On the dispersion management of fluorite whispering-gallery mode resonators for Kerr optical frequency comb generation in the telecom and mid-infrared range

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.

Subharmonic instabilities in Kerr microcombs

We report experimental observation of subharmonic mode excitation in primary Kerr optical frequency combs generated using crystalline whispering-gallery mode resonators. We show that the subcombs can be controlled and span a single or multiple free spectral ranges around the primary comb modes. In the spatial domain, the resulting multiscale combs correspond to an amplitude modulation of intracavity roll patterns. We perform a theoretical analysis based on eigenvalue decomposition that evidences the mechanism leading to the excitation of these combs.

Mid-infrared optical parametric oscillation spanning 3.4-8.2μm in a MgF2microresonator

Mid-infrared optical parametric oscillators (OPOs) offer a compelling route for accessing the 'molecular fingerprint' region and, thus, can find intensive applications such as precision spectroscopy and trace gas detection. Yet it still remains rather a challenge to realize broadband mid-infrared OPOs within a single cavity, usually limited by strict phase-matching conditions for wide spectral coverage and available pump power for adequate frequency generation. Here, we report the mid-infrared parametric oscillation spanning from 3.4 to 8.2μm, based on four-wave mixing in a high-QMgF₂microresonator with optimized dispersion. The center wavelength at 4.78μm is determined by the continuous tunable quantum cascade laser source, which contributes to effective expansion towards longer wavelength, as well as systemic miniaturization with smaller pump module. Such results could not only shed light on new ultimates of crystal and other microresonators, but also inspire explorations on their growing potentials in near future.

Wall-thickness-controlled microbubble fabrication for WGM-based application

We present a wall-thickness-controlled microbubble fabrication model for whispering-gallery-mode (WGM)-based application. The process of fabricating the model is divided into three sequenced steps: geometry size change of the microcapillary during drawing, expanding the process under internal injection air pressure, and microcapillary waist swell into a microbubble. Experiments were carried out to verify the effectiveness of the model. Experiment results show that wall thickness can reach 1.28 µm-1.46 µm at different injection pressure ranges of 50 kPa. The expected wall thickness of the microbubble can be achieved by changing injection pressure while keeping the diameter, which helps to prepare the required microbubble for practical application.

On the transition to secondary Kerr combs in whispering-gallery mode resonators

We demonstrate that extended dissipative structures in Kerr-nonlinear whispering-gallery mode resonators undergo a spatiotemporal instability, as the pumping parameters are varied. We show that the dynamics of the patterns beyond this bifurcation yield specific Kerr comb and sub-comb spectra that can be subjected to a phase of frequency-locking when optimal conditions are met. Our numerical results are found to be in agreement with experimental measurements.

Dispersion engineering of a microsphere via multi-layer coating

Controlling dispersion of a whispering gallery mode resonator is of critical importance for many nonlinear applications, such as frequency comb generation, parametric oscillators, Raman lasers, stimulated Brillouin lasers, and ultrafast optics. Here, we show by numerical and theoretical modeling that dispersion can be strongly engineered in a three-layer-coated microsphere of high, low, and high refractive indices (RIs). We investigate the impact of the coating thickness, the gap between the two high-RI layers, the surrounding medium, and the coating materials on the group-velocity dispersion and discover that the dispersion is controllable over a broad range in both normal and anomalous dispersion regimes. Our approach provides dispersion engineering flexibility in any axisymmetric resonator with a three-layer-coating structure.

Optimization of primary Kerr optical frequency combs for tunable microwave generation

We analyze the condition under which Kerr combs generate the highest microwave output power after photodetection. These optimal comb states correspond to configurations in which the sidemode-to-pump ratio is the highest possible. For the case of primary combs, we show how the interplay between the power and frequency of the pump laser critically influences this ratio, which has a direct influence on the phase noise performance of the generated microwaves. We also experimentally demonstrate primary combs with a sidemode-to-pump ratio as high as -2  dB, thereby leading to efficient energy conversion from the lightwave to the microwave frequency range.

Third-harmonic blue light generation from Kerr clustered combs and dispersive waves

We demonstrated the deterministic generation of blue light emission (438 nm) via the third-harmonic process from an infrared pump by carefully engineering the dispersion of a high-quality-factor whispering gallery mode microcavity. We present two different approaches to obtaining broad bandwidth light. One is based on a clustered comb and the other employs a dispersive wave, and a broad Kerr comb spanning a half-octave is obtained. This allowed frequency conversion over a broad bandwidth ranging from 438 to 612 nm. This approach will enable the development of micro-scale light sources and frequency converters for future optical processing.

Four-wave mixing parametric oscillation and frequency comb generation at visible wavelengths in a silica microbubble resonator

Frequency comb generation in microresonators at visible wavelengths has found applications in a variety of areas such as metrology, sensing, and imaging. To achieve Kerr combs based on four-wave mixing in a microresonator, dispersion must be in the anomalous regime. In this Letter, we demonstrate dispersion engineering in a microbubble resonator (MBR) fabricated by a two-CO₂ laser beam technique. By decreasing the wall thickness of the MBR to 1.4 μm, the zero dispersion wavelength shifts to values shorter than 764 nm, making phase matching possible around 765 nm. With the optical Q-factor of the MBR modes being greater than 10⁷, four-wave mixing is observed at 765 nm for a pump power of 3 mW. By increasing the pump power, parametric oscillation is achieved, and a frequency comb with 14 comb lines is generated at visible wavelengths.

On the phase noise performance of microwave and millimeter-wave signals generated with versatile Kerr optical frequency combs

We investigate the phase noise performance of micro- and millimeter-wave signals generated using a ultra-high Q whispering gallery mode disk-resonator with Kerr nonlinearity. Our study focuses on the stability of the optical spectra and on the performances of the corresponding microwave and millimeter-wave beat notes in terms of power and phase noise. The blue slope of an optical mode of the resonator, allowing for the generation of optical frequency combs, is accurately explored in order to identify various comb patterns. Each of these patterns is characterized in the optical and radio-frequency domains. Phase noise levels below -100 dBc/Hz at 10 kHz offset have been achieved for beat notes in the radio-frequency spectrum at 12 GHz, 18 GHz, 24 GHz, 30 GHz, and 36 GHz with the same resonator.

Phase-locking transition in Raman combs generated with whispering gallery mode resonators

We investigate the mechanisms leading to phase locking in Raman optical frequency combs generated with ultrahigh Q crystalline whispering gallery mode disk resonators. We show that several regimes can be triggered depending on the pumping conditions, such as single-frequency Raman lasing, multimode operation involving more than one family of cavity eigenmodes, and Kerr-assisted Raman frequency comb generation. The phase locking and coherence of the combs are experimentally monitored through the measurement of beat signal spectra. These phase-locked combs, which feature high coherence and wide spectral spans, are obtained with pump powers in the range of a few tens of mW. In particular, Raman frequency combs with multiple free-spectral range spacings are reported, and the measured beat signal in the microwave domain features a 3 dB linewidth smaller than 50 Hz, thereby indicating phase locking.

Universal nonlinear scattering in ultra-high Q whispering gallery-mode resonators

Universal nonlinear scattering processes such as Brillouin, Raman, and Kerr effects are fundamental light-matter interactions of particular theoretical and experimental importance. They originate from the interaction of a laser field with an optical medium at the lattice, molecular, and electronic scale, respectively. These nonlinear effects are generally observed and analyzed separately, because they do not often occur concomitantly. In this article, we report the simultaneous excitation of these three fundamental interactions in mm-size ultra-high Q whispering gallery mode resonators under continuous wave pumping. Universal nonlinear scattering is demonstrated in barium fluoride and strontium fluoride, separately. We further propose a unified theory based on a spatiotemporal formalism for the understanding of this phenomenology.

Properties of fluoride microresonators for mid-IR applications

We study crystalline fluoride microresonators for mid-infrared (mid-IR) applications. Whispering gallery mode resonators were fabricated with BaF₂, CaF₂, and MgF₂ crystals. The quality factors were measured at wavelengths of 1.56 and 4.58 μm. The impacts of post-fabrication processing, impurities, and surface water are highlighted. The mid-IR optical losses due to multiphoton absorption are measured. It is found that MgF₂ resonators have a room temperature Q-factor of 8.3×10⁶ at a wavelength of 4.58 μm, limited by multiphoton absorption.

Dispersion analysis of whispering gallery mode microbubble resonators

This paper examines the opportunities existing for engineering dispersion in non-silica whispering gallery mode microbubble resonators, for applications such as optical frequency comb generation. More specifically, the zero dispersion wavelength is analyzed as a function of microbubble diameter and wall thickness for several different material groups such as highly-nonlinear soft glasses, polymers and crystalline materials. The zero dispersion wavelength is shown to be highly-tunable by changing the thickness of the shell. Using certain materials it is shown that dispersion equalization can be realized at interesting wavelengths such as deep within the visible or mid-infrared, opening up new possibilities for optical frequency comb generation. This study represents the first extensive analysis of the prospects of using non-silica microbubbles for nonlinear optics.

Degenerate four-wave mixing in a silica hollow bottle-like microresonator

A hollow, bottle-like microresonator (BLMR) was fabricated from a microcapillary with a nearly parabolic profile. From simulations at 1.55 μm the fundamental bottle mode is shown to be in the anomalous dispersion regime, while the conventional whispering gallery mode, confined to the center of the BLMR, is in the normal dispersion regime. Therefore, we have experimentally shown that, for a BLMR with a diameter of 102 um, degenerate four-wave mixing can only be observed by judicious selection of the tapered fiber coupling position. Dispersion tuning in such a system is also briefly discussed theoretically. BLMRs are promising devices for the implementation of sparsely distributed, widely spanned frequency combs at the telecommunications C-band.

Broadband mid-infrared frequency comb generation in a Si(3)N(4) microresonator

We demonstrate broadband frequency comb generation in the mid-infrared (MIR) from 2.3 to 3.5 μm in a Si(3)N(4) microresonator. We engineer the dispersion of the structure in the MIR using a Sellmeier equation we derive from experimental measurements performed on Si(3)N(4) films from the UV to the IR. We use deposition-anneal cycling to decrease absorption losses due to vibrational transitions in the MIR and achieve a Q-factor of 1.0×10(6). To our knowledge, this is the highest Q reported in this wavelength range for any on-chip resonator.