Shun Fujii

Broad bandwidth third-harmonic generation via four-wave mixing and stimulated Raman scattering in a microcavity

We generate broad bandwidth visible light ranging from 498 to 611 nm via third-harmonic generation in a silica toroid microcavity. The silica toroid microcavity is fed with a continuous-wave input at a telecom wavelength, where third-harmonic generation follows the generation of an infrared Kerr comb via cascaded four-wave-mixing and stimulated Raman scattering effects. Thanks to these cascaded effects (four-wave mixing, stimulated Raman scattering, and third-harmonic generation) in an ultrahigh-Q microcavity, a broad bandwidth visible light is obtained. The visible light couples with the whispering gallery mode of the cavity by demonstrating the evanescent coupling of the generated visible light with a tapered fiber based on an add-drop configuration.

Transverse mode interaction via stimulated Raman scattering comb in a silica microcavity

Comb generation in different mode families via a stimulated Raman scattering (SRS) process is studied using a silica toroid microcavity. The broad gain bandwidth of SRS in silica allows us to excite longitudinal modes at long wavelengths belonging to mode families that are either the same as or different from the pump mode. We found through experiment and numerical analysis, that an SRS comb in a different mode family with a high quality factor (Q) is excited when we pump in a low-Q mode. No transverse mode interaction occurs when we excite in a high-Q mode resulting the generation of a single comb family. We studied the condition of the transverse mode interaction while varying the mode overlap and Q of the Raman mode. Our experimental results are in good agreement with the analysis and this enables us to control the generation of one- and two-mode combs.

Effect on Kerr comb generation in a clockwise and counter-clockwise mode coupled microcavity

We study the impact of inherent mode coupling between clockwise (CW) and counter-clockwise (CCW) modes on Kerr comb generation in a small whispering-gallery mode microcavity. Our numerical analysis using a coupled Lugiato-Lefever equation reveals the range of the coupling strength in which a soliton pulse can be obtained in the CW direction. It also showed that CCW comb power depends on the coupling strength between the CW and CCW modes. In addition to the simulation, we conducted an experiment to confirm that the power ratio between the CW and CCW comb modes depends on the coupling strength, and the experimental results agree well with the simulation results. This study helps us to understand the relationship between CW and CCW mode coupling and Kerr comb generation, and the effect on soliton formation.

Raman comb generation through broadband gain in a silica microresonator

We study the dynamics of Raman comb formation in silica rod microresonators. We generated a Raman comb with a smooth spectral envelope and observed the center wavelength transition by controlling the detuning and coupling strength.

Transition between Kerr comb and stimulated Raman comb in a silica whispering gallery mode microcavity

We theoretically and experimentally investigated the transition between modulation instability and Raman gain in a small silica microcavity with a large free-spectral range, which reveals that we can selectively switch from a four-wave mixing dominant state to a stimulated Raman scattering dominant state. Both the theoretical analysis and the experiment show that a Raman-dominant region is present between transitions of Kerr combs with different free-spectral range spacings. We can obtain a stable Kerr comb and a stable Raman state selectively by changing the driving power, coupling between the cavity and the waveguide, and laser detuning. Such a controllable transition is achieved thanks to the presence of gain competition between modulation instability and Raman gain in silica whispering gallery mode microcavities.

Broadband gain induced Raman comb formation in a silica microresonator

A high-Q silica whispering gallery mode microresonator is an attractive platform on which to demonstrate a broad and phase-locked Raman comb in various wavelength regimes. Raman combs can be used for applications such as compact pulse laser sources, sensors, optical clocks, and coherence tomography. However, the formation dynamics of a Raman comb has not been well exploited. Here we study the dynamics of the Raman comb formation in silica rod microresonators, which have cavity-free spectral ranges in microwave rates. We generated a Raman comb with a smooth spectral envelope and also observed the transition between two Raman combs located at different center wavelengths. The transition behavior was obtained when we changed the pump detuning and the coupling strength between the microresonator and fiber. We also explain these phenomena by using a simple model based on coupled-mode equations.

Analysis of Mode Coupling Assisted Kerr Comb Generation in Normal Dispersion System

By using nonlinear coupled mode equations, we numerically investigated the generation of a Kerr comb in a normal dispersion microcavity system, where mode coupling between two cavity modes is present. In contrast to previous studies, our model is rigorous in which we fully considered the mode coupling between two modes. We investigated the phase matching condition to obtain the suitable parameters needed to form a free-spectral-range (FSR)-selectable comb. Our calculations are in good agreement with existing experimental results, and enable us to obtain a good understanding of the phenomenon; moreover our model also allows us to reveal new phenomena. We investigated 1-FSR comb generation in detail and found that randomly oscillating behavior will appear when detuning is scanned, which has never before been clearly observed. This modeling approach will be a powerful tool that will assist dispersion engineering for Kerr comb generation and the frequency tuning needed to generate a deterministic mode-locked comb.

Interaction of transverse modes via stimulated Raman scattering on comb generation in a silica toroid microcavity

The use of stimulated Raman scattering (SRS) for Kerr comb generation is very attractive because it allows us to generate long-wavelength light, particularly in the mid-infrared wavelength regime, where the wavelengths are useful for sensing applications. Recently, the generation of comb light and the interaction between transverse modes with the SRS process using a crystalline microcavity have been reported [1, 2], but as yet there is no clear understanding of the condition required for transverse mode interaction [3]. Therefore, to understand the influence of the SRS process, we performed an experimental measurement and numerical analysis using a silica toroid microcavity.

Effect of Raman scattering and mode coupling in Kerr comb generation in a silica whispering gallery mode microcavity

We investigated the effect of Raman scattering in multimode whispering gallery mode (WGM) microcavities. First, we discuss the competition between the effects of four-wave mixing (FWM) and stimulated Raman scattering (SRS). Thanks to the different gain bandwidths of FWM and SRS, we can switch between the FWM and SRS dominant states by changing the pump power or by changing the coupling quality factor (Q). Next, we investigated the transverse mode interaction that occurs during SRS comb formation. We found that transverse mode coupling occurred when we pumped in a low-Q mode but a comb with a single-mode family was generated when we pumped in a high-Q mode. This finding will allow us to obtain or suppress a dual comb in a single WGM microcavity. Finally, we demonstrated broad bandwidth visible light generation by third harmonic generation (THG) following the generation of a broadband SRS comb. The generation and good understanding of the SRS comb will offer us various possibilities such as dual comb generation and broad bandwidth visible light generation.

Raman comb formation in silica rod microresonator

We report smooth Raman comb generation with a linewidth of a few kHz despite a complex Raman gain. The center wavelength was controlled depending on the pump detuning.