Microresonator-based Kerr Frequency Combs Research Articles

Chip-based optical frequency combs for high-capacity optical communications

Abstract Current fibre optic communication systems owe their high-capacity abilities to the wavelength-division multiplexing (WDM) technique, which combines data channels running on different wavelengths, and most often requires many individual lasers. Optical frequency combs, with equally spaced coherent comb lines derived from a single source, have recently emerged as a potential substitute for parallel lasers in WDM systems. Benefits include the stable spacing and broadband phase coherence of the comb lines, enabling improved spectral efficiency of transmission systems, as well as potential energy savings in the WDM transmitters. In this paper, we discuss the requirements to a frequency comb for use in a high-capacity optical communication system in terms of optical linewidth, per comb line power and optical carrier-to-noise ratio, and look at the scaling of a comb source for ultra-high capacity systems. Then, we review the latest advances of various chip-based optical frequency comb generation schemes and their applications in optical communications, including mode-locked laser combs, spectral broadening of frequency combs, microresonator-based Kerr frequency combs and electro-optic frequency combs.

Ultralow-noise photonic microwave synthesis using a soliton microcomb-based transfer oscillator

The synthesis of ultralow-noise microwaves is of both scientific and technological relevance for timing, metrology, communications and radio-astronomy. Today, the lowest reported phase noise signals are obtained via optical frequency-division using mode-locked laser frequency combs. Nonetheless, this technique ideally requires high repetition rates and tight comb stabilisation. Here, a microresonator-based Kerr frequency comb (soliton microcomb) with a 14 GHz repetition rate is generated with an ultra-stable pump laser and used to derive an ultralow-noise microwave reference signal, with an absolute phase noise level below −60 dBc/Hz at 1 Hz offset frequency and −135 dBc/Hz at 10 kHz. This is achieved using a transfer oscillator approach, where the free-running microcomb noise (which is carefully studied and minimised) is cancelled via a combination of electronic division and mixing. Although this proof-of-principle uses an auxiliary comb for detecting the microcomb’s offset frequency, we highlight the prospects of this method with future self-referenced integrated microcombs and electro-optic combs, that would allow for ultralow-noise microwave and sub-terahertz signal generators.

Electro-optic eigenfrequency tuning of potassium tantalate-niobate microresonators

Eigenfrequency tuning in microresonators is useful for a range of applications including frequency-agile optical filters and tunable optical frequency converters. In most of these applications, eigenfrequency tuning is achieved by thermal or mechanical means, while a few non-centrosymmetric crystals such as lithium niobate allow for such tuning using the linear electro-optic effect. Potassium tantalate-niobate ($\mathrm{KTa}_{1-x}\mathrm{Nb}_{x}\mathrm{O}_{3}$ with $0\leq x \leq1$, KTN) is a particularly attractive material for electro-optic tuning purposes. It has both non-centrosymmetric and centrosymmetric phases offering outstandingly large linear as well as quadratic electro-optic coefficients near the phase transition temperature. We demonstrate whispering-gallery resonators (WGRs) made of KTN with quality factors of $Q>10^{7}$ and electro-optic eigenfrequency tuning of more than 100 GHz at $\lambda=1040\,$nm for moderate field strengths of $E=250\,$V/mm. The tuning behavior near the phase transition temperature is analyzed by introducing a simple theoretical model. These results pave the way for applications such as electro-optically tunable microresonator-based Kerr frequency combs.

Reconfigurable optical generation of nine Nyquist WDM channels with sinc-shaped temporal pulse trains using a single microresonator-based Kerr frequency comb.

Sinc-shaped temporal pulse trains have a spectrally efficient, rectangular Nyquist spectrum. We demonstrate the simultaneous and reconfigurable optical generation of multiple Nyquist-shaped wavelength-division-multiplexed (WDM) channels having temporal sinc-shaped pulse trains as data carriers. The channels are generated through the insertion of coherent lines using cascaded continuous-wave amplitude modulation around the spectral lines of a microresonator-based Kerr optical frequency comb. For each of nine Kerr frequency comb lines, we insert sub-groups of uniform and coherent lines to generate nine WDM channels. The deviations from ideal Nyquist pulses for the nine channels at repetition rates of 6 and 2GHz are between 4.2%-6.1% and 2%-4.5%, respectively. Each WDM channel is modulated with on-off keying (OOK) at 6 Gbit/s. In addition, we show the reconfigurability of this method by varying the number of WDM channels, the generated sinc-shaped pulse train repetition rates, the duration, and the number of zero-crossings.

Microresonator Frequency Combs for Integrated Microwave Photonics

Microresonator-based Kerr frequency combs (microcombs) have many outstanding features, including chip-level integration, high repetition rate, and ultrabroad bandwidth. These merits make microcombs very promising for integrated microwave photonic applications. Here, we provide a brief review on recent progress of microcomb generation and its application in microwave photonics. Both coherent and incoherent examples are discussed, including low-phase-noise microwave and optical frequency synthesis, programmable photonic microwave signal processing, true time delay beamforming, and channelized receiver. An outlook on the future research direction and key problems is also given.

Robust cavity soliton formation with hybrid dispersion

Microresonator-based Kerr frequency combs have attracted a great deal of attention in recent years, in which mode locking of the generated combs is associated with bright or dark cavity soliton formation. In this paper, we show that, different from soliton propagation along a waveguide, cavity solitons can be robustly formed under a unique dispersion profile with four zero-dispersion wavelengths. More importantly, such a dispersion profile exhibits much smaller overall dispersion, thus making it possible to greatly reduce the pump power by five to six times.

Power-Efficient Kerr Frequency Comb Based Tunable Optical Source

We designed and demonstrated a power-efficient highly integrated photonic system, requiring a total power consumption of 1.7 W and producing a spectrally pure coherent optical signal with a wavelength range of 23 nm in the C-band. The system consists of a compact, low-power InP-based photonic integrated coherent receiver, microresonator-based Kerr frequency comb, and agile electronic circuits. The photonic coherent receiver contains a 60-nm widely tunable Y-branch local oscillator (LO) diode laser, a coupler, and a pair of photodetectors. It consumes record-low (approximately 184 mW) electrical power. The optical frequency comb reference has excellent spectral purity, $ 4 kHz optical linewidth, and good frequency stability. The spectrally pure tunable optical source was produced by offset locking the on-chip LO laser of the integrated receiver to this frequency comb source. A possibility of further stabilization of the frequency comb repetition rate by locking to an external radio frequency synthesizer was demonstrated.

Normal-dispersion microresonator Kerr frequency combs

Abstract Optical microresonator-based Kerr frequency comb generation has developed into a hot research area in the past decade. Microresonator combs are promising for portable applications due to their potential for chip-level integration and low power consumption. According to the group velocity dispersion of the microresonator employed, research in this field may be classified into two categories: the anomalous dispersion regime and the normal dispersion regime. In this paper, we discuss the physics of Kerr comb generation in the normal dispersion regime and review recent experimental advances. The potential advantages and future directions of normal dispersion combs are also discussed.

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.

Stability and Intrinsic Fluctuations of Dissipative Cavity Solitons in Kerr Frequency Microcombs

The generation of dissipative cavity solitons is one of the most intriguing features of microresonator-based Kerr frequency combs, enabling effective mode locking of comb modes and synthesis of ultrafast pulses. With the Lugiato–Lefever model, here, we conduct detailed theoretical investigations on the transient dynamics of dissipative cavity solitons and describe how several intrinsic effects of the Kerr comb disturb the stability of cavity solitons, including soliton breathing, higher order dispersion, dispersive wave emission, and cavity mode coupling. Our results and analysis agree well with recent measurements and provide insight into some as yet unexplained observations.

Analytical estimates of eigenfrequencies, dispersion, and field distribution in whispering gallery resonators

The problem of accurate calculation of eigenfrequencies in resonators of complex geometry is not only fundamental but also has many practical applications. In particular, a possibility for calculating the eigenfrequencies and geometry dependent dispersion of whispering gallery modes is important for optimization of dielectric microresonator-based Kerr frequency combs. In this case, the required anomalous second-order dispersion may be controlled by means of small shape variations of the resonator. Unfortunately, all uniform approximations for the eigenfrequencies do not reach the required precision for this purpose. We propose new approximations for spheroids, quartics, and toroids with better precision, which also allow for the estimation of the second-order dispersion. We also obtain analytical expressions for field distribution in microresonators and investigate the possibility of achieving better approximations by combining analytical and numerical methods.

Stabilization of a Kerr frequency comb oscillator

We study stability and spectral purity of a microresonator-based Kerr frequency comb oscillator experimentally and observe a correlation between the frequency of the continuous wave laser pumping the nonlinear resonator and the repetition frequency of the comb. This correlation is used in a proof-of-principle demonstration of a Kerr frequency comb stabilized with an optical transition of 87Rb.