Ultra-weak Fiber Bragg Gratings Research Articles

Fiber-optic wireless sensor network using ultra-weak fiber Bragg gratings for vertical subsurface deformation monitoring

Fiber-optic wireless sensor network using ultra-weak fiber Bragg gratings for vertical subsurface deformation monitoring

Multi-Wavelength Ultra-Weak Fiber Bragg Grating Arrays for Long-Distance Quasi-Distributed Sensing

Fiber Bragg grating (FBG) array, consisting of a number of sensing units in a single optical fiber, can be practically applied in quasi-distributed sensing networks. Serious signal crosstalk occurring between large-serial of identical FBGs, however, has limited the further increase in the number of sensing units, thus restricting applications only for short-distance sensing networks. To reduce the signal crosstalk, we design two novel types of 10-kilometer-long FBG arrays with 10 000 equally spaced gratings, written on-line using a customized grating inscription system, which is affiliated to a drawing tower. Main factors causing signal crosstalk, such as spectral shadowing and multiple reflections, are firstly investigated in theory. Consistent with the theoretical findings, experimental results are proving that ultra-weak (the reflectivity of ∼−40 dB) and multi-wavelength gratings of a number more than 10 000 can be readily identified, with satisfied low crosstalk. The maximum attenuation of grating signal and minimum signal-to-noise ratio (SNR) in a single-wavelength array are 10.69 dB and 5.62 dB, respectively. As a comparison, by increasing the number of central wavelengths to three, the attenuation can be effectively reduced to 5.54 dB and the minimum SNR has been improved to 8.14 dB. The current study significantly enhances the multiplexing capacity of FBG arrays and demonstrates promising potentials for establishing large-capacity quasi-distributed sensing networks.

Enlarging Dynamic Strain Range in UWFBG Array-Based Φ-OTDR Assisted With Polarization Signal

Phase-sensitive optical time-domain reflectometry ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR) measures the dynamic strain by extracting the phase of the scattering signal. However, the dynamic strain measurement range in conventional <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR is restricted by the phase unwrapping algorithm. In this letter, a new phase unwrapping algorithm, combining the phase and polarization information, is proposed and demonstrated to enlarge the dynamic strain measurement range in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR based on ultra-weak fiber Bragg grating (UWFBG) array. By correcting the phase signal with the assistance of polarization signal, the large dynamic strain can be recovered. In experiments, the dynamic strain measurement range is demonstrated to increase by about three times, which cannot be achieved by the conventional unwrapping algorithm. Further, the proposed method has the potential to achieve a large amplitude range determined by polarization signal and a good sensitivity determined by phase signal at the same time.

Fiber-end antireflection method for ultra-weak fiber Bragg grating sensing systems

A novel fiber-end antireflection method for reducing the reflection noise in ultra-weak fiber Bragg grating (FBG) sensing systems is proposed in this study. The principle of fiber-end antireflection is analyzed and simulated theoretically, and then an antireflection structure is designed and fabricated. In this method, the fiber end is cut into an inclined surface and covered directly with an ellipsoid photosensitive ultraviolet (UV) glue drop, whose refractive index is similar to that of the fiber core. Under the online monitoring of the fiber-end reflection with the ultra-weak FBG interrogator, the refractive index of the glue drop is adjusted by controlling the UV curing time to achieve optimal reflection suppression. Results show that this method can reduce fiber-end reflection to less than −60 dB and effectively eliminate the influence of fiber-end reflection noise on the last sensor. The proposed method thus provides an excellent solution for the fiber-end processing of ultra-weak FBG sensing systems.

Distributed acoustic sensors with wide frequency response based on UWFBG array utilizing dual-pulse detection

A novel fiber-optic distributed acoustic sensing (DAS) system with wide frequency response of 0.1 Hz ~ 30 kHz is proposed and experimentally demonstrated. The DAS, which is induced by adjacent ultra-weak fiber Bragg gratings (UWFBGs) using dual-pulse and 3 × 3 fiber coupler in direct detection, is capable of quantifying multiple dynamic strain perturbations along 1.5 km UWFBG array with a spatial resolution of 5 m. The delay of dual-pulse equals to the round-trip time of adjacent UWFBGs and will interfere in three photo-detectors. We experimentally demonstrate the effectiveness of the technique by monitoring the dynamic response of vibration events with three different frequencies. The signal-to-noise ratios (SNRs) of DAS at the 0.1 Hz and 30 kHz are 45.92 dB and 43.33 dB, respectively. The minimum detectable strain is as low as 1.21nε. The strain response has good linear characteristics with the linear coefficient of 0.9988. The comparison with the Michelson interferometer shows the dual-pulse detection is free from external environmental perturbation, which it is very helpful for engineering application.

Distributed Vibration Sensing System with High Signal-to-Noise Ratio Based on Ultra-Weak Fiber Bragg Grating

Distributed Vibration Sensing System with High Signal-to-Noise Ratio Based on Ultra-Weak Fiber Bragg Grating

Recent Advancements in Rayleigh Scattering-Based Distributed Fiber Sensors

Recently, Rayleigh scattering-based distributed fiber sensors have been widely used for measurement of static and dynamic phenomena such as temperature change, dynamic strain, and sound waves. In this review paper, several sensing systems including traditional Rayleigh optical time domain reflectometry (OTDR), Φ -OTDR, chirped pulse Φ -OTDR, and optical frequency domain reflectometry (OFDR) are introduced for their working principles and recent progress with different instrumentations for various applications. Beyond the sensing technology and instrumentation, we also discuss new types of fiber sensors, such as ultraweak fiber Bragg gratings and random fiber gratings for distributed sensing and their interrogators. Ultimately, the limitations of Rayleigh-based distributed sensing systems are discussed.

Quasi-distributed acoustic sensing with interleaved identical chirped pulses for multiplying the measurement slew-rate.

Quasi-distributed acoustic sensing (Q-DAS) based on ultra-weak fiber Bragg grating (UWFBG) is currently attracting great attention, due to its high sensitivity and excellent multiplexing capability. Phase-sensitive optical time-domain reflectometry (Φ-OTDR) based on phase demodulation is one of the most promising interrogation schemes for Q-DAS. In this article, a novel interleaved identical chirped pulse (IICP) approach is proposed on the basis of pulse compression Φ-OTDR with coherent detection. Different from the frequency-division-multiplexing (FDM) method, the identical pulses are used for multiplexing in the IICP scheme, and the mixed reflection signals can be demodulated directly, so the inconsistent phase offsets in FDM can be avoided. As a result, this scheme can enlarge the measurement slew-rate (SR) of Q-DAS by times compared with traditional single pulse scheme. In the proof-of-principle experiment, the SR of 28.9 mɛ/s has been achieved with an 860 m sensing range, which is 5 times as that of the traditional single pulse scheme; meanwhile, the response bandwidth has been enlarged by 5 times. The 277 kHz response bandwidth has been achieved, with 5 m spatial resolution and 2.8 pε/Hz strain sensitivity.

Bandwidth-Enhanced Quasi-Distributed Acoustic Sensing With Interleaved Chirped Pulses

In this letter, to break through the response bandwidth limit of quasi-distributed acoustic sensing system, a novel interleaved chirped pulses (ICP) approach is proposed on the basis of pulse compression ${\Phi }$ -OTDR with coherent detection. As shown in the theoretical analysis, for ultra-weak fiber Bragg grating (UWFBG) array with fixed positions and intervals, the space between any two FBGs can be used to multiplex probing pulses, without occupying extra detection bandwidth. The cross-correlation peak of probe pulses can be separated from each other in the time domain with matched filtering. By injecting probe pulses which are evenly interleaved as positive-chirped and negative-chirped, crosstalk of the cross-correlation peaks can be suppressed. In the proof-of-principle experiment, the response bandwidth is 3 times as that of the traditional single pulse probing scheme, all with the same detection bandwidth; 860 m sensing range with 166.4 kHz response bandwidth has been achieved, with 5 m spatial resolution and 1.8 p ${\varepsilon /\sqrt {Hz}}$ strain sensitivity.

Simultaneously distributed temperature and dynamic strain sensing based on a hybrid ultra-weak fiber grating array.

A fiber-optic sensing system based on two types of ultra-weak fiber Bragg gratings (UWFBG) for simultaneous temperature and vibration sensing was proposed. Narrowband and broadband UWFBGs are alternately written into an optical fiber with equal spacing. Distributed temperature sensing is realized by demodulating the wavelength shift of the narrowband UWFBG, while distributed vibration sensing is achieved by detecting phase variation between two adjacent broadband UWFBG interference pulses. The experimental results show that the proposed hybrid UWFBG array can perform temperature and vibration sensing simultaneously. The experimentally conducted temperature measurement ranges from 20°C to 100°C, with the measurement error less than 0.1°C. Vibration signals at different temperatures can be accurately restored, and the signal-to-noise ratio (SNR) is improved by 21.1 dB compared with a normal single-mode fiber (SMF).

Deformation detection of a double-clamped beam according to ultra-weak fiber Bragg grating.

Metal beam strain detection may offer valuable insight into health monitoring for large-scale steel structures. This research presents, to the best of our knowledge, the first implementation of ultra-weak fiber Bragg gratings for the deformation detection of a double-clamped beam because of their higher multiplexing and denser detection points compared to fiber Bragg gratings. The measured values are entirely consistent with those determined by strain gauges applied for reference and demonstrate that the strain of each sensing point increases linearly with the load at the middle of the beam. The errors of different loads at different load points between inversion maximum deflection and measured displacement are less than 9.59%.

An Enhanced Distributed Acoustic Sensor With Large Temperature Tolerance Based on Ultra-Weak Fiber Bragg Grating Array

An enhanced distributed optical fiber sensor based on broadband ultra-weak fiber Bragg grating array (UWFBG) for dynamic strain measurement with large temperature tolerance range was proposed and experimentally demonstrated. The reflectivity of UWFBG is about -50 dB, and the intensity of the reflected signal is 17 dB higher than the Rayleigh back-scattering. Using an unbalanced Michelson interferometer (MI) and 3 × 3 coupler phase demodulation technique, multiple vibration events can be detected simultaneously. We experimentally demonstrated the effectiveness of the technique by monitoring dynamic strain with a large temperature range of 20-200 °C. The minimum phase detection of the system is 1.02 × 10 - 3 rad/√Hz, and the signal-to-noise ratio (SNR) is above 59.2 dB @100 Hz at different temperatures. Meanwhile, the system has a wide frequency response from 10 Hz to 4.5 kHz.

High-sensing-resolution distributed hot spot detection system implemented by a relaxed pulsewidth.

Reliable sensing and accurate location of a weak and small hot spot are critical for applications in industrial infrastructure monitoring. We propose and experimentally demonstrate a practical and reliable distributed hot spot detection method using ultra-weak fiber Bragg gratings (UWFBGs) array and optical time-domain reflectometry (OTDR) based interrogator. To reliably detect the hot spots, the grating spacing of the sensor array is decreased to a similar size of the hot spot. All UWFBGs within a fiber section (FS) are considered as one sensing element, and the wavelength-division multiplexing technique is introduced to reduce crosstalk between adjacent FSs. To retrieve the sensing information, the proposed FS spectrum interrogation method based on OTDR technology is numerically analyzed and experimentally demonstrated. The interrogator exploits the reflection spectrum of each FS instead of each grating, enabling the low-speed hardware implementation of the whole demodulation method. Experimental results show that the expected hot spot can be successfully detected with a sensing resolution of 10 cm and a location resolution of 1 m over a range of 2 km by exploiting 10-ns pulsewidth. Besides, the temperature measurement can be demonstrated with a temperature sensing precision of ± 1°C and a measurement time of 1.5 s, which are meaningful for the early warning of centimeters-sized fire source in some oil and gas pipelines monitoring applications.

Combining SDAE Network with Improved DTW Algorithm for Similarity Measure of Ultra-Weak FBG Vibration Responses in Underground Structures.

Quantifying structural status and locating structural anomalies are critical to tracking and safeguarding the safety of long-distance underground structures. Given the dynamic and distributed monitoring capabilities of an ultra-weak fiber Bragg grating (FBG) array, this paper proposes a method combining the stacked denoising autoencoder (SDAE) network and the improved dynamic time wrapping (DTW) algorithm to quantify the similarity of vibration responses. To obtain the dimensionality reduction features that were conducive to distance measurement, the silhouette coefficient was adopted to evaluate the training efficacy of the SDAE network under different hyperparameter settings. To measure the distance based on the improved DTW algorithm, the one nearest neighbor (1-NN) classifier was utilized to search the best constraint bandwidth. Moreover, the study proposed that the performance of different distance metrics used to quantify similarity can be evaluated through the 1-NN classifier. Based on two one-dimensional time-series datasets from the University of California, Riverside (UCR) archives, the detailed implementation process for similarity measure was illustrated. In terms of feature extraction and distance measure of UCR datasets, the proposed integrated approach of similarity measure showed improved performance over other existing algorithms. Finally, the field-vibration responses of the track bed in the subway detected by the ultra-weak FBG array were collected to determine the similarity characteristics of structural vibration among different monitoring zones. The quantitative results indicated that the proposed method can effectively quantify and distinguish the vibration similarity related to the physical location of structures.

Dynamic Quasi-Distributed Ultraweak Fiber Bragg Grating Array Sensing Enabled by Depth-Resolved Dual-Comb Spectroscopy

Distributed sensing plays an important role in many industrial applications. Sensor arrays consisting of identical ultraweak fiber Bragg gratings can be applied to such scenarios by monitoring the optical spectral response of the sensors. Yet, the spatial resolution, the sampling speed, and the supported number of sensors in the array are mostly determined by the interrogation scheme. While dual-optical frequency comb techniques had been successfully demonstrated as a high-resolution spectroscopic technique in metrology, it had been beyond the reach of industrial sensing applications because of its high complexity and costs. It is proposed and demonstrated here that, enabled by a simple fiber-optic, dual-comb spectroscopy setup consisting of a fiber laser without frequency stabilization, interrogation of an ultraweak fiber sensor array can be realized. High spatial and spectral resolution as well as kilohertz interrogation speed is achieved in both static and dynamic measurements. It is expected that our depth-resolved dual-comb scheme can measure hundreds of sensors response in less than 1 ms. It is also flexible to deal with targets closely spaced or farther apart from each other. This shows the potential of applying dual-comb spectroscopy to more and more industrial application areas.

A visibility enhanced broadband phase-sensitive OTDR based on the UWFBG array and frequency-division-multiplexing

Abstract The phase-sensitive optical time domain reflectometers (Φ-OTDR), which can realize distributed acoustic sensing, has been widely applied in health monitoring of civil structures. To break through the limit of the frequency response bandwidth (FRB) and to reduce the fading positions of traditional Φ-OTDR systems, we propose an Φ-OTDR system based on the ultra-weak fiber Bragg grating (UWFBG) array and the frequency-division-multiplexing (FDM) scheme. Taking advantage of UWFBGs, the probability of fading positions occurred on the Rayleigh backscattering (RBS) waveforms can be dramatically reduced, which means the visibility of the system can be enhanced. The FDM structure can enhance the sampling rate by interleaving algorithm, which helps the Φ-OTDR system to expand its FRB. The experimental results show that the frequency of vibration up to 440 kHz can be detected along 330 m UWFBGs, which is approximately three times higher than the upper limit of the FRB of traditional systems. When compared with the traditional scheme that adopts ordinary single-mode fiber as the sensing part, the probability of fading occurrence in the proposed system has been reduced from 10−2 to 10−10. The proposed sensing system in this paper reveals a promising option for the performance enhancement of the Φ-OTDR system, which can offer wider FRB as well as the enhanced visibility characteristic.

High-Resolution Temperature Distribution Measurement of GIL Spacer Based on OFDR and Ultraweak FBGs

The temperature distribution on the gas-insulated line (GIL) spacer affects surface charge accumulation. However, the spacer temperature distribution is hard to measure because the conventional electric sensors are large dimensions, hard to multiplex, and sensitive to electromagnetic interference (EMI). In this article, a distributed temperature measurement system of GIL spacer based on the optical frequency domain reflectometry (OFDR) was developed to solve the problems. In this system, ultraweak fiber Bragg grating (FBG) was adopted to replace single-mode fiber due to its higher signal-to-noise ratio. The demodulation method was described for compensating the nonlinear frequency tuning errors induced by the unstable tunable laser. In addition, the relationship between the spacer temperature and the wavelength shift was determined in the calibration experiment. Moreover, the temperature distribution on a 126-kV GIL spacer was measured by the proposed OFDR system. The data processing method for 3-D surface temperature on cone-type spacer was also introduced. High-resolution temperature distribution on the spacer surface was successfully obtained using OFDR system, which is unable to accomplish with the conventional electric temperature sensor.

Noise resilient quasi-distributed sensing with an interferometric-noise-suppressing Golay coded optical source

A noise-resilient demodulation technique is proposed and demonstrated with an interferometric-noise-suppressing (INS-) Golay coded optical pulse source for a quasi-distributed sensor array constructed by identical ultra-weak fiber Bragg gratings (UWFBGs).In combination with a medium coherence light source, coding-based time-efficient noise reduction is facilitated for closely-multiplexed UWFBG arrays, for which conventional Golay coding is inapplicable. With 32-bit INS-Golay coded pulse trains, 5.6-dB signal-to-noise ratio improvement is achieved as compared to the case where an uncoded pulse train is adopted. While the performance is almost as good as 32-time averaging, the time consumption for signal acquisition is only ∼1/8. The proposed INS-Golay coding method exhibits linearity up to 0.9986 in temperature sensing with a wavelength demodulation error of ∼±5 pm. The time efficiency of the method increases with increased code length, providing a solution that alleviates the trade-off between the demodulation accuracy and speed in the UWFBG-based sensing systems with closely-multiplexed sensors.

A Novel Monitoring Approach for Train Tracking and Incursion Detection in Underground Structures Based on Ultra-Weak FBG Sensing Array.

Tracking operating trains and identifying illegal intruders are two important and critical issues in subway safety management. One challenge is to find a reliable methodology that would enable these two needs to be addressed with high sensitivity and spatial resolution over a long-distance range. This paper proposes a novel monitoring approach based on distributed vibration, which is suitable for both train tracking and incursion detection. For an actual subway system, ultra-weak fiber Bragg grating (FBG) sensing technology was applied to collect the distributed vibration responses from moving trains and intruders. The monitoring data from the subway operation stage were directly utilized to evaluate the feasibility of the proposed method for tracking trains. Moreover, a field simulation experiment was performed to validate the possibility of detecting human intrusion. The results showed that the diagonal signal pattern in the distributed vibration response can be used to reveal the location and speed of the moving loads (e.g., train and intruders). Other train parameters, such as length and the number of compartments, can also be obtained from the vibration responses through cross-correlation and envelope processing. Experimental results in the time and frequency domains within the selected intrusion range indicated that the proposed method can distinguish designed intrusion cases in terms of strength and mode.

Precision Dynamic Sensing With Ultra-Weak Fiber Bragg Grating Arrays by Wavelength to Frequency Transform

A high-speed interrogator using wavelength to frequency transform is proposed for high-precision demodulation of ultra-weak fiber Bragg gratings (UWFBGs). An amplified spontaneous emission source, modulated by a frequency-swept microwave signal, is adopted as the light source. The signal reflected by the UWFBGs is temporally broadened by a dispersion element, which is then detected and mixed with the driven microwave signal. Through wavelength-dependent time delay introduced by the dispersion element and the frequency sweeping of the microwave signal, the wavelength of the fiber Bragg grating is converted to a beat frequency by the mixing process. Due to the ambient temperature fluctuation, the effective refractive index of the sensing fiber varies over time. Consequently, the induced time delay varies with temperature, generating errors in the wavelength demodulation. By using a dispersion compensating fiber and a single-mode fiber, which introduce opposite group-velocity dispersion (GVD), such error is effectively reduced. Moreover, a dispersion correction model, which considers the wavelength dependence of GVD, is established to replace the traditional dispersion model. Experimental demonstration verifies that the interrogator, operating at 80 kHz, shows high wavelength demodulation accuracy of 6.96 pm.