Franz Kärtner

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.

We demonstrate a diode-pumped Yb:LLF laser that combines multi-watt semiconductor saturable absorber mirror (SESAM) mode locking with record continuous-wave (CW) tuning. Using a compact cavity pumped by a single-emitter 10 W multimode diode at 960 nm, the laser provides up to 3.2 W CW output with a tuning range from 996 to 1073 nm and a slope efficiency of 42%. In mode-locked operation, it delivers up to 2.8 W of average power with 180 fs pulses (10.9 nJ at 257.5 MHz) in a high-power regime, and up to 570 mW with 110 fs pulses (2.6 nJ at 214.1 MHz) in a short-pulse regime. Compared to previous work, this represents more than an order-of-magnitude increase in usable mode-locked power and pulse energy together with a substantially extended CW tuning span, and we outline straightforward routes to further broaden the tuning, shorten the pulses, and scale the usable average power. Taken together with earlier results on Yb:YLF, these findings indicate that Yb:LLF can serve as a competitive sister host and a strong candidate for future high-power and cryogenic Yb-based laser and amplifier studies.

High-power amplifiers are of great importance in many optical systems deployed in optical sensing, ranging, medical surgery and more. Likewise, high-gain, low-noise amplifiers with low polarization dependence are critical components of long-range optical communication systems. Integrated photonic solutions show great potential in challenging application fields thanks to their drastic reduction in size, weight and cost, but coming at the expense of low optical power due to reduced energy storage capacity in small devices. Recently, the large mode area (LMA) technology, which is known for dramatically increasing the energy storage capacity and saturation power of fiber amplifiers, has been brought to the chip-level, allowing for watt-level amplification directly from the chip. Here we demonstrate a silicon photonic LMA amplifier capable of both high-power amplification with output saturation powers > 115 mW and high small-signal gain up to 30 dB with a 3.6 dB noise figure on-chip, which we achieved by suppressing parasitic lasing with index-matching glue on the chip facets. Furthermore, we present a method to tune and completely nullify the polarization dependent gain (PDG). We believe that the PDG-tunability combined with the low-noise, high-gain and high-power amplification positions it as a potentially disruptive technology for next-generation integrated amplifiers across a range of applications.

A chromatic numerical model is presented for simulating gain dynamics in solid-state laser amplifiers. The model is based on a system of rate equations and incorporates key physical effects, including pump saturation, focused beam propagation inside the gain element, and the temperature and wavelength-dependent properties of the gain medium. By accounting for the wavelength-dependent gain, the model enables a detailed analysis of the seed pulse spectral evolution during amplification, which directly influences the duration of the Fourier-transform-limited pulse after compression. The model performance is demonstrated through simulations of cryogenically cooled amplifiers based on Yb:YLF and Yb:YAG gain materials.

Resonant dispersive-wave emission in anomalous-dispersion nonlinear media is widely regarded as intrinsically noisy, owing to strong amplification of pump-laser fluctuations during ultrafast spectral broadening. Here we show that resonant dispersive-wave emission can operate at the quantum limit, producing near-1-µm radiation from a short segment of highly nonlinear fiber pumped by a sub-30-fs, 1.5-µm ultrafast laser. Through combining numerical modeling and experiment, we disentangle the respective roles of pump noise and quantum noise in the dynamics of dispersive-wave and Raman-soliton generation. We show that, when driven by a sufficiently low-noise pump and operated under optimized nonlinear-conversion conditions, dispersive-wave timing jitter approaches its quantum limit. Using a highly sensitive timing-jitter detection scheme, we observe relative timing jitter below one femtosecond, representing the first experimental demonstration sub-femtosecond, quantum-limited dispersive-wave emission.

Multicolor two‐photon microscopy is a powerful tool for simultaneous, high‐resolution imaging of multiple cellular structures and dynamics in complex biological systems. However, its broader adoption remains limited by the complexity of existing excitation light sources, which typically rely on multi‐laser architectures, cascaded parametric conversion schemes, or heuristic broadband fiber sources. Here, we present a deterministic, simulation‐guided spectral‐engineering framework, realized in a compact ultrafast fiber‐laser platform, that enables precise design of efficient and balanced three‐color two‐photon excitation. By systematically designing and numerically co‐optimizing a dispersion‐ and gain‐engineered Yb‐doped fiber laser with subsequent nonlinear spectral shaping in a photonic crystal fiber (PCF), we achieve controlled formation of three energetic and spectrally isolated excitation bands centered at 960, 1080, and 1175 nm. Each band delivers 2.5–5.8 nJ pulse energy with sub‐115 fs duration, well‐matched to the two‐photon excitation bands of widely used fluorescent probes, without the need for multiple laser sources or parametric conversion stages. Multiplexed two‐photon imaging of triple‐stained mouse brain, kidney, and liver tissues demonstrates robust, spectrally independent visualization of fine structures, including astrocytes, neuronal architectures, and nuclei. This work establishes deterministic nonlinear spectral engineering in fiber optics as a predictive and practical route to compact, wavelength‐flexible excitation sources for high‐performance multicolor two‐photon microscopy.

This chapter discusses progress made towards the chip-scale integration of ultra-low jitter mode-locked lasers using rare-earth gain media with CMOS compatible fabrication. Such lasers generate ultra-stable pulse trains and optical frequency combs that are instrumental for signal processing systems such as optically assisted analog-to-digital converters. We also review work made towards the co-integration of optical filters for the interleaving of pulses or the upconversion of the comb’s free spectral range, as a further step towards a complete system integration.

Ultrashort electron bunches with precise pump laser synchronization and adequate charge are fundamental for time-resolved techniques like ultrafast electron diffraction (UED) and advanced light sources. Here, we achieve electron chopping and compression through a single terahertz (THz)–driven device, producing 50-femtosecond, near-femtocoulomb (fC) electron pulses with 1.4-femtosecond arrival time jitter sustained over an hour. This achieves a 35-fold pulse duration reduction and 10-fold jitter improvement compared to cases without the THz device, marking the shortest directly measured pulse duration and smallest timing jitter reported for multielectron pulses (near-fC level) at the kilo–electron volt level. High-quality electron diffraction confirms this technique’s suitability for UED. We further introduce a beam tilting correction via a tilted antenna, enabling bunch compression down to 6 femtoseconds. Furthermore, we have demonstrated its capability of reducing extended timing jitter from several hundred femtoseconds to the few-femtosecond level, offering a promising solution to the long-standing synchronization challenge between radio frequency accelerators and optical lasers. This advancement paves the way for jitter-free UED techniques with attosecond temporal resolution, thereby notably enhancing modern ultrafast applications.

The ability to precisely tailor optical waveforms provides a powerful means of controlling strong-field interactions, including high-harmonic generation (HHG) for attosecond pulse production. Although numerous tailored waveforms have been proposed to extend the harmonic cut-off or enhance conversion efficiency, achieving such improvements in the soft X-ray region remains challenging because plasma formed in the medium can strongly distort the driving field during propagation. Moreover, isolated attosecond pulse (IAP) generation demands sub-cycle waveform shaping to enhance a single attosecond burst while suppressing contributions from neighbouring optical cycles. Here, we demonstrate order-of-magnitude enhancements in IAP generation efficiency up to∼450 eV, by driving HHG in helium with tailored fields spanning the 0.7 - 2.2 μm range, surpassing a 1.5-cycle driver centred at 1.8 μm. Direct waveform measurement and simulations allow us to track the propagation of optimised optical transients through the nonlinear medium and to disentangle the intertwined microscopic and macroscopic contributions that govern soft X-ray HHG. These results establish sub-cycle waveform engineering as a robust strategy for optimising soft X-ray IAP generation and provide an experimentally validated framework for next-generation attosecond sources.

We present a diode-pumped continuous-wave (CW) Yb:YVO ₄ laser exhibiting wavelength tunability from 986 nm to 1119 nm, corresponding to a broad tuning range of 133 nm, one of the widest reported for any Yb-doped laser host. This performance was achieved by employing a low-loss, off-surface optic-axis birefringent filter, high-quality laser crystals, and optimized low-loss cavity optics, enabling smooth and stable wavelength tuning along both principal polarization axes (E//a: 986-1084 nm and E//c: 995-1119 nm). The system delivers CW output powers up to 3.75 W, with slope and optical-to-optical efficiencies reaching 70% and 48%, respectively. These results highlight Yb:YVO ₄ as an attractive gain medium for applications requiring broad tunability, compactness, and efficiency, such as ultrafast pulse generation, broadband spectroscopy, and high-energy amplifier seeding.