The modulation efficiency of a tubular-lattice hollowcore fiber (HCF) employing flexural acoustic waves is investigated in detail for the first time. The main acousto-optic properties of the HCF are evaluated, using 2D and 3D models based on the finite element method. The induced coupling of the fundamental and first higher-order modes is simulated from 743 to 1355 nm. The most relevant acoustic (amplitude, period, strain, energy) and optical (effective index, beat length, birefringence, coupling coefficient) parameters are analyzed. The simulations are compared to experimental results and indicate higher modulation performance in HCFs compared to standard optical fibers. In addition, useful insights into the design and fabrication of all-fiber acousto-optic devices based on HCFs are provided, enabling potential application in tunable spectral filters and mode-locked fiber lasers.

We demonstrate the temporal propagation of high-frequency ultrasonic pulses inside of a suspended core fiber (SCF) for the first time. An SCF composed of a 5 mu m core suspended by nano bridges and four air holes is analytically and numerically investigated using the 2D and 3D finite element method. We reveal the propagation of ultrasonic modes in the SCF and describe the variation of their properties with increasing frequency from 1 to 100 MHz. Ultrasonic Gaussian-modulated pulses traveling with distinct frequencies (25 and 72 MHz) and bandwidths (56 and 10 MHz) are evaluated along the SCF length and time. The induced strain, pressure, and sensing responsivity are reviewed and discussed. Analytical and numerical results show highly confined effective single-mode propagation in the SCF core, amplifying strains along the fiber (up to 18 times) and pressures at the fiber output (up to 3 times). Compared to previous studies, the demonstrated SCF provides the widest 86 MHz high-responsivity bandwidth (with a -218 dB average and -75 dB maximum responsivity at 93 MHz). These achievements are promising for developing smaller, faster, and more efficient acousto-optic modulators for fiber lasers, enhancing simultaneous light-sound guidance and spatial resolution of fiber-based optoacoustic imaging sensors and neurostimulators in biomedicine.

An all-fiber, dual Mach-Zehnder interferometer (MZI) based on an acoustically modulated hollow core fiber (HCF) is experimentally demonstrated for the first time. By attaching an acoustic driver between the fixed ends of an HCF, we fabricated two acousto-optic modulators (AOMs) with distinct driver positions, allowing synchronization of two in-line MZIs inside the HCF. The first MZI is set by two acoustic long-period gratings separated by a second MZI formed at the fiber and driver-attaching region. We show that this setup enables frequency-tuning of the coupling between the fundamental and higher-order modes in the HCF. Additionally, we simulate and analyze the HCF modal couplings and MZIs' modulated spectra under distinct device parameters using the transfer matrix method. The new AOM-MZI enables tuning of the MZI's free spectral range by adjusting 1 Hz of the electrical frequency, which is promising to modulate multiwavelength filters, sensors, and fiber lasers.

We demonstrate the acousto-optic modulation of a hybrid-lattice hollow core fiber (HL-HCF) for the first time. For many years, optical fibers with reduced diameters have been the main solution to increase the interaction of acoustic and optical waves. However, the high drive voltages and large modulator components still employed drastically affect the efficiency and miniaturization of these devices. Here, we experimentally show that combining Kagom & eacute; and tubular lattices in HL-HCFs allows for enhancing the amplification of the acoustic waves and the modulation of the guided optical modes, thus providing high modulation efficiency even when using a fiber with a 240 mu m diameter. To the best of our knowledge, the measured HL-HCF's modulation efficiency (1.3 dB/V) is the highest compared to devices employing reduced diameter fibers. Additionally, we demonstrate a compact acousto-optic modulator with driver dimensions smaller than the HL-HCF diameter. Overall, our results show a promising alternative to solve the compromise of speed, efficiency, and compactness for integration with microscale all-fiber photonic devices.

We demonstrate a broadband acousto-optic notch filter based on a tubular-lattice hollow-core fiber for the first time to our knowledge. The guided optical modes are modulated by acoustically induced dynamic long-period gratings along the fiber. The device is fabricated employing a short interaction length (7.7 cm) and low drive voltages (10 V). Modulated spectral bands with 20 nm half-width and maximum depths greater than 60% are achieved. The resonant notch wavelength is tuned from 743 to 1355 nm (612 nm span) by changing the frequency of the electrical signal. The results indicate a broader tuning range compared to previous studies using standard and hollow-core fibers. It further reveals unique properties for reconfigurable spectral filters and fiber lasers, pointing to the fast switching and highly efficient modulation of all-fiber photonic devices.

High‐frequency broadband ultrasound in nested antiresonant hollow core fibers (NANFs) is investigated for the first time. NANFs have remarkable features enabling high‐resolution microscale optoacoustic imaging sensors and neurostimulators. Solid optical fibers have been successfully employed to measure and generate ultrasonic signals, however, they face issues concerning attenuation, limited frequency range, bandwidth, and spatial resolution. Herein, highly efficient ultrasonic propagation in NANFs from 10 to 100 MHz is numerically demonstrated. The induced pressures and sensing responsivity are evaluated in detail, and important parameters for the development of ultrasonic devices are reviewed. High pressures (up to 234 MPa) and sensing responsivities (up to −207 dB) are tuned over 90 MHz range by changing the diameters of two distinct NANF geometries. To the best of knowledge, this is the widest bandwidth reported using similar diameter fibers. The results are a significant advance for fiber‐based ultrasonic sensors and transmitters, contributing to improve their efficiency and microscale spatial resolution for the detection, diagnosis, and treatment of diseases in biomedical applications.

We demonstrate highly efficient modulation of a tubular lattice hollow core fiber by means of flexural acoustic waves. The acousto-optic parameters required for fluid sensing are numerically and experimentally investigated. The results point out high sensor's sensitivity to characterize fluid properties, such as refractive index, density, and viscosity.

The modulation efficiency of a tubular-lattice hollow-core fiber (HCF) by means of flexural acoustic waves is investigated in detail for the first time. The main acousto-optic properties of the HCF are evaluated employing 2D and 3D models based on the finite element method. The induced coupling of the fundamental and first higher-order modes is simulated in the wavelength range from 743 to 1355 nm. Significant acoustic (amplitude, period, strain, energy) and optical parameters (effective index, beat length, birefringence, coupling coefficient) are analyzed. The simulations are compared to experimental results, indicating higher modulation performance in HCFs compared to standard optical fibers. In addition, useful insights into the design and fabrication of all-fiber acousto-optic devices based on HCFs are provided, enabling potential application in tunable spectral filters and mode-locked fiber lasers.

We demonstrate a new all-fiber electrically tunable modulation method which significantly reduces the response time of a Bragg grating acousto-optic modulator. A 4 cm long device is fabricated with a 1 cm grating inscribed in a suspended core fiber. An acoustic pulse train is switched out of phase along the fiber, damping unwanted natural resonant vibrations inside the grating. The device rise time is decreased from 56 to 9 mu s by tuning the duty cycle of the driven electrical signal, contributing to achieve the shortest switching time of 15.6 mu s. This tunable temporal response reveals unique features to change the profile of optical pulses. High pulse modulation depths are achieved employing a compact acousto-optic modulator, pointing to fast switching of all-fiber photonic devices.

The spectral characteristics of long-period gratings (LPGs) have been researched over the last two decades, and many sensing applications of LPGs have been proposed due to their spectral sensitivity to many surrounding environmental parameters such as the temperature, pressure, and refractive index. However, this sensitivity to many parameters can also be a curse due to cross-sensitivity and the inability to distinguish which environmental parameter is responsible for the LPG's spectral behavior. For the application proposed here-monitoring the progress of a resin flow front, its velocity, and the permeability of the reinforcement mats during the infusion stage of resin transfer molding-the multi-sensitivity of LPGs is a distinct advantage, as it provides the ability to monitor the mold environment at various stages of manufacturing.

A novel numerical study based on the finite element method is developed to demonstrate the beatlengths induced by high frequency acoustic modes inside an optical fiber for the first time. A practical methodology to model, compute and analyze the multimode interaction in the fiber is exemplified with a detailed numerical experiment. The frequency response of 1 mm long standard fiber is evaluated from 30 to 60 MHz, corresponding to the highest attenuation band of experimental fiber optoacoustic devices. The 3D simulated complex ultrasonic fields are decomposed and characterized with the averaged peak-to-peak method and 2D Fourier transform. The resulting dispersion spectra are compared and theoretically validated by the recognized Pochhammer-Chree solutions. The acoustic parameters required to modulate optical fibers are derived from the simulations and discussed. A route to overcome the frequency-induced limitations of the current devices is provided, pointing out new research possibilities for the development of highly efficient and compact all-fiber acousto-optic modulators and fiber-optic ultrasonic sensors.

Rapidly expanding and new applications of liquid crystal materials cover a wide range of technology products. A very incomplete list includes conventional and miniature high-resolution displays, AR/VR glasses, smart windows, dynamic lenses, tunable filters and retarders, electrically controlled sensors, reconfigurable antennas for wireless and space communications, and many other commercially available devices. The aforementioned devices are enabled by the collective reorientation of thermotropic molecular liquid crystals under the action of applied electric fields. The reorientation effects in liquid crystals can be altered by ionic contaminants typically present in mesogenic materials in small quantities. Therefore, information about ions in liquid crystals is very important because it allows for a proper selection of liquid crystal materials and uncompromised performance of liquid crystal devices. This information can be obtained by performing electrical measurements of liquid crystal materials. Measurements of basic electrical parameters (DC conductivity, charge mobility, and ion density) are carried out using sandwich-like liquid crystal cells of finite thickness. Once a cell is filled with liquid crystal materials, interactions between ions and the cell substrates will result in the time dependence of the ion density and DC electrical conductivity until a steady state is reached. In this paper, we show how complementary information about ionic processes in liquid crystal cells can be obtained by analyzing their transient and steady-state electrical properties.

The authors report the use of FBGs to monitor the infusion process of the Resin Transfer Molding (RTM). Composite structure can be very large, ensuring that the resin has reached every part of the mold during the infusion process becomes a critical step to ensure the structural integrity and minimize failure. During the infusion stage of the process, the FBG undergoes a blue wavelength to red wavelength shift. The magnitude of wavelength shifts depends upon the location and depth of the sensors along with the viscosity of resin (we used a resin-like material with similar viscous properties) being used for the infusion. The observed wavelength shifts varied from ~10pm to ~400pm, which is small but still significant and readily measurable also showing reasonable repeatability for all experiments conducted with the same conditions. Evidence will be presented to show that viscous force is a major factor in explaining the observed FBG wavelength shifts. Due to the small wavelength shifts of the FBG sensors, this allows the opportunity to embed a dense population of sensors within a single structure, hence ensuring a satisfactory spatial resolution to monitor the resin flow front to ensure complete impregnation of the reinforcement.

Civil engineering assets and geo-structures continually deteriorate during their lifetime, particularly in harsh environments that may be contaminated with corrosive substances. However, efficient and constant structural health monitoring and accurate prediction of the service-life of these assets can help to ensure their safety, performance, and health conditions and enable proper maintenance and rehabilitation. Nowadays, many of the largest cities throughout the world are situated in coastal zones, leading to a dramatic increase in the construction of nearshore geo-structures/infrastructures which are vulnerable to corrosion attacks resulting from salinity contamination. Additionally, seawater intrusion can threaten the quality and the sustainability of fresh groundwater resources, which are a crucial resource in coastal areas. To address these issues, detection of salinity in soil utilizing a novel polymer optical fibre Bragg grating (POFBG) sensor was investigated in this research. Experiments were carried out at different soil water contents with different salinities to assess the sensor's response in a representative soil environment. The sensitivity of the POFBG sensor to salinity concentrations in water and soil environment is estimated as 58 ± 2 pm/%. The average standard error value in salinity is calculated as 0.43% for the samples with different soil water contents. The results demonstrate that the sensor is a promising and practical tool for the measurement and monitoring with high precision of salinity contamination in soil.

Research, based on both observations and ocean models, has indicated that strong East Pacific El Niños are preceded by the development of unusually low sea levels at the western end of the North Equatorial Trough early in the year. This results in an increase in the strength of the North Equatorial Counter Current which, aided by low sea levels due to the annual Rossby wave, then transports West Pacific Warm Pool water to the central and eastern Pacific. Here, an ocean model is used to investigate the factors affecting sea level prior to the 1982–1983 East Pacific El Niño. The results indicate that during this period the reduction in sea level was caused by Ekman pumping, due to the local winds, acting over a period of many months. The north–south distances involved mean that such upwelling can be connected to the westerly wind phase of Madden–Julian Oscillations. Since the 1980s, the quality and quantity of global wind measurement has steadily improved. So, if the Ekman pumping hypothesis is correct, the better-quality wind data available prior to the 1997–1998 and 2015–2016 East Pacific El Niños should also show unusually large Ekman pumping in the same region, over a similar long period. This is tested and found to be correct. However, a study of the integrated Ekman pumping for the period 1976 to 2015 indicates that in some years similar events did not develop despite a comparable amount of pumping during the first part of the year. The results imply that significant Ekman pumping early in the year is a necessary but not sufficient requirement for the development of a strong East Pacific El Niño.

We demonstrate acousto-optic modulation of a fiber Bragg grating in a birefringent suspended core fiber up to frequencies of 5.2 MHz for the first time. At acoustic resonances lower than 820 kHz, the reflectivity of the orthogonal polarization modes is electrically switched to a superposed reflection band with a maximum modulation depth of 51% at 10 V. The wavelength peak of the polarization modes is dynamically shifted by tuning the driven electrical signal from 1.92 to 5.2 MHz. A novel side-coupled based acousto-optic device is demonstrated, indicating new possibilities for ultracompact, fast and efficient all-fiber integrated devices.

Localized surface plasmons (LSP) excited by optical fields have many potential applications resulting from their ability in detecting ultra‐small, ambient refractive index change. Current methods using surface nano‐patterning by means of lithography have given rise to LSP of limited propagation and interaction lengths, meaning that practical applications remain challenging. This article describes a new all‐optical method of generating LSP by means of a carefully fabricated low‐dimensional nano‐structured material using a direct‐write photochemical lithography. It is shown that the resulting array of localized SPs combine or “Conjoin” to have an unprecedented large interaction length, via coupled evanescent fields, giving rise to superior spectral sensitivities; several orders of magnitude better than those quoted elsewhere and reaching 6 × 103 nm RIU−1 in the aqueous regime and 104 nm RIU−1 in the gaseous regime. Numerical modeling is performed that shows this design of plasmonic platform is capable of producing sensitivities of 105–106 nm RIU−1. It is believed the results achieved in this investigation show that a unique conjoint SP operational mode will significantly impact areas of interest, such as single molecular dynamics, drug delivery systems, etc.

Measuring soil water content is crucially important and can affect soil strength, which is a key parameter in the analysis, design and monitoring of geo-structures. In this study, an optical fibre Bragg grating sensor inscribed in polymer optical fibre was developed, and for the first time, its ability to measure soil water content was investigated. The sensitivity of the sensor to different values of gravimetric soil water content under the different compaction conditions of loose and normal compaction was tested. The effect of soil temperature on the sensor’s performance was considered. To assess the sensor’s implementation, accuracy and reliability, a commercial soil water content probe (SM150), which measures volumetric soil water content was employed. The results indicate that the developed sensor, when calibrated correctly, is able to provide detailed data on any minor variation of soil water content (e.g. 0.5%) with high precision. The outcomes of this study define an additional capability of the polymer optical fibre Bragg grating sensors, which is significantly important for the long-term performance monitoring of geo-structures.

Highly efficient and compact acousto-optic modulator of a fibre Bragg grating is reported for the first time. The device is composed of millimetre scaled components and a 1 cm grating inscribed in a four air holes birefringent suspended core fibre. The reflection of the orthogonal polarization modes is tuned by a sinusoidal electrical signal at the resonance frequency of 469 kHz. A significant modulation depth of 45% is achieved at a maximum drive voltage of 10 V. The demonstrated 4 cm long all-fibre modulator is 60% shorter compared to previous similar setups, indicating new possibilities for stable and fast switching of fibre-integrated photonic devices.

Interest in gas sensing using functionalised carbon nanotubes is a major area of research that up until now have utilised their electrical properties, here the optical properties are exploited and used to detect a specific gas.