Reference Fiber Bragg Grating Research Articles

Temperature and strain monitoring during thermoforming of thermoplastic composite laminates using optical frequency domain reflectometry

In this study, optical frequency domain reflectometry (OFDR) was used to monitor the thermoforming processes of carbon fiber reinforced thermoplastics (CFRTPs) to address the limitations of conventional sensors including large size and low spatial resolution. A bare single-mode fiber with a polyimide coating and a fiber encapsulated by a long metal capillary were cascaded and embedded into composite laminates to withstand the high pressure and temperature during thermoforming, and then connected to the OFDR for monitoring. A fiber encapsulated by a 2 cm short metal capillary was also embedded to demonstrate that a 1 mm resolution of the OFDR is beneficial for reflecting the local change in the composite. After processing by wavelet denoising, signal extraction, and decoupling, the frequency shift along the optical fiber sensor was successfully converted to strain and temperature. In two repeated thermoforming experiments that involved cooling from 340 °C, the average temperature difference measured by the OFDR and reference thermocouple was only 4.64 °C. The strain measured by the OFDR and reference fiber Bragg grating (FBG) decreases in the cooling stage, and has a clear knee point of 250 °C when correlated with the temperature and strain. This knee point is consistent with the liquid–liquid transition temperature of the polyetherimide and indicates the beginning of consolidation when the composite changes its properties significantly. The average strain difference measured by OFDR and the reference FBG was 69 μϵ when the total strain is approximately 1820 μϵ if only considering the consolidation process from 250 °C. The results of 1 mm spatial resolution and high accuracy demonstrate that OFDR is a promising high-resolution sensing solution for the in-situ temperature and strain monitoring of the thermoforming of CFRTPs.

Microwave-photonic Vernier effect enabled high-sensitivity fiber Bragg grating sensors for point-wise and quasi-distributed sensing

This paper reports a sensitivity-improved fiber Bragg grating (FBG) sensor system based on microwave-photonic interferometry and the Vernier effect. An incoherent microwave photonics system based on a broadband light source is employed to interrogate the FBG sensor using the wavelength-to-delay mapping technique combined with interferometry. Specifically, the sensing FBG together with a reference FBG is used to construct a microwave photonics Michelson interferometer (MI). Changes in the Bragg wavelength of the sensing FBG subject to external perturbations are encoded into the spectral shifts of the microwave interferogram of the MI. A virtual interferometer is then generated from the sensing MI based on a computational Vernier effect modality. By superimposing the spectra of the sensing MI and the virtual interferometer, the Vernier effect is generated. By tracking the spectral shift of the Vernier envelope, it is shown that the measurement sensitivity of the sensing FBG is remarkably enhanced with an expected factor. Moreover, a quasi-distributed sensor system with enhanced sensitivity based on cascaded FBGs and the proposed virtual microwave-photonic Vernier effect technique is implemented, representing the first demonstration of a Vernier effect-enhanced FBG array sensor. Additionally, the possibility of employing the harmonic Vernier effect for further sensitivity enhancement is investigated, where a remarkable sensitivity enhancement factor up to 685 with a strain sensitivity of 94 MHz/µε is successfully demonstrated.

In-Bulk Temperature Profile Mapping Using Fiber Bragg Grating in Fluids.

The capabilities of Fiber Bragg Grating (FBG) sensors to measure temperature variations in the bulk of liquid flows were considered. In the first step of our research project, reported in this paper, we investigated to what extent the use of thin glass fibers without encapsulation, which only minimally disturb a flow, can fulfill the requirements for robustness and measurement accuracy. Experimental tests were performed in a benchmark setup containing 24 FBG measuring positions, which were instrumented in parallel with thermocouples for validation. We suggest a special assembly procedure in which the fiber is placed under a defined tension to improve its stiffness and immobility for certain flow conditions. This approach uses a single FBG sensor as a reference that measures the strain effect in real time, allowing accurate relative temperature measurements to be made at the other FBG sensor points, taking into account an appropriate correction term. Absolute temperature readings can be obtained by installing another well-calibrated, strain-independent thermometer on the reference FBG. We demonstrated this method in two test cases: (i) a temperature gradient with stable density stratification in the liquid metal GaInSn and (ii) the heating of a water column using a local heat source. In these measurements, we succeeded in recording both spatial and temporal changes in the linear temperature distribution along the fiber. We present the corresponding results from the tests and, against this background, we discuss the capabilities and limitations of this measurement technique with respect to the detection of temperature fields in liquid flows.

High-precision temperature-compensated fiber Bragg grating axial strain sensing system based on a dual-loop optoelectronic oscillator with the enhanced Vernier effect.

A temperature-compensated fiber Bragg grating (FBG) axial strain sensor based on a two-dual-loop optoelectronic oscillator (OEO) with the enhanced Vernier effect is proposed and experimentally demonstrated. The sensing head consists of two cascaded FBGs, one of which acts as a sensing FBG to measure both the axial strain and temperature and the other as a reference FBG to detect temperature. Acting as the optical carrier, the reflected optical signal of the sensing head is divided into two paths with opposite dispersion coefficients and slightly different lengths to achieve an enhanced Vernier effect. After being divided by a wavelength division multiplexer, the optical signal launches into two electrical paths with different electrical bandpass filters (EBPFs) for frequency division multiplexing. The EBPF I selects the microwave signal generated by the sensing FBG, while the EBPF II selects the microwave signal generated by the reference FBG. Therefore, the axial strain and temperature can be recovered by recording the microwave frequency within EBPF I, and the temperature can be interrogated by tracking the microwave frequency within EBPF II. The axial strain applied on the sensing FBG can be distinguished by solving the cross-sensitivity matrix. The results show that the sensitivity of the dual-loop OEO is much greater than that of the single-loop OEO. The maximum measurement error for the axial strain is 0.112µε, and the maximum temperature compensation error is as low as 0.024°C in the dual-loop OEO, which is far less than that in the single-loop OEO. The enhanced Vernier effect not only improves the sensitivity, but also reduces the temperature compensation error.

Low cross-sensitivity and sensitivity-enhanced FBG sensor interrogated by an OCMI-based three-arm interferometer.

An FBG sensor interrogated by an optical carrier microwave interferometry (OCMI)-based three-arm Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. In our sensing scheme, the interferogram generated by interfering the three-arm-MZI middle arm with the sensing arm and the reference arm respectively is superimposed to produce a Vernier effect to increase the sensitivity of the system. The simultaneous interrogation of the sensing fiber Bragg grating (FBG) and the reference FBG by the OCMI-based three-arm-MZI provides an ideal solution to the cross-sensitivity problems (e.g. temperature vs. strain) associated with conventional sensors that produce the Vernier effect by cascading optical elements. Experimental results show that when applied to strain sensing, the OCMI-three-arm-MZI based FBG sensor is 17.5 times more sensitive compared to the two-arm interferometer based FBG sensor. And the temperature sensitivity is reduced from 371.858 KHz/°C to 1.455 KHz/°C. The prominent advantages of the sensor, including high resolution, high sensitivity, and low cross-sensitivity, make it a great potential for high-precision health monitoring in extreme environments.

Pressure Membrane FBG Sensor Realized by 3D Technology.

The publication describes the design, production, and practical verification of an alternative pressure sensor suitable for measuring the pressure of gas, based on a combination of fiber-optic technology and 3D printing methods. The created sensor uses FBG (Fiber Bragg Grating) suitably implemented on a movable membrane. The sensor is equipped with a reference FBG to compensate for the effect of ambient temperature on the pressure measurement. The sensor is characterized by its immunity to EM interference, electrical passivity at the measuring point, small size, and resistance to moisture and corrosion. The FBG pressure sensor has a pressure sensitivity of 9.086 pm/mbar in the range from 0 to 9 mbar with a correlation coefficient of 0.9982. The pressure measurement in the specified range shows an average measurement error of 0.049 mbar and a reproducibility parameter of 0.0269 ± 0.0135 mbar.

Accelerometer prototype based on enhanced fiber Bragg grating overlapping interrogation method

This paper presents the development and analysis of an optical biaxial accelerometer based on fiber Bragg gratings (FBGs) using an enhanced ratiometric interrogation method: Bragg grating overlapping interrogation. Typically, this method requires two FBGs for each sensing axis in the accelerometer. However, the configuration employed in this study comprises only one grating for each sensing axis and one reference FBG that demodulates the Bragg wavelength changes of both sensing FBGs, in addition to performing temperature compensation. This modified scheme enables the use of fewer gratings without requiring an interrogator, a device often unsuitable for in-field applications. This prototype is also temperature-insensitive, with a usable acceleration range of 0.1–1.0 g (1 g = 9.8 m/s2) for measuring the acceleration amplitudes of objects moving at frequencies of 0.1–20 Hz. These findings show that the developed prototype 2D accelerometer is suitable for real-world seismic applications.

A Fiber Bragg Grating Borehole Deformation Sensor for Stress Measurement in Coal Mine Rock.

A borehole deformation sensor for long-term stress monitoring in coal mine rock based on optical fiber Bragg gratings (FBGs) is presented. The sensor converts borehole deformation into optical fiber strain by using four rings. For each ring, two FBGs are bonded with the ring to measure the borehole deformation, and a reference FBG free from mechanical load is introduced to remove the temperature effect. Two simple checks on the test data can be performed to improve the test accuracy. Laboratory and field tests were conducted to validate the accuracy and long-term performance of the sensor. The results show that the sensor is capable of measuring stress in rock with good accuracy, and it performs well over a long period of time in coal mines. The developed sensor provides an approach for the long-term monitoring of stress changes in coal mine rock.

Static pure strain sensing using dual-comb spectroscopy with FBG sensors.

We propose a method to precisely characterize the optical response of a fiber Bragg grating (FBG) sensor for static strain sensing by using dual-comb spectroscopy (DCS). By digitally post-correcting the mutual noise between the two combs, a robust and pure strain sensing system is achieved by compensating for the temperature-induced frequency shift of the FBG sensor. The comb-resolved radio-frequency (RF) spectra generated by DCS are obtained. Meanwhile, the stability of the central Bragg frequency of the FBG-reflected spectrum in the RF domain is 0.315 kHz, which is better than the difference of repetition rate (1 kHz). A reference FBG is used for detecting and compensating for the temperature-induced central frequency drift. Finally, a spectral sensitivity of 0.85 pm/µɛ with 0.8 µɛ static strain resolution is achieved.

Momentum Change Flow Meter With Pressure Compensation Using FBGs

In this paper, a flow measurement device based on measuring deflections caused by the momentum effect of fluid flowing around a pipe elbow is presented. The movement induced on a cantilever arm as a result of this momentum change is measured using a fiber Bragg grating (FBG) optical sensor. The measured response of the FBG was found to be highly correlated to the flow rate in the system. Temperature compensation of the FBG strain measurement was achieved using a second reference FBG. The effects of static pressure were compensated for with the addition of a dead leg bellows. It has been demonstrated that this flow sensor can effectively measure flow rate independently of both static pressure and temperature variations.

FBG based high sensitive pressure sensor and its low-cost interrogation system with enhanced resolution

A fiber Bragg grating (FBG) pressure sensor with high sensitivity and resolution has been designed and demonstrated. The sensor is configured by firmly fixing the FBG with a metal bellows structure. The sensor works by means of measuring the Bragg wavelength shift of the FBG with respect to pressure change. From the experimental results, the pressure sensitivity of the sensor is found to be 90.6 pm/psi, which is approximately 4000 times as that of a bare fiber Bragg grating. A very good linearity of 99.86% is observed between the Bragg wavelength of the FBG and applied pressure. The designed sensor shows good repeatability with a negligible hysteresis error of ± 0.29 psi. A low-cost interrogation system that includes a long period grating (LPG) and a photodiode (PD) accompanied with simple electronic circuitry is demonstrated for the FBG sensor, which enables the sensor to attain high resolution of up to 0.025 psi. Thermal-strain cross sensitivity of the FBG pressure sensor is compensated using a reference FBG temperature sensor. The designed sensor can be used for liquid level, specific gravity, and static/dynamic low pressure measurement applications.

Magnetic Resonance-compatible needle-like probe based on Bragg grating technology for measuring temperature during Laser Ablation.

Temperature monitoring in tissue undergone Laser Ablation (LA) may be particularly beneficial to optimize treatment outcome. Among many techniques, fiber Bragg grating (FBG) sensors show valuable characteristics for temperature monitoring in this medical scenario: good sensitivity and accuracy, and immunity from electromagnetic interferences. Their main drawback is the sensitivity to strain, which can entail measurement error for respiratory and patient movements. The aims of this work are the design, the manufacturing and the characterization of a needle-like probe which houses 4 FBGs. Three FBGs have sensitive length of 1 mm and are used as temperature sensors; one FBG with length of 10 mm is used as reference and to sense eventual strain. The optical fiber housing the FBGs was encapsulated within a needle routinely used in clinical practice to perform MRI-guided biopsy. Two materials were used for the encapsulation: i) thermal paste for the 3 FBGs used for temperature monitoring, to maximize the thermal exchange with the needle; ii) epoxy resin for the reference FBG, to improve its sensitivity to strain. The static calibration of the needle-like probe was performed to estimate the thermal sensitivity of each FBG; the step response was investigated to estimate the response time. FBGs 1 mm long have thermal sensitivity of 0.01 nm·°C(-1), whereas the reference FBG presents 0.02 nm·°C(-1). For all FBGs, the response time was in the order of 100 ms. Lastly, experiments were performed on ex vivo swine liver undergoing LA to i) evaluate the possible presence of measurement artifact, due to the direct absorption of laser light by the needle and ii) assess the feasibility of the probe in a quasi clinical scenario.

FBG Interrogation Method with High Resolution and Response Speed Based on a Reflective-Matched FBG Scheme.

In this paper, a high resolution and response speed interrogation method based on a reflective-matched Fiber Bragg Grating (FBG) scheme is investigated in detail. The nonlinear problem of the reflective-matched FBG sensing interrogation scheme is solved by establishing and optimizing the mathematical model. A mechanical adjustment to optimize the interrogation method by tuning the central wavelength of the reference FBG to improve the stability and anti-temperature perturbation performance is investigated. To satisfy the measurement requirements of optical and electric signal processing, a well- designed acquisition circuit board is prepared, and experiments on the performance of the interrogation method are carried out. The experimental results indicate that the optical power resolution of the acquisition circuit border is better than 8 pW, and the stability of the interrogation method with the mechanical adjustment can reach 0.06%. Moreover, the nonlinearity of the interrogation method is 3.3% in the measurable range of 60 pm; the influence of temperature is significantly reduced to 9.5%; the wavelength resolution and response speed can achieve values of 0.3 pm and 500 kHz, respectively.

Dual functional performance of fiber Bragg gratings coated with metals using flash evaporation technique

Metallic and other type of coatings on fiber Bragg grating (FBG) sensors alter their sensitivity with thermal and mechanical stress while protecting the fragile optical fiber in harsh sensing surroundings. The behavior of the coated materials is unique in their response to thermal and mechanical stress depending on the thickness and the mode of coating. The thermal stress during the coating affects the temperature sensitivity of FBG sensors. We have explored the thermal response of FBGs coated with Al and Pb to an average thickness of 80nm using flash evaporation technique where the FBG sensor is mounted in a region at room temperature in an evacuated chamber having a pressure of 10−6Torr which will minimize any thermal stress during the coating process. The coating thickness is chosen in the nanometer region with the aim to study thermal behavior of nanocoatings and their effect on FBG sensitivity. The sensitivity of FBGs is evaluated from the wavelengths recorded using an optical sensing interrogator sm 130 (Micron Optics) from room temperature to 300°C both during heating and cooling. It is observed that the sensitivity of the metal coated fibers is better than the reference FBG with no coating for the entire range of temperature. For a coating thickness of 80nm, Al coated FBG is more sensitive than the one coated with Pb up to 170°C and it reverses at higher temperatures. This point is identified as a reversible phase transition in Pb monolayers as the 2-dimensional aspects of the metal layers are dominant in the nanocoatings of Pb. On cooling, the phase transition reverses and the FBGs return to the original state and for repeated cycles of heating and cooling the same pattern is observed. Thus the FBG functions as a sensor of the phase transitions of the coatings also.

Sensitivity Enhancement of Strain Sensing Utilizing a Differential Pair of Fiber Bragg Gratings

In strain measurement applications, the matched fiber Bragg gratings (FBG) method is generally used to reduce temperature dependence effects. The FBG parameters have to be designed to meet the requirements by the particular application. The bandwidth and slope of the FBG has to be balanced well, according to the measurement range, accuracy and sensitivity. A sensitivity enhanced strain demodulation method without sacrificing the measurement range for FBG sensing systems is proposed and demonstrated utilizing a pair of reference FBGs. One of the reference FBGs and the sensing FBG have almost the same Bragg wavelength, while the other reference FBGs has a Bragg wavelength offset relative to the sensing FBG. Reflected optical signals from the sensing FBG pass through two reference FBGs, and subtract from each other after the detection. Doubled strain measurement sensitivity is obtained by static rail load experiments compared to the general matched grating approach, and further verified in dynamic load experiments. Experimental results indicate that such a method could be used for real-time rail strain monitoring applications.

Differential reflectometry of FBG sensors in the wide spectral range

The paper presents a reflectometric technique for interrogation of multiple fiber Bragg grating (FBG) sensors based on conventional optical time domain reflectometry (OTDR). The method proposed rests on the differential measurement of FBGs’ response to a short probing laser pulse. Implementation of differential measurement principle using several reference FBGs allowed us to eliminate the susceptibility of the system to intensity fluctuations as well as to increase the measurement range as compared to the previous developments. The experimental threshold sensitivity amounted to ∼50 microstrain with the measurement range being defined by the number of reference FBGs and limited only by optical fiber tensile strength. Due to its simplicity, efficiency and usage of conventional OTDR equipment the proposed FBG interrogation technique can find a wide range of applications dealing with strain and temperature measurements.

Elimination of environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating sensor array

We present a novel reference compensation method for eliminating environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating (FBG) sensors. By employing a shielded wavelength-division-multiplexed reference FBG in the system, the environmental noise is measured from the reference channel, and then subtracted from the demodulation result of each sensor channel. An approximate 40 dB reduction of the environmental noise has been experimentally achieved over a frequency range from 20 Hz to 2 kHz. This method is also suitable for the elimination of broadband environmental noise. The corresponding FBG sensor array system proposed in this paper has shown a wavelength resolution of 7 × 10 - 4 pm / Hz .

Design and nonlinearity compensation of Fabry-Perot type tunable optical filters for dynamic strain sensing systems

The collected spectrum of the fiber Bragg grating (FBG) and the loss of the detected optical power are discussed with respect to the 3-dB bandwidth of a Fabry-Perot (F-P) type tunable optical filter (TOF), respectively. And the optimized parameters of the TOF are obtained consequently. It is demonstrated that the relationship between the transmission wavelength of the TOF and its drive voltage is nonlinear. A new method to compensate the nonlinearity of the TOF is proposed. The linear sweeping of the transmission wavelength of the TOF is achieved through modifying the drive voltage using interpolation algorithm. It is observed that the average error and the maximum error of the transmission wavelength are reduced sharply under linear fit. The dynamic strain sensing is realized by use of a reference FBG and moving averaging algorithm in this system.

A Novel Opto Electro Mechanical Interrogation System for Measuring Wavelength Shifts in Fiber Bragg Grating (FBG) Sensors

Here, we describe a novel FBG interrogation system in which FBGs are used as both sensing and reference elements. The reference FBGs is bonded to a mechanical flexure system having a linear amplification of 1:3.5, which is actuated using a piezo-actuator by applying a 0–150 V ramp. The lengths of the reference gratings decide the maximum strain that can be applied to the reference grating, which in turn decides that strain range which can be interrogated. The main advantages of the present system are the on-line measurement of the wavelength shifts, small size, good sensitivity, multiplexing capability and low cost.

Investigation of PZT driven tunable optical filter nonlinearity using FBG optical fiber sensing system

The nonlinearity of a piezo-electrical transducer (PZT) driven tunable optical filter (TOF) is investigated and evaluated using FBG optical fiber sensing system including a broadband optical source and a tunable laser (TL) with a group of fiber Bragg gratings (FBGs). Polynomial fit is adopted to model the nonlinearity. Under three-order polynomial fit, the random error in wavelength measurement using this TOF and a reference FBG is minimized to be below 20 pm. At the same time, the absolute error in wavelength measurement is kept below 0.25 nm. Under four-order or higher polynomial fit, a sharp increase of the random measurement error is observed. This research reveals the possibility of using PZT driven TOFs in optical spectroscopy where high resolution, high-accuracy, and fast time-response is required.