Origins of clustered frequency combs in Kerr microresonators

Wideband multimode optical parametric oscillation in a Kerr microresonator

Parametric oscillation in Kerr microresonators provides an attractive pathway for the generation of new optical frequencies in a low-power, small-footprint device. The frequency shift of the newly generated parametric sidebands is set by the phasematching of the underlying four-wave-mixing process, with the generation of large frequency shift sidebands typically placing exacting requirements on a resonator's dispersion profile. In practice, this limits the range of viable pump wavelengths, and ultimately the range of output frequencies. In this paper, we consider a multimode four-wave-mixing process in which the pump and sidebands propagate in different mode families of the resonator. We show that this multimode configuration yields a considerable relaxation in the phasematching requirements needed to generate large frequency shift parametric sidebands, allowing their formation even in resonators with strong second-order dispersion. Experimentally we use a magnesium-fluoride micro-disk resonator to demonstrate this multimode phasematching. By accessing different pump and sideband modes, four distinct multimode parametric processes generating frequency shifts between 118 and 216 THz are reported. The resulting separation between the two sidebands is almost three octaves.

Optical Frequency Combs Generated in Silica Microspheres in the Telecommunication C-, U-, and E-Bands

Optical frequency combs (OFCs) generated in microresonators with whispering gallery modes are demanded for different applications including telecommunications. Extending operating spectral ranges is an important problem for wavelength-division multiplexing systems based on microresonators. We demonstrate experimentally three spectrally separated OFCs in the C-, U-, and E-bands in silica microspheres which, in principle, can be used for telecommunication applications. For qualitative explanation of the OFC generation in the sidebands, we calculated gain coefficients and gain bandwidths for degenerate four-wave mixing (FWM) processes. We also attained a regime when the pump frequency was in the normal dispersion range and only two OFCs were generated. The first OFC was near the pump frequency and the second Raman-assisted OFC with a soliton-like spectrum was in the U-band. Numerical simulation based on the Lugiato–Lefever equation was performed to support this result and demonstrate that the Raman-assisted OFC may be a soliton.

Coupler-induced phase matching of resonant hyperparametric scattering

We show that an evanescent field coupler can break the symmetry of a high quality factor monolithic ring microcavity, enabling generation of strongly nondegenerate frequency harmonics involving a few mode families that are orthogonal in an unperturbed microcavity. Using this property, we explain observed experimental generation of frequency combs in magnesium fluoride whispering gallery mode resonators characterized with strong normal group velocity dispersion.

All-optical thermal control for second-harmonic generation in an integrated microcavity

Nonlinear optical effects in integrated microcavities have been studied extensively with the advantages of strong light-matter interaction, great scalability, and stability due to the small mode volume. However, the pump lasers stimulating nonlinear effects impose obstacles for practical applications, since the material absorption causes thermal resonance drift and instability. Here we experimentally demonstrate an all-optical control of the thermal behavior in optical microcavities for tunable doubly-resonant second-harmonic (SH) generation on an integrated photonic chip. Through an auxiliary control laser, the temperature of a selected microring can be efficiently changed, thus allowing precise frequency tuning of the doubly-resonant wavelength while eliminating the distortion of the lineshape induced by the thermo-optic effect. Although the phase-matching conditions will limit the tuning range of 55GHz, the technique is still potential to achieve a larger tuning range in combination with temperature regulation. Additionally, this approach has the advantage of quick reconfiguration, showing a fast modulation rate up to about 256 kHz. The theoretical model behind our experimental scheme is universal and applicable to other microcavity-enhanced nonlinear optical processes, and our work paves the way for controlling and utilizing the thermal effect in the applications of microcavities.

Widely separated optical Kerr parametric oscillation in AlN microrings

Here, we report $ {\chi ^{(3)}} $χ⁽³⁾-based optical parametric oscillation (OPO) with widely separated signal-idler frequencies from crystalline aluminum nitride microrings pumped at $ 2\,\,\unicode{x00B5}{\rm m} $2µm. By tailoring the width of the microring, OPO reaching toward the telecom and mid-infrared bands with a frequency separation of 64.2 THz is achieved. While dispersion engineering through changing the microring width is capable of shifting the OPO sideband by $ \gt {9}\;{\rm THz}$>9THz, the OPO frequency can also be agilely tuned in the ranges of 1 and 0.1 THz, respectively, by shifting the pump wavelength and controlling the chip's temperature. At high pump powers, the OPO sidebands further evolve into localized frequency comb lines. Such large-frequency-shift OPO with flexible wavelength tunability will lead to enhanced chip-scale light sources.

Octave-wide phase-matched four-wave mixing in dispersion-engineered crystalline microresonators

In this Letter, we report phase-matched four-wave mixing separated by over one octave in a dispersion-engineered crystalline microresonator. Experimental and numerical results presented here confirm that primary sidebands were generated with a frequency shift up to 140 THz, and that secondary sidebands formed a localized comb structure, known as a clustered comb in the vicinity of the primary sidebands. A theoretical analysis of the phase-matching condition validated our experimental observations, and our results agree well with numerical simulations. These results offer the potential to realize a frequency-tunable comb cluster generator operating from 1 μm to mid-infrared wavelengths with a single and compact device.