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.

Chip-scale frequency combs offer massive wavelength parallelization, holding a transformative potential in photonic system integration with potential application in future navigation systems, data center interconnects, and ranging. However, efficient frequency comb solutions have only been reported at the die level. Here, we report the wafer level characterization or soliton microcombs with an average conversion efficiency exceeding 50%, featuring 100 lines at 100 GHz repetition rate with 20 MHz standard deviation. We further illustrate the enabling possibilities of the space multiplicity, i.e., the large wafer-level redundancy, for establishing new sensing applications, and show tri-comb interferometry for broadband phase-sensitive spectroscopy.

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. 

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. 

Microcombs have been an intense area of research in frequency synthesis and metrology over the past decade. The measurement of amplitude and phase of microcombs provides unique insight into the nonlinear cavity dynamics. Different techniques have been reported to this aim, including iterative pulse shaping [1], dual-comb interferometry [2] and lately stepped-laser interferometry [3], resulting in unprecedented sensitivity and bandwidth. Here, we report a dramatic simplification of the latter setup by using another microcomb instead of a stepped tunable laser. This results into the acquisition of complex spectra in a single-scan without requiring additional optical components and high-end detection units.

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. 

Over the last few years, dissipative Kerr solitons (DKS) in microresonators have boosted the development of chip-scale frequency comb sources (microcombs) in a variety of applications, from coherent communications to ultrafast distance ranging [1]. However, the intrinsic large free spectral range (FSR) of microcombs (within the gigahertz regime) is still a drawback for applications such as molecular spectroscopy, in which the comb line spacing dictates the spectral sampling resolution. Overcoming spectral sparsity by scanning the comb modes across a full FSR is challenging for a DKS microcomb, since the soliton operation must be kept while the pump laser is continuously swept. So far, it has been accomplished for a single microresonator by combining a feedback control loop with the thermal tuning of the cavity resonances by means of a microheater [2]. Recently, the use of two linearly coupled cavities (a photonic molecule) has shown to be a promising alternative to generate soliton microcombs with high conversion efficiency and uniform power distribution [3]. In this contribution, we address the challenge of scanning the soliton comb modes of a photonic molecule by thermal tuning. Specifically, we implement a scheme to scan a bright soliton over 60 GHz by tuning simultaneously the pump laser and the resonances of two coupled cavities.

Spectral broadening of optical frequency combs with high repetition rate is of significant interest in optical communications, radio-frequency photonics and spectroscopy. Silicon nitride waveguides (Si3N4) in the anomalous dispersion region have shown efficient supercontinuum generation spanning an octave-bandwidth. However, the broadening mechanism in this regime is usually attained with femtosecond pulses in order to maintain the coherence. Supercontinuum generation in the normal dispersion regime is more prone to longer (ps) pulses, but the implementation in normal dispersion silicon nitride waveguides is challenging as it possesses strong requirements in propagation length and losses. Here, we experimentally demonstrate the use of a Si3N4 waveguide to perform coherent spectral broadening using pulses in the picosecond regime with high repetition rate. Moreover, our work explores the formation of optical wave breaking using a higher energy pulse which enables the generation of a coherent octave spanning spectrum. These results offer a new prospect for coherent broadening using long duration pulses and replacing bulky optical components.

Measuring microcombs in amplitude and phase provides unique insight into the nonlinear cavity dynamics, but spectral phase measurements are experimentally challenging. Here, we report a linear heterodyne technique assisted by electro-optic downconversion that enables differential phase measurement of such spectra with unprecedented sensitivity (−50 dBm) and bandwidth coverage (>110 nm in the telecommunications range). We validate the technique with a series of measurements, including single-cavity and photonic molecule microcombs. © 2022 Optica Publishing Group

Much efforts have been put to elaborate and improve different high precision measurement schemes for characterization of advanced photonic devices and optical fibers with increasing bandwidth requirements. In light of this, swept-wavelength interferometry and dual-comb spectroscopy have been extensively applied in characterization procedures. In this paper we present in detail an experimental scheme that combines these two techniques and overcomes their limitations by using a tunable laser source in order to sweep over the frequency comb spacing and capture all intermediate frequencies. We demonstrate full-field broadband measurements over 1.25 THz comb bandwidth with increased frequency resolution, which can be performed in only 5 ms sweep. We also show that the nonlinearity of the laser sweep can be removed without an auxiliary interferometer in the setup.