Bragg Wavelength Shift Research Articles

Online Gamma Radiation Monitoring Using Few-Mode Polymer CYTOP Fiber Bragg Gratings.

We investigated the gamma radiation response of fiber Bragg gratings (FBGs) inscribed in a few-mode polymer optical fiber. The fiber had a graded-index CYTOP core of 20 µm and XYLEX overclad of 250 µm in diameter. Four FBGs were exposed to gamma radiation during four irradiation sessions at a 5.3 kGy/h dose rate. The FBGs showed a linear Bragg wavelength shift with the received dose with a mean sensitivity of -3.95 pm/kGy at 43 °C. The increased temperature provides a rise in the sensitivity: it reached -10.6 pm/kGy at 58 °C. After irradiation, the FBGs showed partial recovery, which increased with the received dose. Furthermore, the FBG's reflection power decreased with the dose. This attenuation is mainly due to insertion losses caused by the radiation induced attenuation in the CYTOP fiber. Linear response to the received dose makes CYTOP FBGs attractive for gamma radiation dosimetry. However, temperature dependence of the sensitivity should be compensated in practical applications.

Design of two-dimensional sampled Bragg grating for a curved waveguide.

Due to the ability of changing light propagation path direction, curved waveguide Bragg grating (CWG) plays an important role in photonic integrated circuits. In this paper, we proposed a cascaded sampled Bragg grating on tilted waveguide (CSBG-TW) structure to equivalently realize CWG. As an example, by designing two-dimensional (2D) sampled gratings, the direction of +1st sub-grating vector in CSBG-TW can be changed. Then if a curved waveguide is divided into several sections of tilted waveguide, we can keep the grating direction being always parallel to the longitudinal direction of each section of tilted waveguide, while the basic grating is uniform. Hence, the required CWG can be equivalently realized, and the light responses such as reflection Bragg wavelength shift and backward mode convert caused by the tilted grating in curved waveguide can be compensated for. The results show that the sampling structures of CSBG-TW is micro-scale and the difference between reflection intensity between the CSBG-TW with four section tilted waveguide and CWG as design target is less than 0.1 dB. Compared with CWG, the CSBG-TW allows convenient holographic exposure and the wavelength can be accurately controlled. Therefore, the CSBG-TW can be used in various photonic integrated devices that require changing propagation paths.

Radiation Effects on Fiber Bragg Gratings: Vulnerability and Hardening Studies

Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments.

Compact packaged silicon photonic Bragg grating sensor based on a ball lens interface

We demonstrate a ball-lens based optical interface for coupling between a single mode fiber and a silicon grating coupler from the back side of a photonic integrated circuit (PIC). This allows compact packaging solutions and keeps the PIC top side accessible, e.g. for interacting with the environment in case of sensing applications. Different configurations employing 300 μm diameter ball lenses made of both LASF-35 glass and sapphire were investigated. Without any modifications to the grating coupler, the peak coupling efficiency (for TE polarization and operating wavelength around 1550 nm) was experimentally found to be only between 1.4 dB and 2.2 dB lower when interfacing with a ball lens from the back side compared to coupling from the top side using a cleaved fiber. Furthermore, no noticeable change in the bandwidth was found. The potential of the ball lens interface was proven by realizing a compact (diameter less than 1.5 mm) optical temperature sensor probe based on a phased shifted silicon photonic Bragg grating structure on a PIC. The probe consisted of a single mode fiber which was terminated with a 1.25 mm ceramic ferrule onto which a ball lens holder and a PIC was glued. The holder was realized in fused silica glass with a femtosecond laser direct-write technology and served to accurately position the ball lens with respect to the fiber and PIC. The assembly allowed to cleanly read out the Bragg grating spectrum in reflection, using a single fiber interface and was compatible with commercial interrogator systems. A Bragg wavelength shift of 73 pm/°C was found in a tested temperature range between 15 °C and 60 °C.

Color-phase readout radiochromic photonic crystal dosimeter

<h2>Summary</h2> The development of power-free, wide-range, and real-time visible radiation dosimeters is highly desirable but challenging. Here, we report a new dosimetry concept based on the continuous and linear Bragg wavelength shift of photonic crystals in response to X-ray or γ-ray radiation. As a proof of concept, three distinct photonic crystal dosimeters with high spatial resolution and tunable responsive range from 0 up to 225 kGy were demonstrated based on the decrease of lattice distance caused by chain scission or crosslinking of polymer networks. Compared with the conventional radiochromic dosimeters, this design possesses more flexibility in composition or microstructure and exceptional pre-irradiation stability against light and temperature. In addition, their uniqueness leads to convenient device-free readout techniques through naked-eye color-strip calibration and cell phone-based measurement using hue value detection. In conclusion, our work presents the necessary dosimeters for more direct and effective dose verification in industrial and medical applications.

Design of Microstructured Flat Optical Fiber for Multiaxial Strain Monitoring in Composite Materials

An innovative microstructured flat optical fiber is designed to obtain a multiaxial strain sensor for composite material monitoring. The sensing regions are constituted by the two eyelets where Bragg gratings are written. To achieve multiaxial sensing, a suitable microstructure is designed close to only one of the eyelets. The effect of the strain field, evaluated via a 3D finite element method approach, is considered to obtain the change of the refractive index distribution. The electromagnetic modal analysis and the coupled mode theory are exploited to evaluate the Bragg wavelength shift for the slow and fast axis fundamental modes, guided in the two eyelets and affecting the sensor response. The designed microstructured flat optical fiber is technologically feasible and promises sensing performance higher than that obtainable with the conventional optical fibers. In addition, flat optical fiber can be embedded in composite materials reducing the drawbacks related to both orientation and excess resin.

Enhanced sensitivity of bare FBG pressure sensor based on oval-shaped 3D printed structure

A bare Fiber Bragg Grating (FBG) pressure sensor based on an oval-shaped 3D printed structure was proposed and experimentally demonstrated in this study. The whole structure was housed in a well-controlled pressure chamber, and the FBG signals were recorded by an Optical Spectrum Analyzer (OSA). Strains of the oval-shaped structure were simulated numerically using Ansys to optimize its performance. When under pressure, deformation of this 3D printed structure leads to response of FBG sensor, and the experimental results showed that Bragg wavelength shift is linearly proportional to the change in pressure. The proposed sensor shows a good sensitivity of 6.834 nm/MPa, corresponding to 4.43 × 10-3 MPa−1 in the pressure range from 0 to 0.45 MPa. The sensor’s linear fit coefficient is also good, reaching a mean value of 98.57 % and the repeatability with negligible mean hysteresis error is ± 0.16 nm (equivalent to 2.3 %). The proposed pressure sensor has high potential to be used in related industrial applications and is highly customizable to meet the requirements of different pressure ranges.

On estimating axial strains using FBG sensors in single fiber composites

Several factors that impact the calculation of axial strain evolution using fiber Bragg grating (FBG) sensors during the manufacturing of tin/bismuth alloys, ice, and epoxies are investigated. Axial and radial strains in the embedded fiber are impacted by the elastic properties and relative volumes of the fiber and the matrix as well as by the aspect ratio of the fiber. Numerical and analytical composites evaluation approaches are used to determine the relationships between the strains in the fiber and those in the matrix. Radial strains in the fiber can have a significant impact on the Bragg wavelength shift and need consideration to accurately determine matrix strains, especially when the matrix has a high stiffness compared to the glass optical fiber containing the FBG sensors. For stiffer materials such as ice and tin/bismuth alloys investigated in this research, it is shown that the use of a simplified formula that ignores radial strains leads to significant errors.

Fast FBG Interrogator on Chip Based on Silicon on Insulator Ring Resonator Add/Drop Filters

A fast FBG sensor interrogator on chip based on SOI ring resonator (RR) filters is presented. The measurement of the Bragg wavelength shift is based on the detection of multiple signals at the output ports of the RR filter, providing fast dynamic strain measurement capabilities, fundamentally limited by the photodetector bandwidth and signal processing. The ring resonator resonance wavelength is tuned and fixed by an integrated heater and the ring acts as a passband filter for the reflected FBG spectrum. After a theoretical analysis of the interrogation method, which allows the optimization of the RR characteristics depending on the specific application and requirements, we present experimental results on a single RR based interrogation scheme applying high frequency dynamic strain up to 40 kHz. We also theoretically evaluate the performance of multi-ring resonators. The proposed scheme provides low cost, high scalability to mass-volume market applications as well as an extremely fast interrogation method.

Optimum Design Sapphire-Fiber Bragg Grating for High-Temperature Sensing

Fiber Bragg grating (FBG) sensors have limitations in measuring high and extreme temperatures because in general FBG sensors are made of silica fiber, which at high temperatures can interfere with the mechanical performance of the materials. In this paper, we propose an S-FBG (Sapphire Fiber Bragg Grating) sensor which is resistant to extreme environmental influences and high temperatures. By developing S-FBG to measure high temperatures, it is found that S-FBG has high sensitivity, every 10C change is obtained and the Bragg wavelength shifts as far as 30.24 nm, this result is greatly influenced by the thermo-optic coefficient, and the coefficient of expansion-thermal. The design also evaluates the Gaussian apodization profile to improve sensor accuracy in monitoring temperature.

Evaluation of the effectiveness of the effect of photosensitization on the spectral characteristics of the fiber Bragg grating

Currently, fiber Bragg gratings obtained on the basis of photoinduced optical fibers doped with a high concentration of germanium oxide are used as highly sensitive sensors. However, it is worth noting a significant drawback – the manufacturing technology of optical fibers doped with germanium is expensive. When recording Bragg gratings in a standard telecommunication fiber, where the molar concentration of germanium in the fiber core is from 3 % to 5 %, interference occurs due to very low and insufficient light sensitivity. Thus, an important role is played by solving the problem of low photosensitivity of standard telecommunication fibers for recording Bragg gratings. This paper presents the results of studies of the spectral characteristics of fiber Bragg gratings based on standard telecommunication fibers pre-saturated with hydrogen to increase photosensitivity. According to the results obtained, it was found that under the action of UV radiation in the presence of hydrogen, the photosensitivity of the fiber increases and the Bragg wavelength shift is associated with the saturation of the fiber with hydrogen, the effective modulation amplitude of the induced refractive index is equal to 1.2 with a refractive index of 1.438. This work proves that the VBR recorded in the S pre-saturated in hydrogen for 12 days is characterized by increased photosensitivity. The experimental results obtained make it possible to use a Bragg fiber array based on a standard telecommunications optical fiber saturated with hydrogen in the field of telecommunications, seismology, engineering geology as fiber-optic sensors of pressure, deformation, temperature, rotation and rotation, including in extreme environmental conditions

Selective ultrasensitive FBG-SBL chemosensor for detection of copper ions in water resources based on plasmon resonance energy transfer

In this study, the gold (Au) coated fiber Bragg grating (FBG) probe decorated with Schiff base ligands (SBL) with variable denticity is proposed as a susceptible sensor for monitoring the copper ions (Cu2+) concentration in an aqueous solution. The sensing principle can be attributed to the interaction of the SBL coated on Au/FBG, which serves as anionic chelates with Cu2+, altering the environment refractive index and changing the surface plasmon resonance (SPR) condition. The sensitivity and limit of detection (LOD) through a Bragg wavelength shift are measured to be 23.1 pm/µM and 0.07 µM respectively, with a dynamic range of up to 4 µM (µM, 2.5 ppm), which is within the acceptable limit specified by the World Health Organization (WHO) for safe drinking water (1.3 ppm). The probe shows an assertive coordination behaviour towards Cu2+ for selective detection compared to other heavy metals. Excellent reusability (6 trial times) and specificity and portability make FBG-SBL chemosensor suitable for online monitoring of water resources.

Effect of cure shrinkage of epoxy molding compound on warpage behavior of semiconductor package

Cure shrinkage of the epoxy molding compound (EMC) for semiconductor package during molding process, was precisely measured using a fiber Bragg grating (FBG) sensor by measuring a Bragg wavelength (BW) shift. A dielectric sensor was simultaneously used to monitor the curing process of the EMC and define its gelation point. The relationship between internal strain development of the EMC and its degree of cure was investigated by correlating two measured values (The internal strain from the FBG sensor and the degree of cure from the dielectrometry). The viscoelastic properties of the EMC were also measured by a three-point bending stress relaxation test. Finally, the warpage of EMC-aluminum bi-layer strip was measured with digital image correlation (DIC) method and compared with the result from the finite element analysis. As a result, it was confirmed that the cure shrinkage of the EMC should be considered as an important factor in predicting warpage behavior of the semiconductor.

Reversible photonic hydrogel sensors via holographic interference lithography

Continuous monitoring of physiological conditions and biomarkers via optical holographic sensors is an area of growing interest to facilitate the expansion of personalised medicine. Here, a facile laser-induced dual polymerization method is developed to fabricate holographic hydrogel sensors for the continuous and reversible colorimetric determination of pH variations over a physiological range in serum (pH 7–9). Readout parameters simulated through a Finite-difference time-domain Yee's algorithm retrieve the spectral response through expansion. Laser lithography of holographic hydrogel sensor fabrication is achieved via a single 355 nm laser pulse to initiate polymerization of ultrafine hydrogel fringes. Eliminating the requirement for complex processing of toxic components and streamlining the synthetic procedure provides a simpler route to mass production. Optimised pH-responsive hydrogels contain amine bearing functional co-monomers demonstrating reversible Bragg wavelength shifts of 172 nm across the entire visible wavelength range with pH variation from 7.0 to 9.0 upon illumination with broadband light. Photolithographic recording of information shows the ability to convey detailed information to users for qualitative identification of pH. Holographic sensor reversibility over 20 cycles showed minimal variation in replay wavelength supporting reliable and consistent readout, with optimised sensors showing rapid response times of <5 min. The developed sensors demonstrate the application to continuous monitoring in biological fluids, withstanding interference from electrolytes, saccharides, and proteins colorimetrically identifying bovine serum pH over a physiological range. The holographic sensors benefit point-of-care pH analysis of biological analytes which could be applied to the identification of blood gas disorders and wound regeneration monitoring through colorimetric readouts.

FBG Based Optical Weight Measurement System and Its Performance Enhancement Using Machine Learning

Industrial weight measurement is essential to monitoring and optimizing the quality and production cost. Fiber Bragg gratings (FBGs) are among the most widely used fiber optic sensors because they have a high sensitivity to temperature and strain fluctuations, making them ideal for various industrial applications. This paper proposes an optical weight measurement system that includes four FBG based load cells with a detailed analysis of their performance using different machine learning algorithms to improve accuracy and measurement range. Here, the shift in Bragg wavelength caused by strain generated on cantilever type load cell is acquired using an optical interrogator, giving the wavelength encoded FBG data. The data is acquired on PC, where specific Machine Learning models such as Linear Regression (LR), Support vector machines (SVMs), Neural networks (NN), Gaussian Process regression (GPR) are optimally selected and are utilized to directly learn the mapping between changes in load cell (Bragg wavelength) and applied weight. The experimental analysis shows that the GPR algorithm provides more accurate weight measurement than any other standard approach. The model ensures the practicality of the proposed method with improvement in measurement speed and accuracy of the FBG based weight measurement system.

Fiber Bragg grating security fence with temperature compensation based on a tilted cantilever beam

The fiber Bragg grating has been one of the most widely used sensors in the field of optical fiber sensors in recent years. Significant progress has been made in fiber Bragg grating sensor applications to measure environmental parameters such as strain and temperature. The fiber Bragg grating is sensitive to the temperature as well when it is measuring the strain change, which is ignored in most measurement applications, including perimeter protection sensors. Since the fiber Bragg grating is affected by both strain and temperature changes, cross-sensitivity compensation is required to accurately measure and reduce noise. In this paper, what we believe to be a novel intrusion detection technique is proposed based on a tilted cantilever beam, which is very effective for temperature compensation in fiber-Bragg-grating-based security fences. With this technique, temperature and strain can be measured simultaneously and completely independently of each other. For this work, we place a ramp with an angle of θ, similar to a tilted cantilever beam, on the fence and stick a fiber Bragg grating on the ramp. A uniform strain applied to the fence creates a strain gradient along the cantilever beam and of course along the fiber Bragg grating causing a change in the full width at half maximum of the reflection spectrum. This full width at half maximum change along with the Bragg wavelength shift are used to obtain the strain and temperature simultaneously. In our simulation results, the temperature sensitivity of the fiber Bragg grating is 14.2 pm/°C for the Bragg wavelength with no change in the full width at half maximum, and the strain sensitivity is 0.453 pm/µɛ for the Bragg wavelength and a nonlinear sensitivity according to a quadratic function for the full width at half maximum variation.

Femtosecond-Laser-Assisted Fabrication of Radiation-Resistant Fiber Bragg Grating Sensors

This paper demonstrates the fabrication of radiation-resistant fiber Bragg grating (FBG) sensors using infrared femtosecond laser irradiation. FBG sensors were written inside acrylate-coated fluorine-doped single-mode specialty optical fibers. We detected the Bragg resonance at 1542 nm. By controlling the irradiation conditions, we improved the signal strength coming out from the FBG sensors. A significant reduction in the Bragg wavelength shift was detected in the fabricated FBG sensors for a radiation dose up to 105 gray, indicating excellent radiation resistance capabilities. We also characterized the temperature sensitivity of the radiation-resistant FBG sensors and detected outstanding performance.

Short Circuit and Broken Rotor Faults Severity Discrimination in Induction Machines Using Non-invasive Optical Fiber Technology

Multiple techniques continue to be simultaneously utilized in the condition monitoring and fault detection of electric machines, as there is still no single technique that provides an all-round solution to fault finding in these machines. Having various machine fault-detection techniques is useful in allowing the ability to combine two or more in a manner that will provide a more comprehensive application-dependent condition-monitoring solution; especially, given the increasing role these machines are expected to play in man’s transition to a more sustainable environment, where many more electric machines will be required. This paper presents a novel non-invasive optical fiber using a stray flux technique for the condition monitoring and fault detection of induction machines. A giant magnetostrictive transducer, made of terfenol-D, was bonded onto a fiber Bragg grating, to form a composite FBG-T sensor, which utilizes the machines’ stray flux to determine the internal condition of the machine. Three machine conditions were investigated: healthy, broken rotor, and short circuit inter-turn fault. A tri-axial auto-data-logging flux meter was used to obtain stray magnetic flux measurements, and the numerical results obtained with LabView were analyzed in MATLAB. The optimal positioning and sensitivity of the FBG-T sensor were found to be transverse and 19.3810 pm/μT, respectively. The experimental results showed that the FBG-T sensor accurately distinguished each of the three machine conditions using a different order of magnitude of Bragg wavelength shifts, with the most severe fault reaching wavelength shifts of hundreds of picometres (pm) compared to the healthy and broken rotor conditions, which were in the low-to-mid-hundred and high-hundred picometre (pm) range, respectively. A fast Fourier transform (FFT) analysis, performed on the measured stray flux, revealed that the spectral content of the stray flux affected the magnetostrictive behavior of the magnetic dipoles of the terfenol-D transducer, which translated into strain on the fiber gratings.

Selective detection of lead in water using etched fiber Bragg grating sensor

In this work, we demonstrate a maleic acid (MA) functionalized gold nanoparticle (AuNP) coated, clad etched-fiber Bragg grating (e-FBG) sensor, capable of detecting lead metal ion (Pb2+) in water. The self-assembled carboxylic group in MA-AuNP shows an assertive coordination behavior towards Pb2+ for selective detection through a Bragg wavelength shift. The limit of detection (LOD) is found to be 10 femtomolar (fM, 0.0000002 ppb) with a dynamic range up to 100 nanomolar (nM, 20.72 ppb), which is within the acceptable limit stipulated by the World Health Organization (WHO) for safe drinking water (10 ppb). The sensor's recovery concentration is > 90% for spiked real water samples. A good reproducibility (standard error < 10 pm) and specificity along with portability, small footprint and real time response make e-FBG sensor an ideal optical probe for drinking water.

Design and analysis of fiber Bragg grating sensor to monitor strain and temperature for structural health monitoring

In this paper, we have proposed a Bragg grating based sensor to monitor health of civil structures at distinct temperatures. We have considered increased number of gratings with suitable refractive index to enhance sensitivity of fiber Bragg grating sensor. Analysis of Bragg wavelength with respect to load and temperature is successfully studied. The simulation results reveal that when independently strain (50 units per simulation) and temperature (25 °C) are increased uniformly, a linear shift in Bragg wavelength 0.064 and 0.347 nm is observed, respectively. Similarly, when both strain and temperature are increased (ε = 50 & T = 25 °C) concurrently, a directly proportional relation is found in Bragg wavelength (0.403 nm). The results verify the enhanced performance of as-proposed sensor, employing it could be potentially used in civil, bio-medical and military domains.