Two-wavelength phase-sensitive OTDR sensor using perfect periodic correlation codes for measurement range enhancement, noise reduction and fading compensation

Multilayer Structure Damage Detection Using Optical Fiber Acoustic Sensing and Machine Learning

Over the past decade, distributed acoustic sensing has been utilized for structural health monitoring in various applications, owing to its continuous measurement capability in both time and space and its ability to deliver extensive data on the conditions of large structures using just a single optical cable. This work aims to evaluate the performance of distributed acoustic sensing for monitoring a multilayer structure on a laboratory scale. The proposed structure comprises four layers: a medium-density fiberboard and three rigid polyurethane foam slabs. Three different damages were emulated in the structure: two in the first layer of rigid polyurethane foam and another in the medium-density fiberboard layer. The results include the detection of the mechanical wave, comparing the response with point sensors used for reference, and evaluating how the measured signal behaves in time and frequency in the face of different damages in the multilayer structure. The tests demonstrate that evaluating signals in both time and frequency domains presents different characteristics for each condition analyzed. The supervised support vector machine classifier was used to automate the classification of these damages, achieving an accuracy of 93%. The combination of distributed acoustic sensing with this learning algorithm creates the condition for developing a smart tool for monitoring multilayer structures.

Interval-locked dual-frequency φ-OFDR with an enhanced strain dynamic range and a long-term stability

We report on an interval-locked dual-frequency phase-sensitive optical frequency-domain reflectometry relying on a common-reference optical phase-locked loop. With a shared unbalanced interferometry, this design allows for synchronizing the frequency drift of two lasers, leading to a steadily stabilized dual frequency with an arbitrary interval. Equivalently to a longer synthetic wavelength, their phase difference is utilized to demodulate the ambient changes of interest with an enhanced dynamic range and long-term stability. With a stabilized interval of 1 THz, it allows for an enhancement in a strain measurement range of up to 193-fold in theory. Demonstration in terms of distributed strain sensing covering a distance of 500 m with a 10 cm spatial resolution has been verified, showing a significant extension in the achievable strain dynamic range with a preserved sensitivity over 1 h.

Broad-bandwidth and accurate optical vibration sensing by using FDM Φ-OTDR with linear regression analysis of multi-frequency phase responses

We demonstrate Φ-OTDR distributed acoustic sensing (DAS) that realizes both a broad bandwidth for the vibration frequency and wide dynamic range for the vibration amplitude based on optical frequency-division-multiplexing (FDM). We enhance the sampling rate of DAS by using FDM while suppressing waveform distortion in time domain (spurious components in spectral domain) caused by sensor nonlinearity inherent in Φ-OTDR, thus improving dynamic range, with linear regression analysis of multi-frequency phase responses. The proposed method compares the phase offsets and responses of each frequency to those of a common reference frequency and uses the information to calibrate each of the different responses. We clarify the physical origin of the problem and the validity of the proposed method in both simulations and experiments. Experimental results show an improvement in dynamic range by above 8 dB on average for vibration waveforms with nɛ-order amplitudes and kHz-order frequencies over 10-km single-mode fiber.

Coded phase-sensitive OTDR with delayed polarization multiplexing for a WFBG array

Polarization fading degrades the performance of phase-sensitive optical time-domain reflectometry (φ-OTDR) seriously and has to be suppressed. A novel scheme is proposed in this paper to combat polarization fading, which features a quite simple transceiver structure by exploiting both polarization diversity through delayed polarization multiplexing and the aperiodic autocorrelation of pseudorandom binary sequence. The components of Jones matrix of a sensing fiber are then shown at those four peaks of aperiodic autocorrelation and can be obtained directly without complicated computation to give a polarization independent phase variation due to vibration. Moreover, the scheme does not require stringent match between the delayed time and the spacing between sensors. The proposed scheme is demonstrated through experiment on a weak fiber Bragg grating (WFBG) array, which shows a high crosstalk rejection ratio among sensors of more than 50 dB and a high dynamic range of more than 30 dB.

Long Range Raman-Amplified Distributed Acoustic Sensor Based on Spontaneous Brillouin Scattering for Large Strain Sensing

A Brillouin distributed acoustic sensor (DAS) based on optical time-domain refractometry exhibiting a maximum detectible strain of 8.7 mε and a low signal fading is developed. Strain waves with frequencies of up to 120 Hz are measured with an accuracy of 12 με at a sampling rate of 1.2 kHz and a spatial resolution of 4 m over a sensing range of 8.5 km. The sensing range is further extended by using a modified inline Raman amplifier configuration. Using 80 ns Raman pump pulses, the signal-to-noise ratio is improved by 3.5 dB, while the accuracy of the measurement is enhanced by a factor of 2.5 to 62 με at the far-end of a 20 km fiber.

Correlation-based multiplexing of spectral channels and fiber-optic sensors using unmodulated continuous-wave distributed feedback diode lasers

In this Letter, we demonstrate a method for the multiplexing of spectral channels and fiber-optic sensors. The method makes use of the correlation-based demultiplexing technique. In our approach, each light source has an inherent phase noise which we propose to use as a "fingerprint" for correlation-based recognition and demultiplexing of spectral channels as well as signals from different sensors in each spectral channel. The proposed method requires for its implementation an extremely simple and robust scheme and can be attractive for budget-sensitive applications. As active elements, it uses only two standard free-running CW distributed feedback (DFB) diode lasers. No modulators or optical filters are required for operation. Results of proof-of-concept experiments are presented for the interrogation of several ultra-weak fiber Bragg gratings (FBGs) with reflectivity of 0.05% in a 4-km-long fiber for temperature measurements with the resolution of 0.1°C.

Polarization-fading suppression of Φ-OTDR with Rayleigh gray-scale pattern aggregation method

Polarization diversity reception is a widely used method to suppress polarization fading of Φ-optical time domain reflectometer (Φ-OTDR). However, the traditional polarization diversity reception method with multichannel point-to-point signal aggregation causes the discontinuity of the aggregated signal, which affects the subsequent application. In order to decrease the discontinuity of the aggregation signal, an area-to-area signal aggregation method with a Rayleigh gray-scale pattern is proposed. First, the Rayleigh pattern is simulated to verify the large-scale continuity of fading areas. Then, an image-processing method by Rayleigh gray-scale pattern is carried out for the automatic judgment of large-scale fading areas, which includes the operations of normalization, graying, comparison of amplitude threshold using the Otsu algorithm, erosion, and dilation. The experimental results indicate that multichannel area-to-area signal aggregation can reduce the average fading ratio from 6.58% to 2.10% and also realize the signal demodulation with an SNR of 15.83 dB of positioning curve. This aggregation method provides an effective scheme for the polarization-fading suppression for Φ-OTDR.

Frequency multiplexed coherent φ-OTDR

We present a comprehensive analysis of a frequency multiplexed phase-measuring φ-OTDR sensor platform. The system uses a train of frequency-shifted pulses to increase the average power injected into the fiber and provide a diversity of uncorrelated Rayleigh backscattering measurements. Through a combination of simulations, numerical analysis, and experimental measurements, we show that this approach not only enables lower noise and mitigates interference fading, but also improves the sensor linearity. We investigate the sensor dependence on the length of the pulse train and characterize the sensor performance as a function of range, demonstrating operation from 1 to 50 km. Despite its relative simplicity, this platform enables state-of-the-art performance, including low crosstalk, high linearity, and a minimum detectable strain of only 0.6 p[Formula: see text] in a 10 km fiber with 10 m spatial resolution and a bandwidth of 5 kHz.

High dynamic range distributed acoustic sensing based on dual-wavelength fiber Bragg grating pairs

A high dynamic range distributed acoustic sensing (DAS) system is demonstrated based on a dual-wavelength (2λ) interferometric acoustic sensor array, constructed using 2λ fiber Bragg grating pairs (FBGPs). Each FBGP is composed of two weak FBGs presenting -30-dB reflectivity and center wavelengths of 1535 nm and 1565 nm. With the phase demodulation results from the two wavelengths, 2λ linear regression phase unwrapping is implemented, which significantly enhances the dynamic range of the DAS system. By examining the four time-division-multiplexed sensors, an average background noise level of -105.2dBrad/Hz at 1 kHz and a cross talk level lower than -70dB at 1 kHz are achieved.

Hybrid Brillouin/Rayleigh sensor for multiparameter measurements in optical fibers

We present a hybrid Brillouin/Rayleigh sensor for multiparameter sensing in optical fibers. The system makes use of a single laser pulse to excite both Rayleigh and Brillouin backscattering in the same optical fiber. In the detection path, the backscattered signals are separated based on their different wavelengths. The system is capable of determining simultaneously the Brillouin frequency shift (BFS) of the fiber, as well as the frequency contents of any vibration acting on the same fiber as recovered by phase sensitive OTDR (ϕ-OTDR) measurements. The reported experiments show the possibility to perform simultaneous temperature and vibration measurements, as well as to perform dynamic strain measurements combining the information provided by slope-assisted Brillouin scattering measurements, with those provided by amplitude-based ϕ-OTDR measurements.