Bottle microresonator broadband and low-repetition-rate frequency comb generator

Agile THz-range spectral multiplication of frequency combs using a multi-wavelength laser

A breakthrough technology, on-chip frequency comb sources offer broadband combs while being compact, energy-efficient, and cost-effective solutions for various applications from lidar to telecommunications. Yet, these sources encounter a fundamental trade-off between controllability and bandwidth: broadband combs, generated in microresonators, lack free-spectral range or spectral envelope control, while combs generated with electro-optic modulators can be carefully tailored but are limited in bandwidth. Here, we overcome this trade-off through agile spectral multiplication of narrowband combs. Exploiting the nonlinear dynamics of a multi-wavelength laser under modulated optical injection, we achieve spectral multiplication at frequency offsets from 26 GHz to 1.3 THz. Moreover, on-chip control allows for nano-second switching of the frequency offset. Compatible with generic platforms, our approach can be scaled up to cover several THz. When combined with THz photomixers, our system could enable low-cost, compact, and power-efficient THz comb sources, paving the way towards a new generation of THz applications.

Frequency comb generation in SNAP fiber resonator based on axial-azimuthal mode interactions

We study nonlinear mode interaction in the cylindrical microresonator with nanoscale radius variation. We obtain a model which describes the interplay between azimuthal modes and investigate the nonlinear dynamics of axial-azimuthal modes. We also define regimes of generation depending on the pump parameters. Using a properly designed radius longitudinal profile, we obtain broadband generation of stable low-repetition-rate frequency combs based on axial-azimuthal modes.

Microresonator devices lithographically introduced at the optical fiber surface

We present a simple lithographic method for fabrication of microresonator devices at the optical fiber surface. First, we undress the predetermined surface areas of a fiber segment from the polymer coating with a focused CO₂ laser beam. Next, using the remaining coating as a mask, we etch the fiber in a hydrofluoric acid solution. Finally, we completely undress the fiber segment from coating to create a chain of silica bottle microresonators with nanoscale radius variation [surface nanoscale axial photonics (SNAP) microresonators]. We demonstrate the developed method by fabrication of a chain of five 1 mm long and 30 nm high microresonators at the surface of a 125 µm diameter optical fiber and a single 0.5 mm long and 291 nm high microresonator at the surface of a 38 µm diameter fiber. As another application, we fabricate a rectangular 5 mm long SNAP microresonator at the surface of a 38 µm diameter fiber and investigate its performance as a miniature delay line. The propagation of a 100 ps pulse with 1 ns delay, 0.035c velocity, and negligible dispersion is demonstrated. In contrast to previously developed approaches in SNAP technology, the developed method allows the introduction of much larger fiber radius variation ranging from nanoscale to microscale.

SNAP structures fabricated by profile design of in-fiber inscribed regions with a femtosecond laser

Fabricating a surface nanoscale axial photonics (SNAP) microresonator with a specific profile is a challenging and important issue since its advent. We propose a powerful approach for the flexible fabrication of the SNAP structures with arbitrary profiles by a femtosecond laser. Our method is to design the profile of the length distribution of the inscribed lines to match the profile of the required SNAP microresonator, and to combine it with other fabrication parameters to precisely control the radius variation of the SNAP structure. In experiments, we demonstrate the design and fabrication of the SNAP structures with the parabolic, semi-parabolic, and bat profiles. The developed approach is expected to be universal for the fabrication of complex high Q-factor SNAP structures, which lays the groundwork for exploring the versatile performances of the SNAP devices.

Reflection of whispering gallery modes propagating on a surface of an optical fiber from its cleave

We experimentally and numerically study the dynamics of whispering gallery modes slowly propagating within the cladding of an optical fiber near its end facet. We demonstrated that modes reflect from the fiber cleave. The reflection coefficient appears to reach 70% for the sample under study. Use of a facet provides a simple method to confine the axial propagation of the modes.

Discovery of parabolic microresonators produced via fiber tapering

We demonstrate a new method for the creation of surface nanoscale axial photonics (SNAP) microresonators with harmonic profiles via fiber tapering in a laser-heated microfurnace. This simple procedure makes microresonators that support hundreds of axial modes with good spacing uniformity, yielding a promising prospective method for fabricating miniature frequency comb generators and dispersionless delay lines.

Frequency comb generation in a resonantly pumped exciton-polariton microring resonator

We develop a scheme for generation of a regular sequence of narrow spectral lines (optical frequency comb) in semiconductor micro-ring resonators operating in the strong-coupling regime. A strong optical nonlinearity of exciton-polaritons, forming as mixed states between the microcavity photons and quantum-well excitons, allows for a low-threshold operation. This work demonstrates visibility of using the exciton-polaritons for the purposes of generation of GHz combs and trains of picoseconds pulses for future all-polariton information processing schemes.

Tunable SNAP microresonators via internal ohmic heating

We demonstrate a thermally tunable surface nanoscale axial photonics (SNAP) platform. Stable tuning is achieved by heating a SNAP structure fabricated on the surface of a silica capillary with a metal wire positioned inside. Heating a SNAP microresonator with a uniform wire introduces uniform variation of its effective radius which results in constant shift of its resonance wavelengths. Heating with a nonuniform wire allows local nanoscale variation of the capillary effective radius, which enables differential tuning of the spectrum of SNAP structures, as well as the creation of temporary SNAP microresonators that exist only when current is applied. As an example, we fabricate two bottle microresonators coupled to each other and demonstrate differential tuning of their resonance wavelengths into and out of degeneracy with precision better than 0.2 pm. The developed approach is beneficial for ultra-precise fabrication of tunable ultralow loss parity-time symmetric, optomechanical, and cavity quantum electrodynamics (QED) devices.

Two-dimensional nonlinear modes and frequency combs in bottle microresonators

We theoretically investigate frequency comb generation in a bottle microresonator accounting for the azimuthal and axial degrees of freedom. We first identify a discrete set of the axial nonlinear modes of a bottle microresonator that appear as tilted resonances bifurcating from the spectrum of linear axial modes. We then study azimuthal modulational instability of these modes and show that families of two-dimensional (2D) soliton states localized both azimuthally and axially bifurcate from them at critical pump frequencies. Depending on detuning, 2D solitons can be stable, form persistent breathers or chaotic spatio-temporal patterns, or exhibit collapse-like evolution.

Frequency comb generation in SNAP bottle resonators

We develop a theory of optical frequency comb generation in ultra-compact surface nanoscale axial photonic (SNAP) bottle microresonators, employing the nonlinear interaction of whispering gallery modes which are confined along an optical fiber with nanoscale radius variation. We predict that a SNAP microresonator with a radius of a few micrometers can generate a frequency comb with an ultra-fine sub-gigahertz spectral spacing, which would require traditional ring resonators of centimeter radius. We identify regimes of stable or quasi-periodic comb dynamics due to soliton excitation, and show that special engineering of the SNAP radius profile can be used to compensate for nonlinearity-induced dispersion.

Multiple nonlinear resonances and frequency combs in bottle microresonators

We introduce the generalized Lugiato-Lefever equation describing nonlinear effects in the bottle microresonators. We demonstrate that the nonlinear modes of these resonators can form multiple coexisting and overlapping nonlinear resonances and that their instabilities lead to the generation of the low repetition rate frequency combs.

Optical bottle versus acoustic bottle and antibottle resonators

The theory of slow acoustic modes propagating along the optical fiber and being controlled by the nanoscale variation of the effective fiber radius (analogous to the theory of slow optical whispering gallery modes) is developed. Surprisingly, it is shown that, in addition to acoustic bottle resonators (which are similar to optical bottle resonators), there exist antibottle resonators, the neck-shaped deformations of the fiber that can fully confine acoustic modes. It is also shown that an eigenfrequency of the mechanical vibrations of a silica parabolic bottle resonator can match the separation between the eigenfrequencies of a series of its optical modes, thereby enabling the resonant mechanical excitation of these series. The developed theory paves the groundwork for slow-mode optomechanics in an optical fiber.