Octave-spanning, self-referenced frequency combs are applied in diverse fields ranging from precision metrology to astrophysical spectrometer calibration. The octave-spanning optical bandwidth is typically generated through nonlinear spectral broadening of femtosecond pulsed lasers. In the past decade, Kerr frequency comb generators have emerged as novel scheme offering chip-scale integration, high repetition rate and bandwidths that are only limited by group velocity dispersion. The recent observation of Kerr frequency combs operating in the dissipative Kerr soliton (DKS) regime, along with dispersive wave formation, has provided the means for fully coherent, broadband Kerr frequency comb generation with engineered spectral envelope. Here, by carefully optimizing the photonic Damascene fabrication process, and dispersion engineering of Si3N4 microresonators with 1 THz free spectral range, we achieve bandwidths exceeding one octave at low powers (O(100 mW)) for pump lasers residing in the telecom C-band (1.55 µm), as well as in the O-band (1.3 µm). Precise dispersion engineering enables emission of two dispersive waves, increasing the power in the spectral ends of the comb, down to a wavelength as short as 850 nm. Equally important, we find that for THz repetition rate comb states, conventional criteria applied to identify DKS comb states fail. Investigating the coherence of generated, octave-spanning Kerr comb states we unambiguously identify DKS states using a response measurement. This allows to demonstrate octave-spanning DKS comb states at both pump laser wavelengths of 1.3 µm and 1.55 µm including the broadest DKS state generated to date, spanning more than 200 THz of optical bandwidth. Octave-spanning DKS frequency combs can form essential building blocks for metrology or spectroscopy, and their operation at 1.3 µm enables applications in biological and medical imaging such as Kerr comb based optical coherence tomography or dual comb coherent anti-stokes Raman scattering.

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We report on fabrication of high-confinement and low loss silicon nitride (Si 3 N 4 ) waveguides using the photonic Damascene process. This process scheme represents a novel fabrication approach enabling reliable, wafer-scale fabrication of highconfinement optical waveguides. A reflow step of the silica preform reduces sidewall scattering to values not attainable with conventional etching, and reduces losses and backscattering significantly, resulting in a waveguide attenuation of 5.5 dB/m. We discuss the critical aspects of the process in detail and demonstrate the fabrication of high stress Si 3 N 4 waveguides with unprecedentedly large dimensions (1.75 µm × 1.425 µm) providing high-confinement at midinfrared wavelengths. A device characterization strategy allowing for systematic extraction of statistically relevant loss values is discussed and reveals the effects of the sidewall smoothing. Index Terms-Optical waveguides, optical losses, optical resonators. I. INTRODUCTION L OW loss, planar optical waveguides have the potential to be a key enabling technology for a wide range of applications, such as delay lines [1], optical gyroscopes [2], [3], ultra narrow linewidth lasers [4], compact chipscale reference cavities [5], nonlinear photonic devices [6] such as soliton Kerr frequency combs [7] or quantum photonic circuits [8], [9]. While optical fibers have attained linear losses as low as 0.2 dB/km [10] more than 40 years ago, today most chip-based planar optical

Directly accessing the middle infrared, the molecular functional group spectral region, via supercontinuum generation processes based on turn-key fiber lasers offers the undeniable advantage of simplicity and robustness. Recently, the assessment of the coherence of the mid-IR dispersive wave in silicon nitride (Si 3 N 4 ) waveguides, pumped at telecom wavelength, established an important first step towards mid-IR frequency comb generation based on such compact systems. Yet, the spectral reach and efficiency still fall short for practical implementation. Here, we experimentally demonstrate that large cross-section Si 3 N 4 waveguides pumped with 2 μm fs-fiber laser can reach the important spectroscopic spectral region in the 3–4 μm range, with up to 35% power conversion and milliwatt-level output powers. As a proof of principle, we use this source for detection of C 2 H 2 by absorption spectroscopy. Such result makes these sources suitable candidate for compact, chip-integrated spectroscopic and sensing applications.

Efficient light coupling to integrated photonic devices is of key importance to a wide variety of applications. "Inverse nanotapers" are widely used, in which the waveguide width is reduced to match an incident mode. Here, we demonstrate novel "double inverse" tapers, in which we reduce both the waveguide height and width. We demonstrate >45% chip-through coupling efficiency for both the transverse electric and transverse magnetic polarizations in SiN tapers of >500  nm width, in comparison to regular inverse tapers that necessitate <100  nm width. The double inverse tapers show polarization-independent coupling and allow the fabrication using photolithography, relevant for applications at near-infrared and visible wavelengths, e.g., supercontinuum and soliton microcomb generation.

We present an ultrafast thin-disk based multipass amplifier operating at a wavelength of 1030 nm, designed for atmospheric research in the framework of the Laser Lightning Rod project. The CPA system delivers a pulse energy of 720 mJ and a pulse duration of 920 fs at a repetition rate of 1 kHz. The 240 mJ seed pulses generated by a regenerative amplifier are amplified to the final energy in a multipass amplifier via four industrial thin-disk laser heads. The beam quality factor remains ∼ 2.1 at the output. First results on horizontal long-range filament generation are presented.

Lightning discharges between charged clouds and the Earth’s surface are responsible for considerable damages and casualties. It is therefore important to develop better protection methods in addition to the traditional Franklin rod. Here we present the first demonstration that laser-induced filaments—formed in the sky by short and intense laser pulses—can guide lightning discharges over considerable distances. We believe that this experimental breakthrough will lead to progress in lightning protection and lightning physics. An experimental campaign was conducted on the Säntis mountain in north-eastern Switzerland during the summer of 2021 with a high-repetition-rate terawatt laser. The guiding of an upward negative lightning leader over a distance of 50 m was recorded by two separate high-speed cameras. The guiding of negative lightning leaders by laser filaments was corroborated in three other instances by very-high-frequency interferometric measurements, and the number of X-ray bursts detected during guided lightning events greatly increased. Although this research field has been very active for more than 20 years, this is the first field-result that experimentally demonstrates lightning guided by lasers. This work paves the way for new atmospheric applications of ultrashort lasers and represents an important step forward in the development of a laser based lightning protection for airports, launchpads or large infrastructures.

Lightning is highly destructive due to its high power density and unpredictable character. Directing lightning away would allow to protect sensitive sites from its direct and indirect impacts (electromagnetic perturbations). Up to now, lasers have been unable to guide lightning efficiently since they were not offering simultaneously terawatt peak powers and kHz repetition rates. In the framework of the Laser Lightning Rod project, we develop a laser system for lightning control, with J-range pulses of 1 ps duration at 1 kHz. The project aims at investigate its propagation in the multiple filamentation regime and its ability to control high-voltage discharges. In particular, a field campaign at the Säntis mountain will assess the laser ability to trigger upward lightning.