoscale photonic structures. Here a carefully designed air-mode silicon photonic crystal nanobeam cavity for efficient control through 2D materials is reported. By systematically investigating various types and thicknesses of 2D materials, the authors are able to show that enhanced responsivity allows for giant shifts of the resonant wavelength. With atomically precise thickness over a macroscopic area, few-layer flakes give rise to quantization of the mode shifts. The dielectric constant of the flakes is extracted and found to be independent of the layer number down to a monolayer. Flexible reconfiguration of a cavity is demonstrated by stacking and removing ultrathin flakes. With an unconventional cavity design, these results open up new possibilities for photonic devices integrated with 2D materials.
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Deterministic generation of a perfect soliton crystal microcomb with a saturable absorber

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

Optics Letters （Optics Letters) 47 （ 6 ） 1458 - 1461 2022.03

ISSN 01469592

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We numerically investigate the deterministic generation of a perfect soliton crystal (PSC) in an optical microresonator functionalized with a saturable absorber (SA). The SAallows the direct formation of a PSC from an initial, periodic Turing roll. It prevents passage through a chaotic state, which induces a stochastic nature with regard to the number of generated dissipativeKerr solitons.We showthat PSCs form deterministically, and the number is controlled by adjusting the input power and SA parameter. Our work provides a simple approach for obtaining a stable PSC that offers an ultrahigh repetition rate and a high comb output power.
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Dissipative Kerr soliton microcombs for FEC-free optical communications over 100 channels

Fujii S., Tanaka S., Ohtsuka T., Kogure S., Wada K., Kumazaki H., Tasaka S., Hashimoto Y., Kobayashi Y., Araki T., Furusawa K., Sekine N., Kawanishi S., Tanabe T.

Optics Express （Optics Express) 30 （ 2 ） 1351 - 1364 2022.01

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The demand for high-speed and highly efficient optical communication techniques has been rapidly growing due to the ever-increasing volume of data traffic. As well as the digital coherent communication used for core and metro networks, intensity modulation and direct detection (IM-DD) are still promising schemes in intra/inter data centers thanks to their low latency, high reliability, and good cost performance. In this work, we study a microresonator-based frequency comb as a potential light source for future IM-DD optical systems where applications may include replacing individual stabilized lasers with a continuous laser driven microresonator. Regarding comb line powers and spectral intervals, we compare a modulation instability comb and a soliton microcomb and provide a quantitative analysis with regard to telecom applications. Our experimental demonstration achieved a forward error correction (FEC) free operation of bit-error rate (BER) <10−9 with a 1.45 Tbps capacity using a total of 145 lines over the entire C-band and revealed the possibility of soliton microcomb-based ultra-dense wavelength division multiplexing (WDM) with a simple, cost-effective IM-DD scheme, with a view to future practical use in data centers.
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