Xiaoxiao Xue

Mode-locked dark pulse Kerr combs in normal-dispersion microresonators

Kerr frequency combs from microresonators are now extensively investigated as a potentially portable technology for a variety of applications. Most studies employ anomalous dispersion microresonators that support modulational instability for comb initiation, and mode-locking transitions resulting in coherent bright soliton-like pulse generation have been reported. However, some experiments show comb generation in normal dispersion microresonators; simulations suggest the formation of dark pulse temporal profiles. Excitation of dark pulse solutions is difficult due to the lack of modulational instability in the effective blue-detuned pumping region; an excitation pathway has been demonstrated neither in experiment nor in simulation. Here we report experiments in which dark pulse combs are formed by mode-interaction-aided excitation; for the first time, a mode-locking transition is observed in the normal dispersion regime. The excitation pathway proposed is also supported by simulations.

Intracavity characterization of micro-comb generation in the single-soliton regime.

We use a drop-port geometry to characterize the intracavity waveform of an on-chip microcavity soliton. In contrast to the through-port output, the intracavity field shows efficient power transfer from the pump into the comb.

Normal-dispersion microcombs enabled by controllable mode interactions

We demonstrate a scheme incorporating dual coupled microresonators through which mode interactions are intentionally introduced and controlled for Kerr frequency comb (microcomb) generation in the normal dispersion region. Microcomb generation, repetition rate selection, and mode locking are achieved with coupled silicon nitride microrings controlled via an on-chip microheater. Our results show for the first time that mode interactions can be programmably tuned to facilitate broadband normal-dispersion microcombs. The proposed scheme increases freedom in microresonator design and may make it possible to generate microcombs in an extended wavelength range (e.g., in the visible) where normal material dispersion is likely to dominate.

Programmable Single-Bandpass Photonic RF Filter Based on Kerr Comb from a Microring

Microresonators with a high Kerr nonlinearity show great potential to generate optical frequency combs with ultrabroad spectra, high repetition rate, and high coherence between comb lines. The compact size and possibility of chip-level integration make the Kerr combs attractive for many applications, especially including photonic radiofrequency (RF) filters. In this paper we report the first demonstration of a programmable photonic RF filter based on the Kerr comb from a silicon nitride microring. A novel scheme enabled by the large frequency spacing of the Kerr comb is introduced in order to suppress unwanted RF passbands including the image and periodic passbands. As a result, a single passband is achieved. To the best of our knowledge, this is the first demonstration of a single-bandpass photonic RF filter employing a discrete-wavelength comb source.

Highly reconfigurable microwave photonic single-bandpass filter with complex continuous-time impulse responses

We propose a novel structure of complex-tap microwave photonic filter (MPF) employing an incoherent broadband optical source (BOS) and a programmable optical spectrum processor. By tailoring the optical spectral amplitude and phase, arbitrary complex continuous-time impulse responses of the MPF can be constructed. Frequency responses with a single flat-top, highly chirped, or arbitrary-shape passband are demonstrated, respectively. The passband center can also be tuned in a wide range only limited by the opto-electrical devices. To the best of our knowledge, it is the first demonstration of an incoherent-BOS-based MPF which is single-bandpass, widely tunable, and highly reconfigurable with complex taps.

Drop-port study of microresonator frequency combs: power transfer, spectra and time-domain characterization

We use a drop-port geometry to characterize frequency combs generated from silicon nitride on-chip microresonators in the normal group velocity regime. In sharp contrast with the traditional transmission geometry, we observe smooth output spectra with comparable powers in the pump and adjacent comb lines. The power transfer into the comb may be explained to a large extent by the coupling parameters characterizing the linear operation of the resonances studied. Furthermore, comparison of thru- and drop-port spectra shows that much of the ASE noise is filtered out by transmission to the drop-port. Autocorrelation measurements are performed on the drop-port output, without the need to filter out or suppress the strong pump line as is necessary in thru-port experiments. Passively mode-locked pulses with low background are observed in a normal dispersion microcavity.

High-Q silicon nitride microresonators exhibiting low-power frequency comb initiation

Optical resonators with high quality factors (Qs) are promising for a variety of applications due to the enhanced nonlinearity and increased photonic density of states at resonances. In particular, frequency combs (FCs) can be generated through four-wave mixing in high-Q microresonators made from Kerr nonlinear materials such as silica, silicon nitride, magnesium fluoride, and calcium fluoride. These devices have potential for on-chip frequency metrology and high-resolution spectroscopy, high-bandwidth radiofrequency information processing, and high-data-rate telecommunications. Silicon nitride microresonators are attractive due to their compatibility with integrated circuit manufacturing; they can be cladded with silica for long-term stable yet tunable operation, and allow multiple resonators to be coupled together to achieve novel functionalities. Despite previous demonstrations of high-Q silicon nitride resonators, FC generation using silicon nitride microresonator chips still requires pump power significantly higher than those in whispering gallery mode resonators made from silica, magnesium, and calcium fluorides, which all have shown resonator Qs between 0.1 and 100 billion. Here, we report on a fabrication procedure that leads to the demonstration of “finger-shaped” Si3N4 microresonators with intrinsic Qs up to 17 million at a free spectrum range (FSR) of 24.7 GHz that are suitable for telecommunication and microwave photonics applications. The frequency comb onset power can be as low as 2.36 mW and broad, single FSR combs can be generated at a low pump power of 24 mW, both within reach of on-chip semiconductor lasers. Our demonstration is an important step toward a fully integrated on-chip FC source.

Thermal tuning of Kerr frequency combs in silicon nitride microring resonators.

Microresonator based Kerr frequency comb generation has many attractive features, including ultrabroad spectra, chip-level integration, and low power consumption. Achieving precise tuning control over the comb frequencies will be important for a number of practical applications, but has been little explored for microresonator combs. In this paper, we characterize the thermal tuning of a coherent Kerr frequency comb generated from an on-chip silicon nitride microring. When the microring temperature is changed by ~70 °C with an integrated microheater, the line spacing and center frequency of the comb are tuned respectively by -253 MHz (-3.57 MHz/°C) and by -175 GHz (-2.63 GHz/°C); the latter constitutes 75% of the comb line spacing. From these results we obtain a shift of 25 GHz (362.07 MHz/°C) in the comb carrier-envelope offset frequency. Numerical simulations are performed by taking into account the thermo-optic effects in the waveguide core and cladding. The temperature variation of the comb line spacing predicted from simulations is close to that observed in experiments. The time-dependent thermal response of the microheater based tuning scheme is characterized; time constants of 30.9 μs and 0.71 ms are observed.

Analysis and Compensation of Third-Order Dispersion Induced RF Distortions in Highly Reconfigurable Microwave Photonic Filters

A scheme to perform high-quality-factor (Q) reconfigurable microwave photonic filtering with the optical third-order dispersion (TOD) induced radiofrequency (RF) distortions compensated is demonstrated. A detailed investigation of the TOD induced RF distortions is presented both theoretically and experimentally. By programming the optical spectrum in one arm of the Mach-Zehnder structure, the distortions can be completely eliminated, giving rise to ideal RF transfer function which can achieve very high Q factors. The experimentally demonstrated Q factors are 634 and 76 when the RF center is tuned to 18 GHz and the passband shape is configured to be sinc-shape and flat-top respectively, the highest values ever reported for a single-bandpass, complex-tap, and reconfigurable microwave photonic filter.

Deterministic single soliton generation and compression in microring resonators avoiding the chaotic region.

A path within the parameter space of detuning and pump power is demonstrated in order to obtain a single cavity soliton (CS) with certainty in SiN microring resonators in the anomalous dispersion regime. Once the single CS state is reached, it is possible to continue a path to compress it, broadening the corresponding single free spectral range (FSR) Kerr frequency comb. The first step to achieve this goal is to identify the stable regions in the parameter space via numerical simulations of the Lugiato-Lefever equation (LLE). Later, using this identification, we define a path from the stable modulation instability (SMI) region to the stable cavity solitons (SCS) region avoiding the chaotic and unstable regions.