Fiber-optic resonator sensors enable high-sensitivity measurements of various physical parameters. Here, we demonstrate a quantum-enhanced interrogation scheme of an all-fiber-optic Fabry-Pèrot resonator, employing entangled two-photon interference to probe directly strain-induced optical phase shifts. The resonator is interrogated by a narrow-linewidth laser frequency-locked to its resonance mode, thereby exploiting the cavity’s inherent sensitivity to variations in the fiber length and refractive index. To access the phase fluctuations transferred to the interrogation field due to external perturbations on the sensor, we implement an in-fiber Mach–Zehnder interferometer readout. This configuration allows the sensor to operate both with classical coherent light and with entangled photon pairs generated via spontaneous parametric down-conversion. Harnessing two-photon entangled states corresponding to an N = 2 N00N state, we experimentally demonstrate a reduction of the measurement noise below the standard quantum limit. These results provide clear evidence of the quantum advantage in enhancing fiber-optic sensors under practical constraints on optical power

Over the last decades, dielectric Whispering-Gallery Mode (WGM) micro-resonators, such as spherical or toroidal structures, have demonstrated their effectiveness for highly sensitive biochemical sensing [1]. When a WGM resonance is excited, light is tightly concentrated within a small modal volume, greatly enhancing the interaction with analytes. Conversely, Surface-Enhanced Raman Spectroscopy (SERS) is a powerful technique that merges the molecular fingerprinting capabilities of Raman spectroscopy with the optical near-field enhancement provided by localized surface-plasmon resonances (LSPRs) on metallic nanostructures. This combination enables the highly sensitive and selective detection of various substances, even in liquid environments [2].

Inertial sensors, such as gyroscopes and accelerometers, are widely used in aerospace and navigation and play a vital role in seismic monitoring and safety for urban areas and large infrastructures. In particular, fiber-optic inertial sensors, being passive, lightweight, and immune to electromagnetic interference, can be ideally integrated into complex structures for localized and distributed sensing. Optical fibers are also a well-established technology, today widely used in a wealth of applications in physics, engineering and aerospace. However, limitations due to classical noise prevent a significant improvement in the performance currently achievable with fiber sensors.

Whispering-gallery-mode (WGM) microresonators are amongst the most promising optical sensors for detecting bio-chemical targets. A number of laser interrogation methods have been proposed and demonstrated over the last decade, based on scattering and absorption losses or resonance splitting and shift, harnessing the high-quality factor and ultra-small volume of WGMs. Actually, regardless of the sensitivity enhancement, their practical sensing operation may be hampered by the complexity of coupling devices as well as the signalprocessing required to extract the WGM response. Here, we use a silica microsphere immersed in an aqueous environment and efficiently excite optical WGMs with a free-space visible laser, thus collecting the relevant information from the transmitted and back-scattered light without any optical coupler, fiber, or waveguide. We show that a 640-nm diode laser, actively frequency-locked on resonance, provides real-time, fast sensing of dielectric nanoparticles approaching the surface with direct analog readout. Thanks to our illumination scheme, the sensor can be kept in water and operate for days without degradation or loss of sensitivity. Diverse noise contributions are carefully considered and quantified in our system, showing a minimum detectable particle size below 1 nm essentially limited by the residual laser microcavity jitter. Further analysis reveals that the inherent laserfrequency instability in the short, -mid-term operation regime sets an ultimate bound of 0.3 nm. Based on this work, we envisage the possibility to extend our method in view of developing new viable approaches for detection of nanoplastics in natural water without resorting to complex chemical laboratory methods.

Interferometric fiber-optic gyroscopes (IFOGs) are usually interrogated by broadband incoherent light sources instead of narrow linewidth coherent lasers. One of the main reasons is the problem of Rayleigh backscattering into the fiber, whose components sum up coherently for lasers and incoherently for broadband sources. In this work we investigate the possibility to use an amplitude modulated laser for interrogating an IFOG, which brings to a strong Rayleigh backscattering noise suppression. For this purpose, light is sent as short pulses of 1.86 mu s width and 18.6 mu s period in a 2-km fiber loop, which ensures the least temporal overlap between the forward-propagating and backscattered light. This technique reduces the cumulative backscattered power and its negative effect on system performance. The experimental investigation into pulse modulation presented here clearly demonstrate that pulse modulation improves stability and precision up to an order of magnitude, with critical timescales of noise levels lowered by a factor of ten, improving the performance of IFOG systems.

Fiber-optic gyroscopes are widespread sensors used in different fields, such as inertial navigation, seismology and positioning systems. In particular, for applications where high stability and dynamic range are necessary, FOGs are used in closed loop configuration. Here we show an all-electronic closed loop FOG based on a stand-alone phase-locked loop (PLL) system. This approach, is applied directly at the signal processing level without changing the existing optical setup. The PLL stabilizes the signal's frequency reference and suppresses low-frequency fluctuations. A thorough analysis provides a characterization of the gyroscope's performance which shows that this PLL-based technique remarkably reduce the residual noise at low frequency as well as improve the global stability of the system. The gyroscope shows a closedloop residual noise in the range of 8 x 10(-6) - 2 x 10(-9) rad/s/vHz over the frequency bandwidth 1mHz - 100 Hz. Future upgrades and perspectives are discussed.

Whispering-gallery mode (WGM) microresonators are amongst the most promising optical sensors for detection of biochemical targets. A number of laser interrogation methods have been proposed and demonstrated over the last decade, based on scattering and absorption losses or resonance splitting and shift, harnessing the high quality factor and light confinement in the ultra-small volume of WGMs. Regardless of the sensitivity gain, in chemical sensing applications where the chemical targets are present only in rare traces, WGM sensors operation may be augmented by plasmonic interaction, e.g. metallic nanostructures or substrates that provide impressive local field amplification via the so-called Surface-Enhanced Raman Scattering (SERS). Here, we use a silica microsphere and efficiently excite WGMs collecting the relevant information from the transmitted and back-scattered light, which is then analyzed to extract the Raman spectral signature a chemical analyte. We show that a near-infrared diode laser locked on a WGM resonance provides real-time, fast sensing of dielectric nanoparticles approaching the surface with no need for signal post-processing. Also, the Raman spectrum of light scattered from the microsphere surface clearly shows the enhancement of Purcell effect due to the oscillating WGMs. The implementation of a SERS-modified spectroscopic scheme is underway.

The real-time monitoring of densely populated areas with high seismic and volcanic risk is of crucial importance for the safety of people and infrastructures. When an earthquake occurs, the Earth surface experiences both translational and rotational motions. The latter are usually not monitored, but their measurement and characterization are essential for a full description of the ground motion. Here we present preliminary observational data of a highsensitivity rotational sensor based on a 2 -km -long fiber-optic Sagnac gyroscope, presently under construction in the middle of the Campi Flegrei Volcanic Area (Pozzuoli, Italy). We have evaluated its performance by analyzing data continuously recorded during an acquisition campaign of five months. The experimental setup was composed of a digital nine -component seismic station equipped with both a rotational sensor and conventional seismic sensors (seismometers, accelerometers, and tiltmeters). During this experiment we detected seismic noise and ground rotations wavefield induced by small to medium local earthquakes (M-D < 3). The prototype gyroscope shows a very promising sensitivity in the range of 5 x 10 (- 7) - 8 x 10( - 9) rad / s /root Hz over the frequency bandwidth 5 mHz-50 Hz. Future upgrades and perspectives are discussed. (c) 2024 Optica Publishing Group

We demonstrate a sensing scheme for liquid analytes that integrates multiple optical fiber sensors in a near-infrared spectrometer. With a simple optofluidic method, a broadband radiation is encoded in a time-domain interferogram and distributed to different sensing units that interrogate the sample simultaneously; the spectral readout of each unit is extracted from its output signal by a Fourier transform routine. The proposed method allows performing a multiparametric analysis of liquid samples in a compact setup where the radiation source, measurement units, and spectral readout are all integrated in a robust telecom optical fiber. An experimental validation is provided by combining a plasmonic nanostructured fiber probe and a transmission cuvette in the setup and demonstrating the simultaneous measurement of the absorption spectrum and the refractive index of water–methanol solutions.

Portable optical spectrometers are crucial devices for bio-chemical sensing and spectroscopic applications whereby robust, compact and cost-effective set-ups are desirable. However, existing miniaturized instruments typically struggle to achieve broad wavelength operation and high spectral resolution at the same time. Here, an all-fiber optical spectrometer based on two cascaded Bragg gratings is devised and demonstrated, showing a record resolution and a wavelength span-to-resolution ratio larger than that of most miniature broadband spectrometers reported to date. Thanks to a synchronous control of the grating lengths and to a unique combination of their reflection features, spectral analysis of incoherent light within 1 pm is achieved. On the other hand, fast and reproducible wavelength tuning over several nanometers on a millisecond-timescale is ensured by mechanical stretching of the internal fiber, limited only by the actuator’s dynamic range. A striking evidence of the spectrometer capabilities is provided with Doppler-limited spectroscopy of gas absorption bands performed with a near-infrared LED source. The observed spectra exhibit lineshapes comparable with those obtained by laser-based set-ups and the retrieved gas-line parameters are in agreement with existing spectroscopic databases. The spectrometer lends itself to applications in high-resolution interrogation of multiple fiber-optic sensors as well as broadband imaging with supercontinuum light.

We describe the behavior of a beam balance used for the measurement of small forces, in macroscopic samples, in tens of mHz frequency band. The balance, which works at room temperature, is the prototype of the cryogenic balance of the Archimedes experiment, aimed at measuring the interaction between electromagnetic vacuum fluctuations and the gravitational field. The balance described has a 50-cm aluminum arm and suspends an aluminum sample of 0.2 Kg and a lead counterweight. The read-out is interferometric, and the balance works in closed loop. It is installed in the low seismic noise laboratory of SAR-GRAV (Sardinia—Italy). Thanks to the low sensing and actuation noise and finally thanks to the low environmental noise, the sensitivity in torque τ̃ ̃_̃ñ is about τ̃ ̃_̃ñ≈ 2*10^-12Nm/√(Hz) at 10 mHz and reaches a minimum of about τ̃ ̃_̃ñ≈ 7*10^-13Nm/√(Hz) at tens of mHz, corresponding to the force sensitivity F̃_̃ñ of F̃_̃ñ≈ 3*10^ - 12N/√(Hz) . The achievement of this sensitivity, which turns out to be compatible with thermal noise estimation, on the one hand, demonstrates the correctness of the optical and mechanical design and on the other paves the way to the completion of the final balance. Furthermore, since the balance is equipped with weight and counterweight made of different materials, it is sensitive to the interaction with dark B-L photons. A first very short run made to evaluate constraints on B-L dark photon coupling shows encouraging results that will be discussed in view of next future scientific runs.

Whispering-Gallery Mode (WGM) microresonators have become popular in photonic systems thanks to their ease of fabrication, high optical Q-factor and ultra-small mode volume. Here, we illustrate the modelling and experimental activities derived from light coupling mechanisms to passive WGM microresonators based on free-space scattering without using any prism or fiber waveguide. This has been carried out for cavities made of liquid and solid materials, for which we report applications and potential use in optical sensing, machine learning and spectroscopy. In particular, angular momentum matching, i.e. light coupling via scattering, is obtained only in a strict interval of alignment conditions exhibiting WGM spectra populated with a variety of peaks with diverse quality factors. We devised an optical feedback loop based on a spatial light modulator that tailors the phase of a laser beam and, thanks to a random algorithm optimizes the alignment maximizing the scattered light-ring pattern. This allows to harness the strong mode confinement and power amplification of the microresonator to observe opto-mechanical and radiation-pressure effects. Free-space WGMs is also an appealing platform for artificial intelligence architectures. Taking a further step from this optical setup, we started implementing a photonic learning machine whereby the SLM acts as an optical encoder while the WGM spectrum provides the optical readout. Finally, we developed a novel room-temperature radiation sensor based on a free-space laser locked on the resonance of a silica microsphere. Thanks to silica strong absorption in the IR, we showed that the microsphere element enables detection of electromagnetic radiation from the mid-IR (MIR) up to the THz spectral range proving also suitable for absorption spectroscopy.

This paper presents an all-fiber telecom-range optical gyroscope employing a spontaneous parametric down conversion crystal to produce ultra-low intensity thermal light by tracing-out one of the heralded photons. The prototype exhibits a detection limit on photon delay measurements of $249$ zs over a $72$ s averaging time and $26$ zs in differential delay measurements at $t=10^4$ s averaging. The detection scheme proves to be the most resource-efficient possible, saturating $>99.5\%$ of the Cram\'er-Rao bound. These results are groundbreaking in the context of low-photon regime quantum metrology, holding great promise for astrometry and paving the way to novel experimental configurations to bridge quantum optics with special or general relativity.

Whispering-gallery mode microresonators have gained wide popularity as experimental platforms for different applications, ranging from biosensing to nonlinear optics. Typically, the resonant modes of dielectric microresonators are stimulated via evanescent wave coupling, facilitated using tapered optical fibers or coupling prisms. However, this method poses serious shortcomings due to fabrication and access-related limitations, which could be elegantly overcome by implementing a free-space coupling approach; although additional alignment procedures are needed in this case. To address this issue, we have developed a new algorithm to excite the microresonator automatically. Here, we show the working mechanism and the preliminary results of our experimental method applied to a home-made silica microsphere, using a visible laser beam with a spatial light modulator and a software control.

The world of photonics is experiencing, like many other research fields, the growing impact of machine learning techniques. Several neuromorphic photonic scheme have been proposed, and experimentally demonstrated over the last years [1] as novel tools able to boost classification and feature extraction processes.

Infrared (IR) and far-infrared detectors are crucial components in diverse research fields, such as optical metrology, nonlinear optics and spectroscopy, as well as civil and military applications. They are usually distinguished as photoconductive or photovoltaic devices and thermal devices [1]–[3]. In particular, thermal detectors are the prominent type adopted for long-wavelength infrared radiation (THz) and rely on changes in physical properties of the sensor material caused by photon absorption. However, the frequency window from 0.1 to 10 THz is a largely underexploited section of the electromagnetic spectrum due the current lack of room-temperature detectors. Whispering-gallery mode (WGM) micro-resonators [4] have become popular in photonic systems thanks to ease of fabrication, high optical Q-factor and ultra-small mode volume [5], [6]. Their inherent sensitivity to temperature and refractive index variations makes them ideal tools for various applications, such as biosensing and spectroscopy [7].

We present a new method for detecting single nanoparticles using a sensor based on a whispering gallery mode resonator submerged in aqueous solutions. A free-space diode laser excites whispering-gallery mode resonances by focusing it on the edge of the microresonator. Its emission frequency is then locked to a resonant mode in order to track any change induced by the interaction of the microsphere with nanoparticles, which can be suspended in the surrounding liquid medium. A theoretical analysis based on some seminal work, together with preliminary noise source evaluation, indicates that frequency shifts down to the order of hundreds of kHz are measurable, thus allowing to detect single nanoparticles. Further upgrades of the experimental scheme aimed at precise nanoparticle sizing and positioning are discussed.

A novel room‐temperature photodetector based on a silica‐microsphere optical resonator operating from the mid‐infrared to the terahertz spectral region is reported. The sensor relies on a free‐space visible laser whose frequency is locked to the center of a whispering‐gallery mode (WGM). The devised scheme automatically tracks any small change in the microresonator's size and refractive index that occurs upon illumination with radiation absorbed by silica. In this way, direct detection of quantum‐cascade laser beams around 4.3 and 111 μm wavelengths is shown, down to minimum noise‐equivalent power levels of ≈1 pW √Hz−1 with a dynamic range >100 dB. The microsphere resonator is then exploited, for the first time, as a photodetector in absorption spectroscopy of the 12CO2 fundamental ro‐vibrational band and of very‐weak 13CH3OH rotational lines.

We provide a novel methodological approach to the estimate of the change of the Quantum Vacuum electromagnetic energy density in a High critical Temperature superconducting metal bulk sample, when it undergoes the transition in temperature, from the superconducting to the normal phase. The various contributions to the Casimir energy in the two phases are highlighted and compared. While the TM polarization of the vacuum mode allows for a macroscopic description of the superconducting transition, the changes in the TE vacuum mode induced by the superconductive correlations are introduced within a microscopic model, which does not explicitly take into account the anisotropic structure of the material.