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High-coherence visible and near-visible laser sources are centrally important to the operation of advanced position/navigation/timing systems as well as classical/quantum sensing systems. However, the complexity and size of these bench-top lasers are an impediment to their transition beyond the laboratory. Here, a system-on-chip that emits high-coherence near-visible lightwaves is demonstrated. The devices rely upon a new approach wherein wavelength conversion and coherence increase by self-injection locking are combined within a single nonlinear resonator. This simplified approach is demonstrated in a hybridly-integrated device and provides a short-term linewidth of around 4.7 kHz (10 kHz before filtering). On-chip converted optical power over 2 mW is also obtained. Moreover, measurements show that heterogeneous integration can result in a conversion efficiency higher than 25% with an output power over 11 mW. Because the approach uses mature III–V pump lasers in combination with thin-film lithium niobate, it can be scaled for low-cost manufacturing of high-coherence visible emitters. Also, the coherence generation process can be transferred to other frequency conversion processes, including optical parametric oscillation, sum/difference frequency generation, and third-harmonic generation.

We demonstrate generation of a broadband quantum optical frequency comb using spontaneous parametric down-conversion in a dispersion-engineered thin-film lithium niobate resonator spanning 30-THz bandwidth and 600 comb lines at 50-GHz mode spacing.

We report microwave-rate soliton microcombs on a chip-scale lithium niobate platform, where the repetition rate can be tuned by electro-optic means.

By integrating two periodic poling structures into a single high-Q lithium niobate microresonator, cascaded SHG and SFG achieved third-harmonic generation on-chip at a wavelength of 520 nm with an efficiency of 0.6%.

We demonstrate narrow linewidth, second-harmonic generation using a semi-conductor DFB laser self-injection locked with a high-Q lithium niobate microresonator. A second-harmonic linewidth of 31kHz is achieved from 36dB pump frequency noise suppression.

We report an on-chip high-speed visible-band electro-optic modulator that can operate over the full visible spectrum of 400–700 nm, with a record low V π ⋅ L of 0.48, 0.25, and 0.17 V ⋅ c m at red, green, and blue wavelengths of 630, 520, and 450 nm, respectively, and an operation bandwidth of > 20 G H z .

Nanophotonic entangled-photon sources are a critical building block of chip-scale quantum photonic architecture and have seen significant development over the past two decades. These sources generate photon pairs that typically span over a narrow frequency bandwidth. Generating entanglement over a wide spectral region has proven to be useful in a wide variety of applications including quantum metrology, spectroscopy and sensing, and optical communication. However, generation of broadband photon pairs with temporal coherence approaching an optical cycle on a chip is yet to be seen. Here we demonstrate generation of ultra-broadband entangled photons using spontaneous parametric down-conversion in a periodically-poled lithium niobate nanophotonic waveguide. We employ dispersion engineering to achieve a bandwidth of 100 THz (1.2 - 2 $��$m), at a high efficiency of 13 GHz/mW. The photons show strong temporal correlations and purity with the coincidence-to-accidental ratio exceeding $10^5$ and $>$ 98\% two-photon interference visibility. These properties together with the piezo-electric and electro-optic control and reconfigurability, make thin-film lithium niobate an excellent platform for a controllable entanglement source for quantum communication and computing, and open a path towards femtosecond metrology and spectroscopy with non-classical light on a nanophotonic chip., 10 pages, 7 figures

We report lithium niobate soliton microcombs with spectral bandwidths exceeding one octave and spanning 125 THz to 268 THz.

We demonstrate ultra-broadband spontaneous parametric down-conversion over a 100 THz bandwidth using an on-chip dispersion-engineered periodically-poled lithium niobate waveguide. The source produces photon-pairs with an unprecedented 13 GHz/mW efficiency and a coincidence-to-accidental ratio exceeding 105.

High-fidelity periodic poling over long lengths is required for robust, quasi-phase-matched second-harmonic generation using the fundamental, quasi-TE polarized waveguide modes in a thin-film lithium niobate (TFLN) waveguide. Here, a shallow-etched ridge waveguide is fabricated in x-cut magnesium oxide doped TFLN and is poled accurately over 5 mm. The high fidelity of the poling is demonstrated over long lengths using a non-destructive technique of confocal scanning second-harmonic microscopy. We report a second-harmonic conversion efficiency of up to 939 %.W−1 (length-normalized conversion efficiency 3757 %.W−1.cm−2), measured at telecommunications wavelengths. The device demonstrates a narrow spectral linewidth (1 nm) and can be tuned precisely with a tuning characteristic of 0.1 nm/°C, over at least 40 °C without measurable loss of efficiency.

We report LiNbO3 photonic-crystal electro-optic modulators (EOMs) with a modulation efficiency up to 11 pm/V and a bandwidth up to 17.5 GHz, the smallest LiNbO3 EOM ever demonstrated while with the highest modulation efficiency.

Recent advance of soliton microcombs has shown great promise to revolutionize many important areas such as optical communication, spectroscopic sensing, optical clock, and frequency synthesis. A largely tunable comb line spacing is crucial for the practical application of soliton microcombs, which unfortunately is challenging to realize for an on-chip monolithic microresonator. The recently discovered perfect soliton crystal (PSC) offers a convenient route to tune the comb line spacing. However, excitation of a PSC is generally stochastic by its nature and accessing a certain PSC state requires delicate tuning procedure. Here we demonstrate the on-demand generation of PSCs in a lithium niobate microresonator. The unique device characteristics allow us to produce a variety of PCSs and to switch between different PSC states, deterministically and repetitively. We utilize the device to show arbitrary dialing of the comb line spacing from 1 to 11 times of the free-spectral range of the resonator. The demonstration of PCSs on demand may now open up a great avenue for flexibly controlling the repetition rate of soliton pulses, which would significantly enhance and extend the application potential of soliton microcombs for communication, signal processing, and sensing.

We demonstrate a soliton microcomb in a monolithic high-Q lithium niobate resonator. Second-harmonic generation of the soliton spectrum is also observed.

Suppmentary material

We report high-Q lithium niobate microring resonators cladded with a TiO2 thin film, which exhibit nearly zero thermo-optic coefficient around room temperature, while the photorefractive effect is permanently quenched by cladding.

Energy-efficient optical control of photonic device properties is crucial for diverse photonic signal processing. Here we demonstrate extremely efficient optical tuning of photonic nanocavities, with only photon-level optical energy. With a lithium niobate photonic crystal nanocavity with an optical Q up to 1.41 million and an effective mode volume down to 0.78$(\lambda/n)^3$, we are able to achieve a resonance tuning rate of about 88.4~MHz/photon (0.67~GHz/aJ), which allows us to tune across the whole cavity resonance with only about 2.4 photons on average inside the cavity. Such a photon-level resonance tuning is of great potential for energy-efficient optical switching, wavelength routing, and reconfiguration of photonic devices/circuits that are indispensable for future photonic interconnect.

Composite coatings of poly(vinyl pyrrolidone) (PVP)-polyaniline (PANI) were prepared on 304 stainless steel (SS) for corrosion protection application by one-step electrodeposition. The composition and structure of the coatings were obtained by Fourier transform infrared spectra (FTIR), Raman spectroscopy, X-ray diffraction (XRD), UV–vis spectrum, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electropolymerization rate of aniline is decreased and large aggregation of polyaniline particles can be significantly inhibited in the presence of PVP. It is suggested that 1.0 wt% PVP-PANI coating provides enhanced corrosion protection for 304 SS in 1 M H2SO4 solution. The interaction between PANI and PVP favors the formation of the compact structure and consequent high corrosion performance.

Lithium niobate (LN), possessing wide transparent window, strong electro-optic effect, and large optical nonlinearity, is an ideal material platform for integrated photonics application. Microring resonators are particularly suitable as integrated photonic components, given their flexibility of device engineering and their potential for large-scale integration. However, the susceptibility to temperature fluctuation has become a major challenge for their implementation in a practical environment. Here, we demonstrate an athermal LN microring resonator. By cladding an x-cut LN microring resonator with a thin layer of titanium oxide, we are able to completely eliminate the first-order thermo-optic coefficient (TOC) of cavity resonance right at room temperature (20°C), leaving only a small residual quadratic temperature dependence with a second-order TOC of only 0.37 pm/K2. It corresponds to a temperature-induced resonance wavelength shift within 0.33 nm over a large operating temperature range of (−10 – 50)°C that is one order of magnitude smaller than a bare LN microring resonator. Moreover, the TiO2-cladded LN microring resonator is able to preserve high optical quality, with an intrinsic optical Q of 5.8 × 105 that is only about 11% smaller than that of a bare LN resonator. The flexibility of thermo-optic engineering, high optical quality, and device fabrication compatibility show great promise of athermal LN/TiO2 hybrid devices for practical applications, elevating the potential importance of LN photonic integrated circuits for future communication, sensing, nonlinear and quantum photonics.

Optical parametric generation is important for the creation and control of classical and quantum light. Here LiNbO${}_{3}$ microresonators have not yet lived up to their potential, due to the need for careful design of both phase matching and light extraction in high-$Q$ resonators. This study achieves cavity-enhanced second-harmonic generation with a conversion efficiency of 1500% W${}^{\ensuremath{-}1}$, by using a high-$Q$ Z-cut LiNbO${}_{3}$ microring resonator with a single bus waveguide to couple the phase-matched modes. Difference-frequency generation is also observed. This work is an important step toward efficient wavelength conversion and optical signal processing in integrated photonic circuits.

We demonstrate a stable ultrafast all-fiber laser based on filter-driven four wave mixing using a microfiber resonator which serves both as filter and a high nonlinear element.

The wide range of functions that are possible with lithium niobate (LN) waveguide devices, including phase and intensity modulation, second-harmonic generation, and difference-frequency generation, makes it attractive as a potential microcomb material. LN microcombs would combine essential comb self-referencing and control functions with the pulse generation process in a single microresonator device. Here, we demonstrate a soliton microcomb in a monolithic high-Q LN resonator. Direct frequency doubling of the soliton spectrum is observed inside the same cavity. The LN soliton mode-locking process also self-starts and allows bi-directional switching of soliton states, effects that are shown to result from the LN photorefractive effect. The Kerr solitons exhibit a self-frequency shift resulting from the Raman effect of LN. This microcomb platform can dramatically simplify miniature time keeping, frequency synthesis/division, and spectroscopy systems. Moreover, direct generation of femtosecond timescale pulses within LN microresonators can benefit quantum photonics and signal processing systems.

A soft-sensing scheme for electrical conductivity is proposed by using the sine wave amplitude-phase characteristic parameters as secondary variable. The system adopts sine wave to excite conductivity cell and wages multi-sampling for the response signal. The nonlinear least squares method is adopted to wage optimization calculation for system model, which can obtain system amplitude-phase characteristic parameters and inhibit the measurement interference at the same time. By utilizing the equivalent model of conductivity cell, we derived the functional relation between resistance-capacitance (RC) parameters and the amplitude-phase characteristics, thus the RC parameters decoupling value can be obtained and the conductivity can be calculated. We also designed automatic temperature compensation and intelligent shift function, which further improve the system precision and practicality. The scheme could have a certain measurement accuracy, measurement speed and anti-interference ability, which can meet the requirement of industrial online real-time measurement for electrical conductivity.