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A novel method to reduce the nonlinearity effect of a wavelength scanning diode laser interrogating a low-reflectivity Fabry-Perot interferometer is presented. The interferometer is used for refractive index measurement. The proposed method allows improving the resolution of the system from $3\times10$ −4 to $2\times10$ −5; and generates a temperature insensitive sensing system. Using the resolution improvement and the temperature insensitivity, the proposed system was used to measure the thermo-optic coefficient of two liquid samples with well-known refractive index. To demonstrate the reliability of the proposed system, experimental results of the long-term performance of a six-month interval, using the same samples but different sensing heads, are presented. The proposed method allows the implementation of a standard four-pin diode laser as an optical source and a standard PIN photodetector for signal detection; permitting the implementation of a high-performance fiber sensor system based on small, light, and standard optoelectronic components. These characteristics are important for different applications such as medical and aerospace, for example, to monitor strain, curvature, pressure, and refractive index using fiber interferometers. [ABSTRACT FROM AUTHOR]

We propose an ultra-compact full-angle-range direction-distinguishable tilt sensor based on a polymer column working as a Fabry–Perot (FP) microcavity fixed between two sections of single-mode fibers (SMFs). The directional tilt angle from −90 deg to +90 deg can be demodulated by monitoring the wavelength shift of interference fringe, based on the deformation of the polymer column under the effect of the gravity force. The sensitivity of the sensor can be adjusted by changing the shape and size of the polymer column that is fabricated by the single beam illumination with high efficiency and low cost. The sensor can satisfy the engineering requirements owing to its small size, flexible structure, cost-effective performance, large measurement range, and direction-distinguishable ability. [ABSTRACT FROM AUTHOR]

The dynamic range of optical fiber refractive index sensors is mainly determined by the refractive index (RI) of the sensor surface in contact with the sample under test. In case of the refractive index sensor based on the hybrid fiber Fabry-Perot interferometer (HFFPI), the largest measurable refractive index value is equal to that of the fiber core. In this work, we demonstrate that it is possible to extend the refractive index dynamic range of a HFFPI by simply adjusting the optical path length (OPL) of the air and solid cavity to be equal or differ by just a small amount. Two isometric versions of the HFFPI (i-HFFPI) with a total length of 100 and 172 µm, where the OPL of the air and solid cavity are very similar, were fabricated and tested. The interferometers were immersed in different samples with a refractive index ranging from 1.000 to 1.733. The response of the interferometers was analyzed in the Fourier domain, and it was possible to establish a one-to-one relationship between the refractive index of the liquid sample and the amplitude of one of the peaks in the Fourier spectra. The amplitude of this peak experienced a linear increment when the RI of the surrounding medium was increased. Tracking the amplitude changes of a Fourier spectrum peak is straightforward which simplifies the online monitoring of the sensor. These features make this compact refractive index fiber sensor very appealing for biosensing applications. [ABSTRACT FROM AUTHOR]

High-temperature accelerometers have been widely used in aerospace, nuclear reactors, automobile technologies, etc. In this paper, a fiber-optic Fabry–Perot accelerometer (FOFPA) with a cantilever beam for high temperature is designed and experimentally demonstrated. The FOFPA is formed by bonding an all-silica in-line fiber Fabry–Perot etalon (ILFFPE) to one surface of the uniform cantilever beam with the lumped mass at the free end for acceleration measurement. The all silica in-line fiber FP etalon is made by welding two gold-coat single-mode fiber (GSMF) and a hollow silica glass tube (HST). The research results indicate that the sensitivity of the FOFPA is 0.02328rad/g, and the resonance frequency is 1146.6 Hz in the range of 1 g ~ 10 g. The high-temperature performance of the FOFPA was also evaluated. From 20 °C to 800 °C, the temperature drift is about 0.3178 nm/°C. The FOFPA has the potential of being applicable in higher temperatures compared to conventional accelerometers. [ABSTRACT FROM AUTHOR]

• Interferometric fiber-optic sensors are widely used in structural health monitoring. • 2-Channel coherent correlation OTDR is a simple technique for sensor demodulation. • Demultiplexing of channels and sensor signals is achieved by correlation techniques. • The number of interrogation channels can easily be extended to 3 or 4 wavelengths. • The experimentally measured temperature resolution was 3 × 10−3 °C. This paper presents a novel and simple technique for passive quadrature demodulation of multiplexed low-finesse fiber-optic Fabry-Perot interferometers. The approach proposed here uses two quadrature channels with very close wavelengths generated by two thermostabilized DFB diode lasers operating in continuous-wave mode. A coherent light correlation technique was used to demultiplex the wavelength channels and signals from interferometers at different positions along the fiber. The proposed technique does not require optical filters for spectral channel demultiplexing, making it suitable for demodulating relatively long interferometers with any free spectral range. An in-line array of four identical 13.1 cm long interferometers was used to demonstrate the method's performance. Experimental results demonstrate a phase resolution of 1.38 phase degrees, corresponding to a temperature resolution of 3 × 10−3 °C. Numerical simulations indicate that the main factor limiting the length of the sensing interferometer, and thus the resolution of the system, is the wavelength variation of the temperature-stabilized, independently operating lasers. The number of spectral channels can be easily extended to realize three or four-wavelength demodulation schemes. [ABSTRACT FROM AUTHOR]

A fiber-optic temperature sensor based on the harmonic Vernier effect (HVE) generated by a cascade of a Sagnac interferometer (SI) and a Fabry–Perot interferometer (FPI) is proposed, in which the FPI is used as a reference, while the SI is for the sensing. Moreover, the free spectral range (FSR) of the FPI is about an integer multiple of that of the SI. By adjusting the FSR of the SI, the normal Vernier effect (NVE), 1st–, 2nd–, and 3rd–order HVEs were achieved, respectively. Experimental results show that when the HVEs have the same magnification factor as the NVE, the sensors based on the HVEs have almost the same temperature sensitivity as those based on the NVE. However, the detunings of the HVEs are much larger than that of the NVE, and the higher the order of the HVEs, the greater the detuning. We know that the larger the detuning, the easier the magnification factor is to be controlled, that is, the easier the sensor is to be prepared, so from the point of view, the sensors based on the HVEs are better than those based on the NVE. This conclusion is also suitable for other fiber-optic sensors based on Vernier effect. • Fiber-optic temp sensor based on harmonic Vernier effect (HVE) using SI and FPI cascade. • Achieves 1st, 2nd, and 3rd-order HVEs by adjusting SI FSR; higher orders show larger detunings. • HVEs nearly match NVE sensitivity with larger detunings and easier magnification control. • HVE requires much less processing precision than NVE. • Advances understanding of HVE in fiber-optic temp sensing, enhancing sensitivity and control. [ABSTRACT FROM AUTHOR]

Most integrated evanescent-field photonic sensors measure changes in either the real part or the imaginary part of the complex refractive index of the sample, i.e., refraction or absorption. Here we propose and experimentally demonstrate a near-infrared sensor based on a silicon nitride Mach–Zehnder interferometer which provides a direct measurement of the complex refractive index. Our architecture exhibits a high sensitivity, achieving limits of detection below 2 ⋅ 1 0 − 6 RIU for both the real and imaginary parts of the refractive index. We furthermore show that our sensor can be employed as an integrated dispersion spectrometer. • Simultaneous refraction and absorption measurement for enhanced photonic sensing. • Integrated Mach–Zehnder interferometer with coherent phase and intensity readouts. • Phase signal is more robust against system non-idealities than the intensity signal. • State-of-the-art limit of detection for both refraction and absorption measurements. • Promising dispersion spectroscopy results encourage an extension to the mid-infrared. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a novel plug-and-play integrated optical fiber sensing structure based on femtosecond laser micromachining and Vernier effect for label-free detection of bovine serum albumin (BSA) concentration. A rectangular microgroove is etched in the hollow-core fiber (HCF), and a U-shaped fiber is formed to design an open microcavity Fabry-Perot interferometer (FPI), in which the liquid can flow sufficiently into the microcavity as the sensing medium. Through simulation and experimental testing, it has been proven that the sensor can achieve high sensitivity and precision detection of BSA concentration. The refractive index measurement sensitivity is as high as 12308.97 nm/RIU, and the BSA concentration measurement sensitivity is 1.972 nm/(mg/ml), with an average absolute percentage error of 2.03 %. The performance test shows that the sensor has the advantages of strong stability, good repeatability, high measurement accuracy and fast liquid exchange, it can be used as a new sensing method to achieve real-time, online and fast detection of weakly changing biomass. • A novel plug-and-play integrated optical fiber sensing structure is design. • A rectangular microgroove is etched to design an open microcavity FPI. • A measurement sensitivity enhancement method of BSA is proposed. • High sensitivity and precision measurement of BSA is achieved. [ABSTRACT FROM AUTHOR]

An improved interference method is proposed to determine ordinary and extraordinary refractive index dispersions of nematic liquid crystals (LCs). In this method, an LC cell coated with a thin metal layer is used as a Fabry-Perot interferometer, which shows us a sharp transmission fringe. To ensure high reliability, the wavelength dispersion of the refractive index of the metal is taken into account in fitting calculation. In spite of measuring ordinary and extraordinary components, the LC cell, polarizers, and other equipment are not rotated during the experiment. The index evaluation from a single spectrum avoids errors depending on the measurement position owing to nonuniformities of molecular orientation and cell thickness because we can obtain the two indices at exactly the same position. This system can adapt to a wide frequency range and does not require any specific wavelength light source or laser. We demonstrate the determination of ordinary and extraordinary refractive index dispersions of a nematic liquid crystal in the visible and nearinfrared regions. Furthermore, we quantitatively reproduce the measured spectrum by calculation using the measured refractive indices. [ABSTRACT FROM AUTHOR]

• The compact cascaded FPI fiber optic temperature sensors are proposed. • The sensor based on UV glue and HVE exhibits high sensitivities of 2.24 nm/°C. • Increasing the harmonic order of HVE improves the sensor's temperature sensitivity. • The sensors exhibits good linear response, excellent repeatability and stability. In this paper, compact cascaded Fabry-Perot interferometers (FPI) for fiber-optic temperature sensors are proposed and verified. The sensors are prepared by combining the properties of temperature-sensitive material ultraviolet glue (UV glue) and the basic principle of the traditional Vernier effect (TVE), first-order harmonic Vernier effect (HVE), and second-order HVE. The sensing cavity FPI s is made up of UV glue filling the gap between two single-mode fibers (SMF) with flat cut ends. The reference cavity FPI r is composed of SMF-capillary-SMF structure. The temperature sensitivity of the FPI s is up to 1.01 nm/°C by the high thermal expansion coefficient of the UV glue. By changing the free spectral range (FSR) of the FPI r , high detection sensitivities achieved are −10.14 nm/°C, 15.22 nm/°C, and 22.24 nm/°C, corresponding the situation of TVE, first-order HVE, and second-order HVE, respectively. The experimental results indicate that temperature sensors based on UV glue possess a simple and compact structure and strong resistance to electromagnetic interference, making them suitable for temperature measurement in various environments. [ABSTRACT FROM AUTHOR]

• An optical fiber temperature sensor employing the vernier effect is presented. • The sensor has low production cost, simple fabrication, and small volume. • The sensor has good stability and repeatability. • It provides a new approach for in-depth research on temperature monitoring. This paper introduces a optical fiber temperature sensor based on vernier effect. The sensor's design consists of two integrated interferometric structures: the Mach-Zehnder interferometer (MZI) and the Fabry-Perot interferometer (FPI). Both interferometric structures exhibit symmetry, but they employ different types of optical fibers. Specifically, the MZI employs dual side hole fiber (DSHF), while the FPI is constructed using hollow-core optical fibers.Given the MZI's superior sensitivity to temperature changes compared to the FPI, it is chosen as the sensing arm. The paper conducts theoretical research and experimental validation of the optical fiber sensor's sensing principles. The findings reveal a sensor sensitivity of −305.42 pm/°C when measuring temperatures within the 32–50 °C range. Stability testing, repeatability testing, and response time testing are carried out using a constant temperature chamber and a water bath, affirming the sensor's excellent repeatability and stability, along with a response time faster than that of conventional thermometers. [ABSTRACT FROM AUTHOR]

• Design and fabrication of the single-mode fiber based Fabry-Perot interferometric biosensor. • Surface functionalization by graphene oxide & protamine. • The proposed sensor demonstrates label-free, selective, sensitive and real-time heparin detection. Heparin (Hep) is the most commonly used anticoagulant drug in clinical settings, and precise dosage control is crucial. To enable rapid and accurate quantification of Hep concentration in clinical practice, a label-free and selective fiber-optic Fabry-Perot interferometric (FPI) biosensor detection method based on graphene oxide (GO) was proposed. This method utilizes protamine (PRTM) sulfate as the modified layer and achieves Hep detection through layer-by-layer (LBL) assembly. In this study, we developed a label-free and selective fiber optic FPI biosensor for the detection of Hep using GO. The GO on the surface layer of the fiber optic FPI biosensor was treated with specific functionalization, it enables specific interaction with Hep. The binding of Hep to the functionalized surface induces a change in the luminal light signal, which can be monitored within the cavity for rapid and accurate detection of Hep. The results demonstrate that this biosensor exhibits good sensitivity, with a value of 10.82 nm/mM in the low-concentration control group, and an average limit of detection (LOD) calculated as 31 μ g/mL in the low-concentration group. This study presents a label-free, selective and efficient method for biomedical science testing that holds great potential for widespread application in clinical medicine. [ABSTRACT FROM AUTHOR]

• A high sensitivity optical fiber FPI humidity sensor is proposed and applied to human respiration monitoring. • The liquid glycerol can absorb 60 per cent of its weight moisture, significantly improving humidity sensitivity. • The closed liquid cavity senses RH in the air by utilizing the permeability of the UV adhesive network structure. • The sensor is used to monitor fast and slow breaths/deep and shallow breaths via its fast response/recovery time. A novel highly humidity-sensitive with a fast response time Fabry-Perot sensor based on a sealed liquid glycerol microcavity is proposed and experimental lly applied for breath monitoring. The interference cavity structure is composed of single-mode fiber and hollow-core fiber. The hollow-core fiber is filled with glycerol and ultraviolet adhesive through capillary action to form a liquid–solid microcavity. The porous nature of the ultraviolet adhesive enables the liquid glycerol to detect external humidity. With the increase of humidity, the high-water absorption glycerol and ultraviolet adhesive absorb water and expand, the effective cavity length of the FP cavity changes and then affecting affects the sensitivity of the sensor. The results show that the humidity sensitivity of the sensor reaches 1.06917 nm/%RH. Due to the high thermo-expansion coefficient of glycerol and ultraviolet adhesive, the sensor also has high-temperature sensitivity, reaching up to 0.73857 nm/°C. Temperature and humidity parameters can be monitored simultaneously by cascading fiber Bragg grating, it is possible to monitor temperature and humidity parameters simultaneously. What's more, the sensor has fast response time (180 ms ∼ 230 ms). We applied the sensor to human respiration monitoring, and tested the response of the sensor under fast breathing and slow breathing. Compared with the gas–solid cavity structure of the optical fiber Fabry-Perot humidity sensor, the proposed sensor has the advantages of high sensitivity, simple manufacturing, and good robustness. [ABSTRACT FROM AUTHOR]

• A novel FPI based on a tapered two-mode fiber cantilever inserted into a capillary can enhance strain sensitivity. • The VE sensor and HVE sensor achieved ultra high strain sensitivity of −2.30 nm/µε and −2.98 nm/µε respectively. • The proposed sensor is simple, cost-effective and easy to fabricate. An ultra-sensitive strain sensor has been designed using two parallel Fabry-Perot interferometers (FPI) as well as the Vernier effect (VE) and harmonic vernier effect (HVE). This new FPI is formed between tapered two-mode fiber cantilever beam inserted into quartz capillary and single-mode fiber. We prepared two similar cantilever beam structures, namely FPI 1 and FPI 3. Due to the cantilever beam inserted into the capillary tube increasing the effective length of strain acting on FPI, the strain sensitivities of FPI 1 and FPI 3 are as high as 167 pm/µε and 83 pm/µε, respectively. In order to further improve the strain sensitivity of FPI, based on the principles of optical VE and HVE, we prepared FPI 2 and FPI 4 with air-cavity using capillaries. FPI 1 and FPI 2 were matched to form a VE sensor (named S 1), while FPI 3 and FPI 4 were matched to form a first-order HVE sensor (named S 2). The strain sensitivities of S 1 and S 2 are as high as −2.30 nm/µε and −2.98 nm/µε respectively. This is currently the highest reported strain sensitivity. Additionally, their temperature cross-sensitivity is only 35 nε/℃ and 23 nε/℃, respectively. The proposed sensor has excellent repeatability and stability in measuring strain. In short, the proposed strain sensor has a simple structure, cost-effective, easy to manufacture, and has extremely high sensitivity, making it one of the best candidate structures for strain measurement. [ABSTRACT FROM AUTHOR]

• FBG-based fiber F-P interferometer strainmeter arrays for field observation. • Large-scale, real-time multiplexed strainmeter array features ultrahigh-resolution. • The array in Lijiang has 4.80-pε/√Hz @ 30 Hz resolution with 0.22-m cavity length. • The array in Heqing has 476-fε/√Hz @ 30 Hz resolution with 1.4-m cavity length. • The arrays have successfully recorded daily earth tides and over 100 earthquakes. Mining mass data of the earth's surface deformation is a promising approach for earthquake early-warning, raising an urgent demand for developing large-scale optical fiber strainmeters. This paper presents the implementation and results of large-scale, ultrahigh-resolution, real-time multiplexed fiber Bragg grating-based Fabry-Perot interferometer (FBG-FFPI) strainmeter arrays for field observation. The Pound-Drever-Hall technique locks master and slave single-sideband lasers onto the FBG-FFPI resonance peaks for precise strain measurement. A 10-channel array was successfully utilized for field observation at Lijiang National Seismic Station, which accurately recorded over 100 earthquakes and the earth's daily tide signal. The FBG-FFPIs have a strain resolution of 4.80 pε/√Hz @ 30 Hz with a 0.22-m cavity length. A 4-channel array with a longer cavity length of 1.4 m was also established at Heqing Seismic Station, showing advantageous performances. These results demonstrate the significant potential of FBG-FFPI strainmeter arrays in providing geophysical data for earthquake early-warning and hazard mitigation. [ABSTRACT FROM AUTHOR]

Etalon-based calibrators have rapidly gained popularity over the past decade in the field of high-precision radial velocity and high-resolution spectroscopy studies. Solid etalons are compact, pressure insensitive, commercially available alternatives to customized air spaced Fabry–Perot etalons. For tight-budget projects and weight-constricted missions, calibration system built from solid etalon is an interesting option to explore. For those, achievable spectral stability becomes the biggest question due to increased thermal sensitivity of the cavity material. Here, the design and performance of a low-cost solid-etalon calibrator is presented. A dual-loop temperature control system keeps the temperature fluctuations to within 1 mK rms when fully stabilized. Drift performance was tracked simultaneously with a laser frequency comb and the chromatic thermal response is measured through temperature tuning. The results indicate that a thermally controlled solid-etalon system can demonstrate sufficient short-term stability (

The refractive index and its variation with temperature, i.e. the thermo-optic coefficient, are basic optical parameters for all those semiconductors that are used in the fabrication of linear and non-linear opto-electronic devices and systems. Recently, 4H single-crystal silicon carbide (4H-SiC) and gallium nitride (GaN) have emerged as excellent building materials for high power and high-temperature electronics, and wide parallel applications in photonics can be consequently forecasted in the near future, in particular in the infrared telecommunication band of λ = 1500–1600 nm. In this paper, the thermo-optic coefficient (dn/dT) is experimentally measured in 4H-SiC and GaN substrates, from room temperature to 480 K, at the wavelength of 1550 nm. Specifically, the substrates, forming natural Fabry–Perot etalons, are exploited within a simple hybrid fiber free-space optical interferometric system to take accurate measurements of the transmitted optical power in the said temperature range. It is found that, for both semiconductors, dn/dT is itself remarkably temperature-dependent, in particular quadratically for GaN and almost linearly for 4H-SiC. [ABSTRACT FROM AUTHOR]

Micro-channels (μ-channels) in microstructure fibres can be regarded as natural in-fibre flow channels. Thus, the advent of microstructure fibres with μ-channels makes it possible to realize in-fibre integrated microfluidic devices, and microstructure fibres with μ-channels provide the possibility of creating new online monitoring systems and define new concepts. Herein, we developed a novel compact in-fibre detection platform, which is combined with a μ-channel in a new-type microstructure fibre, side-hole fibre, for in-fibre detection. The optical component of this proposed in-fibre detection platform is made of a simple cross-axial open-cavity Fabry–Perot interferometer. This miniaturized cross-axial open-cavity Fabry–Perot interferometer is formed by a 45°-angled side-hole fibre, which is fabricated by a simple end-face polishing process. For a 45°-angled fibre, the incident optical axis can be steered based on total internal reflection at the oblique fibre–air interface, and the reflected light will enter the side-hole μ-channel, and the front and rear inner μ-channel walls form the cavity; the natural in-fibre μ-channel functions as a (liquid/gas) flow channel. Experimentally, this proposed in-fibre detection platform was fabricated, and its spectral characteristics were investigated. Its relative humidity characteristics and potential application in breath sensing were calibrated by measuring the evolution of the reflection spectrum. As a whole, the proposed detection platform demonstrates the advantages of simple structure, easy fabrication without additional sensitive materials, and potential application in breath sensing or lab-in-fibre technology. [ABSTRACT FROM AUTHOR]

High-throughput single-cell analysis based on physical properties (such as morphology or mechanics) is emerging as a powerful tool to inform clinical research, with a great potential for translation towards diagnosis. Here we present a novel microfluidic approach adopting acoustic waves to manipulate and mechanically stimulate single cells, and interferometry to track changes in the morphology and measure size, deformability, and refractive index of non-adherent cells. The method is based on the integration within the acoustofluidic channel of a low-finesse Fabry–Perot resonator, providing very high sensitivity and a speed potentially suitable to obtain the high-throughput necessary to handle the variability stemming from the biological diversity of single cells. The proposed approach is applied to a set of different samples: reference polystyrene beads, algae and yeast. The results demonstrate the capability of the acoustofluidic interferometric device to detect and quantify optomechanical properties of single cells with a throughput suitable to address label-free single-cell clinical analysis. [ABSTRACT FROM AUTHOR]

Abstract—Two modifications of the Fabry–Perot interferometer were developed that use oblique incidence planar fronts from the source: Fabry–Perot interferometer with an open beam input and Fabry–Perot interferometer with an open beam output. The modifications differ only in the direction of the rays. The first modification has already been used as a basic element of the solar interference spectrograph layout (Kulagin, 1980) and the stellar interference spectrograph layout (Kulagin and Panchuk, 2017). The second modification has no analogues. Estimates of the increase in the illumination of interference patterns in the modifications are given. The same formula describes the dependence of the intensity of the outgoing radiation on the interference phase in both devices. However, the properties of the modifications differ dramatically. Possible applications of the two modifications combined with a diffraction grating are described. [ABSTRACT FROM AUTHOR]

The sensing characteristics of irradiated fiber Bragg gratings (FBGs) and Fabry-Perot interferometers (FPIs) were investigated under a 2 MGy dose of gamma radiation. The study found that the pressure sensitivity of FP sensors after irradiation was stable, while the temperature sensitivity of FBG sensors was unstable, and both wavelengths displayed a shift. These findings offer the possibility for the application of FP pressure sensors in the gamma radiation environments, and FBG sensors require further research to be suitable for application in the nuclear radiation environments. [ABSTRACT FROM AUTHOR]

Simultaneously and independently measuring two-dimensional (2D) displacement, one-dimensional (1D) for out-of-plane measurement and 1D for in-plane measurement, is important for practical applications. Although extrinsic fiber Fabry-Perot interferometers (EFPIs) have been reported for highly precise out-of-plane displacement measurements for many years, simultaneous measurement of 2D displacement with sub-micrometer accuracy has not been realized by a monolithic EFPI as far as we know. In this paper, a monolithic dual-cavity EFPI to realize 2D displacement measurement of a target is reported. By quantitatively analyzing and experimentally calibrating the linear dependences of the in-plane displacement sensitivity and interception on the cavity length, the effect of out-of-plane displacement on the in-plane displacement measurement is removed, which is the basis of the 2D displacement detection. 2D random movement detection and the repeatability of the system are investigated experimentally. Within the range of ±7 μm for in-plane displacement, and 352 μm-358 μm for cavity length, demodulation errors better than 96 nm are achieved. The 2D displacement measurement system based on the dual-cavity EFPI with the advantages of noncontact, high accuracy and compact size is promising to be applied in the 2D acceleration measurement, 2D rotor vibration measurement, 2D precise positioning in semiconductor manufacture, thermal expansion coefficient measurement, and so on. [ABSTRACT FROM AUTHOR]

The phase is one of the fundamental properties of a wave that allows to control interference effects and can be used to efficiently encode information. We examine numerically a magnonic resonator of the Gires–Tournois interferometer type, which enables the control of the phase of spin waves (SWs) reflected from the edge of the ferromagnetic film. The considered interferometer consists of a Py thin film and a thin, narrow Py stripe placed above its edge, both coupled magnetostatically. We show that the resonances and the phase of the reflected SWs are sensitive for a variation of the geometrical parameters of this bi-layered part of the system. The high sensitivity to film, stripe, and non-magnetic spacer thicknesses offers a prospect for developing magnonic metasurfaces and sensors. [ABSTRACT FROM AUTHOR]

The thermo-optic coefficient (TOC) of sapphire wafers at temperatures ranging from 25 °C to 1500 °C is measured by an extrinsic Fabry-Perot interferometer capable of operating at high temperatures. Sapphire wafers with two thicknesses are compared in the experiment. The TOC of the sapphire wafer is retrieved based on the measured temperature response of the optical path length of the Fabry-Perot cavity and the known thermal expansion coefficient of sapphire wafer. Experimental results demonstrate that the TOC of sapphire wafer at 1550 nm wavelength has a quadratic dependence on temperature up to 1500 °C. The first and second-order TOCs of the sapphire wafer are calculated as $8.163\times 10$ −6 and $1.1075\times 10$ −8, respectively. The temperature response and sensitivity of the sapphire-wafer-based high-temperature sensors with the same structure can be inferred using the obtained sapphire TOC. The reported approach and results significantly simplify the design and calibration procedures of the high-temperature sensors made of sapphire wafers. [ABSTRACT FROM AUTHOR]

This study proposes an experimental implementation of temperature sensing system to study the enhanced Vernier effect using two polydimethylsiloxane (PDMS)-cavity Fabry-Perot interferometers (FPIs) arranged in parallel. However, these devices exhibited opposite responses to temperature; that is, with an increase (or decrease) in temperature, the two FPIs shifted in red and blue wavelength directions, respectively. This indicated that the sensitivity amplification of their parallel configuration was considerably larger than that generated by the traditional Vernier effect. Experimental results showed that the temperature sensitivities of the two FPIs were −0.42 and 1.36 nm/°C while that of their parallel structure was −14.6 nm/°C corresponding to an amplification factor of 10.7, which was much larger than that (8.2) of the traditional Vernier effect. [ABSTRACT FROM AUTHOR]

We present a climatology of nighttime thermospheric neutral winds between February 2018 and January 2019 measured by a Fabry-Perot interferometer (FPI) located in Sutherland, South Africa (32.2°S, 20.48°E; geomagnetic latitude: 40.7°S). This FPI measures the nighttime oxygen airglow emission at 630.0 nm, which has a peak intensity at an altitude of roughly 250 km. The annual meridional and zonal winds at this location vary between −100 and 120 m/s and show typical midlatitude nocturnal and seasonal variations. During local summer months (December-February), the meridional wind is predominantly equatorward from dusk to predawn. However, during the winter months, the meridional wind is poleward from dusk, turns equatorward around midnight, and either remains in this direction for the rest of the night (June) or turns poleward again just before dawn (July and August). The zonal wind velocity is generally eastward during the evening until just before midnight, changing westward post-midnight. The zonal wind peaks at higher velocities during the winter months compared to the summer months. The eastward-to-westward transition occurs later during the winter months compared to the summer months. We compare results from HWM14 with the FPI measurements and find a better agreement between FPI measured winds and HWM14 modeled winds for the meridional component compared to the zonal component. In addition, the HWM14 zonal wind consistently peaked several hours (∼ 3 h) prior to the measured wind. Furthermore, the HWM14-modeled eastward-to-westward transitions times were a couple of hours (∼ 2 h) earlier during the months of November to January creating what looks like a phase shift compared to the measured wind. This phase shift was apparent in all months and we suggest this can be attributed to a phase shift of the terdiurnal tide between the model and measurements. [ABSTRACT FROM AUTHOR]

In this study, a Vernier effect based temperature sensor with ultra-sensitivity and high-resolution detection is presented. The structure of the proposed temperature sensor is based on dual cascaded Fabry-Perot interferometers (FPIs), which consists of polymer and air cavity FPIs. The polymer cavity works as the sensing part, whereas the air cavity works as the reference part. The slight difference between the Free Spectral Range (FSR) of the sensing and the reference FPIs can establish the Vernier effect, which improves the sensitivity of the cascaded FPIs structure compared to the single FPI structure. The experimental results show that the proposed structure can provide the ultra-high temperature sensitivity of 67.69 nm/°C that is 20 times higher than the single FPI, which is 3.36 nm/°C in the testing range of 26°C-28°C. In addition, the structure is simple to fabricate, compact, inexpensive, along with ultra-sensitivity and high-resolution. Therefore, the proposed sensor is a suitable choice for the applications demanding high resolution temperature detection in different fields of engineering and science. [ABSTRACT FROM AUTHOR]

Portable mid-infrared (mid-IR) spectroscopy and sensing applications require widely tunable, chip-scale, single-mode sources without sacrificing significant output power. However, no such lasers have been demonstrated beyond 3 μm due to the challenge of building tunable, high quality-factor (Q) on-chip cavities. Here we demonstrate a tunable, single-mode mid-IR laser at 3.4 μm using a tunable high-Q silicon microring cavity and a multi-mode Interband Cascade Laser. We achieve single-frequency lasing with 0.4 mW output power via self-injection locking and a wide tuning range of 54 nm with 3 dB output power variation. We further estimate an upper-bound effective linewidth of 9.1 MHz and a side mode suppression ratio of 25 dB from the locked laser using a scanning Fabry-Perot interferometer. Our laser platform based on a tunable high-Q microresonator can be expanded to higher wavelength quantum-cascade lasers and lead to the development of compact, high-performance mid-IR sensors for spectroscopic applications. The availability of integrated mid-infrared light sources is restricted by a lack of suitable materials and increasingly complicated fabrication requirements. Here, a single-mode silicon microresonator laser is demonstrated, who's emission in the mid-infrared can be tuned by integrated microheaters. [ABSTRACT FROM AUTHOR]

A probe-shaped sensor for simultaneous temperature and pressure measurement was reported in this article. The effective length of the sensor was ∼2mm, consisting of a fiber Bragg grating (FBG) and a Fabry-Perot interferometer (FPI) with a nano silica diaphragm. The response sensitivities of the sensor for pressure and temperature were measured as −0.98 nm/MPa and 11.10 pm/°C, respectively. This sensor had an extremely low cross-sensitivity between pressure and temperature, which provided a significant potential in dual-parameter sensing. [ABSTRACT FROM AUTHOR]

In this work, a multi-parameter point sensor based on the combination of Fabry-Perot (FP) and the anti-resonant (AR) reflecting guidance in cascade configuration is proposed and experimentally demonstrated. This structure, based on FP interference and AR reflecting guidance, was fabricated with two different air micro-cavities. The attained experimental results showed different strain and temperature sensitivities for the antiresonance contribution. However, when analyzing the FP interference, only strain sensitivity was observed, demonstrating that this air micro-cavity was also insensitive to temperature variations. [ABSTRACT FROM AUTHOR]

A methodology to design a multipoint fiber refractometer based on the Coherent Optical Frequency Domain Reflectometry (C-OFDR) technique is presented; in this refractometer, the nonlinearity effect generated by the scanning in optical frequency is reduced by a proper design of each sensing head; eliminating the need for the auxiliary interferometer commonly used for reducing this effect. Experimental demonstration on the nonlinearity effect in the sensing signal is presented. This effect is reduced by cutting the length of the processed signal in a cycle of scanning; to demonstrate this behavior, 100%, 50%, and 25% of the sensing signal from a scanning cycle are processed. However, it is not only about reducing the length of the sensing signal, but proper sensing head design also allows the reduced signal to be form by an integer number of cycles from each interference signal generated at the sensing heads, which in turns permit concatenation of the reduced signal without losing information. These three considerations, reduction of the signal in a scanning cycle, an integer number of cycles, and concatenation, generate stable and strong sensing signals. In this sense, the design of the sensing heads allows reducing the nonlinearity effect and possibility of a multipoint refractometer as well. Theoretical description and experimental results to demonstrate the effectiveness of the proposed method are presented. Design, construction, characterization and simultaneous implementation of three sensing heads are demonstrated. [ABSTRACT FROM AUTHOR]

A tip thin-wall hybrid-structure Fabry–Perot interferometer (THFPI) based on zero cross-sensitive, compact, and high robustness is proposed and demonstrated. The tip THFPI was fabricated by stretching and discharging simultaneously when the hollow-core fiber (HCF) and single-mode fiber (SMF) are spliced. The new preparation process and thinking are provided to FPI production. Using the method of stretching into a tapered optical fiber, the material volume of the silica cavity is reduced, which can significantly improve the sensitivity of the lateral load sensor. We exhibit the experimental results of using the Vernier effect to measure the lateral load. The lateral load sensitivity obtained by monitoring the envelope interference wavelength is 3.1 times the lateral load sensitivity obtained by tracking the dip of the high-frequency component. In addition, a parameter decoupling algorithm is proposed to achieve the precise measurement of lateral load in a variational temperature environment. The parametric decoupling algorithm can be extended to similar sensors with cross-sensitivity. The high sensitivity, high stability, and non-cross-sensitive sensors proposed in this paper have important practical application value. [ABSTRACT FROM AUTHOR]

Ultrahigh-resolution fiber-optic sensing has found a wide range of potential applications. However, the techniques reported so far are all based on highly specialized fiber structures and interrogation lasers, which are not widely available. In this paper, we report the demonstration of ultrahigh strain resolutions using only off-the-shelf commercial components. Our method leverages the high wavelength discrimination of long, high-finesse fiber Fabry-Perot interferometers (FFPI), using two 1 m-long FFPIs, one as the sensor and the other as a frequency reference. By locking the interrogation laser to the reference interferometer, which is co-packaged with the sensor interferometer, large, environment-induced sensing background is removed. This allows the laser to reliably probe the strains applied on the sensor with very high resolutions. A nominal, noise-limited strain resolution of 800 fε/√Hz has been achieved within 1-100 Hz. Strain resolution further improves to 75 fε/√Hz at 1 kHz, 60 fε/√Hz at 2 kHz and 40 fε/√Hz at 23 kHz, demonstrating better resolutions than proven techniques such as π-phase-shifted and slow-light fiber Bragg gratings. The work lays out a cost-effective scheme to achieve ultrahigh-resolution fiber-optic sensing.

The continuous advances in sensing and telecommunications fields have boosted the development of new technologies towards the improvement of healthcare systems. This drive of knowledge is required to address the rise of life expectancy of an ageing population with increased associated physical impairments, in order to ease the burden on already stressed healthcare systems. Towards such objective, this paper explores the use of a wearable optical fiber based solution for the ankle plantar-dorsi-flexion monitoring, to be used in the evaluation of the progress of physical rehabilitation therapies. The proposed device is a non-invasive, small size and easy to use solution, based on a cost-effective in-line Fabry-Perot interferometer, complemented with new dynamic interrogation techniques that allow the angular monitoring of the ankle-shank joint during gait (walking). The designed and produced wearable solution was calibrated and tested in a laboratory environment, with promising results that prove the accuracy of the wearable device, as it falls within the expected pattern of an ankle plantar-dorsi-flexion movement during gait. The developed system can be used for rehabilitation therapies monitoring, to be integrated in exoskeletons or applied for athletes' performance analysis and optimization, during injury recovery.

Mirror-on-mirror nanoplasmonic metamaterials, formed on the basis of voltage-controlled reversible self-assembly of sub-wavelength-sized metallic nanoparticles (NPs) on thin metallic film electrodes, are promising candidates for novel electro-tunable optical devices. Here, we present a new design of electro-tunable Fabry–Perot interferometers (FPIs) in which two parallel mirrors – each composed of a monolayer of NPs self-assembled on a thin metallic electrode – form an optical cavity, which is filled with an aqueous solution. The reflectivity of the cavity mirrors can be electrically adjusted, simultaneously or separately, via a small variation of the electrode potentials, which would alter the inter-NP separation in the monolayers. To investigate optical transmittance from the proposed FPI device, we develop a nine-layer-stack theoretical model, based on our effective medium theory and multi-layer Fresnel reflection scheme, which produces excellent match when verified against full-wave simulations. We show that strong plasmonic coupling among silver NPs forming a monolayer on a thin silver-film substrate makes reflectivity of each cavity mirror highly sensitive to the inter-NP separation. Such a design allows the continuous tuning of the multiple narrow and intense transmission peaks emerging from an FPI cavity via electro-tuning the inter-NP separation in situ – reaping the benefits from both inexpensive bottom-up fabrication and energy-efficient tuning.

A highly sensitive strain sensor based on tunable cascaded Fabry–Perot interferometers (FPIs) is proposed and experimentally demonstrated. Cascaded FPIs consist of a sensing FPI and a reference FPI, which effectively generate the Vernier effect (VE). The sensing FPI comprises a hollow core fiber (HCF) segment sandwiched between single-mode fibers (SMFs), and the reference FPI consists of a tunable air reflector, which is constituted by a computer-programable fiber holding block to adjust the desired cavity length. The simulation results predict the dispersion characteristics of modes carried by HCF. The sensor’s parameters are designed to correspond to a narrow bandwidth range, i.e., 1530 nm to 1610 nm. The experimental results demonstrate that the proposed sensor exhibits optimum strain sensitivity of 23.9 pm/με, 17.54 pm/με, and 14.11 pm/με cascaded with the reference FPI of 375 μm, 365 μm, and 355 μm in cavity length, which is 13.73, 10.08, and 8.10 times higher than the single sensing FPI with a strain sensitivity of 1.74 pm/με, respectively. The strain sensitivity of the sensor can be further enhanced by extending the source bandwidth. The proposed sensor exhibits ultra-low temperature sensitivity of 0.49 pm/°C for a temperature range of 25 °C to 135 °C, providing good isolation for eliminating temperature–strain cross-talk. The sensor is robust, cost-effective, easy to manufacture, repeatable, and shows a highly linear and stable response for strain sensing. Based on the sensor’s performance, it may be a good candidate for high-resolution strain sensing.

In this paper, we show that Inkjet Printing can be successfully applied to external-cavity vertically emitting thin-film organic lasers and can be used to generate a diffraction-limited output beam with an output energy as high as 33.6 μJ with a slope efficiency S of 34%. Laser emission shows to be continuously tunable from 570 to 670 nm using an intracavity polymer-based Fabry-Perot etalon. High-optical quality films with several μm thicknesses are realized, thanks to ink-jet printing. We introduce a new optical material where EMD6415 commercial ink constitutes the optical host matrix and exhibits a refractive index of 1.5 and an absorption coefficient of 0.66 cm-1 at 550-680 nm. Standard laser dyes like Pyrromethene 597 and Rhodamine 640 are incorporated in solution to the EMD6415 ink. Such large size "printed pixels" of 50mm² present uniform and flat surfaces, with roughness measured as low as 1.5 nm in different locations of a 50 μm × 50 μm AFM scan. Finally, as the gain capsules fabricated by Inkjet printing are simple and do not incorporate any tuning or cavity element, they are simple to make, have a negligible fabrication cost, and can be used as fully disposable items. This work opens the way towards the fabrication of really low-cost tunable visible lasers with an affordable technology that has the potential to be widely disseminated. [ABSTRACT FROM AUTHOR]

We have theoretically studied the Goos-Hänchen-like shift of spinor-unpolarized beams tunneling through various gate-biased silicene nanostructures. Following the stationary-phase method, lateral displacement in single-, dual-, and multiple-gated silicene systems has been systematically demonstrated. It is shown for simple single-gated silicene that lateral displacement can be generally enhanced by Fabry-Perot interference, and near the transition point turning on the evanescent mode a very large lateral shift could be observed. For the dual-gated structure, we have also shown the crucial role of localized modes like quantum well states in enhancing the beam lateral displacement, while for the multiple gate-biased systems the resulting superlattice subbands are also favorable for lateral displacement enhancement. Importantly, including the degeneracy-broken mechanisms such as gate-field and magnetic modulations, a fully spinor-resolved beam can be distinguished from the rest counterparts by aligning the incident beam with a proper spinor-resolved transition point, localized state, and subband, all of which can be flexibly modulated via electric means, offering the very desirable strategies to achieve the fully spinor-polarized beam for functional electronic applications. [ABSTRACT FROM AUTHOR]

A pure-mode performance is critical for elastic diodes, but difficult to achieve because of the rich polarization characteristics of elastic waves. In this work, we propose a type of elastic mode-converting diode for an efficient and pure-mode asymmetric transmission. The diode consists of two components, one purely for mode-selecting and the other solely for mode-converting. The former is made of fluid-like metamaterials, which only support the transmission of longitudinal waves in a certain frequency range with a nearly 100% power efficiency. The latter works as a transmodal Fabry–Perot interferometer, and can convert the wave energy between longitudinal and transverse modes solely and maximally. With the designed diode, a pure-mode one-way transmission with a power efficiency of nearly 90% is realized in the ultralow frequency range. We expect that this effective and efficient device can offer a wide range of options for manipulating elastic waves, including mode isolation, selection, conversion, rectification, etc. [ABSTRACT FROM AUTHOR]

Low‐finesse fiber‐optic Fabry–Perot (F‐P) interferometric sensor has the advantages of high sensitivity, anti‐electromagnetic interference, and compact size, and is one of the most widely used optical fiber sensors. With the development of sensor design and demodulation algorithms, F‐P sensing technique plays an increasingly important role in high‐speed dynamic measurement, such as dynamic temperature or pressure, acoustic vibration, dynamic displacement, and distance. Signal demodulation methods largely determine the performance of the entire sensing system. Compared with quasi‐static measurement, high‐precision measurement of dynamic F‐P cavity is more challenging. The advancement of fast tunable laser technology in telecommunication field provides novel perspectives for demodulation methods and dynamic applications based on F‐P sensors. This article reviews the latest developments in fast interrogation techniques for dynamic low‐finesse F‐P sensors, including intensity demodulation, quadrature phase demodulation and absolute cavity length demodulation. In addition, array multiplexing and multi‐parameter measurement techniques are also involved. [ABSTRACT FROM AUTHOR]

In this article, a reflective-type optical fiber probe, which can measure salinity and temperature simultaneously, is proposed. The proposed probe integrates Mach–Zehnder interferometer and Fabry–Perot interferometer in the same channel skillfully and effectively, which makes it possible to integrate more information into the sensor. The reflected spectrum is improved greatly by adopting special sputtering technology. The spectrums of Mach–Zehnder and Fabry–Perot are separated by applying the frequency division multiplex technique. The simulation and experimental results prove that the proposed probe has a highly sensitive response to temperature and salinity. The experiment result verifies that the salinity sensitivity is up to 2.7 nm/‰ and the temperature sensitivity is better than 0.4 nm/°C. The reflective probe makes the sensor easy to use and paves the way for practical application. [ABSTRACT FROM AUTHOR]

The strain- and temperature-sensing characteristics of fiber ring laser (FRL) sensors are studied comprehensively. The reflected sensing head, made up of spheroidal Fabry–Perot interferometer (FPI) concatenated with fiber Bragg grating (FBG), is inserted in the cavity of FRL acting as the laser intensity modulator and wavelength selector. The laser intensity is sensitive to temperature change while insensitive to strain, making the dual-parameter demodulation realizable without multivariate matrix by detecting the laser wavelength shift and the laser intensity variation, for the first time in our best knowledge. The comparable sensitivities of 1 pm/ $\mu \varepsilon $ and 24.9 pm/°C are detected in experiment for strain and temperature, respectively, with low detection limit (DL) of 0.87 $\mu \varepsilon $ and 0.035 °C, high $Q$ factor of 1140 and 7.07 $\times \,\,10^{5}$. The high intensity and narrow linewidth of the sensor make it potentially great for application in long-distance sensing systems. [ABSTRACT FROM AUTHOR]

Abstract—The scanning Fabry–Perot interferometer (FPI) is the oldest method of optical 3D spectroscopy. It is still in use because of the high spectral resolution it provides over a large field of view. The history of the application of this method for the study of extended objects (nebulae and galaxies) and the technique of data reduction and analysis are discussed. The paper focuses on the performing observations with the scanning FPI on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The instrument is currently used as a part of the SCORPIO-2 multimode focal reducer. The results of studies of various galactic and extragalactic objects with the scanning FPI on the 6-m telescope—star-forming regions and young stellar objects, spiral, ring, dwarf and interacting galaxies, gas subsystems associated with the ionization cones of active galactic nuclei, galactic winds, etc. are briefly discussed. Further prospects for research with the scanning FPI of the Special Astrophysical Observatory of the Russian Academy of Sciences are discussed. [ABSTRACT FROM AUTHOR]

The increased interest in developing new photonic devices that can support high data rates, high sensitivity and fast processing capabilities for all optical communications, motivates a pre stage pulse compressor research. The pre-stage research was based on cascading single mode fiber and polarization maintaining fiber to get pulse compression with compression factor of 1.105. The demand for obtaining more précised photonic devices; this work experimentally studied the behavior of Polarization maintaining fiber PMF that is sandwiched between two cascaded singe mode fiber SMF and fiber Bragg gratings FBG. Therefore; the introduced interferometer performed hybrid interference of both Mach-Zehnder and Fabry-Perot interferometers. The hybrid interference is resulted from the interference of the forward, backward and X,Y polarization components of the propagating light along the cascaded fibers. In conclusion spectral pulse compression with maximum compression factor in range of 3-6 was obtained by tuning applied stress on the shortest etched PMF segment of the interferometer (i.e. 8cm length). Such interferometer can support fine tunable all optical band pass filter [ABSTRACT FROM AUTHOR]

Silicon nanocrystals embedded in SiC are studied by spectrophotometry and photoluminescence (PL) spectroscopy. Absorptivities are found to be affected by residual Fabry-Pérot interference arising from measurements of reflection and transmission at locations of different film thickness. Multiple computational and experimental methods to avoid these errors in thin film measurements, in general, are discussed. Corrected absorptivity depends on the quantity of Si embedded in the SiC but is independent of the Si crystallinity, indicating a relaxation of the k-conservation criterion for optical transitions in the nanocrystals. Tauc gaps of 1.8-2.0 and 2.12 eV are determined for Si nanoclusters and SiC, respectively. PL spectra exhibit a red-shift of ~100 nm per nm nominal Si nanocluster diameter, which is in agreement with quantum confinement but revealed to be an artifact entirely due to Fabry-Pérot interference. Several simple experimental methods to diagnose or avoid interference in PL measurements are developed that are applicable to all thin films. Corrected PL is rather weak and invariant with passivation, indicating that non-paramagnetic defects are responsible for rapid non-radiative recombination. They are also responsible for the broad, sub-gap PL of the SiC, and can wholly account for the form of the PL of samples with Si nanoclusters. The PL intensity of samples with Si nanoclusters, however, can only be explained with an increased density of luminescent defects in the SiC due to Si nanoclusters, efficient tunneling of photogenerated carriers from Si nanoclusters to SiC defects, or with emission from a-Si nanoclusters. Films prepared on Si exhibit much weaker PL than the same films prepared on quartz substrates. [ABSTRACT FROM AUTHOR]

We present an advanced multimodality characterization platform for simultaneous optical and electrochemical measurements of ferrocyanides. Specifically, we combined a fiber-optic Fabry–Perot interferometer with a three-electrode electrochemical setup to demonstrate a proof-of-principle of this hybrid characterization approach, and obtained feasibility data in its monitoring of electrochemical reactions in a boron-doped diamond film deposited on a silica substrate. The film plays the dual role of being the working electrode in the electrochemical reaction, as well as affording the reflectivity to enable the optical interferometry measurements. Optical responses during the redox reactions of the electrochemical process are presented. This work proves that simultaneous opto-electrochemical measurements of liquids are possible. [ABSTRACT FROM AUTHOR]

A cavitation bubble can be generated in polymer-cap on the fiber end-face by using femtosecond laser in less than 1 s. The position and diameter of the bubble can be adjusted by changing the focus position and average power of the laser beam, respectively. This efficient fabrication method was used to build cascaded Fabry–Perot interference cavities to increase the wavelength demodulation range. The evaluation method based on length matching analysis was proposed to obtain the optimized length of each cascaded cavity. Simulation results showed that the dominant peak/valley can always be highly distinguishable when the bubble diameter was located at 17.4 ~21.5 μm. A larger fabrication tolerance was allowed and the manufacturing difficulty can be reduced tremendously. Finally, a sensor with a bubble diameter of about 20 μm was fabricated. Experimental results showed that the contrast of the dominant valley was increased to 20 dB, and the wavelength demodulation range of proposed sensor was nearly three times over that of the structure without bubble. [ABSTRACT FROM AUTHOR]