The generation of electro-optic (EO) combs in Thin-Film Lithium Niobate (TFLN) based photonic integrated circuits (PICs) has been demonstrated in several publications [1]–[4]. Despite a solid understanding of the design of the optical resonator [3], the optimization of the Radio-Frequency (RF) electrodes is less explored. Here we inves-tigate how electrodes lengths and feeding points influence the EO comb modulation efficiency. In fact, contrary to EO fiber cavities, PIC based EO combs are often operated generating RF standing (and not traveling) waves. This results in a non-trivial spatial and time dependency of the local phase shift experienced by light along the ring. Here, we report an analytical model, based on transmission line theory, that allows to simulate the EO comb conversion efficiency for different electrode length and feeding point. The model only requires parameters (RF losses, RF index, characteristic impedance) that can be either simulated from the cross-section or directly measured on the sample. Fig. 1 a), b) and c) show the simulated accumulated phase by the optical wave-packet propagating over one round-trip in the resonator. The $X$ axis corresponds to the electrodes length $L_{e}$, normalized to the minimal resonator's length $(L_{\min})$ that has free-spectral-range (FSR) equal to the driving RF frequency (fixed). The insets show the three different RF feeding configurations studied. The dotted line corresponds to RF frequency $(\nu_{m})$ being an even multiple $(k)$ of the ring's FSR, while the solid one corresponds to odd multiples. Grey area represents allowed electrode length choice for $k=3$. From the model we infer that: it is generally advantageous to use longer rings driven at a higher multiple of their FSR, while for standing-wave configuration (b) and c)), a well-defined electrode lengths maximize the phase shift, and this is shorter than the ring length (e.g. for $k=3,4L_{e}/L_{\min}=2\rightarrow L_{e}=L_{\min}/2)$, for central RF feeding (configuration a)), driving the resonator at even multiples of its FSR always gives zero phase accumulated during one round-trip. The model was validated experimentally by measuring 5 sets of 5 rings with the same FSR but different electrodes length (Fig 1 g)). The samples have been fabricated at ETH Zurich following the process described in [4]. The measured modulation efficiencies $\beta$ defined as in [5], are extracted from the EO comb bandwidth (Fig. 1 e)) and plotted after renormalization in Fig. 1f). The error bars come from measurements of different samples. A good agreement is present with the analytical model (dashed lines). In conclusion: we have developed and experimentally validated an analytical model to predict the EO comb bandwidth for different feeding configurations, electrodes length and excitation regimes. The model helps in optimizing design of the RF co-planar waveguide, to increase conversion efficiency for a given excitation frequency and feeding scheme. This work is funded by the Swiss-National-Science-Fundation: project number 194693.

Gallium Nitride (GaN) has high nonlinear coefficients $(\chi^{2}=20 p m / V, \chi^{3}=2.2 \cdot 10^{-20} p m^{2} / V^{2})$ that, in combination with its mature fabrication process and its space graded qualification, make it extremely interesting for nonlinear optical devices based on Photonics Integrated Circuits (PICs). Here we present the design, fabrication and testing of a GaN waveguides for detecting the carrier envelope offset frequency $(f_{ceo})$ of a mode-locked laser within a single PIC. Dispersion engineering is crucial for the efficiency of the $f_{ceo}$ generation process. The final goal is to ensure the frequency overlap between the dispersive wave (DW) generated through the Supercontinuum and the second harmonic (SH) of the signal. We selected by numerical Finite Element Method (FEM) simulations an optimized waveguide cross-section height of $725nm$, etch depth of 435nm and length of 1cm. We then fabricated and tested 29 waveguides with different widths to fine tune the DW position and compensate for fabrication tolerances. The waveguides are fabricated by deep UV lithography and reactive ion etching. The setup used for the experiment is shown in Fig. 1a). A mode locked laser (repetition rate 100MHz, pulse length 120 $fs$) is injected into the system from right hand side. A half-wave plate in combination with a polarizer controls the input pulse energy. Light is coupled into the quasi-TM00 mode of the waveguide with 5.3dB loss. The chip output is directed either to an OSA, to measure the optical spectrum, or to a fast Photodiode where the $f_{ceo}$ signal is recorded, filtered, amplified and then measured with an electrical spectrum analyzer (ESA). The spectra from all the waveguides are reported (interpolated) in the spectrogram of Fig. 1 b). We clearly see how the DW location changes with waveguide width (left side of the plot). Also, we see the broadening of the pump centered around 1560nm due to self-phase modulation. The SH signal at 780nm is not distinguishable and burried in DW signal. We benchmarked the recorded data by simulating the Supercontinuum dynamics with a generalized nonlinear Schrödinger equation (GNLSE) code [1]. Fig. 1c reports a good agreement between the experimental and theoretical location of the DW position showing the capability to effectively engineer DW location and increase $f_{ceo}$ efficiency. Octave-spanning Supercontinuum generation with good $f_{ceo}$ signal has been detected for most of waveguides while the waveguide featuring a width of 1.2um showed $f_{ceo}$ detection with the highest SNR and the lowest pulse energy. The position of the DW at 710nm, suggests that SH is generated from a blue-shifted part of the broadened input pulse. In Fig. 1 d) we plot three spectra for such waveguide corresponding to pulse energies of 30 $pJ$, 62 $pJ, 118pJ$. The 62 $pJ$ pulse, equivalent to a pulse power of 517W generates an $f_{ceo}$ shown in Fig. 1e), featuring an SNR of 30dB measured at a resolution bandwidth of 100KHz and at video bandwidth of 100H z. Such values are sufficient to use the $f_{ceo}$ signal for laser stabilization [2]. In conclusion, dispersion engineered waveguides on GaN PICs for low-energy octave spanning supercontinuum generation was shown. Pulse energy is on pair with state-of-the art f-2f scheme based on $\chi^{3}$ non linearity in one single waveguide [3] and are eight times lower than results reported in previous demonstration in GaN [4]. Building on the feedback of this experiment, further improvements in the design and fabrication are under study. The aim is to obtain a lower power $f_{ceo}$ measurements with higher SNR by enhancing SH generation and optimizing the spectral overlap with the DW. This work was supported by European Space Agency (contract no. 4000144309/24/NL/GLC/ov).

The detection of exoplanets by measurement of periodic Doppler shifts in the stellar spectrum [1] marked a significant breakthrough in our understanding of star system compositions. After the success of radial velocity measurements in the visible spectral range with spectrographs such as HARPS [1] and ESPRESSO [2], observations expand to the near-infrared region. The Near-Infrared Planet Searcher (NIRPS) consortium [3], developed a spectrograph operating at the La Silla Observatory in Chile, aiming to achieve radial velocity precision better than 1 m/s [4]. Among various critical factors, accurate and stable calibration of the spectrograph is essential to mitigate system drifts. Common calibration sources include atomic lamps, Fabry-Perot etalons, and laser frequency combs [5]–[7].

We have implemented machine learning techniques into a mid-infrared gas spectrometer for two specific goals: the improvement of chemometric analysis using artificial neural networks and geostatistical analysis over a geographic area using Kriging.

Poling the bottom of etched LNOI waveguides enables modal phase matching with an anti-symmetric mode and a measured second harmonic generation efficiency of 1980 %W -1 cm -2 . The simple design allows for reproducible on-chip frequency conversion.

We present the development of a transportable laser frequency stabilization system with application to both optical clocks and a next-generation gravity mission (NGGM) in space. This effort leverages a 5-cm long cubic cavity with crystalline coatings operating at room temperature and with a center wavelength of 1064 nm. The cavity is integrated in a custom vacuum chamber with dedicated low-noise locking electronics. Our vacuum-mounted cavity and control system are well suited for space applications, exhibiting state-of-the-art noise performance while being resilient to radiation exposure, vibration, shock, and temperature variations. Furthermore, we demonstrate a robust means of automatically (re)locking the laser to the cavity when resonance is lost. We show that the mounted cavity is capable of reaching technology readiness level (TRL) 6, paving the way for high-performance ultrastable laser systems and eventually optical atomic clocks amenable to future satellite platforms.

The rubidium two-photon optical atomic clock is a promising technology for applications that require a compact yet stable timekeeping device. However, its long-term frequency stability is often limited by the light shift (or AC Stark effect). In a rubidium two-photon frequency standard, we demonstrate in experiment that the light shift of the 778.1 nm wavelength probe beam can be mitigated with another, 1556.2 nm wavelength beam.

The performance of a two-photon atomic clock currently developed at CSEM in partnership with Rolex is reported. This clock, designed in view of its integration in a 19-inch rack-mount enclosure, is intended for 24/7 operation as Rolex timescales master clock. Its design, based on a standard architecture, takes advantage of the high-reliability and high-availability of telecom C-band components. Long-term stability limited to a few 10 -15 by the cell helium permeation drift is demonstrated. A drift-removed relative frequency stability in the 10 -15 range at 10 5 s and beyond is achieved, limited by the residual AC Stark-shift.

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.

Driven by the realization of a fiber-based mid-IR broadband supercontinuum light source, this study reports on the design and realization of a field-compatible and portable Fourier Transform Spectrometer for comprehensive air pollution monitoring.

Stabilized dual optical frequency transfer is demonstrated through wavelength- division or polarization multiplexing in a 184-meter long polarization-maintaining fiber link. The latter is stabilized at a primary frequency in the telecom C-band using the established Doppler cancellation technique. Simultaneously, a secondary optical frequency is transferred in the same fiber. Out-of-loop characterization demonstrates an indirect Doppler cancellation for the secondary optical frequency. Compared to an unstabilized link, at 1000 seconds integration time we measure an 11.5 dB stability improvement for wavelength-division multiplexing and a 16 dB improvement for polarization multiplexing. Taking advantage of a stabilized link to distribute other wavelengths is useful for applications in frequency metrology. As an example, we are using a cavity-stabilized 1560 nm laser to stabilize the fiber link while a 1556.2 nm two-photon rubidium clock laser is being distributed. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

This paper reports on the experimental characterization by means of optical frequency-domain reflectometry of a White-type multipass gas cell used for trace gas spectroscopy. The fractional Lambertian reflections inevitably arising from the three high reflectivity mirrors of this multipass cell is precisely detected due to the high sensitivity of the reflectometer. Each bounce of light on the mirror surface generates backscattered light, which returns to the sensing system. Then, using the measured distribution of multiple back-reflections as a function of distance the position of the 3mm-thick CaF2 entrance window is clearly identified, thanks to the spatial resolution of 731μm. In addition, the physical distance between mirrors at both sides of the cavity is accurately assessed to be 40.72cm, delivering the exact optical path length of light inside the multipass cell of 30.9853m, which is an important parameter for improving the accuracy of the computation to retrieve the gas concentration from the measured light absorption spectrum.

We applied an artificial neural network to a mid-infrared trace gas sensing system to completely compensate the detrimental thermally-induced spectral shift of the spectrometer, improving the accuracy of the retrieved gas concentration.

Quantum sensing devices such as atomic clocks enable unmatched precision in various area of metrology. Initially bulky laboratory devices, it is of great interest to miniaturize them to lower their energy consumption and deploy them in many embedded and mobile systems. To allow a dramatic cost reduction and miniaturization, CSEM developed and tested with success miniature atomic clocks based on wafer-level processes. On top of the control electronics and the atomic vapor cells, the optical design, the optical components and their assembly have been fully redesigned to be wafer-level fabricated. To achieve low aspect ratio and integrated optical management, thin glass planar waveguides have been implemented allowing to transport and beam shape the interrogating beam going from and to the atomic vapor gas cell. This proved a much simpler wafer scale assembly process, a monolithic construction less prone to single component alignment issues and provide much more compact atomic clocks

We demonstrate the successful implementation of an artificial neural network (ANN) to eliminate detrimental spectral shifts imposed in the measurement of laser absorption spectrometers (LASs). Since LASs rely on the analysis of the spectral characteristics of biological and chemical molecules, their accuracy and precision is especially prone to the presence of unwanted spectral shift in the measured molecular absorption spectrum over the reference spectrum. In this paper, an ANN was applied to a scanning grating-based mid-infrared trace gas sensing system, which suffers from temperature-induced spectral shifts. Using the HITRAN database, we generated synthetic gas absorbance spectra with random spectral shifts for training and validation. The ANN was trained with these synthetic spectra to identify the occurrence of spectral shifts. Our experimental verification unambiguously proves that such an ANN can be an excellent tool to accurately retrieve the gas concentration from imprecise or distorted spectra of gas absorption. Due to the global shift of the measured gas absorption spectrum, the accuracy of the retrieved gas concentration using a typical least-mean-squares fitting algorithm was considerably degraded by 40.3%. However, when the gas concentration of the same measurement dataset was predicted by the proposed multilayer perceptron network, the sensing accuracy significantly improved by reducing the error to less than ±1% while preserving the sensing sensitivity.

The efficiency and quality of ultrashort pulse amplification decisively depends on the gain medium of the power amplifier. In order to achieve high average and peak powers while guaranteeing good beam quality and optimal pulse recompression, the chosen gain material must have both a wide gain bandwidth and good thermo-optical properties. One approach to obtain high power ultrashort pulses is to use the Chirped Pulse Amplification (CPA) technique with ytterbium doped crystals having a wide gain bandwidth. In the last amplification stage of the CPA chain, the optical spectrum must not suffer from gain narrowing and still be wide enough to have the conditions for an optimal recompression. Additionally, to reach high pulse energy and/or average power the crystal must be strongly pumped with a laser diode and a good thermal management is crucial to keep a good beam quality. It is not easy to find the right compromise to have at the same time good optical and thermal properties in a crystal. These two criteria are directly dependent on the crystal structure but need two antagonistic properties: a good phonon propagation ensuring an optimal thermal conductivity takes place in an ordered crystal structure but, conversely, a certain disorder in the host lattice facilitates a wide gain bandwidth [1]. The compromise to obtain both these optical and thermal properties can be achieved by using (mixed) sesquioxide crystals, such as YScO 3 doped with ytterbium, whose maximum effective gain cross section is at 1037 nm. The effective gain cross-section of this material is a little lower than that of YAG for example, but its larger gain bandwidth makes it a good candidate for the amplification of ultrashort pulses [2]. Naturally the thermal conductivity of Yb:YScO 3 is reduced compared to Yb:YAG by roughly a factor 2.

Space-borne quantum technologies, particularly those based on atom interferometry, are heralding a new era of strategic and robust space exploration. The unique conditions of space, characterized by low noise and low gravity environments, open up diverse possibilities for applications ranging from precise time and frequency transfer to Earth Observation and the search of new Physics. In this paper, we summarise the M-class mission proposal in response to the 2022 call in ESA's science program: Space-Time Explorer and Quantum Equivalence Principle Space Test (STE-QUEST). It consists in a satellite mission featuring a dual-species atom interferometer operating over extended durations. This mission aims to tackle three of the most fundamental questions in Physics: (i) testing the universality of free fall with an accuracy better than one part in 10^-17, (ii) exploring various forms of Ultra-Light Dark Matter, and (iii) scrutinizing the foundations of Quantum Mechanics.