Spontaneous soliton mode-locking of a microcomb assisted by Raman scattering.

We successfully control the interaction dynamics between optical parametric oscillation (OPO) and stimulated Raman scattering, leading to the generation of distinct frequency comb states in a microresonator. Through Raman-scattered photons, a Raman comb with a sech2 envelope is demonstrated having a broad RF beat note linewidth of several hundred kHz. Moreover, under a specific coupling regime, we successfully generate self-locked Raman single-solitons which is confirmed by a narrow RF beat note of 25 Hz. Remarkably, this spontaneous Raman soliton is deterministically generated through adiabatic pump frequency detuning without the requirement of external locking mechanisms. Additionally, we identify a frequency comb with an unconventional envelope that can be fitted with a Lorentzian × sech2 function, generated via an anti-Stokes process with respect to the Raman comb.

Ultrastable microwave and soliton-pulse generation from fibre-photonic-stabilized microcombs.

The ability to generate lower-noise microwaves has greatly advanced high-speed, high-precision scientific and engineering fields. Microcombs have high potential for generating such low-noise microwaves from chip-scale devices. To realize an ultralow-noise performance over a wider Fourier frequency range and longer time scale, which is required for many high-precision applications, free-running microcombs must be locked to more stable reference sources. However, ultrastable reference sources, particularly optical cavity-based methods, are generally bulky, alignment-sensitive and expensive, and therefore forfeit the benefits of using chip-scale microcombs. Here, we realize compact and low-phase-noise microwave and soliton pulse generation by combining a silica-microcomb (with few-mm diameter) with a fibre-photonic-based timing reference (with few-cm diameter). An ultrastable 22-GHz microwave is generated with -110 dBc/Hz (-88 dBc/Hz) phase noise at 1-kHz (100-Hz) Fourier frequency and 10-13-level frequency instability within 1-s. This work shows the potential of fully packaged, palm-sized or smaller systems for generating both ultrastable soliton pulse trains and microwaves, thereby facilitating a wide range of field applications involving ultrahigh-stability microcombs.

A squeezed quantum microcomb on a chip.

The optical microresonator-based frequency comb (microcomb) provides a versatile platform for nonlinear physics studies and has wide applications ranging from metrology to spectroscopy. The deterministic quantum regime is an unexplored aspect of microcombs, in which unconditional entanglements among hundreds of equidistant frequency modes can serve as critical ingredients to scalable universal quantum computing and quantum networking. Here, we demonstrate a deterministic quantum microcomb in a silica microresonator on a silicon chip. 40 continuous-variable quantum modes, in the form of 20 simultaneously two-mode squeezed comb pairs, are observed within 1 THz optical span at telecommunication wavelengths. A maximum raw squeezing of 1.6 dB is attained. A high-resolution spectroscopy measurement is developed to characterize the frequency equidistance of quantum microcombs. Our demonstration offers the possibility to leverage deterministically generated, frequency multiplexed quantum states and integrated photonics to open up new avenues in fields of spectroscopy, quantum metrology, and scalable, continuous-variable-based quantum information processing.

Supercontinuum generation in As2S3 waveguides fabricated without direct etching.

We report a supercontinuum generation (SCG) in a waveguide that spontaneously forms without an etching process during the deposition of a core material on a preformed ${\rm{Si}}{{\rm{O}}_2}$ substructure. The mechanism of dispersion control for this new, to the best of our knowledge, type of waveguide is analyzed by numerical simulation, which results in a design rule to achieve a target dispersion profile by adjusting the substructure geometry. SCG is experimentally demonstrated with a waveguide made of ${\rm{A}}{{\rm{s}}_2}{{\rm{S}}_3}$, chalcogenide glass, which has low material absorption over the mid-IR range. A dispersion-controlled waveguide with a length of 10 mm pumped with 77 pJ pulses at a telecommunication wavelength of 1560 nm resulted in a supercontinuum that extends by more than 1.5 octaves.

Self-stabilized soliton generation in a microresonator through mode-pulled Brillouin lasing.

Reducing the complexity required for starting and maintaining a soliton state has been a major task to fully miniaturize soliton microcombs including the accompanying external operating systems. Here we experimentally examine the generative process of a self-stabilized soliton in which a continuous-wave pump detuned on the thermally stable blue side of a resonance generates a Brillouin lasing signal that relays the pump power to the soliton pulses via intracavity mode-coupling without breaking thermal self-stability. Based on a simple setup consisting of a free-running laser and a microcavity without any external feedback systems by virtue of internal thermal locking, single-soliton pulses of 11 GHz repetition rate were deterministically generated. We demonstrate that the single-soliton pulses can be passively maintained over several days in a laboratory environment with a phase noise performance of -137dBc/Hz at 100 kHz.

Hydrophobic passivation of ultra-high-Q silica wedge resonators using hexamethyldisilazane.

Dissipative Kerr solitons in ultra-high-Q resonators are extremely sensitive to the thermal behavior of the resonators. Especially for resonators with hydrophilic surfaces, moisture continuously adsorbs on their surfaces and causes additional absorption loss that results in an excessive thermal shift of resonance frequency. This change makes soliton mode locking more challenging or even impossible. Here, we report hydrophobic monolayer passivation using hexamethyldisilazane on ultra-high-Q silica wedge resonators. It was experimentally confirmed that the Q-factor and dispersion were maintained after passivation, and excess thermal shift by moisture was inhibited for more than three days in the atmosphere. Soliton mode locking was successfully performed with the resonator one month after passivation.

Universal light-guiding geometry for on-chip resonators having extremely high Q-factor.

By providing an effective way to leverage nonlinear phenomena in integrated devices, high-Q optical resonators have led to recent advances in on-chip photonics. However, developing fabrication processes to shape any new material into a resonator with extremely smooth surfaces on a chip has been an exceptionally challenging task. Here, we describe a universal method to implement ultra-high-Q resonators with any new material having desirable properties that can be deposited by physical vapor deposition. Using this method light-guiding cores with surface roughness on the molecular-scale are created automatically on pre-patterned substrates. Its efficacy has been verified using As2S3, a chalcogenide glass that has high-nonlinearity. The Q-factor of the As2S3 resonator so-developed approached the propagation loss record achieved in chalcogenide fibers which were limited by material losses. Owing to the boosted Q-factor, lasing by stimulated Brillouin scattering has been demonstrated with 100 times lower threshold power than the previous record.