We present, experimentally and numerically, the observation of Fermi-Pasta-Ulam recurrence induced by breather solitons in a high-Q SiN microresonator. Breather solitons can be excited by increasing the pump power at a relatively small pump phase detuning in microresonators. Out of phase power evolution is observed for groups of comb lines around the center of the spectrum compared to groups of lines in the spectral wings. The evolution of the power spectrum is not symmetric with respect to the spectrum center. Numerical simulations based on the generalized Lugiato-Lefever equation are in good agreement with the experimental results and unveil the role of stimulated Raman scattering in the symmetry breaking of the power spectrum evolution. Our results shows that optical microresonators can be exploited as a powerful platform for the exploration of soliton dynamics.The Fermi-Pasta-Ulam (FPU) recurrence was first raised by Fermi and his colleagues in the 1950s [1]. In a numerical simulation of string oscillation with nonlinear coupling between different modes to test thermalization theory, they found at a certain point the energy will return to the fundamentally excited mode, rather than distributing homogenously among different modes. This discovery triggered the rigorous investigation on plasma physics by Zubusky and Kruskal [2], which led to the discovery of solitons. Solitons and their related theory have revolutionized the research in diverse arenas, including fluid dynamics [3], optics [4,5], Bose-Eistein condensation [6,7].In optics, the FPU recurrence was first demonstrated based on the modulation instability (MI) in optical fibers [8]. As a feature of the FPU recurrence, the powers of the pump mode and the signal mode in MI evolves periodically with a phase delay of π. The collision between solitons in fibers also facilitated the observation of FPU recurrence in an active cavity [9]. Furthermore, optical breathers, e.g., the Akhmediev breather (AB) in the nonlinear schrödinger equation (NLSE) [10][11][12], are an important manifestation of FPU recurrence. Since collisions between breathers and solitons can result in optical rogue waves [13][14][15], studying FPU recurrence and the control of the transition between solitons and breathers may contribute to the understanding of optical rogue waves.Recently, maturity in the fabrication of high-Q microresonators [16] has fueled rapid progress on Kerr frequency comb generation [17][18][19][20][21][22]. In the frequency domain, microresonators based frequency comb synthesis has promising applications in optical clock [23], optical arbitrary waveform generation [24], and microwave photonics [25,26] etc. In the time domain, microresonators provide a new and important approach to realize optical solitons [21,[27][28][29][30]. Different from mode-locked lasers, passive microresonators have no active gain or saturable absorber, making them free from the influence of the complex gain dynamics. Hence, soliton generation in microresonators can exhibit excellent predicta...

Kerr nonlinearity-based frequency combs and solitons have been generated from on-chip microresonators. The initiation of the combs requires global or local anomalous dispersion which leads to many limitations, such as material choice, film thickness, and spectral ranges where combs can be generated, as well as fabrication challenges. Using a concentric racetrack-shaped resonator, we show that such constraints can be lifted and resonator dispersion can be engineered to be anomalous over moderately broad bandwidth. We demonstrate anomalous dispersion in a 300 nm thick silicon nitride film, suitable for semiconductor manufacturing but previously thought to result in waveguides with high normal dispersion. Together with a mode-selective, tapered coupling scheme, we generate coherent mode-locked frequency combs. Our method can realize anomalous dispersion for resonators at almost any wavelength and simultaneously achieve material and process compatibility with semiconductor manufacturing.

Microwave phased array antennas (PAAs) are very attractive to defense applications and high-speed wireless communications for their abilities of fast beam scanning and complex beam pattern control. However, traditional PAAs based on phase shifters suffer from the beam-squint problem and have limited bandwidths. True-time-delay (TTD) beamforming based on lowloss photonic delay lines can solve this problem. But it is still quite challenging to build large-scale photonic TTD beamformers due to their high hardware complexity. In this paper, we demonstrate a photonic TTD beamforming network based on a miniature microresonator frequency comb (microcomb) source and dispersive time delay. A method incorporating optical phase modulation and programmable spectral shaping is proposed for positive and negative apodization weighting to achieve arbitrary microwave beam pattern control. The experimentally demonstrated TTD beamforming network can support a PAA with 21 elements. The microwave frequency range is 8 ∼ 20 GHz; and the beam scanning range is ±60.2 • . Detailed measurements of the microwave amplitudes and phases are performed. The beamforming performances of Gaussian, rectangular beams and beam notch steering are evaluated through simulations by assuming a uniform radiating antenna array. The scheme can potentially support larger PAAs with hundreds of elements by increasing the number of comb lines with broadband microcomb generation.

Acquisition of laser frequency with high resolution under continuous and abrupt tuning conditions is important for sensing, spectroscopy and communications. Here, a single microresonator provides rapid and broad-band measurement of frequencies across the optical C-band with a relative frequency precision comparable to conventional dual frequency comb systems. Dual-locked counter-propagating solitons having slightly different repetition rates are used to implement a Vernier spectrometer. Laser tuning rates as high as 10 THz/s, broadly step-tuned lasers, multiline laser spectra and also molecular absorption lines are characterized using the device. Besides providing a considerable technical simplification through the dual-locked solitons and enhanced capability for measurement of arbitrarily tuned sources, this work reveals possibilities for chipscale spectrometers that greatly exceed the performance of table-top grating and interferometerbased devices.Frequency-agile lasers are ubiquitous in sensing, spectroscopy and optical communications 1-3 and measurement of their optical frequency for tuning and control is traditionally performed by grating and interferometerbased spectrometers, but more recently these measurements can make use of optical frequency combs 4-6 . Frequency combs provide a remarkably stable measurement grid against which optical signal frequencies can be determined subject to the ambiguity introduced by their equally spaced comb lines. The ambiguity can be resolved for continuously frequency swept signals by counting comb teeth 7 relative to a known comb tooth; and this method has enabled measurement of remarkably high chirp rates 8 . However, signal sources can operate with abrupt frequency jumps so as to quickly access a new spectral region or for switching purposes, and this requires a different approach. In this case, a second frequency comb with a different comb line spacing can provide a Vernier scale 9 for comparison with the first comb to resolve the ambiguity under quite general tuning conditions 9-11 . This Vernier concept is also used in dual comb spectroscopy 12,13 , but in measuring active signals the method can be significantly enhanced to quickly identify signal frequencies through a signal correlation technique 9 . The power of the Vernier-based method relies upon mapping of optical comb frequencies into a radio-frequency grid of frequencies, the precision of which is set by the relative line-by-line frequency stability of the two frequency combs. This stability can be guaranteed by self-referencing each comb using a common high-stability radio-frequency source or through optical locking of each comb to reference lasers whose relative stability is ensured by mutual locking to a common optical cavity.Here, a broad-band, high-resolution Vernier soliton microcomb spectrometer is demonstrated using a single miniature comb device that generates two mutuallyphase-locked combs. The principle of operation relies upon an optical phase locking effect observed in the generation of counter-pr...

Soliton formation in on-chip micro-comb generation balances cavity dispersion and nonlinearity and allows coherent, low-noise comb operation. We study the intracavity waveform of an on-chip microcavity soliton in a silicon nitride microresonator configured with a drop port. Whereas combs measured at the through port are accompanied by a very strong pump line which accounts for >99% of the output power, our experiments reveal that inside the microcavity, most of the power is in the soliton. Time-domain measurements performed at the drop port provide information that directly reflects the intracavity field. Data confirm a train of bright, close to bandwidth-limited pulses, accompanied by a weak continuous wave (CW) background with a small phase shift relative to the comb.

High optical quality (Q) factors are critically important in optical microcavities, where performance in applications spanning nonlinear optics to cavity quantum electrodynamics is determined. Here, a record Q factor of over 1.1 billion is demonstrated for on-chip optical resonators. Using silica whispering-gallery resonators on silicon, Q-factor data is measured over wavelengths spanning the C/L bands (100 nm) and for a range of resonator sizes and mode families. A record low sub-milliwatt parametric oscillation threshold is also measured in 9 GHz free-spectral-range devices. The results show the potential for thermal silica on silicon as a resonator material.

We investigate, numerically and experimentally, the effect of thermo-optical (TO) chaos on soliton generation dynamics in microresonators. Numerical simulations that include the thermal dynamics show that the generated solitons can either survive or annihilate when the pump laser is scanned from blue to red and then stop at a fixed wavelength; the outcome is stochastic and is strongly related to the number of solitons generated. The random fluctuations of the cavity resonance occurring under TO chaos are also found to trigger delayed spontaneous soliton generation after the laser scan ends, which could enable soliton excitation with slow laser tuning speed. Stochastic soliton annihilation/survival, as well as delayed spontaneous soliton generation, is observed experimentally in a silicon-nitride microresonator.

We demonstrate an experimental observation of soliton rains in an all normal dispersion Yb-fiber laser. The cavity consists of a narrow bandwidth filter and a birefringent plate (BP) filter. Soliton rain is obtained in the weak mode-locking regime, while multisolitons behaving as soliton bunches and harmonic mode-locking under strong mode-locking are also observed. Distinctive multisoliton interactions are observed via changing the pump power and adjustment of the waveplates as well as the BP. To the best of our knowledge, this is the first demonstration of soliton rains in normal dispersion fiber lasers.