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. © Optica Publishing Group 2024

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. 

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. 

In the past years, major scientific and technological progress has been made in the field of dissipative Kerr solitons (DKS) frequency combs on microresonators (microcombs) [1]. The generation of microcombs in $\text{Si}_{3}\mathrm{N}_{4}$ microresonators exhibit broad bandwidth, high repetition rate, and low power consumption. These capabilities have opened a venue for applications within optical communication [2]. However, continuous operation and high power per comb line are needed in order to address the demands of system-level applications. Recent works have demonstrated the generation of bright solitons with unprecedented high power conversion efficiency using photonic molecules [3]. In this work, we report the continuous operation of a super-efficient DKS microcomb over 25 hours using a packaged module. The soliton state is maintained by the stabilization of thermal drifts and the on-chip optical power in the cavity. Our photonic molecule is composed of two coupled cavities with heaters placed on top. The fabricated device has been robustly packaged into a fiber-connectorized module to prevent random variations of the power coupling (Fig. 1(a)). The experimental setup used for the feedback control is shown in Fig 1. (b). The comb is generated with 20 mW of on-chip optical power covering a bandwidth of $\sim 100$ nm, see Fig. 1(c). The power conversion efficiency of the bright soliton corresponds to 32 percent. The long-term operation is achieved by harnessing the thermal drifting of the main resonance via a feedback loop implemented on an FPGA board. The soliton power is used as a control parameter to maintain a fixed pump detuning. This indicates that the coupling between these parameters holds in the super-efficient configuration of the photonic molecule as in [4]. Fig. 1(d) shows the spectral envelope of the microcomb with constant power over 25 hours. Since the pump laser is free-running, the active control forces the cavity to follow in order to maintain a fixed pump detuning. As a result, the frequency of the repetition rate of the microcomb drifts over 800 kHz towards higher frequencies (Fig. 1(e)). The frequency drifting of the pump laser (Toptica CTL) was monitored over the last 5 hours of the soliton existence by beating versus a frequency comb (Menlo FC1500), see Fig. 1(f). During the first 100 minutes of recording the resulting beat note drifts, increasing the frequency until it leaves the set span window. As the drifting continues, the beating with a neighboring line of the frequency comb is observed. The soliton power is stabilized as the photo-detected converted power remains constant (Fig. 1(f)). Nevertheless, the drifts exhibited are very minor, and we expect increased stability by feeding back into the laser piezo-control.

An updated version of an Invar-based dual Fabry-Perot cavity refractometer utilizing the gas modulation methodology has been characterized with regard to its ability to assess gas pressure in the low pressure regime, defined as the regime in which the instrumentation is mainly limited by the constant term a in the [ ( a ) 2 + ( b × P ) 2 ] 1 / 2 expression for the uncertainty. It is first concluded that this ability is predominantly limited by three entities, viz., the empty cavity repeatability, the residual gas pressures in the evacuated (measurement) cavity, and the contamination of the gas residing in the measurement cavity that originates from leaks and outgassing. We then present and utilize methods to separately estimate the uncertainty of the updated refractometer from these entities. It was found that, when utilizing gas modulation cycles of 100 s and when addressing nitrogen, the system can assess pressure in the low pressure regime with an expanded uncertainty ( k = 2 ) of 0.75 mPa, mainly limited by the empty cavity repeatability and outgassing of hydrogen. This is more than 1 order of magnitude below the previously assessed low pressure performance of the instrumentation.

Fabry–Pérot-based refractometry has demonstrated the ability to assess gas pressure with high accuracy and has been prophesized to be able to realize the SI unit for pressure, the pascal, based on quantum calculations of the molar polarizabilities of gases. So far, the technology has mostly been limited to well-controlled laboratories. However, recently, an easy-to-use transportable refractometer has been constructed. Although its performance has previously been assessed under well-controlled laboratory conditions, to assess its ability to serve as an actually transportable system, a ring-type comparison addressing various well-characterized pressure balances in the 10–90 kPa range at several European national metrology institutes is presented in this work. It was found that the transportable refractometer is capable of being transported and swiftly set up to be operational with retained performance in a variety of environments. The system could also verify that the pressure balances used within the ring-type comparison agree with each other. These results constitute an important step toward broadening the application areas of FP-based refractometry technology and bringing it within reach of various types of stakeholders, not least within industry.

Coherent dissipative structures known as platicons can be reliably generated in photonic molecules, resulting in deterministic and reproducible microcombs derived from a continuous-wave pump. However, the supermode spectrum of standard photonic molecules displays numerous avoided mode crossings, distorting the spectral envelope of platicon microcombs. Here, we obtain a platicon microcomb using a photonic molecule configuration based on two coupled microcavities, whose size differs by an order of magnitude. This results in an engineered microcomb spectrum that closely resembles the one generated in an ideal single microresonator with just one frequency mode shift. We observe the coupling between the repetition rate of the platicon microcomb with the frequency of the pump laser, an effect originating from the dispersive-wave recoil induced by mode crossings. Using two identical platicon microcombs, we make use of such coupling to realize dual-comb interferometry. These results contribute to understanding dissipative structures in normal-dispersion microresonators and offer an alternative to applications such as spectroscopy and metrology.