Tobias Herr

Synchronized laser oscillators are essential for probing the fastest processes in chemistry, materials science, and biology down to attosecond timescales. Tight synchronization is also crucial at scientific facilities such as free-electron lasers or radio telescopes and is increasingly relevant to communication and information technologies in multi-node networks. Current synchronization approaches based on mode-locked lasers achieve the required performance, but their complexity, cost, and size hinder deployment in multi-node networks. Here, we demonstrate attosecond-level synchronization between chip-integrated microresonator soliton combs operating at either 25 or 300 GHz pulse repetition rate. For synchronization, each oscillator receives over fiber a pair of continuous-wave lasers as a two-tone timing reference. The lasers power the microcombs, and Kerr-nonlinear synchronization results in integrated relative timing jitter below 400 as (1 kHz to 1 MHz), without any active stabilization. This approach enables scalable precision timing for large facilities, data centers, disaggregated computing, navigation, and quantum networks; ultimately, it may lead to chip-integrated attosecond photonics.

Self-injection-locked (SIL) narrow-linewidth lasers based on high-Q microresonators are promising sources for frequency-modulated continuous-wave (FMCW) LiDAR, but the SIL mechanism, as well as its key characteristics, such as the frequency sweep range and the noise performance, are often determined by uncontrolled backscattering in the resonator. Here, we investigate a tunable SIL laser based on a corrugated photonic-crystal (PhC) microresonator in which the feedback strength is set by design. Numerical and experimental results show that stronger SIL feedback expands the sweep range accessible through resonator modulation while also impacting the phase-noise and linewidth during sweeping, revealing a trade-off between frequency tunability and noise performance. Using CMOS-compatible microheater tuning (sub-1 V driving voltage), we demonstrate linearized up- and down-chirps with 224 THz/s over approximately 3 GHz and, in a proof-of-concept ranging experiment, measure a 10 m fiber length with a standard deviation below 3 mm. These results establish PhC microresonators with engineered SIL feedback as robust, compact, CMOS-compatible LiDAR engines.

Periodically poled thin film lithium niobate waveguides provide simultaneous access to efficient second- and third-order nonlinear processes, enabling broadband generation of coherent laser light. Here, we demonstrate the generation of a broadband mid-infrared continuum in a nanophotonic lithium niobate waveguide pumped by a telecom-wavelength femtosecond source. Specifically, our dual-stage design includes third-order nonlinear spectral broadening followed by a dedicated periodically poled waveguide section performing efficient broadband intrapulse difference-frequency generation. Driven by sub-100 fs pulses with an estimated on-chip pulse energy of around 200 pJ, the generated mid-infrared light covers wavelengths from 3200 to 4800 nm. Cascaded harmonic generation also extends the spectrum into the visible and ultraviolet domains, resulting in an overall spectral bandwidth ranging from 350 to 4800 nm.

Astronomical precision spectroscopy supports searches for exoplanets and may enable the direct observation of the Universe’s expansion. A key challenge in these efforts is the precise and long-term calibration of astronomical spectrographs. White light-illuminated Fabry–Pérot cavities, cross-calibrated with hollow-cathode lamps, are reliable calibrators. However, they exhibit temporal variability and nontrivial dispersion of the resonance frequencies, and the calibration lamps provide only sparse spectra with complex line shapes, which are also prone to aging. In contrast, laser frequency combs (“astrocombs”) can offer SI-referenced accuracy and precision, but the requirement of resolvable comb lines implies pulse repetition rates >10 GHz, which is challenging to implement. Here, we explore an alternative technique where lower-repetition rate dual-frequency comb lasers (comb lines unresolvable by the spectrograph) measure the modes of a 12 GHz Fabry–Pérot cavity across a spectral span of 30 THz (175–205 THz). This effectively transfers the comb’s accuracy to the cavity and enables precise near real-time tracking of the cavity modes. These measurements are of immediate relevance to currently used calibration techniques, and with further advances, this approach could potentially support spectrograph calibration at the level needed to search for Earth-like exoplanets.

Squeezed states of light are essential for emerging quantum technology in metrology and information processing. Chip-integrated photonics offers a route to scalable and efficient squeezed light generation, however, parasitic nonlinear processes and optical losses remain significant challenges. Here, we demonstrate single-mode quadrature squeezing in a photonic crystal microresonator via degenerate dual-pump spontaneous four-wave mixing. Implemented in a scalable, low-loss silicon-nitride photonic-chip platform, the microresonator features a tailored nano-corrugation that modifies its resonances to suppress parasitic nonlinear processes. In this way, we achieve an estimated 7.8 dB of on-chip squeezing in the bus waveguide, with potential for further improvement. These results open a promising pathway toward integrated squeezed light sources for quantum-enhanced interferometry, Gaussian boson sampling, coherent Ising machines, and universal quantum computing.

Optical frequency combs and their spectra of evenly spaced discrete laser lines are essential to modern time and frequency metrology. Recent advances in integrated photonic waveguides enable efficient nonlinear broadening of an initially narrowband frequency comb to multi-octave bandwidth. Here, we study the nonlinear dynamics in the generation of such ultrabroadband spectra where different harmonics of the comb can overlap. We show that a set of interleaved combs with different offset frequencies extending across the entire spectrum can emerge, which can be arranged into a practically evenly spaced ultrabroadband frequency comb when the initial comb is offset-free.

Optical frequency combs and their spectra of evenly spaced discrete laser lines are essential to modern time and frequency metrology. Recent advances in integrated photonic waveguides enable efficient nonlinear broadening of an initially narrowband frequency comb to multi-octave bandwidth. Here, we study the nonlinear dynamics in the generation of such ultra-broadband spectra where different harmonics of the comb can overlap. We show that a set of interleaved combs with different offset frequencies extending across the entire spectrum can emerge, which transform into a single evenly spaced ultra-broadband frequency comb when the initial comb is offset-free.

Squeezed states of light are essential for emerging quantum technology in metrology and information processing. Chip-integrated photonics offers a route to scalable and efficient squeezed light generation, however, parasitic nonlinear processes and optical losses remain significant challenges. Here, we demonstrate single-mode quadrature squeezing in a photonic crystal microresonator via degenerate dual-pump spontaneous four-wave mixing. Implemented in a scalable, low-loss silicon-nitride photonic-chip platform, the microresonator features a tailored nano-corrugation that modifies its resonances to suppress parasitic nonlinear processes. In this way, we achieve an estimated 7.8 dB of on-chip squeezing in the bus waveguide, with potential for further improvement. These results open a promising pathway toward integrated squeezed light sources for quantum-enhanced interferometry, Gaussian boson sampling, coherent Ising machines, and universal quantum computing.

Nonlinear optics is the precursor for many of the modern-day applications of photonics, including femtosecond pulse synthesis, precision spectroscopy, and metrology. In the last decade, nanophotonic waveguides have not only boosted the efficiencies of nonlinear effects but also unlocked new degrees of freedom in the design process and enabled the monolithic integration of multiple nonlinear devices. Now, the advent of thin-film variants of platforms with a strong second-order nonlinearity such as lithium niobate-on-insulator (LNOI) enables entirely new applications while further improving efficiency for the existing ones. However, suitable fabrication processes are needed to exploit the full potential of these new platforms. Here, we introduce a process for fabricating high-confinement lithium niobate waveguides with periodic poling. Our waveguide designs enable both third-order nonlinear χ(3) broadening and sum frequency generation (SFG) up to the fourth harmonic through a quasi-phase-matched χ(2) section. In supercontinuum (SC) experiments, our devices produce multi-octave SC spectra when pumped with an 80 fs mode-locked laser at 1560 nm.

Self-injection locking (SIL) of laser diodes to microresonators is a powerful technique that enables compact narrow-linewidth lasers. Here, we extend this technique to chip-integrated Fabry–Perot (FP) microresonators, which offer high-quality factors and large mode volumes in a compact footprint, reducing fundamental thermorefractive noise (TRN). The resonators consist of a silicon nitride waveguide terminated by two photonic crystal reflectors fabricated via scalable ultraviolet lithography. Evanescent side-coupling allows precise tuning of the SIL feedback mechanism. We present a theoretical model and experimentally demonstrate SIL, resulting in a fundamental thermorefractive-noise-limited laser. The experiments and the theoretical model are in excellent agreement. These results complement current SIL techniques and are relevant to chip-scale low-noise laser systems.