Core Optical Fiber Research Articles

Temperature-Compensated Interferometric High-Temperature Pressure Sensor Using a Pure Silica Microstructured Optical Fiber

We present a high-temperature interferometric pressure sensor using a simple-design and easy-to-fabricate pure silica four-hole novel microstructured optical fiber. The asymmetric geometry of the fiber allows hydrostatic pressure to induce stress at the optical fiber core, which converts to an interferometric shift. The large core of the fiber supports the propagation of several modes. Multimode interference created between different pairs of modes is used to sense the temperature and pressure change. The use of pure silica fiber is motivated by the ability of this fiber to operate up to high temperature as dopant diffusion is avoided. The sensor is demonstrated to measure pressure at a temperature up to 800 °C. We demonstrate temperature compensation using a Fourier approach to monitor different interference pairs and their phase response to pressure and temperature change. Experimental results show that the sensor has a linear response and excellent stability with a detection limit of 8.86 kPa at 800 °C temperature. This simple, compact, and potentially low-cost sensor is promising for harsh environment applications to improve quality control, operation efficiency, and safe working conditions.

Optical Fiber-Based Monitoring of X-ray Pulse Series from a Linear Accelerator

We investigated in this work the radioluminescence properties of a Ce-doped multimode silica-based optical fiber (core diameter of 50 µm) manufactured by the sol–gel technique when exposed to the high-energy X-rays (~600 keV) of the ORIATRON facility of CEA. We demonstrated its potential to monitor in real-time the beam characteristics of this facility that can either operate in a pulsed regime (pulse duration of 4.8 µs, maximum repetition rate of 250 Hz) or in a quasi-continuous mode. The radiation-induced emission (radioluminescence and a minor Cerenkov contribution) linearly grew with the dose rate in the 15–130 mGy(SiO2)/s range, and the afterglow measured after each pulse was sufficiently limited to allow a clear measurement of pulse trains. A sensor with ~11 cm of sensitive Ce-doped fiber spliced to rad-hard fluorine-doped optical fiber, for the emitted light transport to the photomultiplier tube, exhibited interesting beam monitoring performance, even if the Cerenkov emission in the transport fiber was also considered (~5% of the signal). The beam monitoring potential of this class of optical fiber was demonstrated for such facilities and the possibilities of extending the dose rate range are discussed based on possible architecture choices such as fiber type, length or size.

Environmental Durability of an Optical Fiber Cable Intended for Distributed Strain Measurements in Concrete Structures.

The present study investigates the environmental durability of a distributed optical fiber sensing (DOFS) cable on the market, commonly used for distributed strain measurements in reinforced concrete structures. An extensive experimental program was conducted on different types of specimens (including samples of bare DOFS cable and plain concrete specimens instrumented with this DOFS cable) that were exposed to accelerated and natural ageing (NA) conditions for different periods of up to 18 months. The instrumentation of both concrete specimens consisted of DOFS cables embedded at the center of the specimens and bonded at the concrete surface, as these two configurations are commonly deployed in the field. In these configurations, the alkalinity of the surrounding cement medium and the outdoor conditions are the main factors potentially affecting the characteristics of the DOFS component materials and the integrity of the various interfaces, and hence impacting the strain transfer process between the host structure and the core optical fiber (OF). Therefore, immersion in an alkaline solution at an elevated temperature or freeze/thaw (F/T) and immersion/drying (I/D) cycles were chosen as accelerated ageing conditions, depending on the considered configuration. Mechanical characterizations by tensile and pull-out tests were then carried out on the exposed specimens to assess the evolution of the mechanical properties of individual component materials as well as the evolution of bond properties at various interfaces (internal interfaces of the DOFS cable, and interface between the cable and the host structure) during ageing. Complementary physico-chemical characterizations were also performed to better understand the underlying degradation processes. The experimental results highlight that immersion in the alkaline solution induced a significant and rapid decrease in the bond properties at internal interfaces of the DOFS cable and at the cable/concrete interface (in the case of the embedded cable configuration), which was assigned to chemical degradation at the surface of the cable coating in contact with the solution (hydrolysis and thermal degradation of the EVA copolymer component). Meanwhile, F/T and I/D cycles showed more limited effects on the mechanical properties of the component materials and interfaces in the case of the bonded cable configuration. A comparison with the same specimens exposed to outdoor NA suggested that the chosen accelerated ageing conditions may not be totally representative of actual service conditions, but provided indications for improving the ageing protocols in future research. In the last part, an analysis of the distributed strain profiles collected during pull-out tests on instrumented concrete specimens clearly illustrated the consequences of ageing processes on the strain response of the DOFS cable.

In-Plane Strain Measurement in Composite Structures with Fiber Bragg Grating Written in Side-Hole Elliptical Core Optical Fiber.

In this paper, the application of a fiber Bragg grating written in a highly birefringent side-hole elliptical core optical fiber for two-axial strain measurement is presented. Hybrid optical fiber structures achieved by combining large side-holes and elliptical core result in a very high birefringence of 1 × 10−3 and thus high initial Bragg peak spectral separation of 1.16 nm, as well as a very high transverse force sensitivity, of up to 650 pm/(N/mm) or even −1150 pm/(N/mm), depending on the fiber orientation with respect to the applied force. Due to the ~22 %m/m GeO2 concentration in the core the fiber being highly photosensitive, which significantly simplifies FBG fabrication by UV illumination without the need for prior hydrogen loading, which worsens thermal stability. Finally, the developed FBGs written in the highly birefringent side-hole elliptical core optical fiber were embedded in the square composite plates and applied for strain measurements. Tests of two-directional four-point bending have shown usability of such FBG for two-axial in-plane strain measurement with a single FBG in iso-thermal conditions.

Long-period fiber grating based on inner microholes in optical fiber.

A new, to the best of our knowledge, type of long-period fiber grating based on inner microholes in an optical fiber core is proposed and demonstrated. The device is fabricated by first using a femtosecond laser to write a series of vertical short line structures in the core of a single-mode fiber, followed by discharge of the short line areas with a fusion splicer. Owing to the high temperature generated, the line structure rapidly expands and forms an inner microhole. The device is found to have good high temperature sustainability up to 1000°C, with a sensitivity of approximately 13 pm/°C and its strain and bending sensitivities are approximately -1.57 pm/µɛ and -5.198 dB/m-1, in the ranges of 0-800 µɛ and 0-0.77 m-1, respectively. It is expected that the device can contribute to the multi-parameter sensing and detection of high temperature in harsh environments.

Application of Forced Modulation Function Mathematical Model in the Characteristic Research of Reflective Intensity Fibre Sensors

Abstract In order to discuss the application and mode of the forced modulation function in a sensor, the optical fibre emphasis function was established by referring to the geometric method, the tilt factor and the shape factor of the reflecting surface. These were introduced for the first time, and the corresponding mathematical model was established. The method of numerical simulation is systematically studied and multimode optical fibre parameters (including optical fibre of axial spacing, optical fibre core diameter and numerical aperture) are adopted. The reflective surface inclination and shape factors on the RIM–FOS intensity modulation characteristics are studied according to the obtained light quasi-Gaussian distribution model, establishing a general three-intensity modulation function of fibre optic sensor. The results show that the intensity modulation characteristic of specular reflection is obviously better than that of the diffuse reflection surface, and the peak value of the modulation function is five times that of diffuse reflection. The intensity modulation characteristic decreases with increase in the roughness of the reflection surface. The system can not only complete the RIM–FOS characteristic simulation and characteristic testing functions, but can also start-up the test and not be affected by the ambient light interference and power fluctuation of the light source. The test stability is good with high repeatability.

Tapered coated coreless optical fiber pollution sensor

This project is centered on the design and implementation of a pollution-sensitive optical fiber using a No core optical fiber. The sensors are used for estimating refractive indices and concentrations of various contaminated water (dirty pond water (contains mud and animal excrement), chemically contaminated water, oiled water, drainage water is an environmental pollutant, Sodium chloride water and sucrose water) and distilled water (The refractive index was measured in the laboratory with an Abbe refractometer device) as well as for evaluating the performance parameters such as sensitivity (11.11μm/RIU for tapered sensor 10min and 11.13μm/RIU for tapered sensor 20min) and resolution (0.000069 RIU for tapered No core 10min and 0.000079 RIU for 20min tapered) of sensor (multimode fiber-no core fiber-multimode fiber), with 40 nm thick gold (Au) metal film of the exposed sensing region).

Effect of optical fiber core diameter on Brillouin scattering loss

This paper reports the effect of core diameter of optical fiber cables on stimulated Brillouin scattering loss, which is one of the major loss characteristics of an optical fiber communication system. Analysis of this loss characteristic at three windows of the operating wavelength of a laser has been carried out through a numerical approach. Among different types of optical fiber cables, multi-mode step index silica fiber, multi-mode graded index silica fiber and plastic fibers have been considered for the numerical analysis. The numerical analysis has been performed using MATLAB in this research work. Through the comparative analysis, it has been ascertained that the Brillouin scattering loss is not only affected by the operating wavelength, but also by the core diameter of the different type of the cable. From the investigation of the comparative analysis, it is revealed that Brillouin scattering loss declines with the application of multi-mode graded index silica fiber. However, in the plastic fiber category, plastic step index fiber offers better performance.

Specialty optical fiber fabrication: fiber draw tower based on a CO laser furnace

An experimental, laboratory-scale optical fiber drawing tower based on CO laser heating has been developed and used to fabricate speciality optical fiber. The CO laser was utilized in a symmetric four beam heating system. The localized and responsive heating time of the laser-based furnace was beneficial for manufacturing crystalline core fibers, specifically, silicon core optical fibers. Moreover, the specific absorption properties of the CO laser radiation in silica have been evaluated with the aid of finite element modeling. In comparison to the more traditional C O 2 laser, CO lasers were found to improve temperature uniformity and heating times while minimizing surface evaporation.

Photobleaching Effect on the Radiation‐Induced Attenuation of an Ultralow Loss Optical Fiber at Telecommunication Wavelengths

An unexpected high sensitivity to radiation has recently been discovered in a new generation of ultralow loss pure‐silica core optical fiber, the Corning Vascade EX1000. It is investigated how the injected power level of the probing signal at 1310 and 1550 nm affects the radiation‐induced attenuation (RIA) during and after steady‐state X‐ray exposure up to a total ionizing dose (TID) of 250 kGy (SiO2) at room temperature. The experiments confirm this fiber's high sensitivity to radiations, mainly due to the generation of metastable defects absorbing in the infrared domain. RIA kinetics are characterized by a non‐monotonic behavior, reaching a maximum of the radiation‐induced losses at 3.3 kGy (SiO2). Through successive radiation tests, the RIA dependence on the signal power level at the dose corresponding to the maximum of the RIA is studied, finding out that, for the two investigated wavelengths, the RIA follows a power law dependence in the 100 nW–1 mW injected power range, whose exponent seems independent on the dose rate (between 1 and 6 Gy s−1). The results highlight a clear photobleaching phenomenon, with the RIA associated to these metastable defects strongly reduced at higher injected powers of mW levels.

Dissolved gas sensing using an anti-resonant hollow core optical fiber

Sensors that measure dissolved gases directly are needed for environmental, industrial, and biomedical applications. Here we present a hollow core fiber optic sensor capable of measuring dissolved methane gas in liquids using only nanoliters of sample gas. The sensor is based on an anti-resonant hollow core fiber combined with a permeable capillary membrane inlet that extracts gas from the liquid for analysis. Using a small capillary inlet for gas extraction is only possible due to the small amount of sample gas needed for analysis, and it presents new possibilities for dissolved gas analysis in a simple, robust, and compact sensor configuration. We demonstrate the sensing technique using wavelength modulation spectroscopy and measure methane dissolved in water with a 1σ lower detection limit of 230 ppb, a resolution of 45 ppb, and a response time of ∼8min.

Optical Fiber Interferometer Based on Inner Air-Cavity With an Open Channel for Gas Pressure Sensing

An optical fiber Mach-Zehnder interferometer based on an inner air-cavity with an open micro-channel structure is presented. The device is fabricated by using femtosecond laser to inscribe a cavity structure in the optical fiber core, followed by discharging the cavity area with a fusion splicer to form an inner air-cavity in the fiber core, and finally create an open micro-channel with the help of hydrofluoric acid corrosion. The fiber in-line interferometric device is miniature, robust, and stable in operation and exhibits a high pressure sensitivity of ~3351 pm/MPa, with an extremely low temperature cross-sensitivity of 2.58 kPa/°C.

Optical properties enhancement with multilayer coating technique of additive‐enhanced zinc oxide nanostructure for fiber Bragg grating humidity sensor

AbstractA humidity sensing material is usually hygroscopic in nature and can allow sites for water adsorption. The hygroscopic material is usually coated in various devices for sensing enhancement. A zinc oxide additive‐enhanced nanostructure (ZnO‐HMT) was coated repeatedly on fiber Bragg grating (FBG) have been fabricated. The sensing material was synthesized for the purpose of characterizing its capability in enhancement of relative humidity sensing of uniformly grated core optical fibers with multiple layers of sensing layer. The coating and annealing process was repeated for at least three times resulting three layers of ZnO‐HMT coated on an FBG fiber. Observed microscopy images showed that the thickest coating began to form larger lumps on the sensor surface, but absence of any gaps present. Reflectivity power was observed to be greatest for triple layered samples due to the coating possibly containing the light propagation and prevent evanescent leakages. Analysis with optical spectrum analyzer detects notable wavelength shifts at sensing range of 60–80 %RH and a significant increase of sensitivity for triple layered samples.

Optical Fiber In-Line Mach-Zehnder Interferometer Based On an Inner Air-Cavity With Long Cavity Length

An optical fiber in-line Mach-Zehnder interferometer based on an inner air-cavity with long cavity length is demonstrated. The device is fabricated by using femtosecond laser to inscribe a waveguide structure in the optical fiber core, followed by discharging the waveguide area with a fusion splicer. The inner air-cavity structure is highly robust, and the relatively long cavity length corresponds to a small free spectral range, which makes it possible to implement accurate measurement. Such an inner air-cavity device fabricated in single mode fiber has good high temperature sustainability. The device is also “open” to the external environment when being fabricated in multimode fiber, thus supporting refractive index measurement. The proposed device is attractive in many optical fiber sensing applications.

A novel plus shaped cavity based optical fiber sensor for the detection of Escherichia-Coli

In this work, we have demonstrated the refractive index (RI) based sensing of Escherichia Coli (E-Coli) by introducing a novel plus shaped cavity in optical fiber. This plus-shaped cavity increases the interaction between the signal and the analytes, that improves the sensitivity. The proposed sensor model is achieved by combining the lateral and vertical slots in optical fiber core structure. The sensing ability of the plus shaped cavity has been examined in visible range, where the RI of E-Coli ranges from 1.38 to 1.395. The numerical investigation of sensor structure has been done by employing the finite difference time domain (FDTD) method. The geometrical analysis of plus shaped cavity indicates that; the maximum output power is achieved at the slot width of 600 nm. The proposed cavity is capable of sensing the presence of E-Coli with the autocorrelation function and sensitivity of 99.54 % and 2125 nm/RIU, respectively.

Engineering Entangled Photons for Transmission in Ring-Core Optical Fibers

The capacity of optical communication channels can be increased by space division multiplexing in structured optical fibers. Radial core optical fibers allows for the propagation of twisted light–eigenmodes of orbital angular momentum, which have attracted considerable attention for high-dimensional quantum information. Here we study the generation of entangled photons that are tailor-made for coupling into ring core optical fibers. We show that the coupling of photon pairs produced by parametric down-conversion can be increased by close to a factor of three by pumping the non-linear crystal with a perfect vortex mode with orbital angular momentum ℓ, rather than a gaussian mode. Moreover, the two-photon orbital angular momentum spectrum has a nearly constant shape. This provides an interesting scenario for quantum state engineering, as pumping the crystal with a superposition of perfect vortex modes can be used in conjunction with the mode filtering properties of the ring core fiber to produce simple and interesting quantum states.

Study of dissolved oxygen sensing characteristics of hydrogel optical fiber based on fluorescence quenching method

In this paper, hydrogel optical fiber doped with fluorescent indicator was used to detect dissolved oxygen (DO) based on fluorescence quenching method. Polyethylene glycol diacrylate (PEGDA) was used as precursor and 2-hydroxy-2-methyl-phenylacetone as photoinitiator to fabricate the hydrogel optical fiber. Fluorescent indicator was doped into the core and cladding of the hydrogel optical fiber separately to optimized the DO sensing characteristics. Experiment results show that the two kinds of hydrogel optical fiber have good repeatability. Compared with the core doped fiber, the loss of the cladding doped fiber decreased from 4.2468±0.1396 dB/cm to 1.3193±0.0355 dB/cm, the response time to DO dropped from 15 min to 10 min, and the detection limit of dissolved oxygen decreased from 0.18 mg/L to 0.17 mg/L. However, due to the weak interaction between the fluorescent indicator and the evanescent wave in the cladding, the sensitivity decreased and the value of I0/I decreased from 3.47 to 2.12. Due to excellent permeability and biocompatibility of PEGDA, the prepared hydrogel optical fiber is very promising for detection dissolved oxygen in biomedical area.

Fabrication of Humidity-Resistant Optical Fiber Sensor for Ammonia Sensing Using Diazo Resin-Photocrosslinked Films with a Porphyrin-Polystyrene Binary Mixture.

Ammonia gas sensors were fabricated via layer-by-layer (LbL) deposition of diazo resin (DAR) and a binary mixture of tetrakis(4-sulfophenyl)porphine (TSPP) and poly(styrene sulfonate) (PSS) onto the core of a multimode U-bent optical fiber. The penetration of light transferred into the evanescent field was enhanced by stripping the polymer cladding and coating the fiber core. The electrostatic interaction between the diazonium ion in DAR and the sulfonate residues in TSPP and PSS was converted into covalent bonds using UV irradiation. The photoreaction between the layers was confirmed by UV-vis and Fourier transform infrared spectroscopy. The sensitivity of the optical fiber sensors to ammonia was linear when exposed to ammonia gases generated from aqueous ammonia solutions at a concentration of approximately 17 parts per million (ppm). This linearity extended up to 50 ppm when the exposure time (30 s) was shortened. The response and recovery times were reduced to 30 s with a 5-cycle DAR/TSPP+PSS (as a mixture of 1 mM TSPP and 0.025 wt% PSS in water) film sensor. The limit of detection (LOD) of the optimized sensor was estimated to be 0.31 ppm for ammonia in solution, corresponding to approximately 0.03 ppm of ammonia gas. It is hypothesized that the presence of the hydrophobic moiety of PSS in the matrix suppressed the effects of humidity on the sensor response. The sensor response was stable and reproducible over seven days. The PSS-containing U-bent fiber sensor also showed superior sensitivity to ammonia when examined alongside amine and non-amine analytes.

Flexible and directional fibre optic ultrasound transmitters using photostable dyes.

All-optical ultrasound transducers are well-suited for use in imaging during minimally invasive surgical procedures. This requires highly miniaturised and flexible devices. Here we present optical ultrasound transmitters for imaging applications based on modified optical fibre distal tips which allow for larger transmitter element sizes, whilst maintaining small diameter proximal optical fibre. Three optical ultrasound transmitter configurations were compared; a 400 µm core optical fibre, a 200 µm core optical fibre with a 400 µm core optical fibre distal tip, and a 200 µm core optical fibre with a 400 µm core capillary distal tip. All the transmitters used a polydimethylsiloxane-dye composite material for ultrasound generation. The material comprised a photostable infra-red absorbing dye to provide optical absorption for the ultrasound transduction. The generated ultrasound beam profile for the three transmitters was compared, demonstrating similar results, with lateral beam widths <1.7 mm at a depth of 10 mm. The composite material demonstrates a promising alternative to previously reported materials, generating ultrasound pressures exceeding 2 MPa, with corresponding bandwidths ca. 30 MHz. These highly flexible ultrasound transmitters can be readily incorporated into medical devices with small lateral dimensions.

Performance analysis of transmissive modified cladding optical fibre sensors

Optical fibre sensors with modified cladding have been a topic of great interest for decades. Thanks to the new techniques of fabricating nanostructured films, this interest has been renewed. Although there have been many increases in published experimental research, no comprehensive theoretical formulation of the structure is reported in the literature. In this study, a general theory for transmissive modified cladding optical fibre sensors is presented. The model is formulated based on the electromagnetic fields and optical power in the core and cladding of a step-index fibre. The model considers various parameters, especially the radius of the optical fibre core, modified cladding refractive index, and light wavelength. The sensitivity, response, and optical transmission of the sensor are analysed as a function of the parameters. The analyses show that optical transmission reduces dramatically when the modified cladding refractive index approaches the refractive index of the core of the fibre and results in higher sensor response and sensitivity. Increasing the core radius of the fibre will result in lower sensor response with higher sensitivity. Both parameters will increase for a light with a larger wavelength. The results are in good agreement with experimental evidence.