Coherent Optical Time Domain Reflectometry Research Articles

Chirped-pulse coherent-OTDR with predistortion

In this paper, a novel method for generating high-quality chirped pulses with IQ modulator is studied theoretically and experimentally, which is a crucial building block for high-performance coherent optical time-domain reflectometry (COTDR). In order to compensate the nonlinearity of the modulator transfer function, we present a predistortion technique for chirped-pulse coherent optical time-domain reflectometry (CP-COTDR), the arcsin predistortion method and the single sideband with a suppressed carrier analog modulation used to generate the high quality chirped optical pulse. The high order sidebands, due to the large amplitude of the modulation signal and the nonlinear transfer function of the IQ modulator, can be relieved by the predistortion process, which means the power and the quality of the generated chirped pulse has been improved. In the experiment, this method increases the peak power of the chirped pulse by 4.2 dB compared to the case without predistortion process, as for the CP-COTDR system, this method increases the signal-to-noise ratio of the demodulated phase variation by 6.3 dB.

High-resolution distributed strain sensing system for landslide monitoring

A novel fiber-optics landslide monitoring system is proposed to achieve distributed,high-sensitivity and high-resolution strain measurement. The whole system adopts Rayleigh backscatter-based coherent optical time-domain reflectometry (COTDR) technique to realize long-distance, real-time monitoring with a resolution of 0.1με and a spatial resolution of one meter.

Multipoint Refractometer Based on Combined Correlation and Frequency Multiplexing

We demonstrate a new simple multi-point refractometer using Fresnel reflection at the fiber tip with closely spaced interferometer for intensity reference. A combined technique for simultaneous interrogation and multiplexing of identical sensor heads was developed to obtain multipoint operation with a standard continuous-wave DFB diode laser as light source. The technique is based on the combination of coherent optical frequency division reflectometry and coherent correlation optical time-domain reflectometer. Feasibility of accurate operation for multipoint refractive index measurements is demonstrated in proof-of-concept experiments. The proposed refractometer could be useful for sensing in chemical and biological applications.

Intensity noise limit in a phase-sensitive optical time-domain reflectometer with a semiconductor laser source

In the present paper we perform, for the first time, the analysis of the average intensity noise power level at the output of a coherent phase-sensitive optical time-domain reflectometer (phase-OTDR) with a semiconductor laser source. The origin of the considered intensity noise lies in random phase fluctuations of a semiconductor laser source field. These phase fluctuations are converted to intensity noise in the process of interference of backscattered light. This intensity noise inevitably emerges in every phase-OTDR spatial channel and limits its sensitivity to external phase actions. The analysis of intensity noise in a phase-OTDR was based on the study of a fiber scattered-light interferometer (FSLI) which is treated as the constituent part of OTDR. When considered independently, FSLI has a broad intensity noise spectrum at its output; when FSLI is treated as a part of a phase-OTDR, due to aliasing effect, the wide FSLI noise spectrum is folded within the spectral band, determined by the probe pulse repetition frequency. In the analysis one of the conventional phase-OTDR schemes with rectangular dual-pulse probe signal was considered, the FSLI, which corresponds to this OTDR scheme, has two scattering fiber segments with additional time delay introduced between backscattered fields. The average intensity noise power and resulting noise spectrum at the output of this FSLI are determined by the degree of coherence of the semiconductor laser source, the length of the scattering fiber segments and by the additional time delay between the scattering segments. The average intensity noise characteristics at the output of the corresponding phase-OTDR are determined by the analogous parameters: the source coherence, the lengths of the parts constituting the dual-pulse and the time interval which separates the parts of the dual-pulse. In the paper the expression for the average noise power spectral density (NPSD) at the output of FSLI was theoretically derived and experimentally verified. Based on the found average NPSD of FSLI, a simple relation connecting the phase-OTDR parameters and the limiting level of full average intensity noise power at its output was derived. This relation was verified by experimental measurement of the average noise power at the output of phase-OTDR. The limiting noise level, considered in the paper, determines the fundamental noise floor for the phase-OTDR with given parameters of the source coherence, probe pulse length and time delay between two pulses constituting the dual-pulse.

Coherent optical time domain reflectometry by logarithmic detection and timed random frequency hopping

A scheme using timed random frequency hopping and signal logarithmic mean method is proposed and experimentally demonstrated in a coherent optical time domain reflectometry (OTDR) system to reduce the fading noise of the OTDR trace and simplify the signal processing procedure. The timed random frequency hopping is realized by randomly changing the driving current of the laser at certain time points. By this method, the fading noise of OTDR trace can be reduced to be 1/5 of that without using it. Also, a radio frequency power detector (RFPD), whose output voltage has linear relationship with the input logarithmic RF power, is used to extract the power of the RF signals from the balanced photodetector. Then, a data acquisition card directly captures and adds the digital voltage signals from the RFPD to reduce the fading noise and improve the measurement dynamic range. Compared with synchronous and asynchronous frequency hopping scheme, the proposed method is of high efficiency.

Steady γ-Ray Effects on the Performance of PPP-BOTDA and TW-COTDR Fiber Sensing.

We investigated the evolution of the performances of Pulse Pre Pump-Brillouin Time Domain Analysis (PPP-BOTDA) and Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR) fiber-based temperature and strain sensors when the sensing optical fiber is exposed to two γ-ray irradiation conditions: (i) at room temperature and a dose rate of 370 Gy(SiO2)/h up to a total ionizing dose (TID) of 56 kGy; (ii) at room temperature and a dose rate of 25 kGy(SiO2)/h up to a TID of 10 MGy. Two main different classes of single-mode optical fibers have been tested in situ, radiation-tolerant ones: fluorine-doped or nitrogen-doped core fibers, as well as Telecom-grade germanosilicate ones. Brillouin and Rayleigh Sensitivities of N-Doped fibers were not reported yet, and these characterizations pave the way for a novel and alternative sensing scheme. Moreover, in these harsh conditions, our results showed that the main parameter affecting the sensor sensitivity remains the Radiation Induced Attenuation (RIA) at its operation wavelength of 1550 nm. RIA limits the maximal sensing range but does not influence the measurement uncertainty. F-doped fiber is the most tolerant against RIA with induced losses below 8 dB/km after a 56 kGy accumulated dose whereas the excess losses of other fibers exceed 22 dB/km. Both Rayleigh and Brillouin signatures that are exploited by the PPP-BOTDA and the TW-COTDR remain unchanged (within our experimental uncertainties). The strain and temperature coefficients of the various fibers under test are not modified by radiations, at these dose/dose rate levels. Consequently, this enables the design of a robust strain and temperature sensing architecture for the monitoring of radioactive waste disposals.

Relative change measurement of physical quantities using dual-wavelength coherent OTDR.

We propose the use of alternating pulse wavelengths in a direct-detection coherent optical time domain reflectometry (C-OTDR) setup not only to measure strain and temperature changes but also to determine the correct algebraic sign of the change. The sign information is essential for the intended use in distributed mode shape analysis of civil engineering structures. Correlating relative backscatter signal shifts in the temporal/signal domain allows for measuring with correct magnitude and sign. This novel approach is simulated, experimentally implemented and demonstrated for temperature change measurement at a spatial resolution of 1 m.

Yb,Er:glass Microlaser at 1.5 µm for optical fiber sensing: development, characterization and noise reduction

A fiber-pumped single-frequency microchip erbium laser was developed and characterized with the aim of using it in coherent Optical Time Domain Reflectometry (OTDR) measurements and sensing. The laser is pumped by a fiber-coupled 976 nm laser diode and provides 8 mW TEM00 single frequency output power at 1.54 µm wavelength, suitable for efficient coupling to optical fibers. The amplitude and phase noise of this 200 THz oscillator were experimentally investigated and a Relative Intensity Noise (RIN) control loop was developed providing 27 dB RIN peak reduction at the relaxation oscillation frequency of 800 kHz

Random Quadrature Demodulation for Direct Detection Single-Pulse Rayleigh C-OTDR

We present a demodulation method that partially compensates for the nonlinear and random transfer function of fiber-optic direct detection coherent Rayleigh optical time-domain reflectometry (C-OTDR). Specifically, the proposed method is shown to improve the detection of small amplitude, high-frequency dynamic optical fiber strain as it occurs in acoustic emission sensing. The method is applicable provided that the dynamic fiber strain to be sensed is known to affect a longer section of sensing fiber in a spatially homogeneous way. It is shown that this knowledge can be used to extract more quantitative information from the measured C-OTDR signal by averaging signal components from the affected fiber section in a suitable and efficient way. The theoretical basis of the method is developed and supporting experimental results are presented.

Distributed Fiber-Optic Vibration Sensing Based on Phase Extraction From Optical Reflectometry

Distributed fiber-optic vibration sensing based on phase extraction from optical reflectometry is discussed in this paper. The principles on the phase extraction from coherent phase-sensitive optical time-domain reflectometry and time-gated digital optical frequency-domain reflectometry are introduced, and the experimental results of distributed vibration measurements based on the two techniques are demonstrated. To have a better understanding on the performance of the techniques, the phase extraction noise in the phase extraction process is analyzed. To solve the problem of existing measurement dead zones, the influence of Rayleigh fading phenomenon is investigated, and a solution of using statistical analysis to overcome this drawback is introduced and experimentally verified. With these improvements, long-range nondead-zone distributed vibration sensing techniques with linear response to external vibration signals based on phase extraction from optical reflectometry are realized.

Fading reduction in a phase optical time-domain reflectometer with multimode sensitive fiber

In the present paper we propose a novel type of a coherent phase-sensitive optical time-domain reflectometer (OTDR) that utilizes a multimode optical fiber as a sensitive element and is capable of considerable reduction of signal fading. Elimination of OTDR signal fading consequently removes randomly occurring insensitivity of the fiber regions to an external phase action. The backscattered light field at the input of OTDR sensitive multimode optical fiber is represented by a speckle-like pattern, due to a so called modal noise phenomenon. This speckle pattern randomly changes when an optical probe pulse propagates in the fiber line. The backscattered field intensity in every single speckle changes in time statistically independently from the intensity change in every other speckle remote enough from the first one. Thus, on the output of a multimode sensitive fiber, there exist several statistically independent reflectograms, and every single reflectogram contains the same information about external action. The joint independent analysis of these reflectograms can result in reduced or complete fading elimination.

Comparison of various approaches to multi-channel Information fusion in c-otdr systems for remote monitoring of extended objects

The paper presents new results concerning selection of optimal information fusion formula for ensembles of COTDR channels. Here C-OTDR is a coherent optical time domain reflectometer. Each of these channels provides data for appropriate automatic classifier which is designed to classify the elastic vibration sources in the multiclass case. Those classifiers form a so-called classifiers ensemble. Ensembles of Lipschitz Classifiers were considered. In this case the goal of information fusion is to create an integral classificator designed for effective classification of seismoacoustic target events. The Matching Pursuit Optimization Ensemble Classifiers (MPOEC), the Linear Programming Boosting (LP-Boost) (LP-β and LP-B variants), the Multiple Kernel Learning (MKL), and Weighing of Inversely as Lipschitz Constants (WILC) approaches were compared. The WILC is a brand new approach to optimal fusion of Lipschitz Classifiers Ensembles. The basics of these methods have been briefly described along with intrinsic features. All of those methods are based on reducing the task of choosing convex hull parameters to a solution of an optimization problem. All of the mentioned approaches can be successfully used for using in the C-OTDR system data processing. Results of practical usage are presented.

Mathematical analysis of marine pipeline leakage monitoring system based on coherent OTDR with improved sensor length and sampling frequency

A system based on coherent optical time domain reflectometry (OTDR) for subsea pipeline monitoring is described. The fiber sensor length is increased using erbium-doped fiber amplifier (EDFA) cascades. The sampling frequency is increased by dividing the fiber sensor into separate sensitive areas, with parallel scanning. The calculation of the erbium amplifier cascade spontaneous noise influence on the signal-to-noise ratio (SNR) is carried out.

Simulation of Noise Within BOTDA and Cotdr Systems to Study the Impact on Dynamic Sensing

Abstract Real-time structural health monitoring requires dynamic sensing of distributed strain and temperature. Brillouin Optical Time Domain Analysis (BOTDA) and Rayleigh Coherent Optical Time Domain Reflectometry (COTDR) are promising candidates to achieve dynamic sensing. A noise model with specific parametric simulation of independent laser and detector noise sources has been developed. Although ensemble averaging significantly enhances the signal-to-noise ratio (SNR) in both systems, its time-consuming accumulation procedure prevents dynamic sensing. The sequence of averaging in the signal processing workflow varies the SNR for both systems. The system components should be optimized to reduce averaging times and achieve the required system specifications, including dynamic sensing.

Pulse Waveform Manipulation in FDM-OTDR for Suppressing Inter-Channel Crosstalk

The inter-channel crosstalk (XT) in frequency division multiplexing coherent optical time domain reflectometry (FDM-OTDR) is described theoretically. The inter-channel XT degrades the spatial resolution of the OTDR trace at the position, where the reflectivity changes precipitously, and is dependent on the frequency channel spacing, pulse waveform, and weighting function of the signal processing. We show that the deterioration of the OTDR trace can be fully simulated by using impulse response analysis, and that apodization of the probe pulse shape and weighting function effectively suppress the inter-channel XT. Moreover, we propose a novel nonlinear self-phase modulation (SPM) suppression scheme to adapt apodized probe pulse in our experiment. The experimental results for the impulse response, OTDR trace, and SPM suppression are discussed.

Fast digital envelope detector based on generalized harmonic wavelet transform for BOTDR performance improvement

We propose a fast digital envelope detector (DED) based on the generalized harmonic wavelet transform to improve the performance of coherent heterodyne Brillouin optical time domain reflectometry. The proposed DED can obtain undistorted envelopes due to the zero phase-shift ideal bandpass filter (BPF) characteristics of the generalized harmonic wavelet (GHW). Its envelope average ability benefits from the passband designing flexibility of the GHW, and its demodulation speed can be accelerated by using a fast algorithm that only analyses signals of interest within the passband of the GHW with reduced computational complexity. The feasibility and advantage of the proposed DED are verified by simulations and experiments. With an optimized bandwidth, Brillouin frequency shift accuracy improvements of 19.4% and 11.14%, as well as envelope demodulation speed increases of 39.1% and 24.9%, are experimentally attained by the proposed DED over Hilbert transform (HT) and Morlet wavelet transform (MWT) based DEDs, respectively. Spatial resolution by the proposed DED is undegraded, which is identical to the undegraded value by HT-DED with an allpass filter characteristic and better than the degraded value by MWT-DED with a Gaussian BPF characteristic.

A BOTDA with break interrogation function over 72 km sensing length

A BOTDA with the capacity of break interrogation is proposed and demonstrated experimentally. In our configuration, coherent detection and double sideband probe method are employed to enhance the signal-to-noise ratio (SNR) and to effectively reduce nonlocal effects, respectively. Without amplification, a 72 km sensing range with 5-meter resolution and an estimated temperature uncertainty of 1.8 °C are obtained. Benefiting from the flexible optical configuration, this sensor system has the capacity of break interrogation as a coherent optical time domain reflectometry (COTDR) if there is a break in the fiber under test (FUT). The sensor achieves a dynamic range of 36 dB with a 100 m spatial resolution, which offers an excellent solution for the requisite of two-end-access in BOTDA, and significantly enhances the robustness of the sensing system.

Time Division Multiplexing Optical Time Domain Reflectometry Based on Dual Frequency Probe

A new frequency coding method is proposed and experimentally demonstrated that employs a phase modulator driven by sequential radio frequency and a synchronously controlled acoustooptic modulator to generate time division multiplexing probe pulses for coherent optical time domain reflectometry (C-OTDR). Each probe pulse contains two probe frequencies that are symmetrical regarding the frequency of local oscillator (LO). Heterodyne between the backscattered Rayleigh light of the dual frequency probe pulse and LO generates the same intermediate frequency. Experimental results show that the new C-OTDR, using ten frequency-coded probe pulses, brings an 8.0-dB dynamic range enhancement, compared with conventional C-OTDR.

Coherent optical time domain reflectometry using three frequency multiplexing probe

This paper proposes a multi-frequency probe based coherent optical time domain reflectometry (MFP C-OTDR). In the new C-OTDR scheme, single frequency light is converted to multi-frequency light and three multiplexing frequencies with the same power level are adopted as the probe, while local oscillator (LO) remains original single frequency. Coherent detection between the backscattered Rayleigh light of the multi-frequency probe and LO generates many intermediate frequencies (IFs), and three IFs corresponding to the three dominant frequencies of the probe are simultaneously measured and processed. Experimental results indicate that the MFP C-OTDR can triple the measurement number compared with conventional single frequency probe based C-OTDR, which brings much quick fading noise reduction capability and a 1.2dB single way dynamic range (SWDR) enhancement.

High-Sensitivity Coherent Optical Time Domain Reflectometry Employing Frequency-Division Multiplexing

This paper proposes a novel parallel detection scheme for enhancing the sensitivity of coherent optical time domain reflectometry (C-OTDR). The proposed scheme is based on frequency-division multiplexing using a phase modulator and software processing. The performance advantages of the proposed method are clarified experimentally by comparison with a conventional C-OTDR. The detection sensitivity performance with the proposed method, operating at 40 frequency-division multiplexing, was improved by about 8 dB compared with the conventional method. We also clarified that the 3 dB resolution and Rayleigh dead zone of conventional C-OTDR and the proposed method were almost the same at a spatial resolution of 1 km.