Fiber Bragg Grating Sensor Interrogation Research Articles

Evaluation of an Erbium-Doped Fiber Ring Laser as an Edge Filtering Device for Fiber Bragg Grating Sensor Interrogation

An easy-to-implement and cost-effective Fiber Bragg Grating (FBG) sensor interrogation technique based on a ring Erbium-Doped Fiber Laser (EDFL) topology is proposed and experimentally assessed. The FBG sensor is part of the EDFL cavity and must have a central wavelength located within the linear region of the EDF’s amplified spontaneous emission (ASE) spectrum, which occurs at between 1530 and 1540 nm. In this manner, the wavelength-encoded response of the FBG under strain is converted to a linear variation in the laser output power, removing the need for spectrum analysis as well as any limitations from the use of external edge-filtering components. In addition, the laser linewidth is significantly reduced with respect to the FBG bandwidth, thus improving the resolution of the system, whereas its sensitivity can be controlled through pumping power. The performance of the system has been characterized by modeling and experiments for EDFs with different lengths, doping concentrations, and pumping power levels. The influence of mode-hopping in the laser cavity on the resolution and accuracy of the system has also been investigated.

Fiber Bragg grating sensor interrogation using edge-filtering with long period grating modulated light source

Fiber Bragg grating sensor interrogation using edge-filtering with long period grating modulated light source

Theoretical modeling and simulation of fiber Bragg grating sensor interrogator based on linear variable filter.

With the increasing frequency of aviation accidents in recent years, aircraft safety has received increasing attention. Aircraft operating condition detection is an important part of aviation safety. Fiber Bragg grating (FBG) sensors, with their excellent characteristics, enable online monitoring of aircraft operating conditions. However, the application of FBG sensors in aviation is currently limited because it is difficult for FBG sensor interrogators to meet the requirements of small size, light weight, and good vibration resistance in the aviation field. Therefore, this paper proposes a linear variable filter (LVF)-based FBG sensor interrogator to meet the requirements. An optical model of the interrogator is established. The parameters which determine the performances of the interrogator are analyzed and the design criteria are discussed. According to the requirements in the aviation field, the optical system of the interrogator is designed. The simulation results show that the LVF-based FBG sensor interrogation system has a bandwidth range of 90 nm (1505 nm-1595 nm), a resolution of 2 pm, and a capacity of 15 FBG sensors.

Fiber Bragg Grating Sensor Interrogation Using Tunable Erbium-Doped Fiber Ring Laser Source

Fiber Bragg gratings (FBG) are ideal sensors for temperature, strain or vibration sensing applications with their advantages over conventional electronics sensor systems. In this paper, two different FBG sensor interrogation schemes based on broadband ASE source and tunable EDFRL source are demonstrated and compared. With both interrogator configurations, the response of the FBG sensor to temperature change is found to be linear and the sensitivities are measured as 11 pm/°C and 10.5 pm/°C, respectively. The experimental results have revealed that the optical power reflected from the FBG sensor is almost 20 dB higher with tunable EDFRL source than broadband ASE source. Thus, an interrogation method with tunable EDFRL can be suitable for remote, quasi-distributed sensor applications requiring high output power and OSNR.

High-speed and high-resolution interrogation of FBG sensors using wavelength-to-time mapping and Gaussian filters.

In this work we report a novel intensity-based technique for simultaneous high-speed and high-resolution interrogation of fiber Bragg grating (FBG) sensors. The method uses a couple of intensity Gaussian filters and the dispersion-induced wavelength-to-time mapping effect. The Bragg wavelength is retrieved by means of the amplitude comparison between the two filtered grating spectrums, which are mapped into a time-domain waveform. In this way, measurement distortions arising from residual power due to the grating sidelobes are completely avoided, and the wavelength measurement range is considerably extended with respect to the previously proposed schemes. We present the mathematical background for the interrogation of FBGs with an arbitrary bandwidth. In our proof-of-concept experiments, we achieved sensitivities of ∼20 pm with ultra-fast rates up to 264 MHz.

Fiber Bragg grating sensor interrogation system based on an optoelectronic oscillator loop.

In this article, we propose and experimentally demonstrate a fiber Bragg grating (FBG) sensor interrogation technique based on an optoelectronic oscillator (OEO). The main components of the OEO loop in this proposed scheme contains an electro-optic modulator (EOM), a section of dispersive element, an electric filter, and a photodiode (PD). The reflection signal of the FBG sensor is functioning as the optical source of the OEO. The oscillating frequency of the OEO is determined by the overall time delay of the OEO loop. Due to the dispersive element in the loop, time delay of the OEO loop is a function of the OEO optical source wavelength. As a result, the wavelength change of the FBG can be converted into the oscillating frequency shift of the OEO. A proof-of-concept FBG based axial strain sensing experiment is carried out. The experimental results show that the frequency of the OEO generated microwave signals have a good linear relationship with the axial strain applied to the FBG. The sensitivity is about 58 Hz/με when using dispersion compensation fiber (DCF) with dispersion of -120 ps/(nm*km) as the dispersive medium and tracking the microwave signal with frequency near 2056.4 MHz, which is consistent with the theoretical calculation. The proposed method can also be applied to interrogate optical sensors based on detecting the wavelength change of the optical signals.

Integrated Dynamic Wavelength Division Multiplexed FBG Sensor Interrogator on a Silicon Photonic Chip

A wavelength division multiplexed fiber Bragg grating (FBG) sensor interrogator using an integrated unbalanced Mach–Zehnder interferometer followed by an arrayed waveguide grating on silicon-on-insulator platform is presented. The Bragg wavelength shift is determined through the phase-generated carrier demodulation technique, allowing accurate dynamic FBG interrogation. The performance of the proposed device was validated by comparing the measurements with a commercial FBG readout unit based on a spectrometer, serving as a reference. Experimental results demonstrate a dynamic strain resolution of 4.56 nϵ/√Hz and minimum detectable strain down to ∼0.49 μ ϵ at 1-kHz bandwidth.

A Fast Linearly Wavelength Step-Swept Light Source Based on Recirculating Frequency Shifter and Its Application to FBG Sensor Interrogation.

A wavelength step-swept light source (WSSL) using a recirculating frequency shifter loop (RFSL) based on a single-side-band (SSB) modulator is proposed, in order to achieve a linear and fast wavelength-sweeping. The swept step can be tuned from 1.2 pm to 128 pm by adjusting a precise and stable radio frequency (RF) signal that is applied to the SSB modulator. The swept rate can be tuned up to 99 kHz in a range of over 5.12 nm. Wavelength-to-time mapping is used to measure static strain-induced or temperature-induced shifting of the reflected central wavelength of a fiber Bragg grating (FBG). Because of the high linearity of the light source, the interrogation linearity of the strain and the temperature are as high as 0.99944 and 0.99946, respectively. When a dynamic periodic strain applied to FBG sensor, the dynamic performance of the FBG sensor is successfully recorded in the time domain and its power spectral density of a fast Fourier transform (FFT) is calculated. The signal-to-noise ratio (SNR) of the power spectral density is over 40 dB for a 100 Hz dynamic strain and the calculated sensitivity is 0.048 με/Hz1/2. A sharp change in the strain frequency from 100 Hz to 500 Hz is captured in real time.

A highly precise FBG sensor interrogation system with wavemeter calibration

Abstract Fiber Bragg grating (FBG) demodulators based on tunable lasers have been extensively applied in many fields. However, the tunable lasers can hardly avoid wavelength nonlinear tuning. Such problems and unexpected wavelength fluctuations may cause serious measurement errors. In this study, we propose a wavelength calibration scheme to solve this problem by incorporating a magneto-optic wavemeter into the interrogation system. We present experimental verification of real-time wavelength calibration of tunable lasers and measurement corrections. In the temperature measurement demonstrations, the experimental results show that the FBG interrogation accuracy was raised from ±3 pm to ±1 pm, and better long-term stability was achieved, corresponding to the temperature accuracy of ±0.1 °C.

Optical Fiber Bragg Grating Instrumentation Applied to Horse Gait Detection

This paper presents two in vivo instrumentation techniques to study the different types of gait of horses performing athletics using fiber Bragg gratings (FBG). These techniques can be used as an auxiliary tool in the early diagnosis of injuries related to the horse’s locomotor system, mainly in the distal portion of the digit, one of the most common causes of retirement when they are athletes. Therefore, the first technique presented consists of the fixation of FBGs without encapsulation, directly on the dorsal wall of the hoof in each of the limbs. In the second technique presented, the FBG sensor is encapsulated in a prototype developed using a composite material reinforced with carbon fiber in a horseshoe shape. The second technique is associated with digital image processing (DIP) for better visualization of the deformation and compression forces that act upon the limbs. The first method showed sensitivity to detection of the digit compression against the ground, being able to identify walking patterns. The second technique, with the encapsulated sensor elements, also allows the capture of characteristic signals of gait, such as step walk, trot, and gallop under training conditions. Both, FBG sensor interrogation and DIP, analysis techniques have proven good performance and promising results for the clinical and biomechanical study and medical evaluations of horses even during dynamic training and competitions.

Effect of polarization sensitivity on ultrasmall silicon-on-insulator-based arrayed waveguide grating for fiber Bragg grating sensor interrogation

Polarization sensitivity is an important factor that affects the interrogation of ultrasmall arrayed waveguide grating (AWG) for fiber Bragg grating (FBG) sensor. An ultrasmall 1 × 8 silicon-on-insulator (SoI) AWG with a core size of less than 530 μm × 480 μm is proposed in this study. This ultrasmall SoI AWG exhibits good transmission spectra and high polarization sensitivity. The increased channel numbers and tight structure increase the polarization sensitivity of AWG. Temperature interrogation experiments show that the FBG sensor interrogation is drastically affected when the effect of polarization sensitivity on the ultrasmall AWG is sufficiently large.

Evaluation of fiber Bragg grating sensor interrogation using InGaAs linear detector arrays and Gaussian approximation on embedded hardware.

Fiber Bragg Grating (FBG) sensors have become popular for applications related to structural health monitoring, biomedical engineering, and robotics. However, for successful large scale adoption, FBG interrogation systems are as important as sensor characteristics. Apart from accuracy, the required number of FBG sensors per fiber and the distance between the device in which the sensors are used and the interrogation system also influence the selection of the interrogation technique. For several measurement devices developed for applications in biomedical engineering and robotics, only a few sensors per fiber are required and the device is close to the interrogation system. For these applications, interrogation systems based on InGaAs linear detector arrays provide a good choice. However, their resolution is dependent on the algorithms used for curve fitting. In this work, a detailed analysis of the choice of algorithm using the Gaussian approximation for the FBG spectrum and the number of pixels used for curve fitting on the errors is provided. The points where the maximum errors occur have been identified. All comparisons for wavelength shift detection have been made against another interrogation system based on the tunable swept laser. It has been shown that maximum errors occur when the wavelength shift is such that one new pixel is included for curve fitting. It has also been shown that an algorithm with lower computation cost compared to the more popular methods using iterative non-linear least squares estimation can be used without leading to the loss of accuracy. The algorithm has been implemented on embedded hardware, and a speed-up of approximately six times has been observed.

Current Status and Future Trends of Photonic-Integrated FBG Interrogators

In this paper, we present an overview of the current efforts toward integration on chip of fiber Bragg grating (FBG) sensor interrogators. Different photonic-integration platforms are discussed, including monolithic planar lightwave circuit technology, silicon on insulator (SOI), indium phosphide, and gallium arsenide material platforms. Furthermore, various possible techniques for wavelength metering and methods for FBG multiplexing are discussed and compared in terms of resolution, dynamic performance, multiplexing capabilities, and reliability. The use of linear filters, array waveguide gratings (AWG) as multiple linear filters, and AWG-based centroid signal processing techniques are presented as well as interrogation techniques based on tunable microring resonators and Mach-Zehnder interferometers for phase sensitive detection. FBG sensor interrogation based on SOI platform using active and passive phase sensitive detection is also described, demonstrating specifically the potential of passive phase demodulation for high-speed dynamic strain measurements. This paper finally presents the challenges and perspectives of photonic integration to address the increasing requirements of several industrial applications.

Characterization of a FBG sensor interrogation system based on a mode-locked laser scheme.

This paper is focused on the characterization of a fiber Bragg grating (FBG) sensor interrogation system based on a fiber ring laser with a semiconductor optical amplifier as the gain medium, and an in-loop electro-optical modulator. This system operates as a switchable active (pulsed) mode-locked laser. The operation principle of the system is explained theoretically and validated experimentally. The ability of the system to interrogate an array of different FBGs in wavelength and spatial domain is demonstrated. Simultaneously, the influence of several important parameters on the performance of the interrogation technique has been investigated. Specifically, the effects of the bandwidth and the reflectivity of the FBGs, the SOA gain, and the depth of the intensity modulation have been addressed.

Crosstalk and Ghost Gratings in a Large-Scale Weak Fiber Bragg Grating Array

Large-scale distributed fiber Bragg grating (FBG) sensor networks have a wide range of applications. However, serious crosstalk could occur in a near-identical serial FBG sensor network because of multiple reflections. Since crosstalk cannot be observed directly and separately from signal, experimental investigation of crosstalk is challenging. In this work, we point out the similarity and symmetry between crosstalk and ghost gratings, and by way of measuring the spectrum of ghost gratings, we experimentally study the characteristics of crosstalk in a serial array of 3010 near-identical FBGs using a commercial FBG sensor interrogation system. We show that ghost gratings drop much more rapidly in power as one goes downstream, and they are more homogeneous in spectral distribution than real gratings. The peak wavelengths of real gratings vary within ±100 pm while the peak wavelengths of the ghost gratings they generate vary within ±20 pm only. Spectral ripples are observed for ghost gratings and real gratings severely affected by crosstalk. These ripples reduce the FBG wavelength reading accuracy to ±5 pm. The work constitutes the first experimental study on crosstalk in large-scale FBG arrays.

Fiber-optic temperature profiling for thermal protection system heat shields

To achieve better designs for spacecraft heat shields for missions requiring atmospheric aero-capture or entry/reentry, reliable thermal protection system (TPS) sensors are needed. Such sensors will provide both risk reduction and heat-shield mass minimization, which will facilitate more missions and enable increased payloads and returns. This paper discusses TPS thermal measurements provided by a temperature monitoring system involving lightweight, electromagnetic interference-immune, high-temperature resistant fiber Bragg grating (FBG) sensors with a thermal mass near that of TPS materials together with fast FBG sensor interrogation. Such fiber-optic sensing technology is highly sensitive and accurate, as well as suitable for high-volume production. Multiple sensing FBGs can be fabricated as arrays on a single fiber for simplified design and reduced cost. Experimental results are provided to demonstrate the temperature monitoring system using multisensor FBG arrays embedded in a small-size super-light ablator (SLA) coupon which was thermally loaded to temperatures in the vicinity of the SLA charring temperature. In addition, a high-temperature FBG array was fabricated and tested for 1000°C operation, and the temperature dependence considered over the full range (cryogenic to high temperature) for which silica fiber FBGs have been subjected.

Fiber-MZI-based FBG sensor interrogation: comparative study with a CCD spectrometer.

We present an experimental comparative study of the two most commonly used fiber Bragg grating (FBG) sensor interrogation techniques: a charge-coupled device (CCD) spectrometer and a fiber Mach-Zehnder interferometer (F-MZI). Although the interferometric interrogation technique is historically known to offer the highest sensitivity measurements, very little information exists regarding how it compares with the current commercially available spectral-characteristics-based interrogation systems. It is experimentally established here that the performance of a modern-day CCD spectrometer interrogator is very close to a F-MZI interrogator with the capability of measuring Bragg wavelength shifts with sub-picometer-level accuracy. The results presented in this research study can further be used as a guideline for choosing between the two FBG sensor interrogator types for small-amplitude dynamic perturbation measurements down to nano-level strain.

Heterogeneous integration of a III-V VCSEL light source for optical fiber sensing.

We propose a fiber Bragg grating (FBG) sensor interrogation system utilizing a III-V vertical cavity surface emitting laser (VCSEL) as the on-chip light source. Binary blazed grating (BBG) for coupling between III-V VCSEL and silicon-on-insulator (SOI) waveguides is demonstrated for interrogation of the FBG sensor. The footprint size of the BBG is only 5.62 μm×5.3 μm, and each BBG coupler period has two subperiods. The diameter of the VCSEL's emitting window is 5 μm, which is slightly smaller than that of the BBG coupler, to be well-matched with the proposed structure. Results show that the coupling efficiency from vertical cavities of the III-V VCSEL to the in-plane waveguides reached as high as 32.6% when coupling the 1550.65 nm light. The heterogeneous integration of the III-V VCSEL and SOI waveguides by BBG plays a fundamental role in inducing a great breakthrough to the miniaturization of an on-chip light source for optical fiber sensing.

Integrated FBG Sensors Interrogation Using Active Phase Demodulation on a Silicon Photonic Platform

The authors experimentally demonstrate the feasibility of interrogating Fiber Bragg Grating (FBG) sensors using an integrated unbalanced Mach–Zehnder Interferometer and active phase demodulation on silicon-on-insulator platform. The use of an external arrayed waveguide grating at the output of the circuit allows the interrogation of multiple FBGs through wavelength division multiplexing. Signal processing employing the phase-generated carrier demodulation technique is used to extract the wavelength shift from the signal patterns, allowing accurate dynamic FBG interrogation. The performance of the proposed integrated FBG interrogator is validated by comparing it with a commercial FBG readout unit based on a spectrometer and used as a reference. Experimental results demonstrate a dynamic strain resolution of 72.3 nϵ/√Hz.

Elimination of Light Intensity Noise Using Dual-Channel Scheme for Fiber MZI-Based FBG Sensor Interrogation

Fiber Mach–Zehnder interferometers (F-MZI) are widely used for fiber Bragg grating (FBG) sensor interrogation purpose due to their advantages in terms of wide bandwidth, high resolution, and tunable sensitivity. They are appropriate for dynamic strain measurement applications in the areas of vibration analysis, hydrophones, and acoustic emission studies. The interferometer converts the Bragg wavelength shift of an FBG sensor into a corresponding phase shift which is finally recorded as intensity variations at the output. Power fluctuations of the light source as well as disturbances in the light guiding path and/or dependence of light source intensity on wavelength introduce fictitious output intensity variations resulting into significant measurement errors. We propose and experimentally demonstrate a dual-channel F-MZI approach that eliminates the light intensity disturbance noise. Furthermore, the proposed method also improves the demodulated signal quality by a factor of $\sim 3$ dB.