Kerr optical frequency comb generation in strontium fluoride whispering-gallery mode resonators with billion quality factor

Optofluidic microbubble Fabry-Pérot cavity

An optofluidic microbubble Fabry-Pérot (OMBFP) cavity was investigated. In contrast to plane-plane FP (PPFP) cavities, the optical mode confinement and stability in an OMBFP were significantly enhanced. The optical properties of the OMBFP cavity, including the quality (Q) factor, effective mode area, mode distribution as a function of the core refractive index, microbubble position, and mirror tilt angle, were investigated systematically using the finite element method. In optofluidic lasing experiments, a low lasing threshold of 1.25 µJ/mm², which was one order magnitude lower than that of the PPFP, was achieved owing to improved modal lateral confinement. Since the microbubble acts as a micro-lens and microfluidic channel in the parallel FP cavity, mode selection and cell-dye laser were easily realized in the OMBFP cavity.

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.

On-chip dual-comb source for spectroscopy

Dual-comb spectroscopy is a powerful technique for real-time, broadband optical sampling of molecular spectra, which requires no moving components. Recent developments with microresonator-based platforms have enabled frequency combs at the chip scale. However, the need to precisely match the resonance wavelengths of distinct high quality-factor microcavities has hindered the development of on-chip dual combs. We report the simultaneous generation of two microresonator combs on the same chip from a single laser, drastically reducing experimental complexity. We demonstrate broadband optical spectra spanning 51 THz and low-noise operation of both combs by deterministically tuning into soliton mode-locked states using integrated microheaters, resulting in narrow (<10 kHz) microwave beat notes. We further use one comb as a reference to probe the formation dynamics of the other comb, thus introducing a technique to investigate comb evolution without auxiliary lasers or microwave oscillators. We demonstrate high signal-to-noise ratio absorption spectroscopy spanning 170 nm using the dual-comb source over a 20-μs acquisition time. Our device paves the way for compact and robust spectrometers at nanosecond time scales enabled by large beat-note spacings (>1 GHz).

Comparison of various excitation and detection schemes for dye-doped polymeric whispering gallery mode micro-lasers

We compare different excitation and collection configurations based on free-space optics and evanescently coupled tapered fibers for both lasing and fluorescence emission from dye-doped doped polymeric whispering gallery mode (WGM) micro-disk lasers. The focus of the comparison is on the lasing threshold and efficiency of light collection. With the aid of optical fibers, we localize the pump energy to the cavity-mode volume and reduce the necessary pump energy to achieve lasing by two orders of magnitude. When using fibers for detection, the collection efficiency is enhanced by four orders of magnitude compared to a free-space read-out perpendicular to the resonator plane. By enhancing the collection efficiency we are able to record a pronounced modulation of the dye fluorescence under continuous wave (cw) pumping conditions evoked by coupling to the WGMs. Alternatively to fibers as a collection tool, we present a read-out technique based on the detection of in-plane radiated light. We show that this method is especially beneficial in an aqueous environment as well as for size-reduced micro-lasers where radiation is strongly pronounced. Furthermore, we show that this technique allows for the assignment of transverse electric (TE) and transverse magnetic (TM) polarization to the observed fundamental modes in a water environment by performing polarization-dependent photoluminescence (PL) spectroscopy. We emphasize the importance of the polarization determination for sensing applications and verify expected differences in the bulk refractive index sensitivity for TE and TM WGMs experimentally.

Dependence of quality factor on surface roughness in crystalline whispering-gallery mode resonators

We present an experimental study of the variation of quality factor (Q-factor) of WGM resonators as a function of surface roughness. We consider mm-size whispering-gallery mode resonators manufactured with fluoride crystals, featuring Q-factors of the order of 1 billion at 1550 nm. The experimental procedure consists of repeated polishing steps, after which the surface roughness is evaluated using profilometry by white-light phase-shifting interferometry, while the Q-factors are determined using the cavity-ring-down method. This protocol permits us to establish an explicit curve linking the Q-factor of the disk-resonator to the surface roughness of the rim. We have performed measurements with four different crystals, namely, magnesium, calcium, strontium, and lithium fluoride. We have thereby found that the variations of Q-factor as a function of surface roughness is universal, in the sense that it is globally independent of the bulk material under consideration. We also discuss our experimental results in the light of theoretical estimates of surface scattering Q-factors already published in the literature.

Effect of crystalline family and orientation on stimulated Brillouin scattering in whispering-gallery mode resonators

Ultra-high Q whispering-gallery mode resonators pumped by a continuous-wave laser are known to enhance stimulated Brillouin scattering when optimal resonance and phase-matching conditions are met. In crystalline resonators, this process depends critically on the crystal orientation and family, which impose the elastic constants defining the velocity of the acoustic waves. In this article, we investigate the effect of crystalline orientation and family on this velocity which is proportional to the Brillouin frequency down-shift. In particular, the study is based on the development of a model and numerical simulations of acoustic wave velocities that propagate along the periphery of four fluoride crystals, namely calcium, magnesium, lithium and barium fluoride. We find that depending on the crystal and its orientation, the frequency excursion around the Brillouin offset can vary from few tens of kHz to more than a GHz.

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.

Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials

The unavailability of highly transparent materials in the mid-infrared has been the main limitation in the development of ultra-sensitive molecular sensors or cavity-based spectroscopy applications. Whispering gallery mode microresonators have attained ultra-high-quality (Q) factor resonances in the near-infrared and visible. Here we report ultra-high Q factors in the mid-infrared using polished alkaline earth metal fluoride crystals. Using an uncoated chalcogenide tapered fibre as a high-ideality coupler in the mid-infrared, we study via cavity ringdown technique the losses of BaF₂, CaF₂, MgF₂ and SrF₂ microresonators. We show that MgF₂ is limited by multiphonon absorption by studying the temperature dependence of the Q factor. In contrast, in SrF₂ and BaF₂ the lower multiphonon absorption leads to ultra-high Q factors at 4.5 μm. These values correspond to an optical finesse of , the highest value achieved for any type of mid-infrared resonator to date.

Highly sensitive trace-gas detection is possible in the mid-infrared range with transparent microresonators. Here, the authors directly measure the necessary ultra-high quality factors of microresonators made from fluoride crystal materials using a tapered chalcogenide fibre.

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.

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.