Tomoya Kobatake

Dispersion tailoring of a crystalline whispering gallery mode microcavity for a wide-spanning optical Kerr frequency comb

We fabricated a crystalline whispering gallery mode microcavity by using a computer-controlled ultraprecision cutting process to control the cavity cross section. We used a numerical simulation to show that a wide-spanning optical Kerr frequency comb is generated by tailoring the dispersion of a crystalline whispering gallery mode microcavity. To control the dispersion, we designed the cross-sectional shape of the device and fabricated it by using ultraprecision cutting. Both the measured value and the numerical result show that the microcavity has an anomalous dispersion over one octave.

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.

Thermal Effects on Kerr Comb Generation in a CaF 2 Whispering-Gallery Mode Microcavity

We numerically investigate the dynamics of Kerr comb generation in a calcium fluoride whispering gallery mode (WGM) microcavity taking thermal effects into account. Soliton pulse generation is investigated in the presence of thermooptic (TO) and thermal expansion (TE) effects and particularly when the TO effect is negative. We found that, with optimized cavity parameters, self-induced cavity resonance shifts resulting from a negative TO effect allow the system to generate a soliton pulse with no wavelength scanning or dynamic power optimization, which are usually needed to achieve a soliton pulse with silica and other WGM microcavity systems. A negative TO effect automatically tunes the cavity resonance so that the cavity will achieve a soliton state. The competition between the TO and TE effects is also investigated in detail.

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.

Hysteresis behavior of Kerr frequency comb generation in a high-quality-factor whispering-gallery-mode microcavity

A numerical and experimental study of Kerr frequency comb generation in a silica toroid microcavity is presented. We use a generalized mean-field Lugiato–Lefever equation and solve it with the split-step Fourier method. We observe that a stable mode-locked regime can be accessed when we reduce the input power after strong pumping due to the bistable nature of the nonlinear cavity system used. The experimental results agree well with the results of the numerical analysis, where we obtain a low-noise Kerr comb spectrum by gradually reducing the pumping input after strong pumping. This finding complements the results obtained by a previous wavelength scanning method and clarifies the procedure for achieving mode-locked states in such high-Q microcavity systems.

Suppression of optomechanical parametric oscillation in a toroid microcavity assisted by a Kerr comb

We demonstrate the suppression of cavity optomechanical parametric oscillations with the generation of a Turing pattern comb (Kerr comb) in an anomalous dispersion toroidal microcavity. If the pump light alone influences the optomechanical oscillation, it should be amplified because the strong pump light is blue-detuned from the cavity resonance. However, once the Kerr comb was generated, all the comb lines contributed to the cavity optomechanical behavior and the optomechanical oscillation was suppressed. A calculation taking account of the effective damping rate from all the comb lines is in good agreement with the experimental observation.

Harmonic mode locking in a high- Q whispering gallery mode microcavity

We present a numerical and experimental study of the generation of harmonic mode locking in a silica toroid microcavity. We use a generalized mean-field Lugiato-Lefever equation and solve it with the split-step Fourier method. We found that a stable harmonic mode-locking regime can be accessed when we reduce the input power after strong pumping even if we do not carefully adjust the wavelength detuning. This is due to the bistable nature of the nonlinear cavity system. The experiment agrees well with the numerical analysis, where we obtain a low-noise Kerr comb spectrum with a narrow longitudinal mode spacing by gradually reducing the input pump power after strong pumping. This finding clarifies the procedure for generating harmonic mode locking in such high-Q microcavity systems.

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.

Cavity optomechanical coupling to multiple resonances assisted by kerr comb generation in toroid microcavity

A high quality factor (Q) microcavity is an efficient device for generating a Kerr comb via a four-wave mixing process inside the cavity [1]. Kerr comb platforms are candidates for such applications as optical frequency combs, optical communications, arbitrary waveform generation, and microwave oscillators. Although a silica toroid microcavity (Fig. 1(a)) also has good characteristics for Kerr comb generation (high Q and small mode volume), the problem is that the cavity is prone to optomechanical oscillation, which causes amplitude and phase noises [2, 3]. Here, we study cavity optomechanical couplings between generated Kerr comb lines and multiple resonances with a view to generating a comb with low noise from a toroid microcavity. And we observed a Turing pattern comb [4] whose mechanical oscillation was suppressed with blue detuned pumping and red detuned comb lines that was induced by optomechanical couplings.

The effect of Raman scattering in Kerr comb generation in a silica toroidal microcavity

We investigated the interaction between the stimulated Raman scattering (RS) and four-wave mixing (FWM) occurring during the Kerr comb generation in a silica toroidal microcavity. While both Raman lasing and frequency comb generation have already been demonstrated, the effect of the interaction between these two phenomena is still unclear. Due to the broad Raman gain bandwidth in silica, multiple longitudinal modes are excited from single wavelength pump. Since the multiple modes interact each other via FWM, the behavior becomes complex. Figs. 1(a) and (b) show the experimental results of the cascaded generation of RS light, where we obtained 4th order RS light. As shown Fig. 1(c), two different sets of longitudinal modes were excited in a cavity. To understand this result, we developed a model based on the Lugiato-Lefever to simulate the mode interaction.