Coherent Optical Time Domain Reflectometry Research Articles

Phase Demodulation under Ultra-low sampling rate in heterodyne coherent [formula omitted]-OTDR

Heterodyne coherent phase-sensitive optical time-domain reflectometry(Φ-OTDR) requires a higher data sampling rate to demodulate phase in the “distance” direction effectively. Aiming at the problem of large amount of demodulated data, we propose a phase demodulation method that can effectively recover disturbance information from ultra-low sampling data. This method demodulates the phase of the two-dimensional reconstructed signal in the “time” direction. Thus, the influence of spectrum aliasing in the “distance” direction caused by undersampling is avoided. The undersampling rate is not limited by the spectrum aliasing effect of the detected signal when using this method to demodulate phase. Therefore, accurate phase information can be retrieved under ultra-low sampling rates, significantly reducing the data for phase demodulation in heterodyne coherent Φ-OTDR. In the experiment, three sampling rates (100 MSa/s, 10 MSa/s, 1 MSa/s) within the restricted area of the traditional undersampling demodulation method were selected to acquire data, and the proposed method can also accurately demodulate phase information.

Shape Sensing of Resin Pipe with a Helical-winding Optical Fiber in Bending Test

In this study, we developed a shape-sensing technique for a pipe. An optical fiber was helically bonded on the outer surface of a resin pipe whose length, outer diameter and inner diameter are 3200 mm, 34 mm, and 26.6 mm, respectively. Firstly, four grooves were made on the surface along the pipe in a helical pattern with the same pitch of 360 mm. At the one pipe end, adjacent grooves were offset by 90 degrees and the offset was maintained along the pipe. The single-mode fiber was put into a groove and fixed by epoxy and then it was embedded into the adjacent groove. Consequently, the optical fiber was going back and forth twice along the pipe. At the pipe ends, there were stress-free fibers between the grooves. Shape sensing for the pipe is conducted as follows. 1. Strain distributions along the grooves are measured by a distributed fiber-optic strain sensing system. 2. The bending strain is extracted from the measured strain to calculate the curvature and bending angle. 3. The shape of the pipe is reconstructed based on the curvature and bending angle using the Frenet-Serret formulas. Three-point bending tests were conducted. In the tests, the pipe ends were clamped to be the fixed ends and a point load was applied to the pipe center. Strain distributions were measured by tunable wavelength coherent optical time domain reflectometry (TW-COTDR) or optical frequency domain reflectometry (OFDR). Then the pipe shape was reconstructed as described above. We compared the reconstructed shape with the deformation calculated by finite element analysis and the agreement was excellent. If we insert the developed pipe into a bedrock and reconstruct the pipe shape continuously, it is expected that we can detect cracks or damage in the bedrock. This approach is expected to make rock excavation constructions safer.

Water Pipeline Leakage Detection Based on Coherent φ-OTDR and Deep Learning Technology

Leakage in water supply pipelines remains a significant challenge. It leads to resource and economic waste. Researchers have developed several leak detection methods, including the use of embedded sensors and pressure prediction. The former approach involves pre-installing detectors inside pipelines to detect leaks. This method allows for the precise localization of leak points. The stability is compromised because of the wireless signal strength. The latter approach, which relies on pressure measurements to predict leak events, does not achieve precise leak point localization. To address these challenges, in this paper, a coherent optical time-domain reflectometry (φ-OTDR) system is employed to capture vibration signal phase information. Subsequently, two pre-trained neural network models based on CNN and Resnet18 are responsible for processing this information to accurately identify vibration events. In an experimental setup simulating water pipelines, phase information from both leaking and non-leaking pipe segments is collected. Using this dataset, classical CNN and ResNet18 models are trained, achieving accuracy rates of 99.7% and 99.5%, respectively. The multi-leakage point experiment results indicate that the Resnet18 model has better generalization compared to the CNN model. The proposed solution enables long-distance water-pipeline precise leak point localization and accurate vibration event identification.

A high-fidelity numerical model of coherent Φ-OTDR

The existing coherent phase-sensitive optical time-domain reflectometry (φ-OTDR) model does not systematically consider the impact of various disturbances and noises. More importantly, it cannot effectively simulate low-frequency vibrations. To address this issue, a high-fidelity model is proposed. Unlike the existing models, the proposed model comprehensively considers interference fading, polarization fading, the frequency drift and phase noise of the laser, and the balanced photodetector (BPD) noise, which can effectively simulate the impact of various disturbances and noises on the Rayleigh backscattering (RBS) signals during actual measurement. Quantitative analysis and comparison between the demodulation results of the measured and the numerically generated RBS signals based on the existing and proposed models with 1 kHz, 5 Hz and 1 Hz vibrations show that the proposed model is closer to the experimental system in terms of vibration demodulation results than the existing model.

Improved Rotational Vector Summation Algorithm for Fade Suppression in Distributed Acoustic Sensing

Signal fading severely affects the performance of optical fiber distributed acoustic sensing (DAS) systems based on coherent optical time domain reflectometry (COTDR), especially in terms of differentiated responses between channels. To eliminate fading, a polarization diversity COTDR system is established, which generates two pulse feedbacks with different fading components for P and S polarization. The rayleigh backscattered (RBS) signals from the two separate probing pulses exhibit different fading positions under the influence of external interference but have the same phase change rate. Through an improved rotational vector summation (IRVS) algorithm, the complex RBS signals of the two polarization states are effectively synthesized, simultaneously suppressing fading phenomena caused by orthogonal polarization. Experimental results demonstrate that the demodulated signal-to-noise ratio (SNR) can reach 65 dB@200 Hz. Moreover, by using 20% of the mean amplitude as a threshold, the probability of fade events decreases from 4.9% to 0.12%. The proposed fade mitigation distributed sensing system exhibits excellent performance and plays a crucial role in practical DAS systems.

Noise analysis in direct detection and coherent detection phase-sensitive optical time-domain reflectometry systems.

This study compares noise and signal-to-noise ratio (SNR) in direct detection and coherent detection fiber-based distributed acoustic sensing (DAS) systems. Both detection schemes employ the dynamic analysis of Rayleigh-backscattered light in phase-sensitive optical time-domain reflectometry (ΦOTDR) systems. Through theoretical and experimental analysis, it is determined that for photodetection filters with a sufficiently narrow bandwidth, the SNR performance of both detection schemes is comparable. However, for filters with poor selectivity, coherent detection was found to exhibit superior performance. These findings provide crucial guidelines for the design of high-performance time-domain DAS systems.

Frequency Response Enhanced Quantitative Vibration Detection Using Fading-Free Coherent $\varphi$-OTDR With Randomized Sampling

A frequency response enhanced quantitative vibration detection scheme for interference fading suppressed coherent phase-sensitive optical time domain reflectometry (ϕ-OTDR) is proposed and demonstrated. By utilizing randomized sampling, the proposed scheme not only breaks the inherent limitation between the sensing range and the maximum detectable frequency in conventional ϕ-OTDR systems, but also allows for quantitative measurement of the vibration amplitude by tracking the peak intensity of the discrete Fourier transformation spectrum of the phase signals. The operation principle and the measurement accuracy are mathematically analyzed through the frequency spectrum estimator unbiasedness and the estimation standard deviation, which provides a comprehensive guidance for parameter configuration in practical applications. For proof of concept, quantitative detection of vibration signals up to 384 kHz is realized over a 5.4 km fiber, while the conventional solutions only reach 18.5 kHz as restricted by the Nyquist sampling theorem. The proposed sensing scheme paves the way to enable ultra-high frequency quantitative vibration detection using ϕ-OTDR.

Compensation of Phase Noise Impairments in Distributed Acoustic Sensors Based on Optical Pulse Compression Time-Domain Reflectometry

We introduce a method to compensate for the deleterious effects of the phase noise of the laser source on long-range distributed acoustic sensors (DAS) that implement optical pulse compression (OPC). Pulse compression can be used in coherent optical time-domain reflectometry (COTDR) sensors to extend the measurement range without compromising spatial resolution. In fact, OPC-COTDR sensors have demonstrated the longest measurement range to date in passive sensing links that do not require distributed amplification to compensate fiber attenuation. However, it has been found that the limited coherence of the laser source has a degrading effect on the actual performance enhancement that pulse compression can bring because it constrains the maximum duration of the compression waveforms that can be used and makes the use of lasers with extremely low phase noise necessary. We introduce a technique to compensate for the effects of phase noise on OPC-COTDR sensors so that they can demonstrate their full potential for long-range measurements using lasers with less stringent phase noise requirements. The method is based on sampling the phase noise of the laser with an auxiliary interferometer and using this information in a simple signal processing technique to mitigate its deleterious effect on the signal measured. We test our method in an OPC-COTDR sensor that uses 500- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> s linear frequency modulated pulses to demonstrate 100-km range measurements with 200 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\rm{p}\epsilon /\sqrt{\rm{Hz}}$</tex-math></inline-formula> of strain sensitivity at 2-m initial spatial resolution that becomes 10-m after applying the gauge length. To our knowledge, this is the longest compression waveform demonstrated to date in an OPC-COTDR sensor. Its use provides an extra 20-km range compared to previous demonstrations using laser sources of comparable linewidth. Furthermore, comparable performance is also demonstrated when using a laser source with an order of magnitude larger linewidth.

Coherent OTDR with large dynamic range based on double-sideband linear frequency modulation pulse.

Rayleigh scattering-based distributed optical fiber sensors with long sensing distance and large dynamic range are highly desired for application scenarios such as vehicle tracking, structure health monitoring, and geological survey. To enlarge the dynamic range, we propose a coherent optical time domain reflectometry (COTDR) based on double-sideband linear frequency modulation (LFM) pulse. By utilizing I/Q demodulation, both the positive and negative frequency band of the Rayleigh backscattering (RBS) signal can be properly demodulated. Consequently, the dynamic range is doubled without increasing the bandwidth of signal generator, photodetector (PD), and oscilloscope. In the experiment, the chirped pulse with 10 μs pulse width and 498 MHz frequency sweeping range is launched into the sensing fiber. Single-shot strain measurement is achieved over 5 km single-mode fiber with a spatial resolution of 2.5 m and a strain sensitivity of 7.5 pε/H z. A vibration signal with 3.09 με peak-to-peak amplitude (corresponding to 461 MHz frequency shift) is successfully measured with the double-sideband spectrum, which cannot be properly recovered with the single-sideband spectrum.

190km Φ-OTDR with bidirectional Raman and relay erbium-doped fiber hybrid amplification

In this paper, a long-distance coherent detection phase-sensitive optical time-domain reflectometer (Φ-OTDR) scheme with bidirectional Raman and relay erbium-doped fiber hybrid amplification is proposed. The bidirectional Raman pump light acting on the erbium-doped fiber (EDF) can realize the passive amplification of pulsed signal light and Rayleigh backward scattering (RBS) light at the same time. The scheme utilizes bidirectional 1455 nm pumps as the pump source of relay amplification to achieve a longer sensing distance. Experimental results demonstrate that, an ultra-long Φ-OTDR system with 190 km sensing distance and 22 m spatial resolution is realized. Multi-point vibration signals are located and restored at 69.67 km, 120.13 km and 190.79 km, respectively. The average signal-to-noise ratio (SNR) of positioning curve is 10.32 dB, the average SNR of demodulated phase signal is 26.02 dB, and the frequency response range is from 5 Hz to 150 Hz. Furthermore, the sensing system has good linear characteristics of phase demodulation with a correlation coefficient of 0.997. The actual disturbance events are also detected at the far-end of sensing fiber. To the best of our knowledge, this system achieves the longest-distance Φ-OTDR without relay power supply. This passive hybrid amplification method can avoid the intermediate power supply along sensing fiber, which provides a reference for long-distance vibration monitoring of Φ-OTDR system.

Distributed optical fiber sensing in coherent Φ-OTDR with a broadband chirped-pulse conversion algorithm.

We propose a broadband chirped-pulse conversion algorithm (BCPCA) to convert a finite-step scanning probe pulse into an equivalent broadband chirped probe pulse by convolving a chirp factor on the received signal in coherent phase-sensitive optical time domain reflectometry (Φ-OTDR). Combined with Rayleigh interference pattern (RIP) demodulation in chirped-pulse Φ-OTDR (CP-ΦOTDR), environmental perturbations, such as strain and temperature along the sensing fiber, can be quantitatively measured. The equivalent broadband chirped pulse is generated by digital processing, and its bandwidth can be increased by changing the composition of the scanning pulse. Thus, the measurable perturbation range of the system can be expanded. As a proof-of-concept experiment, a high-performance distributed strain measurement was realized on a 10 km fiber, the frequency response was 5 kHz, which is only limited by the fiber length, and the strain resolution was 8.04 p ε/H z. The proposed method of generating equivalent broadband chirped pulse through the digital domain can be used as a supplement to CP-ΦOTDR.

Comparison of Raman, Brillouin, and Rayleigh Distributed Temperature Measurements in High-Rate Wells

With the maturity of and demand for fiber-optic sensing technology growing steadily over the last few years across multiple basins, operators are seeking fiber-optic sensing solutions that address the technology challenges associated with the life-of-field monitoring of subsea developments. Single-ended distributed temperature sensing (DTS) measurements have been acquired for decades now, typically using Raman optical time-domain reflectometry (OTDR) on multimode fiber. However, for topside interrogation of subsea completions, Raman DTS performs poorly. This is due to the available optical power budget and the potential wavelength dependency of optical attenuation across multiple connectors and splices comprising the optical subsea infrastructure. Any wavelength-dependent attenuation as the signals pass through connectors, splices, and optical feedthrough systems will generate step changes in the measured Raman DTS temperature profile. Brillouin OTDR can provide a DTS alternative that overcomes these challenges and operates on single-mode fiber. Brillouin OTDR operates with a large dynamic range to measure a wavelength (frequency) shift of the Stokes/anti-Stokes components that is proportional to both strain and temperature. Since downhole cables are manufactured with optical fibers suspended in a gel and with appropriate extra fiber length (EFL), any fiber strain relaxes, and the Brillouin wavelength shift is an absolute temperature measurement. An additional alternative is also explored here. We typically associate coherent Rayleigh OTDR with distributed acoustic sensing (DAS) on single-mode fibers, but low frequencies also contain a relative temperature dependence. The low-pass filtering of DAS data can then be used as a form of Rayleigh DTS with appropriate data processing. In this paper, we report on a comparison of Raman, Brillouin, and Rayleigh DTS simultaneously acquired in the same high-rate producer and injector wells. We validate that, with appropriate cable design, Brillouin DTS can be simultaneously operated on the same single-mode fiber with DAS and can deliver absolute temperature measurements suitable for production analysis. We also use a laboratory experiment to show that Rayleigh DTS provides an accurate measure of temperature changes comparable in precision to a traditional thermocouple. We conclude with a discussion about the implementation of this DAS-DTS solution for sensing subsea completions.

Random coding method for coherent detection φ-OTDR without optical amplifier

In this article, a coherent detection phase-sensitive optical time-domain reflectometry (φ-OTDR) without optical amplifier using random coding method is proposed. A series of pulses modulated by random codes are injected into the optical fiber to enhance the signal-to-noise ratio (SNR) and extend the sensing distance. The code only needs one sequence for decoding, which maintains the original sensing bandwidth. Simultaneously, the coding probe pulse does not pass through any optical amplifier to avoid the transient effect. Experimental results show that the SNR of the demodulated signal is improved 14.19 dB by using 128-bit random coding pulse. In the traditional single pulse scheme with erbium doped fiber amplifier (EDFA), the intensity SNR at 25.521 km is only 5.39 dB with a 40 ns pulse width, and the vibration signal cannot be effectively demodulated. In the proposed system with the same pulse width, the external disturbance can be located successfully at 42.338 km, and the SNR of the demodulated signal is 21.5 dB.

Study on the Efficiency of Temperature/Strain Measurement for Ultra-Long-Distance Optical Fiber Composite Overhead Power Transmission Lines

The coherent optical time domain reflectometer (COTDR) is a very important instrument for distributed temperature/strain measurement along the sensing fiber with a large dynamic range and high accuracy. The length of the sensing fiber and the temperature/strain measurement range limit the system performance, especially the measurement efficiency. So, a COTDR system is constructed, and the characteristics of the obtained coherent Rayleigh noise (CRN) are analyzed. Then, in consideration of the temperature/strain measurement range, accuracy, and time efficiency, the temperature/strain demodulation algorithm in noise conditions is studied. With different noise coefficients, the array length with 11, 21, 31, 51, and 101 independent frequency sweeping points are adopted to calculate the cross-correlation coefficients along a standard reference array with 301 independent frequency sweeping points. The results demonstrate that the array length has little influence on the signal processing time, but it can decide the measurement accuracy. To balance the system measurement efficiency and accuracy, it is inferred that, for a sensing fiber with a length of 100 km or more, the optimal independent frequency sweeping points are 101 and the trace average number is 1000.

Advantages of using few-mode and multimode fibers in Phase Sensitive OTDR measurement system due to the modulation instability deferment

We discuss the possible advantages of using few-mode and multimode fibers in coherent optical time-domain reflectometry sensing systems. We show that the application of few-mode and multimode fibers decreases nonlinear effects and impedes modulation instability. In comparison to standard single-mode optical fiber, few-mode and multimode fibers may help to increase signal power at the receiver. We also show that few-mode fibers may potentially increase sensing distance.

Diagnostic Tool for Unconventionals Evaluates Near-Wellbore Fractures

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 2021-5408, “New Fracture Diagnostic Tool for Unconventionals: High-Resolution Distributed Strain Sensing by Rayleigh Frequency Shift During Production in Hydraulic Fracture Test 2,” by Gustavo A. Ugueto, SPE, Shell; Magdalena Wojtaszek, SPE, Brunei Shell; and Somnath Mondal, Shell, et al. The paper has not been peer reviewed. _ Fiber-optic (FO) monitoring in unconventional reservoirs has proved to be an invaluable diagnostic tool. Unfortunately, gaining detailed understanding of the near-wellbore fracture geometry or cluster or stage productivity through FO has proved to be more difficult. A new FO diagnostic method, distributed strain sensing based on Rayleigh frequency shift (DSS-RFS), provides insights about the characteristics of the near-wellbore-region (NWR) during production. The authors write that this novel FO technique can significantly improve understanding of near-wellbore hydraulic fracture characteristics and the relationships between stimulation and production from unconventional oil and gas wells. Introduction Successful production profiling through distributed acoustics sensing (DAS) has been reported in gas-producing wells. Regrettably, many wells do not generate strong DAS signals while flowing. Recently developed FO production-profiling data-acquisition protocols and work flows rely on collecting signals during short shut-in reopening cycles lasting only a few minutes. Therefore, a need exists to develop new work flows and algorithms using high-fidelity FO data that can be related to the production characteristics of individual perforation clusters (PCs). DSS-RFS Method DSS-RFS uses Rayleigh backscatter in a nonengineered single-mode fiber to measure strain changes along the fiber. When an optical fiber is manufactured, random inhomogeneities of the glass density are created in the fiber core. The random density heterogeneities manifest as a variation of refractive index along the fiber. For a certain laser frequency, the interferences between the Rayleigh backscatters cause irregular but unique amplitude fluctuations in the coherent optical time-domain reflectometer along the fiber length. For each discrete fiber segment, a unique Rayleigh scattering spectrum is obtained by scanning the fiber with a coherent optical time-domain reflectometer with a range of laser frequencies using a tunable-wavelength laser system. This unique Rayleigh scattering spectrum shifts in frequency if the temperature or strain of the fiber section changes. DSS-RFS measures relative strain changes instead of absolute strain along the fiber. However, DSS-RFS has a higher measuring sensitivity than strain measured using Brillouin optical time-domain reflectometry. DSS-RFS has higher spatial resolution than low-frequency DAS. Finally, DSS-RFS does not have the fiber sensing length restrictions of commercially available fiber-Bragg-grating-based DSS systems. First DSS-RFS During Shut-in and Reopening Test In February 2020, DSS-RFS data was acquired at one of the two FO-instrumented wells in Hydraulic Fracture Test Site 2 (HFTS2). The spatial resolution of the measurement was 20 cm, with a time-sampling interval of 150 seconds. The test began after establishing a baseline measurement during normal flowing conditions. The well was shut in for 4 days and opened again in a series of 2-hour shut-in and 1-hour producing cycles. Finally, the well was reopened and DSS-RFS was acquired for approximately 6 hours. This DSS-RFS data allowed for the definition of intervals where extension has occurred corresponding to active clusters with positive strain-change signals, slightly compressed intervals with negative strain-change signals between the clusters, and intervals with inactive clusters. These signals were interpreted to be caused by fracture-aperture changes that, in turn, were caused by pressure increases during shut-in within the fracture network in the NWR.

The Noise Lower-Bound of Rayleigh-Scattering-Pattern-Based Distributed Acoustic Sensing With Coherent Detection

As a Rayleigh-scattering-pattern-based distributed sensing scheme, recently coherent optical time-domain reflectometry (COTDR) exhibits high performance in distributed acoustic sensing, particularly for those demonstrations with coherent detection. As the noise lower-bound (NLB) determines the minimal detectable strain, the dedicated study is of prime importance. Straightforwardly, key parameters like pulse width, sweeping bandwidth and signal-to-noise ratio would affect NLB, however, so far there has been no explicit expression between key parameters and NLB in the COTDR with coherent detection. This paper derives the Cramér-Rao lower bound (CRLB) of the COTDR with coherent detection, which illustrates the quantitative relationship between the minimal detectable strain and the related parameters. Besides, the method to reach the CRLB is discussed, and the experimental results exhibit good consistent to the theoretical analysis. This work gives a guidance for the sensing system design based on coherent detection COTDR.

Application of Intensity-Based Coherent Optical Time Domain Reflectometry to Bridge Monitoring.

Although distributed fiber sensing techniques have been widely used in structural health monitoring, the measurement results of bridge monitoring, particularly under destructive testing, have rarely been reported. To the best of our knowledge, this paper is the first report of distributed vibration measurement results, which we obtained during a three-day destructive test on an abolished bridge. A coherent optical time domain reflectometry (COTDR) was used to acquire the vibration information while the bridge was being sawed. The obtained signal was analyzed in time and frequency domain. Some characteristics of the sawing-induced vibration were retrieved by the short-time Fourier transform; the vibration exhibited several high frequency components within the measured range up to 20 kHz and all the components appeared in the same time slot. Some unexpected signals were also detected. Thorough analysis showed that they are quite different from the sawing-induced vibration and are believed to originate from internal damage to the bridge (probably the occurrence of cracks).

Envelope Extraction for Vibration Locating in Coherent Φ-OTDR.

In a coherent phase-sensitive optical time-domain reflectometry (Φ-OTDR) sensing system, a frequency shift of hundreds of MHz generated by the pulse modulation of an acoustic optic modulator results in a high central frequency of a beating signal spectrum. In order to reduce the high-performance hardware requirement of signal acquisition, the coherent Φ-OTDR based on envelope extraction is proposed in this paper. Firstly, a theoretical model of a quasi-sinusoidal amplitude-modulated signal is built for the beating signal between local oscillator light and Rayleigh backward scattering light. An envelope detector is then utilized to realize the envelope extraction of beating signals with advantages of a simple structure and quick response. The extracted envelope can be directly used for vibration locating without the conventional orthogonal demodulation. Experiment results present that the sampling rate can be reduced to 10 MHz under the spatial resolution of 10 m and the sensing distance of 31 km. This scheme proves that envelope extraction is a reliable technical route for vibration locating, which can effectively reduce the sampling rate and simplify the data demodulation.

Range and Bandwidth Extension in Direct-Detection C-OTDR Using Optical Repeaters

Sensing range and response bandwidth are the main parameters that define the performance of distributed optical fiber sensors. In this work, a novel scheme based on the use of in-line optical repeaters is proposed to extend the sensing range of distributed acoustic sensors (DAS) based in direct-detection coherent optical time-domain reflectometry (DD C-OTDR). The proposed regenerators comprise three different stages that reamplify the incoming pulse amplitude, synchronize the acquisition using the incoming pulse and also detect the backscattered signal from the next span. With repeaters placed every 40 km along a fiber under test (FUT), we have achieved a total sensing range above 120 km while keeping a pulse frequency of 2.5 kHz (1.25 kHz bandwidth). At the end of each segment of the FUT we experimentally demonstrate the correct measurement of a 15 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{n}\varepsilon $ </tex-math></inline-formula> stimulus, with and an average degradation lower than 3 dB per fiber span. The experimental results show that an extension of the sensing range in DD C-OTDR can be achieved without reducing the frequency response bandwidth or increasing the scheme complexity.