Microcombs are steadily advancing toward system-level applications. Recent progress in high-Q silicon nitride microresonators and coupled cavities allows for generating coherent microcombs with an optical conversion efficiency approaching unity. The generation of efficient soliton microcombs in this coupled cavity arrangement (photonic molecule) relies on controlling the strength and location of an avoided mode crossing to compensate for the spectral shift introduced by the temporal soliton. While resulting in exceptionally high efficiency and reliability, it is challenging to attain continuous broadband tuning of the pump, limiting deployment in some practical applications. In this work, we demonstrate offset-tunable super-efficient microcombs in photonic molecules featuring low-noise operation and a smooth, constant spectral envelope. This is achieved by thermally tuning in tandem the main and auxiliary cavities, allowing to attain the desired avoided mode crossing across multiple free spectral ranges of the main cavity. Additionally, we analyze the impact of the thermal tuning on the phase noise and repetition rate stability as well as the thermal response of heaters. The demonstrated broadband, post fabrication tunability establishes photonic molecules as a highly flexible and efficient platform for the generation of super-efficient microcombs at arbitrary pump wavelengths, making this configuration a particularly appealing one for applications in metrology and precision spectroscopy

Fabry–Pérot refractometry is a leading technique for high-accuracy pressure realization; however, multigas operation is typically constrained by gas-handling complexity and the risk of cross-contamination. In this work, we investigate whether the pronounced difference in time scales between rapid pressure equilibration and the much slower molecular diffusion in long, narrow connections can be exploited to enable two connected Fabry–Pérot cavity-based refractometers to operate with different gases at the same pressure. Diffusion modeling based on Fick’s second law, together with experiments using two refractometers, shows that gas mixing can remain below 1 ppm for a variety of measurement times and pressures. These results demonstrate a simple and robust approach to simultaneous multigas refractometry at a common pressure without the need for physical separation hardware

Current temperature sensors require regular recalibration to maintain reliable temperature measurement. Photonic/quantum-based approaches have the potential to radically change the practice of thermometry through provision of in situ traceability, potentially through practical primary thermometry, without the need for sensor recalibration. This article gives an overview of the European Partnership in Metrology (EPM) project: Photonic and quantum sensors for practical integrated primary thermometry (PhoQuS-T), which aims to develop sensors based on photonic ring resonators and optomechanical resonators for robust, small-scale, integrated, and wide-range temperature measurement. The different phases of the project will be presented. The development of the integrated optical practical primary thermometer operating from 4 K to 500 K will be reached by a combination of different sensing techniques: with the optomechanical sensor, quantum thermometry below 10 K will provide a quantum reference for the optical noise thermometry (operating in the range 4 K to 300 K), whilst using the high-resolution photonic (ring resonator) sensor the temperature range to be extended from 80 K to 500 K. The important issues of robust fibre-to-chip coupling will be addressed, and application case studies of the developed sensors in ion-trap monitoring and quantum-based pressure standards will be discussed.

When Fabry-Perot (FP) refractometry is used to assess the refractivity of gases, it has so far been assumed that the Gouy phase is independent of the presence of gas in the cavity. Here we show, by both theory and experiments, that this is only correct for a non-deformable cavity. For a deformable one, the pressure can affect the radius of curvature of the mirrors. This gives the Gouy phase a component that is proportional to gas pressure. Although being a small effect (1.6 nrad/Pa), since it affects the Gouy phase only when the cavity contains gas, it affects the refractivity on a 10−6 level. 

Dissipative Kerr soliton (DKS) frequency combs, when generated within coupled cavities, exhibit exceptional performance concerning controlled initiation and power conversion efficiency. Nevertheless, to fully exploit these enhanced capabilities, it is necessary to maintain the frequency comb in a low-noise state over an extended duration. In this study, we demonstrate the control and stabilization of super-efficient microcombs in a photonic molecule. Our findings demonstrate that there is a direct relation between effective detuning and soliton power, allowing the latter to be used as a setpoint in a feedback control loop. Employing this method, we achieve the stabilization of a highly efficient microcomb indefinitely, paving the way for its practical deployment in optical communications and dual-comb spectroscopy applications. 

This work models and experimentally assesses the influence of absorption of laser light in mirrors in Fabry-Pérot based refractometers used for realization of pressure. Model parameters are assessed by experimental characterizations. Characterizations of two refractometers agree well with the predictions of the model. It is shown that, when pressures are assessed in the viscous region, the absorption of laser light in mirrors will give rise to a small alteration in the proportional response and a pressure-independent offset, where the latter is significant for He but considerably smaller for Ar and N2

Microcomb-based phase-sensitive interferometry is demonstrated over a broad bandwidth using power-efficient solitons. This work highlights the possibilities of spatial multi-sensing using chip-scale frequency combs enabled by wafer-scale manufacturing with a high yield. 

Microcomb-based phase-sensitive interferometry is demonstrated over a broad bandwidth using power-efficient solitons. This work highlights the possibilities of spatial multi-sensing using chip-scale frequency combs enabled by wafer-scale manufacturing with a high yield. © Optica Publishing Group 2024

A procedure for automated low uncertainty assessment of empty cavity mode frequencies in Fabry-Pérot cavity based refractometry that does not require access to laser frequency measuring instrumentation is presented. It requires a previously well-characterized system regarding mirror phase shifts, Gouy phase, and mode number, and is based on the fact that the assessed refractivity should not change when mode jumps take place. It is demonstrated that the procedure is capable of assessing mode frequencies with an uncertainty of 30 MHz, which, when assessing pressure of nitrogen, corresponds to an uncertainty of 0.3 mPa. 

Based on a recent experimental determination of the static polarizability and a first-principle calculation of the frequency-dependent dipole polarizability of argon, this work presents, by using a Fabry–Perot refractometer operated at 1550 nm, a realization of the SI unit of pressure, the pascal, for pressures up to 100 kPa, with an uncertainty of [(1.0 mPa)2 + (5.8 × 10−6 P)2 + (26 × 10−12P2)2]1/2. The work also presents a value of the molar polarizability of N2 at 1550 nm and 302.9146 K of 4.396572(26) × 10−6 m3/mol, which agrees well with previously determined ones.