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arXiv:2512.09475

High-resolution broadband characterization of resonance dispersion in an optical microresonator

Romain Dalidet, Adrien Bensemhoun, Gregory Sauder, Anthony Martin, David Medina, Carlos Alonso Ramos, Eric Cassan, Laurent Vivien, Jonathan Faugier Tovar, Baptiste Routier, Quentin Wilmart, Ségolène Olivier, Virginia D Auria, Laurent Labonté, Sébastien Tanzilli

physics.opticsquant-phSubmitted Dec 10, 2025• Announced Dec 11, 2025
arXivHTMLPDF

Abstract

Accurate knowledge of the uneven free spectral range of an optical microresonator, which provides direct insight into group velocity dispersion, is essential for understanding and controlling Kerr frequency comb dynamics. In this work, we present a simple and highly precise method formeasuring the free spectral range over a 5 THz bandwidth in silicon nitride microresonators, leveraging a wavemeter with 0.4 MHz resolution. Our fully fibered plug-and-play experimental setup enables the accurate extraction of resonance frequencies. By carefully analyzing the spectral position of each resonance, we measure both second- and third-order free spectral range expansion coefficients. This approach offers a robust and accessible tool for dispersion characterization in integrated photonic circuits, paving the way for next-generation of Kerr comb sources and quantum photonic technologies.

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Accurate knowledge of the uneven free spectral range of an optical microresonator, which provides direct insight into group velocity dispersion, is essential for understanding and controlling Kerr frequency comb dynamics. In this work, we present a simple and highly precise method formeasuring the free spectral range over a 5 THz bandwidth in silicon nitride microresonators, leveraging a wavemeter with 0.4 MHz resolution. Our fully fibered plug-and-play experimental setup enables the accurate extraction of resonance frequencies. By carefully analyzing the spectral position of each resonance, we measure both second- and third-order free spectral range expansion coefficients. This approach offers a robust and accessible tool for dispersion characterization in integrated photonic circuits, paving the way for next-generation of Kerr comb sources and quantum photonic technologies.
Dissipative Kerr solitons in optical microresonators underpin chip-scale frequency combs with applications ranging from coherent telecommunications to precision spectroscopy. Yet the characterization of their intrinsic femtosecond temporal structure remains challenging, as the low pulse energy and broad spectral bandwidth necessitate optical amplification and careful dispersion compensation in conventional ultrafast diagnostics, both of which can significantly distort the waveform. Here we demonstrate a quantum-interference metrology of microcomb solitons based on Hong-Ou-Mandel interference. By attenuating the soliton stream to the single-photon level and measuring fourth-order interference, we directly retrieve near transform-limited pulse durations without amplification or dispersion management, remaining accurate even after propagation through 25 km of standard fiber. The same interferogram also provides direct access to the temporal separations in multi-soliton states by converting inter-soliton separations into additional interference dips at corresponding delays, enabling sub-picosecond characterization of their intracavity temporal structure. This quantum-inspired paradigm introduces a fundamentally new metrological approach that is immune to amplification and dispersion distortions, offering a powerful tool for the characterization of complex soliton physics.
Photonic structures such as photonic bandgap fibers and high confinement nanowaveguides have proven to be excellent platforms for studying nonlinear optical interactions tailored towards applications in spectroscopy, quantum communication, quantum computation protocols, optical clockwork and precision frequency metrology. This thesis discusses our approach towards exploiting these high confinement media for demonstrating novel few photon nonlinear optical interactions such as two-photon absorption and cross-phase modulation in hot atomic vapors (Rubidium) confined inside hollow-core photonic bandgap fibers (PBGF), generating ultra-broadband optical frequency combs that utilize cascaded parametric four-wave mixing and mode-locked femtosecond pulses in silicon nitride nanowaveguides. We generate large optical depths in such a Rb-PBGF system, and the tight light confinement, high vapor density and long interaction length allow us to perform nonlinear optics at ultralow power. We observe 25% all-optical modulation with <20 photons, i.e., a few attojoules of energy, and large cross-phase shifts of 0.3 mrad per photon with a response time <5 ns in Rb-filled PBGF. This result takes us to within an order of magnitude of single-photon switching, and improves upon previous experiments for freely propagating optical fields, including those in cold-atoms. Using high quality factor silicon nitride optical microcavities we show that the gain from the four-wave mixing process can lead to optical parametric os- cillation, allowing for the generation of multiple new wavelengths as wide as an octave of wavelengths. Next we show that by dispersion engineering the waveguide dimensions, we can generate combs by pumping at 1064 nm. The advantage of this platform is that we can independently tune the free spectral range (FSR) and the dispersion. We exploit this property of the silicon nitride microresonator platform to generate microcombs with various FSRs such as 20, 40-, 80-,100-GHz. Next we go on to characterize the spectral and temporal dynamics of the microresonator based combs and demonstrate that such parametric frequency combs can generate modelocked ultra-short pulses.
Optical frequency combs in integrated photonics have widespread applications in high-dimensional optical computing, high-capacity communications, high-speed interconnects, and other paradigm-shifting technologies. However, quantum frequency combs with high-dimensional quantum states are vulnerable to decoherence, particularly in the presence of perturbations such as sharp bends. Here we experimentally demonstrate the robust on-chip topological transport of quantum frequency combs in valley photonic crystal waveguides. By measuring the time correlations and joint spectral intensity of the quantum frequency combs, we show that both quantum correlations and frequency entanglement remain robust against sharp bends, owing to the topological nature of the quantum valley Hall effect. We also demonstrate that dissipative Kerr soliton combs with a bandwidth of 20 THz maintain their spectral envelope and low-noise properties even in the presence of structure perturbations. These topologically protected optical frequency combs offer robust, complex, highly controllable, and scalable light sources, promising significant advances in high-dimensional photonic information processing.
The optical microresonator-based frequency comb (microcomb) provides a versatile platform for nonlinear physics studies and has wide applications ranging from metrology to spectroscopy. The deterministic quantum regime is an unexplored aspect of microcombs, in which unconditional entanglements among hundreds of equidistant frequency modes can serve as critical ingredients to scalable universal quantum computing and quantum networking. Here, we demonstrate a deterministic quantum microcomb in a silica microresonator on a silicon chip. 40 continuous-variable quantum modes, in the form of 20 simultaneously two-mode squeezed comb pairs, are observed within 1 THz optical span at telecommunication wavelengths. A maximum raw squeezing of 1.6 dB is attained. A high-resolution spectroscopy measurement is developed to characterize the frequency equidistance of quantum microcombs. Our demonstration offers the possibility to leverage deterministically generated, frequency multiplexed quantum states and integrated photonics to open up new avenues in fields of spectroscopy, quantum metrology, and scalable, continuous-variable-based quantum information processing.