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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.

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.

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., Comment: 15 pages, 7 figures

Efficient supercontinuum (SC) generation featuring high spectral intensity across a large bandwidth requires high peak powers of several megawatt from pulsed lasers. Under these conditions and at multi-kilohertz (kHz) repetition rates, the SC generated in most materials is unstable due to thermal effects. In this work, we leverage the superior dispersion properties of water to maximize the spectral width of the SC, while avoiding stability issues due to thermal loading by means of a constant laminar flow of the liquid. This flow is controlled by a differential pressure scheme that allows to precisely adjust the fluid velocity to an optimum value for maximum stability of the SC. This approach is successfully implemented for repetition rates of 50 kHz and 100 kHz and two different pump wavelengths in the visible (VIS) and near infrared (NIR) spectral region with stability of the SC signal only limited by the driving pulses. The resulting water SC spans more than one octave covering the VIS to NIR range. Compared to established materials, such as yttrium aluminum garnet (YAG) and sapphire, the spectral bandwidth is increased by 60 % and 40 % respectively. Our scheme has the potential to be implemented with other liquids such as bromine or carbon disulfide (CS2), which promise even wider broadening and operation up to the mid-infrared.

Dissipative Kerr solitons (DKSs) in high-Q microresonators enable applications in sensing, communication, and signal processing. Until now, DKSs driven by continuous-wave (CW) lasers are exclusively generated in ring-type resonators. Complementary to ring-type resonators, Fabry-Perot resonators could enable new approaches to dispersion engineering, addressing a key challenge of DKS technology. However, DKS generation in a CW-driven Fabry-Perot microresonator has not yet been achieved. Here, we demonstrate for the first time CW-driven DKSs in a high-Q Fabry-Perot microresonator. Fabricated in a wafer-level process, two photonic crystal reflectors in a waveguide form the chip-integrated resonator and define its dispersion. The intrinsic Q-factor of 4 million is propagation-loss limited. In principle, each cell of the photonic crystal reflector can be tailored, opening a design space beyond traditional dispersion engineering, with potential for future extension of DKSs to visible and other currently inaccessible wavelengths. Beyond DKSs, this creates opportunities for filter-driven pulse formation, engineered spectra and broadband phase-matching in microresonators., Comment: 9 pages, 4 figures

Dual-frequency comb spectroscopy permits broadband precision spectroscopic measurements with short acquisition time. A dramatic improvement of the maximal spectral bandwidth and the minimal measurement time can be expected when the lasers' pulse repetition rate is increased, owing to a quadratic dependence (Nyquist criterion). Here, we demonstrate a dual-comb system operating at a high repetition rate of 1 GHz based on mature, digitally-controlled, low-noise mode-locked lasers. Compared to conventional lower repetition rate ($\sim$100 MHz) oscillators, this represents a 100-fold improvement in terms of the Nyquist criterion, while still providing adequate spectral sampling even for trace gas absorption fingerprints. Two spectroscopy experiments are performed with acquisition parameters not attainable in a 100 MHz system: detection of water vapor absorption around 1375 nm, demonstrating the potential for fast and ambiguity-free broadband operation, and real-time acquisition of narrow gas absorption features across a spectral span of 0.6 THz (600 comb lines) in only 5 $\mu$s. Additionally, we show high mutual coherence of the lasers below the Hz-level, generating opportunities for broadband spectroscopy even with low-bandwidth detectors such as mid-infrared, imaging or photo-acoustic detectors.

Femtosecond frequency combs have boosted progress in various fields of precision metrology. Nevertheless, demanding applications such as front-end frequency and time standards, ultrastable microwave generation or high-resolution spectroscopy still necessitate improved stability. The spectral bandwidth and absolute position of individual comb lines are crucial in this context. Typically, both parameters are controlled on short and long time scales by tight locking to external optical and microwave references which represent costly and cumbersome additions to the entire setup. Here, we demonstrate fully self-controlled stabilization of a fibre-based femtosecond frequency comb requiring neither optical nor radio frequency external references. In the first step, this technology allows us to optically eliminate the carrier-envelope phase slip via ultrabroadband difference frequency generation. The resulting amplification of intrinsically quantum-limited phase noise from the mode-locked oscillator is elegantly addressed in the second step. We efficiently suppress these excess fluctuations by a direct transfer of the superior short-time noise properties of the fundamental oscillator to the offset-free comb. Our combined scheme provides a high-precision frequency reference operating completely autonomously, thus marking a new era for fibre-based sources in advanced applications ranging from space exploration to tests of the invariability of fundamental constants., Comment: 8 pages, 4 figures, submitted to: "Light: Sciences & Applications"