No abstract

Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communications, precision metrology, timing and spectroscopy. To date, wide-spanning combs are most often generated by mode-locked lasers or dispersion-engineered resonators with third-order Kerr nonlinearity. An alternative comb generation method uses electrooptic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. In this work, we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large electro-optic response, ultralow optical loss and highly co-localized microwave and optical fields, while enabling dispersion engineering. Our measured EO frequency comb spans more than the entire telecommunications L-band (over 900 comb lines spaced at ∼ 10 GHz), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 Hz to 100 MHz), and utilize this feature to generate dual frequency combs in a single resonator. Our results show that integrated EO comb generators, capable of generating wide and stable comb spectra, are a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy to optical communications.The migration of optical frequency comb generators to integrated devices is motivated by a desire for efficient, compact, robust, and high repetition-rate combs [1,2]. At present, almost all on-chip frequency comb generators rely on the Kerr (third-order, χ (3) ) nonlinear optical process, where a continuous wave (CW) laser source excites a low-loss optical microresonator having a large Kerr nonlinear coefficient. This approach has enabled demonstration of wide-spanning Kerr frequency combs from the near-to mid-infrared in many material platforms [3][4][5][6][7]. Owing to the complex nature of the parametric oscillation process, however, the formation dynamics and noise properties of the Kerr combs are not yet fully understood and are still under active investigation [8,9]. Sophisticated control protocols are typically required to keep Kerr combs stabilized.An alternative frequency comb-generation method uses the electro-optic (EO) effect in materials with secondorder (χ (2) ) nonlinearity. Conventionally, EO frequency comb generators pass a CW laser through a sequence of discrete phase and amplitude modulators [10][11][12]. Such EO comb generators can feature remarkable comb power and flat spectra, and can support flexible frequency spacing. They usually have narrow bandwidth, however, comprising only tens of lines and spanning only a few nan...

We demonstrate an ultralow loss monolithic integrated lithium niobate photonic platform consisting of dry-etched subwavelength waveguides. We show microring resonators with a quality factor of 10 7 and waveguides with propagation loss as low as 2.7 dB/m.Lithium niobate (LN) is a material with wide range of applications in optical and microwave technologies, owing to its unique properties that include large second order nonlinear susceptibility (χ (2) = 30 pm/V), large piezoelectric response (C 33 ∼ 250 C/m 2 ), wide optical transparency window (350 nm − 5 µm) and high refractive index (∼ 2.2) [1]. Conventional LN components, including fiber-optic modulators and periodically poled frequency converters have been the workhorse of the optoelectronic industry. The performances of these components have the potential to be dramatically improved as optical waveguides in bulk LN crystals are defined by ion-diffusion or proton-exchange methods which result in low index contrast and weak optical confinement. Integrated LN platform, featuring sub-wavelength scale light confinement and dense integration of optical and electrical components, has the potential to revolutionize optical communication and microwave photonics [1][2][3][4][5][6][7].The major road-block for practical applications of integrated LN photonics is the difficulty of fabricating devices that simultaneously achieve low optical propagation loss and high confinement. Recently developed thin-film LN-on-insulator technology makes this possible, and has resulted in the development of two complementary approaches to define nanoscale optical waveguides: hybrid and monolithic. The hybrid approach integrates an easyto-etch material (e.g. silicon or silicon nitride) with LN thin films to guide light [2-4] with a relatively low propagation loss (0.3 dB/cm) [4]. However, the resulting optical modes only partially reside in the active material region (i.e. LN), reducing the nonlinear interaction efficiency. The monolithic approach relies on direct etching of LN to achieve high optical confinement in the active region, but has suffered from a relatively high propagation loss. While freestanding LN microdisk resonators have achieved optical quality factors (Q) of ∼ 10 6 [5, 6], integrated microring resonators typically feature Q ∼ 10 5 with waveguide propagation loss on the order of 3 dB/cm [7]. Since LN is perceived as a difficult-to-etch material, it is commonly accepted that low-loss propagation in a monolithic waveguide is not possible, and that therefore it is not the most promising path forward.Here we challenge the status quo and show that subwavelength scale lithium niobate waveguides can be fabricated with a propagation loss as low as 2.7 ± 0.3 dB/m through an optimized etching process. We also demonstrate ultra-high Q factor optical cavities with intrinsic Q = 10 7 . We fabricate waveguide coupled microring and racetrack resonators with a bending radius r = 80 µm, and various straight arm lengths l and waveguide widths w (Fig. 1). The optical modes in these reso...

Since the emergence of optical fiber communications, lithium niobate (LN) has been the material of choice for electro-optic modulators, featuring high data bandwidth and excellent signal fidelity. Conventional LN modulators however are bulky, expensive and power hungry, and cannot meet the growing demand in modern optical data links. Chip-scale, highly integrated, LN modulators could offer solutions to this problem, yet the fabrication of low-loss devices in LN thin films has been challenging. Here we overcome this hurdle and demonstrate monolithically integrated LN electro-optic modulators that are significantly smaller and more efficient than traditional bulk LN devices, while preserving LN's excellent material properties. Our compact LN electro-optic platform consists of low-loss nanoscale LN waveguides, micro-ring resonators and miniaturized Mach-Zehnder interferometers, fabricated by directly shaping LN thin films into sub-wavelength structures. The efficient confinement of both optical and microwave fields at the nanoscale dramatically improves the device performances featuring a half-wave electro-optic modulation efficiency of 1.8 V∙cm while operating at data rates up to 40 Gbps. Our monolithic LN nanophotonic platform enables dense integration of high-performance active components, opening new avenues for future high-speed, low power and cost-effective communication networks.