Sensitivity-enhanced strain sensor based on a shape-modulated multimode fiber.

In this Letter, we demonstrate a sensitivity-enhanced strain sensor based on a shape-modulated multimode fiber (MMF). In contrast to conventional single-mode-multimode-single-mode (SMS) fiber structures, which typically contain a single cylindrical homogeneous MMF section, the shape of the MMF section in this investigation is modulated by lateral offset splicing of multiple MMF segments. Simulation results show that the designed shape-modulated MMF has a higher peak mechanical strain than that of a cylindrical MMF. Experimental results demonstrate that the strain sensitivity achieved by the shaped-modulated MMF-formed SMS fiber structure is as high as -55.63 pm/µÎµ, which is 33 times higher than that for a cylindrical MMF-formed conventional SMS fiber structure at -1.65 pm/µÎµ. This high sensitivity and low-fabrication cost SMS fiber sensor has the potential to be a promising candidate in precise strain measurement applications.

High-sensitivity vector bend sensor based on a fiber directional coupler inscribed by a femtosecond laser.

In this Letter, we demonstrate a high-sensitivity vector bend sensor based on a fiber directional coupler. The fiber directional coupler is composed of two parallel waveguides inscribed within a no-core fiber (NCF) by a femtosecond laser. Since the two written waveguides have closely matched refractive indices and geometries, the transmission spectrum of the fiber directional coupler possesses periodic resonant dips. Such a fiber directional coupler exhibits a good bending-dependent spectral shift response due to its asymmetric structure. Experimental results show that bending sensitivities of -97.11 nm/m-1 and 58.22 nm/m-1 are achieved for the 0° and 180° orientations in the curvature range of 0-0.62 m-1, respectively. In addition, the proposed fiber directional coupler is shown to be insensitive to external humidity changes, thus improving its suitability in high-accuracy bending measurements.

Novel Corrugated Long Period Grating Surface Balloon-Shaped Heterocore-Structured Plastic Optical Fibre Sensor for Microalgal Bioethanol Production.

A novel long period grating (LPG) inscribed balloon-shaped heterocore-structured plastic optical fibre (POF) sensor is described and experimentally demonstrated for real-time measurement of the ultra-low concentrations of ethanol in microalgal bioethanol production applications. The heterocore structure is established by coupling a 250 µm core diameter POF between two 1000 µm diameter POFs, thus representing a large core-small core-large core configuration. Before coupling as a heterocore structure, the sensing region or small core fibre (SCF; i.e., 250 µm POF) is modified by polishing, LPG inscription, and macro bending into a balloon shape to enhance the sensitivity of the sensor. The sensor was characterized for ethanol-water solutions in the ethanol concentration ranges of 20 to 80 %v/v, 1 to 10 %v/v, 0.1 to 1 %v/v, and 0.00633 to 0.0633 %v/v demonstrating a maximum sensitivity of 3 × 106 %/RIU, a resolution of 7.9 × 10-6 RIU, and a limit of detection (LOD) of 9.7 × 10-6 RIU. The experimental results are included for the intended application of bioethanol production using microalgae. The characterization was performed in the ultra-low-level ethanol concentration range, i.e., 0.00633 to 0.03165 %v/v, that is present in real culturing and production conditions, e.g., ethanol-producing blue-green microalgae mixtures. The sensor demonstrated a maximum sensitivity of 210,632.8 %T/%v/v (or 5 × 106 %/RIU as referenced from the RI values of ethanol-water solutions), resolution of 2 × 10-4%v/v (or 9.4 × 10-6 RIU), and LOD of 4.9 × 10-4%v/v (or 2.3 × 10-5 RIU). Additionally, the response and recovery times of the sensor were investigated in the case of measurement in the air and the ethanol-microalgae mixtures. The experimentally verified, extremely high sensitivity and resolution and very low LOD corresponding to the initial rate of bioethanol production using microalgae of this sensor design, combined with ease of fabrication, low cost, and wide measurement range, makes it a promising candidate to be incorporated into the bioethanol production industry as a real-time sensing solution as well as in other ethanol sensing and/or RI sensing applications.

State-of-the-Art Sensors Research in Ireland.

This Special Issue captures a significant portion of the current sensors research excellence in Ireland [...].

A Review of Optical Fibre Ethanol Sensors: Current State and Future Prospects.

A range of optical fibre-based sensors for the measurement of ethanol, primarily in aqueous solution, have been developed and are reviewed here. The sensing approaches can be classified into four groups according to the measurement techniques used, namely absorption (or absorbance), external interferometric, internal fibre grating and plasmonic sensing. The sensors within these groupings can be compared in terms of their characteristic performance indicators, which include sensitivity, resolution and measurement range. Here, particular attention is paid to the potential application areas of these sensors as ethanol production is globally viewed as an important industrial activity. Potential industrial applications are highlighted in the context of the emergence of the internet of things (IoT), which is driving widespread utilization of these sensors in the commercially significant industrial and medical sectors. The review concludes with a summary of the current status and future prospects of optical fibre ethanol sensors for industrial use.

Ultra-compact in-core-parallel-written FBG and Mach-Zehnder interferometer for simultaneous measurement of strain and temperature.

An ultra-compact in-core-parallel-written fiber Bragg grating (FBG) and Mach-Zehnder interferometer (MZI) for simultaneous measurement of strain and temperature is described. The FBG and MZI are written spatially parallel in the same section of fiber core using a femtosecond laser, forming an ultra-compact device, which is different from the previously developed axial cascade of different structures. Due to the weak coupling between the FBG and the MZI, their individual extinction ratios are traded off by optimizing their writing position and separation, and extinction ratios of 5.9 dB for the FBG and 10 dB for the MZI are achieved. Experimental results show that the FBG and MZI have different sensitivities for strain and temperature, allowing this device to measure strain and temperature simultaneously. In addition, since both the FBG and MZI are written in the fiber core, this ultra-compact device is proven to be impervious to ambient humidity, making it a promising candidate for accurate industrial strain and temperature measurements.

Influence of probe geometry on the characteristics of optical fiber gas-liquid two-phase flow measurement signals.

A conical tip-shaped plastic optical fiber sensor for gas-liquid flow measurement is described. Experimental results show that a distinctive spike signal occurs before the output signal when using a conventional conical fiber probe, whereas this spike signal was greatly suppressed when a cleaved probe is used. A full simulation is implemented based on a three-dimensional ray-tracing method providing a means of comparison with theoretical analysis. The results show that an appropriately cleaved tip provides a promising method of enhancing the bubble measurement signal-to-noise ratio.

Influence of Bubble Deformation on the Signal Characteristics Generated Using an Optical Fiber Gas-liquid Two-Phase Flow Sensor.

The use of optical fiber probe in two-phase flow measurements is very frequently encountered, especially in the applications of chemical engineering and petroleum industries. In this work, the influence of bubble piercing signals caused by bubble deformation is studied experimentally using a laboratory-prepared wedge-shaped fiber probe in a lab-scale gas-liquid flow generator. A three-dimensional simulation model is established to study the influence of bubble deformation on the piercing signals. A theoretical analysis of the characteristics of the pre-signal influenced by the bubble deformations is undertaken for a wide range of different modeled bubble shapes. Combining the experimental and simulation results, a promising analytical method to estimate the bubble shapes by analyzing the characteristics of pre-signals is proposed. The results of this investigation demonstrate that it is possible to estimate the bubble shapes before the fiber probe contacts the bubble surface. The method developed in this investigation is therefore highly promising for reducing errors caused by deformation during the probe piercing process.

Miniature Fabry-Perot interferometer based on a movable microsphere reflector.

We propose and demonstrate a miniature Fabry-Perot interferometer (FPI) based on a movable microsphere reflector. The movable microsphere acts as a good reflector, with the reflections occurring at the spliced single-mode fiber/hollow-core fiber interface and the surface of a microsphere, resulting in two-beam interference. The silica microsphere is formed at the tip of a half-tapered optical fiber, and its diameter can be reduced to miniaturize the FPI. The movable microsphere interferometer exhibits a highly linear response to external displacement change, and a high displacement sensitivity of 11.9 pm/nm with a nanoscale resolution of 1.7 nm is achieved. Wide-range displacement can also be measured by monitoring the changes in the free spectral range of the reflection spectrum. Therefore, this miniaturized FPI may find use in applications in nano-displacement measurement fields, and the concept of a movable microsphere reflector is of great significance for the miniaturization of micro-photonic devices.

Intense mid-infrared emission at 3.9 µm in Ho3+-doped ZBYA glasses for potential use as a fiber laser.

Intense mid-infrared emission at 3.9 µm in Ho3+-doped ZBYA glasses with direct upper laser level (Ho3+:5I5) pumping at a wavelength of 888 nm is reported for the first time, to the best of our knowledge. Spectroscopic parameters were determined using the Judd-Ofelt theory and the measured absorption spectrum. The maximum emission cross section of the Ho3+-doped ZBYA glass is estimated to be 2.7×10-21cm2 at 3906 nm. Additionally, fluorescence spectra and lifetimes of ZBYA glasses with different Ho3+ ion doping concentrations were measured. The results provide theoretical and experimental basis for better selection of rare-earth-doped matrix glasses to achieve a fluorescence output centered on a wavelength of 3.9 µm.

Investigation of the characteristics of a fiber-optic gas-liquid two-phase flow sensor.

A laboratory-prepared wedge-shaped fiber probe using step-index multimode plastic optical fiber was described and tested in a lab-scale gas-liquid flow generator. A three-dimensional model was established in order to fully simulate the process of bubble piercing by the optical fiber probe. A theoretical analysis of the luminous intensity distribution of the light transmission in the process of bubble piercing was undertaken under conditions of different relative positions between the fiber probe and the bubble axis. Using this analytical method, it was possible to accurately define the range of the central region of the bubble where the presignal appeared.

All-fiber optic displacement sensing system for an Ilizarov transverse tibial bone transport device.

A single-mode-multimode-single-mode (SMS) optical fiber-based displacement sensing system mounted on an Ilizarov transverse tibial bone transport device for microcirculation reconstruction is reported. Wide-range displacement is approximated as a uniform extension of a spring that is connected to an SMS optical fiber structure acting as the displacement sensor and allowing full displacement characterization. Transmission spectrum changes are measured, providing a displacement range of 24 mm with a sensitivity of $ - {55.42}\;{\rm pm/mm}$-55.42pm/mm and a resolution of 45.2 µm. The experimental results are characterized using a polynomial response curve for measuring the displacement due to transverse distraction of the Ilizarov device. The SMS fiber interrogation system is based on a macrobending fiber edge filter-based ratiometric measurement system. The use of SMS fibers together with the macrobending fiber-based interrogation system eliminates the influence of temperature on the displacement measurement. The implementation of the all-fiber sensing system of this investigation has uniquely facilitated a smart clinical device with a wide displacement range as well as operating in real-time monitoring when attached to the Ilizarov transverse tibial bone transport device. It also means that this fiber-optic sensing device can be made more cost-effective, simpler in construction, and more versatile while providing a high degree of measurement accuracy and resolution.

Estimation of various radiological parameters associated with radioactive contents emanating with fly ash from Sahiwal coal-fuelled power plant, Pakistan.

The release of natural radioactive materials with the emission of fly ash as a result of coal burning in power generation plants is considered amongst the sources that elevate the technologically environmental radioactivity level. This research mainly concerns the assessment of various radiological parameters including excess lifetime cancer risk due to natural radioactive contents associated with fly ash emitted to the surrounding environment from the stack of 1320 Mw Sahiwal coal-fuelled power generation plant (CFPP). For this purpose, fifty-four soil samples were collected in a radius of 4 Km from CFPP and a highly background radiation-shielded HPGe system is used to measure radioactivity in the collected samples. The activity concentrations of radium-226, potassium-40, and thorium-232 in collected samples was found to be in the range of 20 to 138, 43 to 860, and 27 to 127 Bq/kg with average values of 66, 409, and 67 Bq/kg respectively. Activity concentrations of radium-226 and thorium-232 were observed significantly higher than UNSCEAR reported typical global average values. A significant decrease in the level of the aforementioned radionuclides in the collected soil samples was observed with increasing distance from the power plant, which is a clear indication for the elevation of radioactivity concentrations in the surrounding environment as a result of the operation of the CFPP. To assess the radiation dose delivered to the occupational workers and inhabitants living next to Sahiwal CFPP, absorbed γ-dose rate (Dγ), outdoor annual effective dose rate (EOutdoor), and excess lifetime cancer risk (ELCR) were estimated and these were found higher than the UNSCEAR recommended values of 59 nGy/h, 0.07 mSv/y, and 2.9 × 10-4 respectively. The outcome of this first systematic study is the assessment of potential radiological health risk to the occupational workers as well as the inhabitants living in the proximity of this CFPP.

Novel optical fiber SPR temperature sensor based on MMF-PCF-MMF structure and gold-PDMS film: erratum.

Figure 1(c) in [Y. Wang, Optics Express 26, 1910 (2018)] contains an error and is corrected in this erratum.

Nanosecond passively Q-switched fibre laser using a NiS2 based saturable absorber.

Q-switched pulse laser generation is successfully demonstrated in both Erbium-doped fibre laser (EDFL) and Thulium-doped fibre laser (TDFL) cavities by employing Nickel disulfide (NiS2) as a saturable absorber (SA). Q-switched pulse laser operation at 1.55 µm and 2.0 µm is obtained at low pump power levels of 37 mW and 48 mW, respectively. For the EDFL, stable passively Q-switched laser output at a wavelength of 1561.86 nm is achieved, with a minimum pulse duration of 237 ns and a repetition rate of 243.9 kHz. For the TDFL, the centre wavelength of the laser output is 1915.5 nm, with a minimum pulse duration of 505 ns and a repetition rate of 214.68 kHz. NiS2 is used as SA for Q-switched laser generation over a broadband wavelength for the first time.

Temperature-insensitive refractometer based on an RI-modulated singlemode-multimode-singlemode fibre structure.

A temperature-insensitive refractometer based on a refractive index (RI)-modulated singlemode-multimode-singlemode (RMSMS) fibre structure is proposed and experimentally demonstrated. In this investigation, a combination of no-core fibre (NCF) and multimode fibre (MMF) regions provides an RI modulation region due to the difference in RI between the NCF and the MMF. In effect, by periodically embedding the NCF within the MMF section of a singlemode-multimode-singlemode (SMS) fibre structure, a long-period grating (LPG) can be effectively introduced in the MMF section, and the excited cladding modes are therefore able to sense surrounding RI variation. The modulation parameters are determined from the numerical simulations, and the experimental results show the maximum RI sensitivity of the fabricated sample is as high as 206.96 nm/RIU. In addition, the proposed RMSMS fibre structure is proven to be unaffected by external temperature variation (in the wavelength domain), which is a very attractive feature in practical sensing applications.

New model for explaining the over-response phenomenon in percentage of depth dose curve measured using inorganic scintillating materials for optical fiber radiation sensors.

Inorganic scintillating material used in optical fibre sensors (OFS) when used as dosimeters for measuring percentage depth dose (PDD) characteristics have exhibited significant differences when compared to those measured using an ionization chamber (IC), which is the clinical gold standard for quality assurance (QA) assessments. The percentage difference between the two measurements is as high as 16.5% for a 10 × 10 cm2 field at 10 cm depth below the surface. Two reasons have been suggested for this: the presence of an energy effect and Cerenkov radiation. These two factors are analysed in detail and evaluated quantitatively. It is established that the influence of the energy effect is only a maximum of 2.5% difference for a beam size 10 × 10 cm2 compared with the measured ionization chamber values. And the influence of the Cerenkov radiation is less than 0.14% in an inorganic scintillating material in the case of OFS when using Gd2O2S:Tb as the luminescent material. Therefore, there must be other mechanisms leading to over-response. The luminescence mechanism of inorganic scintillating material is theoretically analysed and a new model is proposed and validated that helps explain the over-response phenomenon.

Optical interleaver based on nested multiple knot microfiber resonators.

A novel design of nested optical fiber based multiple knot resonators is presented. The design consists of three knot resonators, two of which share a significant fraction of their optical path. The relationship between the knots' diameter ratio and the transmission spectrum is investigated. The output spectrum is theoretically analyzed using transfer matrix analysis and the calculated results exhibit good agreement with experimental results. The free spectral range (FSR) is varied by simply fine-tuning the diameter of the small knot. The periodic spectrum of this optical microfiber based photonic device has a number of applications in the sensing and communications field, e.g., optical interleavers, frequency combs, filters, and fiber lasers. This Letter demonstrates that the variation of the output spectrum can be implemented simply by changing the knot sizes and coupling coefficients.

Tm3+-Ho3+ codoped tellurite glass microsphere laser in the 1.47 µm wavelength region.

In this Letter, a Tm3+-Ho3+ codoped tellurite glass microsphere laser in the 1.47 µm wavelength region is described. Using a traditional tapered microfiber-microsphere coupling method, multimode and single-mode lasing around the wavelength of 1.47 µm is observed using an 802 nm laser diode as a pump source. This Tm3+-Ho3+ codoped tellurite glass microsphere laser can be used in near-infrared telecommunications, biomedical, and astrophysical applications.

In-fiber temperature sensor based on green up-conversion luminescence in an Er3+-Yb3+co-doped tellurite glass microsphere.

A novel, to the best of our knowledge, in-fiber temperature sensor based on green up-conversion (UC) luminescence in an Er3+-Yb3+ co-doped tellurite glass microsphere is described. The tellurite glass microsphere is located firmly inside a suspended tri-core hollow-fiber (STCHF) structure. The pump light launched via a single-mode fiber (SMF) is passed through a section of multimode fiber, which is fusion spliced between the SMF and the STCHF into the cores suspended inside the hollow fiber and coupled into the microsphere. Green and red UC emissions of the Er3+ ions are observed using 980 nm pump excitation. The temperature-sensing capability of the tellurite glass microsphere is based on the thermally coupled effect between the upper energy levels responsible for green emissions at 528 nm and 549 nm. The resulting fluorescence intensity ratio, depending on the surrounding temperature range from 303 K to 383 K, is experimentally determined, and a maximum sensitivity of 5.47×10-3 K-1 is demonstrated. This novel in-fiber microsphere-resonator-based device is highly integrated and has the additional advantages of ease of fabrication, compact structure, and low fabrication cost and therefore has great application potential in integrated optical sources including lasers.