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Microwave interferometers have been attractive devices for high-precision measurements of displacement-related physical quantities in a variety of scientific and engineering applications. However, due to the long operating wavelength, the displacement measurement resolution of microwave interferometers is limited to the micrometer scale. In this article, we propose and demonstrate a novel technology based on microwave interferometry for displacement measurements with potential picometer resolution. An open-ended (OE) transmission line resonator based on a homemade hollow coaxial cable is explored in proof-of-concept numerical and experimental investigations. The technique involves two stages of displacement sensitivity amplification: one is the open end-based phase sensitivity amplification, and the other is the destructive interference-based phase sensitivity amplification. We experimentally demonstrate a wavelength sensitivity amplification factor of 1800 through spectral interrogation where only the open end-based amplification is involved, and a phase sensitivity amplification factor of 1.4 million through phase interrogation where both the open end-based amplification and the destructive interference-based phase shift amplification are involved, i.e., two-stage phase shift amplified microwave interferometry (TPAMI). The numerical investigations and the experimental results match well. [ABSTRACT FROM AUTHOR]

For the past 50 years, open-ended coaxial lines were commonly employed for the determination of electromagnetic properties of various materials. In this article, the application of an open-ended hollow coaxial cable resonator (OE-HCCR) as a 1-D inclinometer for tilt measurements with 110 nanoradian (nrad) resolution is proposed and demonstrated. The coaxial cable resonant structure is formed between a metal post welded within the coaxial cable at the RF input end and the open end of the coaxial cable. A metal mass block, suspended in proximity to the open end in parallel is used to construct a pendulum structure, serving as the element responsive to a tilt. When the device is tilted, the distance between the mass block and the open end varies due to the displacement of the suspended pendulum. The change in the gap distance modifies the phase reflection coefficient associated with the open end, causing the resonance frequency of the coaxial cable resonator to change. By tracking the resonance frequency, the coaxial line resonator is effectively used as an inclinometer. The experimental results demonstrated that the novel device has a measurement resolution of 110 nrad over a range of 0–3 milliradians (mrad). Additionally, based on the same principle, a 2-D inclinometer based on two perpendicularly aligned OE-HCCRs, useful for simultaneous tilt measurements in two orthogonal dimensions is demonstrated. The open-ended coaxial cable resonator-based inclinometer holds several advantages, including high resolution, robustness, cost effectiveness, ease of signal interrogation, and user-configurable sensitivity and dynamic range. [ABSTRACT FROM AUTHOR]

Herein, the prospects of detecting peritectic behavior in carbon steels (Fe–C) using solidified shell thickness measurements and real‐time mold thermal maps are investigated. Irregular steel shell thickness and localized shell thinning are negative conditions associated with the peritectic transformation in many steel grades. Detailed three dimensional (3D) thickness maps are generated to directly contrast peritectic from nonperitectic solidification features using a laboratory dip test. Characterization of the steel shell also includes measurement of 3D shell surface deviations from a nominally flat surface. The unevenness parameter, which is calculated using the coefficient of variation in shell thickness as a function of the carbon content in the steel, clearly peaks around 0.14%C. A distributed fiber‐optic sensor, embedded 1 mm from the working face of a 100 × 55 × 12 mm3 thick copper chill block, has been used to obtain real‐time mold thermal maps of the solidification event during immersion of the block into molten steel. A programmable drive is used to control the motion of the chill block during immersion into a steel bath to minimize variability in immersion speed, position, and time. The thermal mapping measurements are well correlated with the shell profile measurements. [ABSTRACT FROM AUTHOR]

Metal-organic frameworks (MOFs), a newer class of crystalline nanoporous materials, have been in the limelight owing to their exceptional tunability for structures and physicochemical properties, and have found successful applications in gas storage, gas separation, and catalysis. The mesmerizing properties of MOFs, especially the extensive and tunable porosity and chemical selectivity, also make them an excellent candidate class as chemo-sensory materials. Moreover, MOF-based sensors have attracted considerable attention in the past decade. Recent literature reviews focused on the progress of MOF-based electronic sensors and luminescent MOF sensors, while sensors exploiting the dielectric properties (refractive index) of MOFs were also demonstrated and discussed very recently. The motivation of this report is to provide, for the first time, a general review on such MOF sensors with a particular focus on miniature optical fiber (OF) based MOF sensors and to demonstrate the promising potential of MOFs as dielectric coatings on OF for highly sensitive chemical sensing. The fundamental principle of OF-MOF sensors relies on the tunability of the refractive index of a MOF, which is dependent on the amount and type of adsorbed guest molecules in the MOF pores. MOF sensors based on different optical sensing principles are reviewed; challenges and perspectives on further research into the field of OF-MOF sensors are also discussed. [ABSTRACT FROM AUTHOR]

This paper reports a novel diaphragm-assisted open-ended hollow coaxial cable resonator (OE-HCCR) for highly sensitive pressure measurements at elevated temperatures. The coaxial line resonator is constructed using two highly reflective reflectors in a homemade hollow coaxial cable, including a metal post shorting the two concentric conductors (i.e., the inner conductor and the outer conductor) and the open end of the coaxial line. In the pressure sensor device design, a sealed metal diaphragm is mounted on the open end of the OE-HCCR and is employed as the pressure responsive element. The diaphragm’s deflection can be accurately captured by the coaxial line resonator in real-time, in response to a change in differential pressure. The sensor device’s responses to hydraulic pressures at ambient temperature and air pressures at elevated temperatures up to 1000 °C were investigated in the demonstration experiments, showing user-configurable measurement sensitivity and high-temperature survivability. The device’s pressure measurement resolution is also demonstrated as high as sub-Pascal by probing tiny hydrostatic pressure changes. A positive feedback loop is constructed as an alternative interrogator for the novel device, showing high signal quality and ease of demodulation. The pressure sensor described in this work is robust, easy-to-manufacture, user-configurable, low-cost, highly sensitive, and has great potential for industrial applications involving high-temperature and harsh environments. The presented work opens new avenues to develop a new generation of high-sensitivity and high-temperature microwave sensors based on well-known open-ended coaxial probes. [ABSTRACT FROM AUTHOR]

This article reports a technique to embed optical fiber into a copper mold plate for generating high-density thermal maps of the mold during the process of metal casting. The temperature measurements were based on acquiring and interpreting Rayleigh backscattering (RBS) signals from embedded fiber, using the interrogation technique of optical frequency domain reflectometry (OFDR). The instrumented mold plate was used to perform a cast-iron dip test and a steel dip test in a 200 lb induction furnace. The maximum temperatures recorded by the embedded fiber-optic sensors were 469 °C and 388 °C in the cast-iron and steel dip tests, respectively. The closely spaced and rapidly fluctuating temperature features that were imparted to the mold wall during solidification were successfully mapped with a high spatial resolution (0.65 mm) and a fast measurement rate (25 Hz) using a commercial OFDR interrogator (LUNA ODiSI 6108). Moreover, the thickness of the solidified steel shell was measured, and a thickness map of the shell was generated. A good correlation was observed between the thickness of the solidified shell and the temperature of the mold, as regions with higher and lower temperatures in the thermal profile of the mold corresponded to thicker and thinner areas on the shell, respectively. The dip testing experiments demonstrate that RBS-based fiber-optic sensing is a feasible and effective method for generating information-rich thermal maps of caster molds. The information obtained from thermal maps can be useful for improving the quality of the metal and productivity of the metal casting process. [ABSTRACT FROM AUTHOR]

This article presents an ultrasensitive open-ended hollow coaxial cable resonator (OE-HCCR) that was trained to detect impact events that occurred in the surrounding space through machine learning (ML) techniques. First, inspired by the well-known mass-based accelerometer, for the first time, we propose an OE-HCCR-based accelerometer with the addition of an inertial mass block as the element responsive to acceleration. The OE-HCCR is formed by a metal shorting post welded at the RF input end and the open end of the cable. The instantaneous displacements of the mass block mounted at the endface of the cable due to accelerations lead to variations of the gap distance between the mass block and the endface of the coaxial line, which can be accurately metered by the coaxial resonating structure. Impact events that occur in the surrounding space of the OE-HCCR result in vibrations transmitted to the OE-HCCR system and thereby measured by the OE-HCCR. Second, we demonstrated that employing ML algorithms to analyze the transient responses from the OE-HCCR makes unambiguous mappings between the transient responses of the OE-HCCR and the sources of vibrations possible, due to the ultrahigh sensitivity of the OE-HCCR and the spatial anisotropy of matter in the surrounding space. The preliminary results demonstrate the feasibility of using ML-assisted OE-HCCR system for locating impact events and recognizing different impactor implements that caused the impact events. [ABSTRACT FROM AUTHOR]

This paper presents a thin-core high-numerical aperture (TC-HNA) optical fiber with low macrobending loss for Rayleigh backscattering-based (RBS-based) temperature and strain measurements. The standard single-mode optical fiber (SMF), commonly used in RBS-based sensing, has low macrobending loss-resistance, making the fiber less suited for sensing applications that require sharp bending of the fiber under test. The TC-HNA fiber, on the other hand, offers high macrobending loss-resistance. Experiments were designed and conducted to demonstrate the reliability of a TC-HNA fiber for RBS-based temperature and strain measurements. The macrobending losses in a standard SMF and a TC-HNA fiber were compared in many experiments conducted with different bending radii ranging from 12.5 mm down to 0.9 mm. The RBS signal for a standard SMF, with a single turn of a 5 mm bending radius, degraded severely, exhibiting a 75% drop (6 dB) in signal power. Notably, the RBS signal for a similar test configuration of a TC-HNA fiber did not exhibit any observable loss. We demonstrated that a TC-HNA fiber could be used for RBS-based measurements with a single turn of ~1 mm bending radius. Moreover, an experiment was conducted to demonstrate the spatial thermal mapping capability of the TC-HNA fiber configured with multiple tight bends with radii in the range ~2-3 mm. The high macrobending loss-resistance of the TC-HNA fiber could extend the range of applications for RBS-based measurements to compact structures, such as batteries, robotic fingers, and printed circuit boards, where sharp bending of the test optical fiber is unavoidable. [ABSTRACT FROM AUTHOR]

The fiber-optic extrinsic Fabry–Perot interferometer (EFPI) is one of the simplest sensing configurations and is widely used in various applications. Inspired by the EFPI, we report a novel and universal ultra-sensitive microwave sensing platform based on an open-ended hollow coaxial cable resonator. Two highly-reflective microwave reflectors were fabricated in a coaxial cable to form a microwave Fabry–Perot etalon. Although the operating wavelength of the proposed device is increased by five orders of magnitude compared to the fiber-optic EFPI (e.g., from 1500 nm to 150 mm), the resolution regarding the “pseudo cavity length” of the proposed device is as high as 0.6 nanometer, which is comparable to that of the EFPI. The resolution can be further increased by high-precision machining of the device. Due to its low cost, high sensitivity, all-metal structure, robustness, and ease of signal demodulation, it is envisioned that the proposed device will revolutionize the sensing field and enable many important sensing applications that take place in harsh environments. [ABSTRACT FROM AUTHOR]

High-temperature measurements are of significant importance in various harsh-environment engineering fields, such as fossil fuel production, and the metallurgical and aviation industries. In recent years, there is a steady trend to shift from conventional electronic sensors to optical fiber sensors for high-temperature applications. In particular, optical fiber sensors are small in size, immune to electromagnetic interference, readily applicable for remote sensing, have high elasticity, and incorporate capabilities for multiplexing and distributed sensing. However, commonly used fused silica optical fiber sensors exhibit severe limitations at ultrahigh temperatures due to significantly degraded optical and mechanical properties at temperatures >1000 °C. The excellent optical transparency, thermal and chemical stability, mechanical robustness, and high melting temperature (~2040 °C) of single-crystal sapphire fibers (SFs) make them a strong candidate for sensing applications in high-temperature environments. Translation of the sensing schemes from mature silica fiber sensors to SF sensors has undergone tremendous growth and advancements in the past two decades. However, hurdles to the development of a near-term deployable SF sensing system have proven persistent due to the highly multimodal nature of SFs. This article reviews sensing techniques that have been implemented with SFs recently. The aim is to provide a comprehensive summary of past research on SF sensing systems. Perspectives on further research into the challenging yet promising arena are also discussed. [ABSTRACT FROM AUTHOR]

We report and demonstrate a high-resolution extrinsic Fabry–Perot interferometer (EFPI)-based 2-D fiber optic inclinometer for tilt measurements in two orthogonal dimensions. The inclinometer consists of a pair of perpendicularly arranged EFPI sensors packaged inside a rectangular container box. A triangular-shaped metal prism is attached to the top plate of the container box by three ropes with identical lengths. Two optical fibers are rigidly connected to the top plate of the container box by two supporting rods, respectively. Therefore, two EFPI sensors are formed between the endfaces of two optical fibers and their corresponding side face mirrors. The prism is connected to a cross paddle, which is immersed in a damping fluid to reduce the influence of deleterious vibrations. After tilting, the two reflectors of each EFPI sensor will remain parallel, while the cavity lengths of the EFPI sensors change. Changes in the cavity lengths can be calculated, and the 2-D tilt angles of the inclinometer can be calculated according to the Fabry–Perot principle. The design of the sensor and the measurement principles are discussed. Measurements of the tilt angles of a simply supported beam induced by small loads are presented to verify the tip-angle resolution and uncertainty of the two EFPI sensors in our 2-D inclinometer. The results show the capability for metering tilt angles with high resolution. The 2-D inclinometer was used to continuously monitor the tilt angle of a gravity dam for a one-year period, showing excellent stability and practicability. [ABSTRACT FROM AUTHOR]

This article reports a novel optical fiber extrinsic Fabry–Perot interferometer (EFPI) for probing pressure changes with subpascal (~0.1 Pa, ~ 1.45 × 10−5 psi) resolution. The principal idea of the sensor is to employ a traditional C-shaped Bourdon tube as a mechanical transducer that is coupled to a highly sensitive EFPI device. The cavity of the EFPI device is formed between a gold-coated mirror, mounted to a concave section of the Bourdon tube, and the endface of an optical fiber mounted at the base section of the Bourdon tube. Based on this design, the pressure-induced deflections of the Bourdon tube are directly correlated with the changes in cavity length of the EFPI, which can be determined by analyzing the interference signals. An experiment based on probing hydrostatic pressure changes induced by the addition of single water droplets to a test chamber was performed to quantify the measurement resolution of the proposed sensor because an apparatus for producing exceedingly small, stable, and reproducible pressure changes does not exist. Compared with conventional optical fiber pressure sensors, the proposed pressure sensor requires a simple fabrication process and can be used to measure pressure changes with high sensitivity (~ 23.5 μm/kPa, cavity length change/pressure change). Moreover, the sensitivity and resolution of the pressure sensor can be flexibly adjusted using Bourdon tubes designed for different dynamic ranges (e.g., 0–70 MPa, 0–10152 psi). It is believed that the proposed novel pressure sensor has the potential to find a wide range of applications that require precise instrumentation. [ABSTRACT FROM AUTHOR]