Dissipative coupling in a Bragg-grating-coupled single resonator with Fano resonance for anti-PT-symmetric gyroscopes.

Anti-parity-time-symmetric Hamiltonians show an enhanced sensitivity to external perturbations that can be used for high-performance angular velocity sensing. Dissipative coupling is a valuable way for realizing anti-PT-symmetric Hamiltonians with optical resonators and is usually obtained by means of auxiliary waveguides. Here, we model and experimentally show the dissipative coupling between two counterpropagating modes of a single resonator, by means of a Bragg-grating placed in the feeding bus. The proposed solution enables the possibility of accurately designing the dissipative coupling strength in integrated non-Hermitian gyroscopes, thus providing high flexibility in the design of the proposed sensor. Moreover, we theoretically and experimentally demonstrate that the dissipative coupling between two counterpropagating modes of the same resonant cavity can give rise to an asymmetric Fano resonance.

Feature Papers in Optical Sensors 2022.

Today, optical sensors are the subject of a very significant number of studies and applications [...].

Potentiometric Chloride Ion Biosensor for Cystic Fibrosis Diagnosis and Management: Modeling and Design.

The ion-sensitive field-effect transistor is a well-established electronic device typically used for pH sensing. The usability of the device for detecting other biomarkers in easily accessible biologic fluids, with dynamic range and resolution compliant with high-impact medical applications, is still an open research topic. Here, we report on an ion-sensitive field-effect transistor that is able to detect the presence of chloride ions in sweat with a limit-of-detection of 0.004 mol/m3. The device is intended for supporting the diagnosis of cystic fibrosis, and it has been designed considering two adjacent domains, namely the semiconductor and the electrolyte containing the ions of interest, by using the finite element method, which models the experimental reality with great accuracy. According to the literature explaining the chemical reactions that take place between the gate oxide and the electrolytic solution, we have concluded that anions directly interact with the hydroxyl surface groups and replace protons previously adsorbed from the surface. The achieved results confirm that such a device can be used to replace the traditional sweat test in the diagnosis and management of cystic fibrosis. In fact, the reported technology is easy-to-use, cost-effective, and non-invasive, leading to earlier and more accurate diagnoses.

Non-Hermitian Sensing in Photonics and Electronics: A Review.

Recently, non-Hermitian Hamiltonians have gained a lot of interest, especially in optics and electronics. In particular, the existence of real eigenvalues of non-Hermitian systems has opened a wide set of possibilities, especially, but not only, for sensing applications, exploiting the physics of exceptional points. In particular, the square root dependence of the eigenvalue splitting on different design parameters, exhibited by 2 × 2 non-Hermitian Hamiltonian matrices at the exceptional point, paved the way to the integration of high-performance sensors. The square root dependence of the eigenfrequencies on the design parameters is the reason for a theoretically infinite sensitivity in the proximity of the exceptional point. Recently, higher-order exceptional points have demonstrated the possibility of achieving the nth root dependence of the eigenfrequency splitting on perturbations. However, the exceptional sensitivity to external parameters is, at the same time, the major drawback of non-Hermitian configurations, leading to the high influence of noise. In this review, the basic principles of PT-symmetric and anti-PT-symmetric Hamiltonians will be shown, both in photonics and in electronics. The influence of noise on non-Hermitian configurations will be investigated and the newest solutions to overcome these problems will be illustrated. Finally, an overview of the newest outstanding results in sensing applications of non-Hermitian photonics and electronics will be provided.

Compact resonant 2 × 2 crossbar switch using three coupled waveguides with a central nanobeam.

This theoretical simulation paper presents designs and projected performance of ∼1550-nm silicon-on-insulator (SOI) and ∼2000-nm Ge-on-Si-on-nitride and Ge-on-nitride 2×2 optical crossbar switches based upon a three-waveguide coupler in which the central waveguide is a nanobeam actuated by the thermo-optical (TO) effect. A TO heater stripe is located atop the central nanobeam. To implement accurate and realistic designs, the 3D finite difference time domain approach was employed. The metrics of crossbar switching, insertion loss (IL) and crosstalk (CT) were evaluated for choices of 3-waveguide structure parameters and TO-induced index changes. The predicted ILs and CTs were excellent, enabling the designed devices to be considered as fundamental building blocks in wavelength-division-multiplexed cross-connect (WXC) applications. Proposed here are compact, nonblocking space-and-wavelength routing switches to be constructed in a monolithic, industry-standard SOI chip (and in Ge-on-SON and GON chips). Specifics are given for realizing 16 × 16 × Mλ WXCs as well as reconfigurable, multi-resonant, programmable hexagonal and diamond meshes.

Quartz-enhanced photoacoustic spectroscopy exploiting low-frequency tuning forks as a tool to measure the vibrational relaxation rate in gas species.

We demonstrated that quartz-enhanced photoacoustic spectroscopy (QEPAS) is an efficient tool to measure the vibrational relaxation rate of gas species, employing quartz tuning forks (QTFs) as sound detectors. Based on the dependence of the QTF resonance frequency on the resonator geometry, a wide range of acoustic frequencies with narrow detection bandwidth was probed. By measuring the QEPAS signal of the target analyte as well as the resonance properties of different QTFs as a function of the gas pressure, the relaxation time can be retrieved. This approach has been tested in the near infrared range by measuring the CH4 (nν4 ) vibrational relaxation rate in a mixture of 1% CH4, 0.15 % H2O in N2, and the H2O (ν1 ) relaxation rate in a mixture of 0.5 % H2O in N2. Relaxation times of 3.2 ms Torr and 0.25 ms Torr were estimated for CH4 and H2O, respectively, in excellent agreement with values reported in literature.

Fiber-Coupled Quartz-Enhanced Photoacoustic Spectroscopy System for Methane and Ethane Monitoring in the Near-Infrared Spectral Range.

We report on a fiber-coupled, quartz-enhanced photoacoustic spectroscopy (QEPAS) near-IR sensor for sequential detection of methane (CH4 or C1) and ethane (C2H6 or C2) in air. With the aim of developing a lightweight, compact, low-power-consumption sensor suitable for unmanned aerial vehicles (UAVs)-empowered environmental monitoring, an all-fiber configuration was designed and realized. Two laser diodes emitting at 1653.7 nm and 1684 nm for CH4 and C2H6 detection, respectively, were fiber-combined and fiber-coupled to the collimator port of the acoustic detection module. No cross talk between methane and ethane QEPAS signal was observed, and the related peak signals were well resolved. The QEPAS sensor was calibrated using gas samples generated from certified concentrations of 1% CH4 in N2 and 1% C2H6 in N2. At a lock-in integration time of 100 ms, minimum detection limits of 0.76 ppm and 34 ppm for methane and ethane were achieved, respectively. The relaxation rate of CH4 in standard air has been investigated considering the effects of H2O, N2 and O2 molecules. No influence on the CH4 QEPAS signal is expected when the water vapor concentration level present in air varies in the range 0.6-3%.

Design of an on-Chip Room Temperature Group-IV Quantum Photonic Chem/Bio Interferometric Sensor Based on Parity Detection.

We propose and analyze three Si-based room-temperature strip-guided "manufacturable" integrated quantum photonic chem/bio sensor chips operating at wavelengths of 1550 nm, 1330 nm, and 640 nm, respectively. We propose design rules that will achieve super-sensitivity (above the classical limit) by means of mixing between states of coherent light and single-mode squeezed-light. The silicon-on-insulator (SOI), silicon-on-sapphire (SOS), and silicon nitride-on-SiO2-on Si (SiN) platforms have been investigated. Each chip is comprised of photonic building blocks: a race-track resonator, a pump filter, an integrated Mach-Zehnder interferometric chem/bio sensor, and a photonic circuit to perform parity measurements, where our homodyne measurement circuit avoids the use of single-photon-counting detectors and utilizes instead conventional photodetectors. A combination of super-sensitivity with super-resolution is predicted for all three platforms to be used for chem/bio sensing applications.

Numerical Simulation of Optical Sensing by the Far Field Pattern Radiated by Periodic Grating Strips Over Silica Buffer on the Silicon Wire Waveguide.

This paper presents results of numerical modeling of a modified design of an optical sensor based on segmented periodic silicon oxynitride (SiON) grating evanescently coupled with silicon wire. This segmented grating works as a leaky waveguide, which filters input power from a broadband optical source and radiates it as an outcoming optical beam with both a small wavelength band and a small beam divergence. The radiation angle strongly depends on the refractive index of the grating environment and provides sensor interrogation by measuring the far field pattern in the focal plane of the lens, which is placed near the sensor element. The device concept was verified by direct numerical modeling through the finite difference time domain (FDTD) method and provided moderate intrinsic limit of detection (iLOD) ~ 0.004 RIU with a possible iLOD ~ 0.001 RIU for 10 mm-long structures.

Ultrafast electro-optical disk modulators for logic, communications, optical repeaters, and wavelength converters.

We propose a U-shaped pn junction in a silicon-on-insulator microdisk resonator to effectively double the junction-mode overlap in the state-of-the-art, vertical pn junction microdisk electro-optical (EO) modulators. The U-shaped pn junction promotes the maximum overlap between the junction depletion zone and the whispering gallery optical mode in the microdisk. By fully depleting the p region of the npn-sequenced U-junction, the capacitance is reduced below 3 fF, which significantly improves the speed and power performance. In this work, we implement the high-efficiency, depleted U-junction design to maximize the operating bandwidth of EO modulators, EO logic elements, EO 2 × 2 switches for wavelength-division cross-connects, 2 × 2 reconfigurable optical add-drop multiplexers, optical-to-electrical-to-optical (OEO) repeaters-with-gain, OEO wavelength converters, and 2 × 2 optical-optical logic gates. These devices all operate in the 7.6-to-50 GHz bandwidth range with ultralow energy consumption between 0.4 and 9.8 fJ/bit. By using CMOS-compatible materials and fabrication-feasible design dimensions, our proposed high-performance devices offer a promising potential in next-generation, high-volume electro-optical communications and computing circuits.

Partial Least-Squares Regression as a Tool to Retrieve Gas Concentrations in Mixtures Detected Using Quartz-Enhanced Photoacoustic Spectroscopy.

We report on a statistical tool based on partial least-squares regression (PLSR) able to retrieve single-component concentrations in a multiple-gas mixture characterized by spectrally overlapping absorption features. Absorption spectra of mixtures of CO-N2O and mixtures of C2H2-CH4-N2O, both diluted in N2, were detected in the mid-IR range by exploiting quartz-enhanced photoacoustic spectroscopy (QEPAS) and using two quantum cascade lasers as light sources. Single-gas reference spectra of each target molecule were acquired and used as PLSR-based algorithm training data set. The concentration range explored in the analysis varies from a few parts-per-million (ppm) to thousands of ppm. Within this concentration range, the influence of the gas matrix on nonradiative relaxation processes can be neglected. Exploiting the ability of PLSR to deal with correlated data, these spectra were used to generate new simulated spectra, i.e., linear combinations of the reference ones. A Gaussian noise distribution was added to the created data set, simulating the real QEPAS signal fluctuations around the peak value. Compared with standard multilinear regression, PLSR predicted gas concentrations with a calibration error up to 5 times better, even with absorption features with spectral overlap greater than 97%.

On-Chip Group-IV Heisenberg-Limited Sagnac Interferometric Gyroscope at Room Temperature.

A room-temperature strip-guided "manufacturable" Silicon-on-Insulator (SOI)/GeSn integrated-photonics quantum-gyroscope chip operating at 1550 nm is proposed and analysed. We demonstrate how the entangled photons generated in Si Spontaneous Four Wave Mixing (SFWM) can be used to improve the resolution of a Sagnac interferometric gyroscope. We propose different integrated architectures based on degenerate and non-degenerate SFWM. The chip comprises several beam splitters, two SFWM entangled photon sources, a pump filter, integrated Mach-Zehnder interferometric gyro, and an array of waveguide coupled GeSn/Ge/Si single-photon avalanche detectors. The laser pumped SWFM sources generate the signal-idler pairs, which, in turn, are used to measure the two-photon, four-photon, and higher order coincidences, resulting in an increasing of the gyro resolution by a factor of two and four, with respect to the classical approach.

Environmental Monitoring of Methane with Quartz-Enhanced Photoacoustic Spectroscopy Exploiting an Electronic Hygrometer to Compensate the H2O Influence on the Sensor Signal.

A dual-gas sensor based on the combination of a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and an electronic hygrometer was realized for the simultaneous detection of methane (CH4) and water vapor (H2O) in air. The QEPAS sensor employed an interband cascade laser operating at 3.34 µm capable of targeting a CH4 absorption line at 2988.8 cm-1 and a water line at 2988.6 cm-1. Water vapor was measured with both the electronic hygrometer and the QEPAS sensor for comparison. The measurement accuracy provided by the hygrometer enabled the adjustment of methane QEPAS signal with respect to the water vapor concentration to retrieve the actual CH4 concentration. The sensor was tested by performing prolonged measurements of CH4 and H2O over 60 h to demonstrate the effectiveness of this approach for environmental monitoring applications.

Broadband detection of methane and nitrous oxide using a distributed-feedback quantum cascade laser array and quartz-enhanced photoacoustic sensing.

Here we report on the broadband detection of nitrous oxide (N2O) and methane (CH4) mixtures in dry nitrogen by using a quartz-enhanced photoacoustic (QEPAS) sensor exploiting an array of 32 distributed-feedback quantum cascade lasers, within a spectral emission range of 1190-1340 cm-1 as the excitation source. Methane detection down to a minimum detection limit of 200 ppb at 10 s lock-in integration time was achieved. The sensor demonstrated a linear response in the range of 200-1000 ppm. Three different mixtures of N2O and CH4 in nitrogen at atmospheric pressure have been analyzed. The capability of the developed QEPAS sensor to selectively determine the N2O and CH4 concentrations was demonstrated, in spite of significant overlap in their respective absorption spectra in the investigated spectral range.

Methane Gas Photonic Sensor Based on Resonant Coupled Cavities.

In this paper we report methane gas photonic sensors exploiting the principle of absorption-induced redirection of light propagation in coupled resonant cavities. In particular, an example of implemented architecture consists of a Fabry-Pérot (FP) resonator coupled to a fibre ring resonator, operating in the near IR. By changing the concentration of the methane gas in the FP region, the absorption coefficient of the FP changes. In turn, the variation of the methane gas concentration allows the redirection of the light propagation in the fibre ring resonator. Then, the methane gas concentration can be evaluated by analysing the ratio between the powers of two resonant modes, counter-propagating in the fibre ring resonator. In this way, a self-referenced read-out scheme, immune to the power fluctuations of the source, has been conceived. Moreover, a sensitivity of 0.37 ± 0.04 [dB/%], defined as the ratio between resonant modes at different outputs, in a range of methane concentration included between the 0% and 5%, has been achieved. These results allow a detection limit below the lower explosive limit (LEL) to be reached with a cost-effective sensor system.

Acoustic Coupling between Resonator Tubes in Quartz-Enhanced Photoacoustic Spectrophones Employing a Large Prong Spacing Tuning Fork.

A theoretical model describing the acoustic coupling between two resonator tubes in spectrophones exploiting custom-made quartz tuning forks (QTFs) is proposed. The model is based on an open-end correction to predict the optimal tube length. A calculation of the sound field distribution from one tube exit allowed for the estimation of the optimal radius as a function of the QTF prong spacing and the sound wavelength. The theoretical predictions have been confirmed using experimental studies employing a custom QTF with a fundamental flexural mode resonance frequency of 15.8 kHz and a quality factor of 15,000 at atmospheric pressure. The spacing between the two prongs was 1.5 mm. Spectrophones mounting this QTF were implemented for the quartz-enhanced photoacoustic detection of water vapor in air in the mid-infrared spectral range.

Dual-Gas Quartz-Enhanced Photoacoustic Sensor for Simultaneous Detection of Methane/Nitrous Oxide and Water Vapor.

The development of a dual-gas quartz-enhanced photoacoustic (QEPAS) sensor capable of simultaneous detection of water vapor and alternatively methane or nitrous oxide is reported. A diode laser and a quantum cascade laser (QCL) excited independently and simultaneously both the fundamental and the first overtone flexural mode of the quartz tuning fork (QTF), respectively. The diode laser targeted a water absorption line located at 7181.16 cm-1 (1.392 µm), while the QCL emission wavelength is centered at 7.71 µm and was tuned to target two strong absorption lines of methane and nitrous oxide, located at 1297.47 and 1297.05 cm-1, respectively. Two sets of microresonator tubes were positioned, respectively, at the antinode points of the fundamental and the first overtone flexural modes of the QTF to enhance the QEPAS signal-to-noise ratio. Detection limits of 18 ppb for methane, 5 ppb for nitrous oxide and 20 ppm for water vapor have been achieved at a lock-in integration time of 100 ms.

High-sensitivity real-splitting anti-PT-symmetric microscale optical gyroscope.

Optical gyroscopes measure the angular velocity using the Sagnac effect. However, the resonance splitting due to the Sagnac effect is directly proportional to the linear dimensions of the device. Consequently, integrated optical gyroscopes are still the subject of research. We propose the idea and the design of an anti-parity-time (APT)-symmetric optical gyroscope exhibiting a resonance splitting independent from the dimensions of the device. With a 80 µm×40 µm footprint integrated device, we demonstrated that it is possible to achieve a resonance splitting 106 times higher than the one obtained through the classical Sagnac effect. With respect to the previously proposed parity-time (PT)-symmetric gyroscope, our solution exhibits a real frequency splitting, directly measurable at the output power spectrum. Moreover, it can be kept at its exceptional point more accurately than the PT-symmetric counterpart. Finally, the anti-PT-symmetric gyroscope presented here can detect the sign of the angular velocity differently from the PT-symmetric one.

Stimulated Brillouin Scattering in an AlGaN Photonics Platform Operating in the Visible Spectral Range.

We present Stimulated Brillouin Scattering (SBS) process in AlGaN integrated photonic waveguides. The wide bandgap of this III-Nitride material platform allows operating at visible wavelengths enabling large Stokes shifts. For this study, we employ a multiphysics approach that includes electric-photoelastic, magnetic-photoelastic, material interface displacement effects, and for optimal waveguide dimensions to find the Brillouin-active acoustic modes involved in the SBS process. The SBS power gain and the Stokes frequency shift are investigated for both backward and forward scattering processes, and it is shown that stokes shift larger than 50 GHz with high gain are achievable. Moreover, a parametric analysis is presented in order to demonstrate the possibility of realizing Brillouin lasers operating at blue wavelengths.

Fibre Bragg Grating Based Strain Sensors: Review of Technology and Applications.

Fibre Bragg grating (FBG) strain sensors are not only a very well-established research field, but they are also acquiring a bigger market share due to their sensitivity and low costs. In this paper we review FBG strain sensors with high focus on the underlying physical principles, the interrogation, and the read-out techniques. Particular emphasis is given to recent advances in highly-performing, single head FBG, a category FBG strain sensors belong to. Different sensing schemes are described, including FBG strain sensors based on mode splitting. Their operation principle and performance are reported and compared with the conventional architectures. In conclusion, some advanced applications and key sectors the global fibre-optic strain sensors market are envisaged, as well as the main market players acting in this field.