Papers - Tanabe, Takasumi

Small Spectrometer with Extremely High-resolution (0.07 nm) Realized with an Improved Reconstruction Algorithm

Kokubu J., Sugano R., Ohtsuka M., Yokoyama N., Okano M., Tanabe T.

CLEO: Science and Innovations, CLEO:S and I 2023  2023

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Spectra are reconstructed by capturing wavelength-specific patterns of localized light in a photonic-crystal-waveguide. We show that real-time reconstruction at a wavelength resolution of 0.07 nm is possible by using low-rank approximation with singular-value-decomposition and regularized-least-squares.

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Small Spectrometer with Extremely High-resolution (0.07 nm) Realized with an Improved Reconstruction Algorithm

Kokubu J., Sugano R., Ohtsuka M., Yokoyama N., Okano M., Tanabe T.

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

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Spectra are reconstructed by capturing wavelength-specific patterns of localized light in a photonic-crystal-waveguide. We show that real-time reconstruction at a wavelength resolution of 0.07 nm is possible by using low-rank approximation with singular-value-decomposition and regularized-least-squares.

Passive mode-locking without a saturable absorber using a coupled microresonator

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

CLEO: Fundamental Science, CLEO:FS 2023  2023

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Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

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Passive mode-locking without a saturable absorber using a coupled microresonator

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

CLEO: Applications and Technology, CLEO:A and T 2023  2023

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Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

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Passive mode-locking without a saturable absorber using a coupled microresonator

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

CLEO: Science and Innovations, CLEO:S and I 2023  2023

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Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

• Access to Document (DOI)

Passive mode-locking without a saturable absorber using a coupled microresonator

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

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

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Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

Mode-locking System of Coupled Microcavities with Gain and Nonlinear Loss

Imamura R., Fujii S., Nakashima 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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Ultrahigh repetition rate lasers are vital light sources, and a mode-locked laser with whispering-gallery-mode (WGM) microresonators would provide a cost-effective on-chip solution. We have already reported numerically on the mode-locking in an erbium (Er) ion-doped resonator that was coated with polymer-dispersed carbon nanotubes (CNTs) [1]. Although mode-locking is possible, the required Q is still high, so the experiment remains challenging. In this work, we show that a coupled microresonator system with a gain resonator coupled to a nonlinear loss resonator will solve this problem. It is based on the idea of employing the Q revival of the gain cavity when the loss is increased at the cavity with additional loss, which is known to exist in the vicinity of the exceptional point [2].

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Field Demonstration of Multi-Wavelength Optical Transmission with Microresonator Frequency Combs

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

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

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Data traffic has been increasing with the spread of the internet. Therefore, a network that connects data centers and metropolitan areas requires a high-capacity, low-latency, and low-power-consumption optical communication system [1]. Since microresonator-based optical frequency combs (i.e., microcombs) have many longitudinal modes, they are expected to be applied to wavelength division multiplexing (WDM) communications [2,3]. We describe a field demonstration of a multi-wavelength optical transmission with a soliton microcomb. We employed intensity modulation and direct detection (IM-DD) for a simple and low-latency communication system [3]. We also used a magnesium fluoride (MgF2) microresonator because it has a small free spectral range (FSR) for efficient bandwidth use. Although sophisticated experiments have already been reported in the laboratory, we believe that our demonstration using a commercially installed optical fiber in a metropolitan area is a significant step towards the practical use of a microresonator system.

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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.

CLEO: Science and Innovations, CLEO:S and I 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 μs.

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Carbon Nanotubes Coupled with Silica Toroid Microcavities as Emitters for Silicon-Integrated Photonics

Tokunaga R., Kinoshita K., Imamura R., Nagashima K., Imafuku R., Nakagawa K., Tanabe T., Maki H.

ACS Applied Nano Materials 5 ( 10 ) 14328 - 14335 2022.10

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A light source based on single-walled carbon nanotubes (SWNTs) is one of the promising candidates for a microsized light source on a silicon chip at telecommunication wavelengths in optical communications and optical interconnects. However, SWNT-based light emitters possess the disadvantage of having a very broad emission spectrum. Here, we present an ultranarrow-linewidth photoluminescence (PL) emitter based on a silica toroid resonator, along with SWNTs, on a silicon chip. We simultaneously managed both excitation and emission lights at telecommunication wavelengths on a silicon chip by employing a very simple in-line configuration consisting of a toroid resonator and a tapered fiber for light input and output. Owing to the extremely high Q factor of our silica toroid resonator, we obtained an ultrahigh Q factor (∼2.1 × 104) of C-band PL emission. We also demonstrated strong PL emission under laterally polarized excitation conditions owing to the strong coupling to the toroid resonator, and laterally polarized PL emission can be selectively generated independently of the excitation polarization direction. This SWNT-based PL emitter based on a simple system with a silica toroid resonator can open routes to highly integrated photonics and optoelectronics on silicon-based platforms.

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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 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 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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Theoretical approach on the critical depth of cut of single crystal MgF<inf>2</inf> and application to a microcavity

Hayama Y., Fujii S., Tanabe T., Kakinuma Y.

Precision Engineering 73   234 - 243 2022.01

ISSN  01416359

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Optical microcavity, which can localize light at a certain spot for a short period of time, has a wide range of applications, such as optical signal processing and optical frequency combs. Single crystal CaF2 has excellent optical properties and many research on the manufacturing process to create a microcavity has been done. However, due to its unstable thermal conditions, stable oscillation of Kerr comb could not be done. Single crystal MgF2, which also has excellent optical properties, has stable thermal conditions suitable to create a microcavity. Like CaF2, MgF2 microcavity can only be manufactured by ultra-precision cylindrical turning, yet its appropriate processing conditions is not known. In this study, a method to derive the critical depth of cut theoretically in ductile mode machining was proposed in consideration of specific cutting energy, mechanical properties and slip system of workpiece, and tool geometry. The critical depth of cut calculated by the proposed method roughly agreed with the experimental value on the MgF2 plane. Unlike CaF2, MgF2 has an optical anisotropy; therefore, in order to create a microcavity where the refractive angle is constant, the resonance part needs to be made on a cylinder with (001) plane as end face. Therefore, a microcavity was fabricated according to the critical depth of cut on a cylinder with (001) as end face derived from the proposed method, and the surface integrity was evaluated from the viewpoint of microcavity performance.

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Deterministic Generation of Perfect Soliton Crystal Assisted by Saturable Absorption

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

2022 Conference on Lasers and Electro-Optics, CLEO 2022 - Proceedings  2022

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We found that we can deterministically generate a perfect soliton crystal by incorporating a saturable absorber in a microresonator. The crystal is generated directly from the initial Turing rolls without passing through chaos.

Deterministic Generation of Perfect Soliton Crystal Assisted by Saturable Absorption

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

Optics InfoBase Conference Papers  2022

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We found that we can deterministically generate a perfect soliton crystal by incorporating a saturable absorber in a microresonator. The crystal is generated directly from the initial Turing rolls without passing through chaos.

Active Thermal Tuning of a Soliton Frequency Comb in a Crystalline Optical Microresonator

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

Optics InfoBase Conference Papers  2022

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We report the thermal tuning of a Kerr soliton frequency comb in an ultrahigh-Q magnesium fluoride crystalline microresonator yielding broad pump frequency tuning of up to ~ 50 GHz and a repetition rate of ~ 6 MHz.

Active Thermal Tuning of a Soliton Frequency Comb in a Crystalline Optical Microresonator

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

2022 Conference on Lasers and Electro-Optics, CLEO 2022 - Proceedings  2022

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We report the thermal tuning of a Kerr soliton frequency comb in an ultrahigh-Q magnesium fluoride crystalline microresonator yielding broad pump frequency tuning of up to ~ 50 GHz and a repetition rate of ~ 6 MHz.

Effect of detuning on noise characteristics in a microcomb-based light source

Kogure S., Fujii S., Kumazaki H., Sota S., Hashimoto Y., Kobayashi Y., Araki T., Tanabe T.

2022 Conference on Lasers and Electro-Optics Pacific Rim, CLEO-PR 2022 - Proceedings  2022

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We investigated the pump effective detuning dependence of noise and linewidth characteristics of modulation instability combs. Modulation instability combs are useful for applications if the detuning is smaller than several times the resonace linewidth.

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Stability and mutual coherence measurement of a Raman microcomb in a silica WGM microresonator

Sugawara S., Fujii S., Sota S., Tanabe T.

Optics InfoBase Conference Papers  2022

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We investigated the stability, longitudinal spacing, and mutual coherence of a generated Raman microcomb and found that we can obtain a highly stable and coherent Raman microcomb even without a four-wave mixing process.

Stability and mutual coherence measurement of a Raman microcomb in a silica WGM microresonator

Sugawara S., Fujii S., Sota S., Tanabe T.

2022 Conference on Lasers and Electro-Optics, CLEO 2022 - Proceedings  2022

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We investigated the stability, longitudinal spacing, and mutual coherence of a generated Raman microcomb and found that we can obtain a highly stable and coherent Raman microcomb even without a four-wave mixing process.