arXiv subjects

Ingo Breunig

Publications and source records attributed to Ingo Breunig.

At least 19 recordsLinked to original sources

arXiv2026

Recirculating frequency-shifting loop for flexible optical chirp generation and FMCW LiDAR

A recirculating frequency-shifting loop (FSL) provides a highly flexible platform for generating coherent optically chirped light with tunable bandwidth, duration, chirp rate and repetition rate. The properties of the chirped light are controlled using low-frequency sinusoidal electronic drive signals, enabling deterministic waveform synthesis without complex high-speed electronics. We achieve chirp bandwidths of 10 GHz with a duration in the nanosecond regime, representing one of the fastest tunable laser sources to date. Using such chirped laser pulses, we demonstrate coherent FMCW LiDAR measurements over distances up to 3 m, highlighting the potential of FSL-based sources for compact, scalable and high-performance ranging systems.

physics.optics↗

arXiv2026

Electro-optic frequency combs for multi-wavelength digital holography with high dynamic range

Multi-wavelength digital holography enables surface-shape measurements with an exceptional dynamic range by combining interferometric resolution with synthetic wavelengths spanning multiple length scales. Although the concept promises measurement ranges of many orders of magnitude, its practical implementation is limited by the lack of light sources that allow fast, reliable, and calibration-free switching between synthetic wavelengths over a wide frequency range. Here, we present a synthetic-wavelength generator based on an electro-optic frequency comb with electronically tunable modulation frequency and a set of switchable band-pass filters. By combining discrete selection of comb-lines with continuous radio-frequency tuning, the proposed scheme merges the advantages of single-sideband modulation and filter-based comb extraction. Using only off-the-shelf components, the system provides synthetic frequencies from 0.1-220GHz, corresponding to synthetic wavelengths from meters down to millimeters in the visible. The generator achieves MHz-level frequency accuracy, side-mode suppression exceeding 40dB, and switching times below 25ms, even without active stabilization. We characterize the spectral purity and frequency agility of the source and demonstrate rapid tuning of synthetic wavelengths over 3 orders of magnitude. We apply the light source to multi-wavelength digital holography and reconstruct the surface of an industrially machined metal part featuring height variations from 0.1-100mm. The measurements achieve ten-mum-level precision using 7 single wavelengths covering synthetic wavelengths from 1.36mm to 1.874m within an acquisition time < 2s. The presented architecture combines high dynamic measurement range of 50dB, fast electronic reconfigurability, and intrinsic frequency calibration, making it a promising light source for high-speed interferometric surface metrology.

physics.optics↗

arXiv2025

Influence of mechanical resonances on the linearity of adiabatic frequency conversion in whispering gallery resonators

Adiabatic frequency conversion enables fast and efficient tuning of laser light by coupling it into an optical resonator whose eigenfrequency is varied on a timescale shorter than its photon lifetime. In this regime, the optical frequency follows the cavity resonance, allowing frequency shifts of several hundred gigahertz within sub-microsecond time - independent of optical power and without phase-matching constraints. While a linear dependence of the cavity resonance on a control parameter (e.g., applied voltage) suggests that arbitrary temporal signals could be linearly transferred to optical frequency changes, we show that this assumption fails near mechanical resonances of the resonator. Using a millimeter-sized lithium niobate whispering gallery resonator with a pronounced mechanical mode at 10.5 MHz, we observe strong deviations from linearity even when higher harmonics of the control signal coincide with this resonance. The experimental results are in excellent agreement with theoretical predictions. They demonstrate that mechanical resonances impose intrinsic limits on the linearity of adiabatic frequency conversion and other frequency control schemes based on the variation of the eigenfrequency of an optical cavity.

physics.optics↗

arXiv2025

Electrically-induced resonance shifts of whispering gallery resonators made of barium magnesium fluoride

Barium magnesium fluoride (BMF) is a ferroelectric crystal with a transparency range far beyond the one of other optical materials. In particular, its low loss in the deep ultraviolet makes this material an unique candidate for frequency conversion in this spectral range. Due to its relatively weak second-order nonlinearity, a resonant configuration such as an optical whispering gallery would be beneficial. We show that femtosecond-laser based material processing enables the reliable fabrication of BMF whispering gallery resonators with quality factors beyond $10^7$. Their resonance frequencies can be shifted linearly by applying electric fields between the $+c$ and $-c$ faces of the crystal. The slope of the shift is $-0.8$~MHz/(V/mm). It seems that the origin of this shift is piezoelectricity, while the electro-optic effect is negligible. Our results pave the way for millimeter-sized frequency converters in the deep ultraviolet. Furthermore, they indicate that a careful determination of fundamental material properties is still necessary.

physics.optics↗

arXiv2025

Dynamically reconfigurable multi-wavelength interferometry

Single-wavelength interferometry achieves high resolution for smooth surfaces but struggles with rough, industrially relevant ones due to limited unambiguous measuring range and speckle effects. Multi-wavelength interferometry addresses these challenges by using synthetic waveleths, enabling a balance between extended measurement range and resolution by combining several synthetic wavelengths. This approach holds immense potential for diverse industrial applications, yet it remains largely untapped due to the lack of suitable light sources. Existing solutions are constrained by limited flexibility in synthetic-wavelength generation and slow switching speeds. We demonstrate a light source for multi-wavelength interferometry based on electro-optic single-sideband modulation. It reliably generates synthetic wavelengths with arbitrary values from centimeters to meters and switching times below 30 ms. This breakthrough paves the way for dynamic, reconfigurable multi-wavelength interferometry capable of adapting to complex surfaces and operating efficiently even outside laboratory settings. These capabilities unlock the full potential of multi-wavelength interferometry, offering unprecedented flexibility and speed for industrial and technological applications.

physics.optics↗

arXiv2023

Continuous adiabatic frequency conversion for FMCW-LiDAR

Continuous tuning of the frequency of laser light is the cornerstone for a plethora of applications in basic science as well as in the industrial environment. They range from the hunt for gravitational waves, the realization of optical clocks over health and environmental monitoring to distance measurements. So far, it is difficult to combine a wide tuning range (> 100 GHz) with sub-microsecond tuning times, intrinsic tuning linearity and coherence lengths beyond 10 m. We show that electro-optically driven adiabatic frequency converters using high Q microresonators made of lithium niobate are able to transfer arbitrary voltage signals into frequency chirps on time scales far below 1 {\mu}s. The temporal variation of the frequency nicely agrees with the one of the voltage applied. Superimposing the converted light with the unconverted one, we derived from the beat signal that 200-ns-long linear frequency chirps deviate less than 1 % from perfectness without any additional measures. The Coefficient of Determination is R2 > 0.999. The coherence length of the emitted light is of the order of 100 m. In order to prove these findings, we apply linear frequency sweeps for FMCW LiDAR for distances between 0.5 and 10 m without any signs of nonlinearity. Combined with the demonstrated ns tuning, with the potential to tune the eigenfrequency of lithiumniobate-based resonators by several 100 GHz, our results show that electro-optically driven adiabatic frequency converters can be used in applications that require ultrafast and flexible continuous frequency tuning with intrinsic linearity and large coherence length.

physics.optics↗

arXiv2022

Mid-infrared frequency combs and staggered spectral patterns in $χ^{(2)}$ microresonators

The potential of frequency comb spectroscopy has aroused great interest in generating mid-infrared frequency combs in the integrated photonic setting. However, despite remarkable progress in microresonators and quantum cascade lasers, the availability of suitable mid-IR comb sources remains scarce. Here, we present a new approach for the generation of mid-IR microcombs through cascaded three-wave-mixing. By pumping a CdSiP$_2$ microresonator at 1.55 $μ$m wavelength with a low power continuous wave laser, we generate $χ^{(2)}$ frequency combs at 3.1 $μ$m wavelength, with a span of about 30 nm. We observe ordinary combs states with a line spacing of the free spectral range of the resonator, and combs where the sideband numbers around the pump and half-harmonic alternate, forming staggered patterns of spectral lines. Our scheme for mid-IR microcomb generation is compatible with integrated telecom lasers. Therefore, it has the potential to be used as a simple and fully integrated mid-IR comb source, relying on only one single material.

physics.optics↗

arXiv2022

Electro-optically tunable single-frequency lasing from neodymium-doped lithium niobate microresonators

Tunable light sources are a key enabling technology for many applications such as ranging, spectroscopy, optical coherence tomography, digital imaging and interferometry. For miniaturized laser devices, whispering gallery resonator lasers are a well-suited platform, offering low thresholds and small linewidths, however, many realizations suffer from the lack of reliable tuning. Rare-earth ion-doped lithium niobate offers a way to solve this issue. Here we present a single-frequency laser based on a neodymium-doped lithium niobate whispering gallery mode resonator that is tuned via the linear electro-optic effect. Using a special geometry, we suppress higher-order transverse modes and hence ensure single-mode operation. With an applied voltage of just $68\,$V, we achieve a tuning range of $3.5\,$GHz. The lasing frequency can also be modulated with a triangular control signal. The freely running system provides a frequency and power stability of better than $Δν=20\,$MHz and 6%, respectively, for a 30 minute period. This concept is suitable for full integration with existing photonic platforms based on lithium niobate.

physics.optics↗

arXiv2021

Dual backgrounds and their stability during $χ^{(2)}$ comb generation in microresonators

Light states relevant to the $χ^{(2)}$ comb generation in optical microresonators possess typically dual backgrounds for coupled first-harmonic (FH) and second-harmonic (SH) envelopes. Stability and/or instability of these backgrounds is crucial for realization of stable $χ^{(2)}$ combs and also for an efficient SH generation. We explore the properties of the dual backgrounds and their instability for the cases of FH and SH pumping of the resonator. In contrast to the optical parameteric oscillation, the instability is controlled by a 4th-degree characteristic equation for the increment. Coefficients of this equation depend not only on wavenumbers of the perturbations, the pump power, and dispersion parameters, but also on FH-SH group velocity difference (temporal walk-off). Our results include characterization of the regions and conditions of stability for the FH- and SH-pumping cases and different spectral ranges.

physics.optics↗

arXiv2021

Optical-parametric-oscillation-based $χ^{(2)}$ frequency comb in a lithium niobate microresonator

Microresonator frequency combs based on the $χ^{(3)}$ nonlinearity are nowadays well understood and making their way into different applications. Recently, microresonator frequency combs based on the $χ^{(2)}$ nonlinearity are receiving increasing attention, as they promise certain benefits, but still require further study. Here, we demonstrate the generation of a $χ^{(2)}$ frequency comb, initiated via optical parametric oscillation (OPO) in a lithium niobate mm-sized microresonator. By pumping at 532 nm with 30 $μ$W of power, we observe 1-THz-wide comb spectra around 1064 nm with degenerate and non-degenerate states. We also show that comb generation requires signal and idler waves to be degenerate in mode numbers and how the fulfillment of this condition can be identified from the temperature tuning curves. The results demonstrate the potential to directly generate frequency combs via OPO beyond 3 $μ$m wavelengths in the mid-IR by puming in the near-IR region.

physics.optics↗

arXiv2021

Fine structure of second-harmonic resonances in χ^{(2)} optical microresonators

Owing to the discrete frequency spectrum of whispering gallery resonators (WGRs), the resonance and phase-matching conditions for the interacting waves in the case of second-harmonic generation (SHG) cannot generally be fulfilled simultaneously. To account for this, we develop a model describing SHG in WGRs with non-zero frequency detunings at both the pump and second-harmonic frequencies. Our model predicts strong distortions of the line shape of pump and second-harmonic resonances for similar linewidths at both frequencies; for much larger linewidths at the second-harmonic frequency, this behavior is absent. Furthermore, it describes the SHG efficiency as a function of detuning. Experimentally, one can change the WGR eigenfrequencies, and thus the relative detuning between pump and second-harmonic waves by a number of means, for example electro-optically and thermally. Using a lithium niobate WGR, we show an excellent quantitative agreement for the SHG efficiency between our experimental results and the model. Also, we show the predicted distortions of the pump and second-harmonic resonances to be absent in the lithium niobate WGR, but present in a cadmium silicon phosphide WGR, as expected from the linewidths of the resonances involved.

physics.optics↗

arXiv2020

Frequency comb generation threshold in $χ^{(2)}$ optical microresonators

We investigate the threshold of $χ^{(2)}$ frequency comb generation in lithium niobate whispering gallery microresonators theoretically and experimentally. When generating a frequency comb via second-harmonic generation, the threshold for the onset of cascaded second-order processes leading to a comb is found to be approximately 85 $μ$W. The second-harmonic generation efficiency up to this value is in excellent agreement with a previously known theoretical framework. This framework is extended here, showing that the onset of cascaded $χ^{(2)}$ processes and the maximum of the second-harmonic generation efficiency coincide. Furthermore, we observe that the frequency distance between the comb lines is a function of the pump power. It changes from 4 free spectral ranges at the oscillation threshold to 1 free spectral range at 590 $μ$W.

physics.optics↗

arXiv2020

Walk-off controlled self-starting frequency combs in $χ^{(2)}$ optical microresonators

Investigations of frequency combs in $χ^{(3)}$ optical microresonators are burgeoning nowadays. Changeover to $χ^{(2)}$ resonators promises further advances and brings new challenges. Here, the comb generation entails not only coupled first and second harmonics (FHs and SHs) and two dispersion coefficients, but also a substantial difference in the group velocities - the spatial walk-off. We predict walk-off controlled highly stable comb generation, drastically different from that known in the $χ^{(3)}$ case. This includes the general notion of antiperiodic state, formation of coherent antiperiodic steady states (solitons), where the FH and SH envelopes move with a common velocity without shape changes, characterization of the family of antiperiodic steady states, and the dependence of comb spectra on the pump power and the group velocity difference.

physics.optics↗

arXiv2020

Pockels-effect-based adiabatic frequency conversion in ultrahigh-$Q$ microresonators

Adiabatic frequency conversion has some key advantages over nonlinear frequency conversion. No threshold and no phase-matching conditions need to be fulfilled. Moreover, it exhibits a conversion efficiency of $100\,\%$ down to the single-photon level. Adiabatic frequency conversion schemes in microresonators demonstrated so far suffer either from low quality factors of the employed resonators resulting in short photon lifetimes or small frequency shifts. Here, we present an adiabatic frequency conversion (AFC) scheme by employing the Pockels effect. We use a non-centrosymmetric ultrahigh-$Q$ microresonator made out of lithium niobate. Frequency shifts of more than $5\,$GHz are achieved by applying just $20\,$V to $70$-micrometer-thick crystal. Furthermore, we demonstrate that already with the same setup positive and a negative frequency chirps can be generated. With this method, by controlling the voltage applied to the crystal, almost arbitrary frequency shifts can be realized. The general advances in on-chip fabrication of lithium-niobate-based devices make it feasible to transfer the current apparatus onto a chip suitable for mass production.

physics.optics↗

arXiv2019

Frequency comb generation via cascaded second-order nonlinearities in microresonators

Optical frequency combs are revolutionising modern time and frequency metrology. In the past years, their range of applications has increased substantially, driven by their miniaturisation through microresonator-based solutions. The combs in such devices are typically generated using the third-order $χ^{(3)}$-nonlinearity of the resonator material. An alternative approach is making use of second-order $χ^{(2)}$-nonlinearities. While the idea of generating combs this way has been around for almost two decades, so far only few demonstrations are known, based either on bulky bow-tie cavities or on relatively low-$Q$ waveguide resonators. Here, we present the first such comb that is based on a millimetre-sized microresonator made of lithium niobate, that allows for cascaded second-order nonlinearities. This proof-of-concept device comes already with pump thresholds as small as 2 mW, generating repetition-rate-locked combs around 1064 nm and 532 nm. From the nonlinear dynamics point of view, the observed combs correspond to the Turing roll patterns.

physics.optics↗

arXiv2019

Electro-optic eigenfrequency tuning of potassium tantalate-niobate microresonators

Eigenfrequency tuning in microresonators is useful for a range of applications including frequency-agile optical filters and tunable optical frequency converters. In most of these applications, eigenfrequency tuning is achieved by thermal or mechanical means, while a few non-centrosymmetric crystals such as lithium niobate allow for such tuning using the linear electro-optic effect. Potassium tantalate-niobate ($\mathrm{KTa}_{1-x}\mathrm{Nb}_{x}\mathrm{O}_{3}$ with $0\leq x \leq1$, KTN) is a particularly attractive material for electro-optic tuning purposes. It has both non-centrosymmetric and centrosymmetric phases offering outstandingly large linear as well as quadratic electro-optic coefficients near the phase transition temperature. We demonstrate whispering-gallery resonators (WGRs) made of KTN with quality factors of $Q>10^{7}$ and electro-optic eigenfrequency tuning of more than 100 GHz at $λ=1040\,$nm for moderate field strengths of $E=250\,$V/mm. The tuning behavior near the phase transition temperature is analyzed by introducing a simple theoretical model. These results pave the way for applications such as electro-optically tunable microresonator-based Kerr frequency combs.

physics.optics↗

arXiv2019

Nonlinear solutions for χ^(2) frequency combs in optical microresonators

Experimental and theoretical studies of nonlinear frequency combs in χ^(3) optical microresonators attracted tremendous research interest during the last decade and resulted in prototypes of solitonbased steadily working devices. Realization of similar combs owing to χ^(2) optical nonlinearity promises new breakthroughs and is a big scientific challenge. We analyze the main obstacles for realization of the χ^(2) frequency combs in high-Q microresonators and propose two families of steadystate nonlinear solutions, including soliton and periodic solutions, for such combs. Despite generic periodicity of light fields inside microresonators, the nonlinear solutions can be topologically different and relevant to periodic and antiperiodic boundary conditions. The found particular solutions exist owing to a large difference in the group velocities between the first and second harmonics, typical of χ^(2) microresonators, and to the presence of the pump. They have no zero-pump counterparts relevant to conservative solitons. Stability issue for the found comb solutions remains open and requires further numerical analysis.

physics.optics↗

arXiv2019

Frequency combs in a microring optical parametric oscillator

We report the soliton frequency comb generation in microring optical parametric oscillators operating in the down-conversion regime and with the simultaneous presence of the $χ^{(2)}$ and Kerr nonlinearities. The combs are studied considering a typical geometry of a bulk LiNbO$_3$ toroidal resonator with the normal group velocity dispersion spanning an interval between the pump and the down-converted signal. We have identified critical power signaling a transition between the relatively low pump power predominantly $χ^{(2)}$ combs and the high pump power ones shaped by the competition between the $χ^{(2)}$ and Kerr nonlinearities.

physics.optics↗