David R. Carlson

We demonstrate a self-referenced and optically-stabilized Er:fiber frequency comb that does not require external amplification. The 100 MHz frequency comb operates with only 250 mW of optical pump power. This advance employs a low-noise polarization-maintaining mode-locked laser that outputs 100 pJ, 100 fs pulses to drive octave generation and self-referencing with integrated nanophotonics. Multi-day optical phase locking and outdoor operation in the Colorado mountains highlight the robustness required for future use on transportable and airborne platforms beyond the research lab, where power and space are at a premium.

Choice of material is ubiquitous in integrated photonics to design device properties, whereas changing material composition is much less common. With phase matching as an additional objective, constraints in depositing and patterning thin films limit the use of integrated nonlinear photonics to only select materials. Here, we explore an amorphous metal oxide mixture of titania (TiO ₂ ) and tantala (Ta ₂ O ₅ ) in which material composition is a tool to enhance and customize photonics properties. In particular, the inclusion of titania reduces the oxygen-defect density in a tantala film, while maintaining a comparable Kerr nonlinear index. With ion-beam sputtering at room temperature, we deposit a thick, ultralow-loss titania-tantala film, and we nanopattern it to create microresonator frequency combs. Titania-tantala microresonators offer lower loss, higher index of refraction, and reduced optical absorption and photorefractive effects. Specifically, the titania-tantala mixture enables microresonator quality factor up to 10 ⁷ and a direct factor of 1.7 reduction in optical absorption. This, along with its large transparency window, high nonlinearity, and low processing temperature requirement, positions titania-tantala as a leading material for integrated nonlinear photonics. Our work demonstrates that composition in metal-oxide mixtures is a design parameter alongside the nanofabrication process and photonics design for integrated nonlinear photonics.

The photonics landscape encompasses a wide scope of material platforms, each optimized for specific functionalities, yet no platform meets the demands of all current and evolving photonic applications. Although combining integrated-photonics materials enhances overall capability, such as unifying nonlinear optics, low-loss passive devices and electro-optics, material and process compatibility remains a major challenge. Here we introduce full-wafer, monolithic 3D integration of tantalum pentoxide (Ta₂O₅, hereafter tantala¹) photonics directly onto a patterned substrate, demonstrated here with thin-film lithium niobate². Tantala's unique properties, importantly room-temperature deposition, moderate-temperature annealing and low residual stress in thick films optimized for phase matching, make it well suited for monolithic 3D integration without compromising substrate performance or compatibility. We demonstrate low-loss, high-quality-factor microresonators and nanophotonics in tantala, robust quasi-phase-matching in poled lithium niobate waveguides³, and efficient 3D interlayer routing. These capabilities enable us to demonstrate a rich palette of nonlinear frequency conversion processes, including χ⁽³⁾ four-wave mixing for supercontinuum generation, optical parametric oscillation and dark-pulse microcomb generation in tantala microresonators and photonic crystal resonators, χ⁽²⁾ second-harmonic generation in periodically poled lithium niobate, and combinations thereof.

Many uses of lasers place the highest importance on access to specific wavelength bands. For example, mobilizing optical-atomic clocks for a leap in sensing requires compact lasers at frequencies spread across the visible and near-infrared. Integrated photonics enables high-performance, scalable laser platforms. However, customizing laser-gain media to support wholly new bands is challenging and often prohibitively mismatched in scalability to early quantum-based sensing and information systems. Here, we demonstrate a tantalum pentoxide microresonator optical-parametric oscillator (OPO) that converts a pump laser to an output wave within a frequency span exceeding an octave. We control phase matching for oscillation by nanopatterning the microresonator to open a photonic-crystal bandgap on the mode of the pump laser. The photonic crystal splits only the pump mode and preserves the broader mode structure of the resonator, thus affording a single parameter to control output waves across the octave span using a nearly fixed frequency pump laser. We also demonstrate tuning the oscillator in free-spectral-range steps, more finely with temperature, and minimal additive frequency noise of the laser-conversion process. Our work shows that nanophotonic structures offer control of laser conversion in microresonators, bridging phase-matching of nonlinear optics and application requirements for laser designs.

We demonstrate the first heterogeneously integrated high-speed waveguide photodiode on tantalum pentoxide (Ta $_{2}$ O $_{5}$ , or tantala) for visible light detection. The PDs have 100 pA dark current, more than 56% quantum efficiency (QE) between 635 nm and 780 nm wavelengths, and up to 16 GHz bandwidth. We also measured open eye diagrams at 12 Gbit/s - the upper limit of our experimental setup.

Microresonator frequency combs and their design versatility have revolutionized research areas from data communication to exoplanet searches. While microcombs in the 1550 nm band are well documented, there is interest in using microcombs in other bands. Here, we demonstrate the formation and spectral control of normal-dispersion dark soliton microcombs at 1064 nm. We generate 200 GHz repetition rate microcombs by inducing a photonic bandgap of the microresonator mode for the pump laser with a photonic crystal. We perform the experiments with normal-dispersion microresonators made from Ta2O5 and explore unique soliton pulse shapes and operating behaviors. By adjusting the resonator dispersion through its nanostructured geometry, we demonstrate control over the spectral bandwidth of these combs, and we employ numerical modeling to understand their existence range. Our results highlight how photonic design enables microcomb spectra tailoring across wide wavelength ranges, offering potential in bioimaging, spectroscopy, and photonic-atomic quantum technologies.

Frequency combs with mode spacing of 10–20 GHz are critical for increasingly important applications such as astronomical spectrograph calibration, high-speed dual-comb spectroscopy, and low-noise microwave generation. While electro-optic modulators and microresonators can provide narrowband comb sources at this repetition rate, a significant remaining challenge is a means to produce pulses with sufficient peak power to initiate nonlinear supercontinuum generation spanning hundreds of terahertz (THz) as required for self-referencing. Here, we provide a simple, robust, and universal solution to this problem using off-the-shelf polarization-maintaining amplification and nonlinear fiber components. This fiber-integrated approach for nonlinear temporal compression and supercontinuum generation is demonstrated with a resonant electro-optic frequency comb at 1550 nm. We show how to readily achieve pulses shorter than 60 fs at a repetition rate of 20 GHz. The same technique can be applied to picosecond pulses at 10 GHz to demonstrate temporal compression by 9× and achieve 50 fs pulses with a peak power of 5.5 kW. These compressed pulses enable flat supercontinuum generation spanning more than 600 nm after propagation through multi-segment dispersion-tailored anomalous-dispersion highly nonlinear fibers or tantala waveguides. The same 10 GHz source can readily achieve an octave-spanning spectrum for self-referencing in dispersion-engineered silicon nitride waveguides. This simple all-fiber approach to nonlinear spectral broadening fills a critical gap for transforming any narrowband 10–20 GHz frequency comb into a broadband spectrum for a wide range of applications that benefit from the high pulse rate and require access to the individual comb modes.

A mode-locked laser photonic integrated circuit with a repetition rate of 10 GHz is optically synchronized to a tantala-based photonic crystal resonator comb with a repetition rate of 200 GHz. The synchronization is achieved through regenerative harmonic injection locking using a coupled optoelectronic oscillator loop resulting in an optical frequency division factor of 20. The repetition rate of the photonic crystal resonator comb is stabilized and locked through electro-optic division. This stability is transferred to the mode-locked laser where we measure a fractional frequency instability of $8\times 10^{-11}$ at an averaging time of 10 s for the repetition rate signal of the mode-locked laser. Furthermore, we also measure the near carrier phase noise of the pulse repetition rate and estimate the integrated rms timing jitter of the pulses to be 6 ps.