arXiv subjects

Tobias Herr

Publications and source records attributed to Tobias Herr.

At least 19 recordsLinked to original sources

arXiv2026

Bright solitons in hybrid-dispersion photonic crystal microresonators

Bright dissipative Kerr solitons in optical microresonators provide chip-scale sources of ultrashort pulses and frequency combs. Their properties are defined by the cavity dispersion for which fundamentally conflicting requirements exist: short pulses and broadband spectra require weak dispersion, whereas strong dispersion is associated with predictable dynamics. Here, we resolve this conflict by introducing a localized strong-dispersion section spanning several modes around the pump resonance within an otherwise weakly dispersive system. We implement this hybrid-dispersion scheme in a photonic crystal microresonator and reveal a new soliton attractor of backward-propagating solitons, accessible at low pump power in a thermally stable manner within the blue-detuned regime. The conflicting requirements for broadband spectra and low-noise single-soliton formation are reconciled, even in microwave-repetition-rate resonators, which otherwise are prone to uncontrollable multi-soliton formation. These results highlight the potential to achieve previously incompatible characteristics in nonlinear photonic systems through hybrid-dispersion attractor shaping.

physics.optics↗

arXiv2026

Photonic-crystal microresonator-based LiDAR engine

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.

physics.optics↗

arXiv2026

Frequency combs and coherent dissipative structures in nonlinear optical microresonators

Laser-driven high-Q Kerr-nonlinear optical microresonators enable parametric oscillation with low-power continuous-wave lasers and host a variety of coherent dissipative structures, including dissipative Kerr solitons and switching waves. These time-periodic structures constitute coherent optical frequency combs, and photonic-chip integration has miniaturized them to the chip scale. Such photonic-integrated, microresonator-based frequency combs - often termed 'microcombs' or 'Kerr combs' - have been demonstrated in various system-level and scientific applications. They complement femtosecond-laser-based frequency combs when high repetition rates, broad bandwidths, or high power per comb line are needed. This review introduces the field of microcombs and outlines the fundamental physical principles governing the generation of coherent frequency combs in microresonators.

physics.optics↗

arXiv2026

Test of a 34 GHz EOM laser frequency comb at ESPRESSO

Laser frequency combs (LFCs) are a promising technology for wavelength calibration of astronomical high-resolution spectrographs requiring utmost accuracy and stability, since they directly translate the fundamental SI time standard from the radio frequency regime to optical frequencies. However, they have so far seen limited use in practice, due to their complexity, incomplete wavelength coverage, but also the challenges in the data analysis they imply. Here, we present a detailed test of a 34 GHz electro-optic modulation comb with the ESPRESSO spectrograph. Using thin-film lithum-niobate waveguides for broadening and harmonic generation, the setup provides partial coverage of the IR, visible, and near-UV spectral ranges. We focus on assessing the quality of the delivered spectra and their capability to facilitate accurate and stable wavelength calibration. We present a detailed analysis of the spectrally-diffuse background, the line width, and characterize the line-spread function over a broader width than possible with the ESPRESSO facility LFC. Comparing both combs, we find strong local discrepancies in the wavelength calibration accuracy up to 15m/s , which correlate with the echellogram structure. These do not originate from the lasers, but from misalignments in the ESPRESSO calibration unit, highlighting the strong need to make instrument fiber feeds more robust to light-injection effects. Nevertheless, we demonstrate excellent stability of the wavelength calibration, with a scatter of only 17cm/s . This, however, can only be achieved when accurately modeling the non-Gaussian line-spread function, showcasing the need for advanced data analysis techniques when dealing with LFC spectra.

astro-ph.IM↗

arXiv2025

Breaking the bandwidth-efficiency trade-off in soliton microcombs via mode coupling

Dissipative Kerr solitons in optical microresonators have emerged as a powerful tool for compact and coherent frequency comb generation. Advances in nanofabrication have allowed precise dispersion engineering, unlocking octave-spanning soliton combs that are essential for applications such as optical atomic clocks, frequency synthesis, precision spectroscopy, and astronomical spectrometer calibration. However, a key challenge hindering their practical deployment is the intrinsic bandwidth-efficiency trade-off: achieving broadband soliton generation requires large pump detuning, which suppresses power coupling and limits pump-to-comb conversion efficiencies to only a few percent. Recent efforts using pulsed pumping or coupled-resonator architectures have improved efficiency to several tens of percent, yet their bandwidths remain below one-tenth of an octave, inadequate for applications demanding wide spectral coverage. Here, we overcome this limitation by harnessing mode interactions between spatial modes within a single microresonator. The mode hybridization creates an additional power-transfer channel that supports large pump detuning while maintaining strong pump-to-resonator coupling, enabling broadband soliton formation at substantially reduced pump power. Using this approach, we demonstrate an octave-spanning soliton microcomb with a record pump-to-comb conversion efficiency exceeding 50%. These results resolve the fundamental bandwidth-efficiency dilemma in soliton microcombs and paves the way toward fully-integrated, high-efficiency, ultrabroad comb sources for next-generation photonic systems.

physics.optics↗

arXiv2025

Roadmap: Emerging Platforms and Applications of Optical Frequency Combs and Dissipative Solitons

The discovery of optical frequency combs (OFCs) has revolutionised science and technology by bridging electronics and photonics, driving major advances in precision measurements, atomic clocks, spectroscopy, telecommunications, and astronomy. However, current OFC systems still require further development to enable broader adoption in fields such as communication, aerospace, defence, and healthcare. There is a growing need for compact, portable OFCs that deliver high output power, robust self-referencing, and application-specific spectral coverage. On the conceptual side, progress toward such systems is hindered by an incomplete understanding of the fundamental principles governing OFC generation in emerging devices and materials, as well as evolving insights into the interplay between soliton and mode-locking effects. This roadmap presents the vision of a diverse group of academic and industry researchers and educators from Europe, along with their collaborators, on the current status and future directions of OFC science. It highlights a multidisciplinary approach that integrates novel physics, engineering innovation, and advanced researcher training. Topics include advances in soliton science as it relates to OFCs, the extension of OFC spectra into the visible and mid-infrared ranges, metrology applications and noise performance of integrated OFC sources, new fibre-based OFC modules, OFC lasers and OFC applications in astronomy.

physics.optics↗

arXiv2025

Mid-infrared continua via spectral broadening and difference frequency generation in a nanophotonic lithium niobate waveguide

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 both 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 approximately 200 pJ pulse energy, 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.

physics.optics↗

arXiv2025

Attosecond-level synchronisation of chip-integrated oscillators

Synchronised laser oscillators are essential for probing the fastest processes in chemistry, materials science, and biology down to atto-second timescales. Tight synchronisation is also crucial at scientific facilities such as free-electron lasers or radio-telescopes, and increasingly relevant to communication and information technologies in multi-node networks. Current synchronisation 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 synchronisation between chip-integrated microresonator soliton oscillators operating at either 25 or 300 GHz pulse repetition rate. For synchronisation, each oscillator receives over fibre a pair of continuous-wave lasers as a two-tone timing reference. The lasers power the microcombs and Kerr-nonlinear synchronisation results in integrated relative timing jitter below 400 as (1 kHz to 1 MHz), without any active stabilisation. This approach enables scalable precision timing for large facilities, data centres, disaggregated computing, navigation, and quantum networks; ultimately, it may lead to chip-integrated attosecond photonics.

physics.optics↗

arXiv2025

Spectral dynamics in broadband frequency combs with overlapping harmonics

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.

physics.optics↗

arXiv2025

Quadrature squeezing in a nanophotonic microresonator

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.

physics.optics↗

arXiv2024

Laser diode self-injection locking to an integrated high-Q Fabry-Perot microresonator

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 microresonators, which offer high-quality factors and large mode volumes in a compact footprint, reducing fundamental thermo-refractive noise. 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. We achieve excellent agreement between the experimental results and theoretical model. These results complement current SIL techniques and are relevant to chip-scale low-noise laser systems.

physics.optics↗

arXiv2024

Supercontinua from integrated gallium nitride waveguides

Supercontinua are broadband spectra that are essential to optical spectroscopy, sensing, imaging, and metrology. They are generated from ultrashort laser pulses through nonlinear frequency conversion in fibers, bulk media, and chip-integrated waveguides. For any generating platform, balancing the competing criteria of strong nonlinearity, transparency, and absence of multiphoton absorption is a key challenge. Here, we explore supercontinuum generation in integrated gallium nitride (GaN) waveguides, which combine a high Kerr-nonlinearity, mid-infrared transparency, and a large bandgap that prevents two- and three-photon absorption in the technologically important telecom C-band, where compact erbium-based pump lasers exist. Using this type of laser, we demonstrate tunable dispersive waves and gap-free spectra extending to almost 4 micron in wavelength, relevant to functional group chemical sensing. Additionally, leveraging the material's second-order nonlinearity, we implement on-chip f-2f interferometry to detect the pump laser's carrier-envelope offset frequency, which enables precision metrology. These results demonstrate the versatility of GaN-on-sapphire as a new platform for broadband nonlinear photonics.

physics.optics↗

arXiv2024

Phase-stabilised self-injection-locked microcomb

Microresonator frequency combs (microcombs) hold great potential for precision metrology within a compact form factor, impacting a wide range of applications such as point-of-care diagnostics, environmental monitoring, time-keeping, navigation and astronomy. Through the principle of self-injection locking, electrically-driven chip-based microcombs with minimal complexity are now feasible. However, phase-stabilisation of such self-injection-locked microcombs, a prerequisite for metrological frequency comb, has not yet been attained. Here, we address this critical need by demonstrating full phase-stabilisation of a self-injection-locked microcomb. The microresonator is implemented in a silicon nitride photonic chip, and by controlling a pump laser diode and a microheater with low voltage signals (less than 1.5 V), we achieve independent control of the comb's offset and repetition rate frequencies. Both actuators reach a bandwidth of over 100 kHz, enabling phase-locking of the microcomb to external frequency references. These results establish photonic chip-based, self-injection-locked microcombs as low-complexity yet versatile sources for coherent precision metrology in emerging applications.

physics.optics↗

arXiv2023

Femtosecond pulse amplification on a chip

Femtosecond laser pulses enable the synthesis of light across the electromagnetic spectrum and provide access to ultrafast phenomena in physics, biology, and chemistry. Chip-integration of femtosecond technology could revolutionize applications such as point-of-care diagnostics, bio-medical imaging, portable chemical sensing, or autonomous navigation. However, current chip-integrated pulse sources lack the required peak power and on-chip amplification of femtosecond pulses has been an unresolved challenge. Here, addressing this challenge, we report >50-fold amplification of 1 GHz-repetition-rate chirped femtosecond pulses in a CMOS-compatible photonic chip to 800 W peak power with 116 fs pulse duration. This power level is 2-3 orders of magnitude higher compared to those in previously demonstrated on-chip pulse sources and can provide the power needed to address key applications. To achieve this, detrimental nonlinear effects are mitigated through all-normal dispersion, large mode-area and rare-earth-doped gain waveguides. These results offer a pathway to chip-integrated femtosecond technology with peak power-levels characteristic of table-top sources.

physics.optics↗

arXiv2023

2023 Astrophotonics Roadmap: pathways to realizing multi-functional integrated astrophotonic instruments

Photonics offer numerous functionalities that can be used to realize astrophotonic instruments. The most spectacular example to date is the ESO Gravity instrument at the Very Large Telescope in Chile. Integrated astrophotonic devices stand to offer critical advantages for instrument development, including extreme miniaturization, as well as integration, superior thermal and mechanical stabilization owing to the small footprint, and high replicability offering cost savings. Numerous astrophotonic technologies have been developed to address shortcomings of conventional instruments to date, including for example the development of photonic lanterns, complex aperiodic fiber Bragg gratings, complex beam combiners to enable long baseline interferometry, and laser frequency combs for high precision spectral calibration of spectrometers. Despite these successes, the facility implementation of photonic solutions in astronomical instrumentation is currently limited because of (1) low throughputs from coupling to fibers, coupling fibers to chips, propagation and bend losses, device losses, etc, (2) difficulties with scaling to large channel count devices needed for large bandwidths and high resolutions, and (3) efficient integration of photonics with detectors, to name a few. In this roadmap, we identify 24 areas that need further development. We outline the challenges and advances needed across those areas covering design tools, simulation capabilities, fabrication processes, the need for entirely new components, integration and hybridization and the characterization of devices. To realize these advances the astrophotonics community will have to work cooperatively with industrial partners who have more advanced manufacturing capabilities. With the advances described herein, multi-functional instruments will be realized leading to novel observing capabilities for both ground and space platforms.

astro-ph.IM↗

arXiv2023

Sideband Injection Locking in Microresonator Frequency Combs

Frequency combs from continuous-wave-driven Kerr-nonlinear microresonators have evolved into a key photonic technology with applications from optical communication to precision spectroscopy. Essential to many of these applications is the control of the comb's defining parameters, i.e., carrier-envelope offset frequency and repetition rate. An elegant and all-optical approach to controlling both degrees of freedom is the suitable injection of a secondary continuous-wave laser into the resonator onto which one of the comb lines locks. Here, we study experimentally such sideband injection locking in microresonator soliton combs across a wide optical bandwidth and derive analytic scaling laws for the locking range and repetition rate control. As an application example, we demonstrate optical frequency division and repetition rate phase-noise reduction to three orders of magnitude below the noise of a free-running system. The presented results can guide the design of sideband injection-locked, parametrically generated frequency combs with opportunities for low-noise microwave generation, compact optical clocks with simplified locking schemes and more generally, all-optically stabilized frequency combs from Kerr-nonlinear resonators.

physics.optics↗

arXiv2023

Ultraviolet astronomical spectrograph calibration with laser frequency combs from nanophotonic lithium niobate waveguides

Astronomical precision spectroscopy underpins searches for life beyond Earth, direct observation of the expanding Universe and constraining the potential variability of physical constants across cosmological scales. Laser frequency combs can provide the critically required accurate and precise calibration to the astronomical spectrographs. For cosmological studies, extending the calibration with such astrocombs to the ultraviolet spectral range is highly desirable, however, strong material dispersion and large spectral separation from the established infrared laser oscillators have made this exceedingly challenging. Here, we demonstrate for the first time astronomical spectrograph calibrations with an astrocomb in the ultraviolet spectral range below 400 nm. This is accomplished via chip-integrated highly nonlinear photonics in periodically-poled, nano-fabricated lithium niobate waveguides in conjunction with a robust infrared electro-optic comb generator, as well as a chip-integrated microresonator comb. These results demonstrate a viable route towards astronomical precision spectroscopy in the ultraviolet and may contribute to unlocking the full potential of next generation ground- and future space-based astronomical instruments.

physics.optics↗

arXiv2023

Watt-class CMOS-compatible power amplifier

Power amplifier is becoming a critical component for integrated photonics as the integrated devices try to carve out a niche in the world of real-world applications of photonics. That is because the signal generated from an integrated device severely lacks in power which is due mainly to the small size which, although gives size and weight advantage, limits the energy storage capacity of an integrated device due to the small volume, causing it to rely on its bench-top counterpart for signal amplification downstream. Therefore, an integrated high-power signal booster can play a major role by replacing these large solid-state and fiber-based benchtop systems. For decades, large mode area (LMA) technology has played a disruptive role by increasing the signal power and energy by orders of magnitude in the fiber-based lasers and amplifiers. Thanks to the capability of LMA fiber to support significantly larger optical modes the energy storage and handling capability has significantly increased. Such an LMA device on an integrated platform can play an important role for high power applications. In this work, we demonstrate LMA waveguide based CMOS compatible watt-class power amplifier with an on-chip output power reaching ~ 1W within a footprint of ~4mm2.The power achieved is comparable and even surpasses many fiber-based amplifiers. We believe this work opens up opportunities for integrated photonics to find real world application on-par with its benchtop counterpart.

physics.optics↗