Raman Optical Time Domain Reflectometry Research Articles

Leakage channel outlet detection and diameter estimation for earth-rock dam using ROTDR

In the early stage of leakage in earth-rock dam, detection and quantification of the leakage channel outlet can provide an important reference for leakage hazard assessment and emergency response. In this paper, Raman Optical Time Domain Reflectometry (ROTDR) is used to detect the leakage point and a leakage channel outlet diameter estimation method is proposed. Firstly, the variation law between the sensing fiber of different length and ROTDR measurement results under the condition of different temperature difference is explored. Then, a model is constructed to simulate the leakage of earth-rock dam. The sensing fiber is deployed on the shallow layer of downstream slope and cushion of earth-rock dam to detect leakage. Finally, according to the measurement results, the leakage point is located and the diameter of leakage channel outlet is estimated. The estimated value is very close to the measured value. This study provides a useful reference for promoting the quantitative research of ROTDR in leakage detection.

Submetric Spatial Resolution ROTDR Temperature Sensor Assisted by Wiener Deconvolution.

A submetric spatial resolution Raman optical time-domain reflectometry (ROTDR) temperature sensor assisted by the Wiener deconvolution postprocessing algorithm has been proposed and experimentally demonstrated. Without modifying the typical configuration of the ROTDR sensor and the adopted pump pulse width, the Wiener demodulation algorithm is able to recover temperature perturbations of a smaller spatial scale by deconvoluting the acquired Stokes and anti-Stokes signals. Numerical simulations have been conducted to analyze the spatial resolution achieved by the algorithm. Assisted by the algorithm, a typical ROTDR sensor adopting pump pulses of 20 ns width can realize the distributed temperature sensing with a spatial resolution of 0.5 m and temperature accuracy of 1.99 °C over a 2.1-km sensing fiber.

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.

Slope-assisted Raman distributed optical fiber sensing

Raman distributed optical fiber sensing is required to achieve accurate temperature measurements in a micro-scale area. In this study, we first analyze and demonstrate the pulse transmission feature in the temperature variation area and the superposition characteristics of Raman optical time-domain reflectometry (OTDR) signals by numerical simulation. The equations of superimposed Raman anti-Stokes scattered signals at different stages are presented, providing a theoretical basis for the positioning and physical quantity demodulation of whole optical fiber systems based on the OTDR principle. Moreover, we propose and experimentally demonstrate a slope-assisted sensing principle and scheme in a Raman distributed optical fiber system. To the best our knowledge, this is the first experimental demonstration of Raman distributed optical fiber sensing in a centimeter-level spatial measurement region.

Chaos Raman optical time-domain reflectometry for millimeter-level spatial resolution temperature sensing

The Raman optical time-domain reflectometry (ROTDR) uses the Raman backscattering signal of an optical pulse to obtain environmental information along the sensing fiber, with the pulse width limiting spatial resolution to the meter level in current systems. To solve this problem, we propose a chaos ROTDR system to eliminate the superposition effect of Raman backscattering response caused by pulse width. Here, the chaotic laser is applied as the sensing source instead of a conventional laser. For the random amplitude characteristics of the chaotic laser in time series, the chaotic Raman backscattering signal of individual points along the sensing fiber can be obtained, so that the millimeter-evel spatial resolution is realized. Firstly, the propagation equation of the chaotic Raman backscattering signal is established. Subsequently, the most relevant factors impacting the performance of spatial resolution and the signal-to-noise ratio (SNR) could be addressed based on two methods proposed in this study: one is the time-domain differential reconstruction (TDDR), and the other is the short-scale time-domain correlation compression (SSTDCC). Finally, the spatial resolution of 5 mm and temperature sensitivity of 0.1 K are achieved, which is an unprecedented breakthrough compared to the state-of-the-art technology.

Comparative Experimental Study of a High-Temperature Raman-Based Distributed Optical Fiber Sensor with Different Special Fibers.

An experimental study of a high temperature distributed optical fiber sensor based on Raman Optical-Time-Domain-Reflectometry (ROTDR) (up to 450 °C) and optical fibers with different coatings (polyimide/carbon, copper, aluminum and gold) is presented. Analysis of the distributed temperature sensor (DTS) measurements determined the most appropriate optical fiber to be used in high temperature industrial environment over long periods of time. To demonstrate the feasibility of this DTS for an industrial application, an optical cable was designed with the appropriate optical fiber and it was hermetically sealed to provide the required mechanical resistance and isolate the fiber from environmental degradations. This cable was used to measure temperature up to 360 °C of an industrial furnace during 7 days.

Harnessing oversampling in correlation-coded OTDR.

Coding technique has long been investigated as a solution to improve the signal-to-noise ratio (SNR) without sacrificing spatial resolution in optical time domain reflectometry (OTDR) systems. The past researches have been focusing at the construction of new codes, the combination of coding and other techniques, and the application of coding technique to versatile distributed optical fiber sensing systems such as Raman OTDR and Brillouin optical time domain analyzer, where the results are fruitful. Here, we reveal that oversampling after photodetection opens up a new dimension for coded OTDR other than code length and code type. We demonstrate that the coding gain can be further improved by harnessing the oversampling. Furthermore, the photodetector's bandwidth-limited feature can also be used to select the optimal sampling rate in order to obtain additional SNR enhancement. We believe this principle could be applied to any practical correlation-coded OTDR-based distributed fiber sensing systems with sufficient SNR enhancement. Our findings can serve to update existing instruments based on correlation-coded OTDR in a straightforward manner and at a relatively low cost.

Simultaneous Distributed Vibration and Temperature Sensing Using Multicore Fiber

Distributed optical fiber sensing that enables simultaneous vibration and temperature monitoring is demonstrated in this work, which is achieved by implementing polarization optical time-domain reflectometry (P-OTDR) and Raman optical time-domain reflectometry (ROTDR) respectively through space-division multiplexed (SDM) configuration in different cores of a multicore fiber (MCF). Optimized system setup is obtained in the proposed hybrid system, where only one laser source is used, and the generated pulse is shared by the two reflectometers. Owing to the reason that separate interrogation fiber cores are used by the two reflectometers, the proposed hybrid system allows for simultaneous measurements of spontaneous Raman and Rayleigh scattering signals, thus it no longer suffers from the incompatible pump power levels issue, which results from different pump power demands, and hinders the implementation of the hybrid system in single mode fiber due to the fiber nonlinear effects imposed restriction. In addition, in order to extend the sensing range, we also explore the employment of wavelet transform denoising technique in the improvement of signal-to-noise ratio of ROTDR measurement. Eventually, simultaneous distributed vibration and temperature sensing in 5.76 km sensing range with 3 m spatial resolution and 0.57 °C temperature uncertainty has been demonstrated. The proposed hybrid sensing system has the outstanding advantage of high reliability, since the cross-sensitivity issue is effectively mitigated, which shows great potential for pipeline monitoring in oil and gas industry.

A hybrid distributed optical fibre sensor for acoustic and temperature fields reconstruction

Abstract We have proposed and experimentally demonstrated a distributed optical fibre sensor (DOFS) based on the hybrid of phase-sensitive optical time domain reflectometry ( Φ -OTDR) and Raman optical time domain reflectometry (ROTDR), which could reconstruct the acoustic and temperature fields all along the sensing fibre at the same time. The Rayleigh back scattering (RBS), Raman Stokes (RS) and Raman anti-Stokes (RAS) generated by the probe pulse were separated by a WDM filter. Then the RBS was received by a heterodyne balanced photo detector (BPD) for quantitative vibration measurement and the dynamic acoustic field reconstruction, while the RS and RAS components were received directly by avalanche photodiodes (APDs) for temperature demodulation. The experimental results have shown that the proposed hybrid system could measure the temperature profile along a 12km long standard single-mode fibre with a maximal temperature uncertainty of 0.95 °C; and a linearity response of R 2 = 0 . 9986 , along with a periodical knocking event detected simultaneously. Meanwhile, a 100 Hz sinusoidal vibration at 12 km distance along the same fibre could be clearly identified. Distributed knocking vibration and temperature was achieved at the same time with high accuracy.

Enabling Simultaneous DAS and DTS Through Space-Division Multiplexing Based on Multicore Fiber

We have proposed and demonstrated a hybrid optical-fiber sensor that enables simultaneous distributed acoustic sensing (DAS) and distributed temperature sensing (DTS). The hybrid fiber sensor is realized through space-division multiplexed (SDM) reflectometers in a multicore fiber (MCF), where Raman optical time-domain reflectometry (ROTDR) for DTS is implemented simultaneously with phase-sensitive optical time-domain reflectometry (Φ-OTDR) for DAS through space-division multiplexing. The SDM reflectometers share an identical pulse source, but use separate interrogation fiber cores, allowing simultaneous measurement of ROTDR and Φ-OTDR. The proposed hybrid sensor based on MCF does not suffer from the incompatible pump power levels issue existing in its counterpart based on single mode fiber thanks to the SDM implementation. Thus it effectively eliminates the restriction imposed by fiber nonlinear effects (e.g., modulation instability). Wavelet transform denoising method is employed to reduce the noise of temporal ROTDR traces; as a result, the worst temperature uncertainty is reduced from 4.1 to 0.5 °C over 5.76 km sensing range. The proposed SDM hybrid fiber sensor can realize simultaneous distributed intrusion detection and temperature monitoring. It offers great potential in long-term real-time pipeline monitoring for oil and gas industry.

Simultaneous optical fibre strain and temperature measurements in a hybrid distributed sensor based on Rayleigh and Raman scattering

A new type of sensor for simultaneous measurements of strain and temperature changes in an optical fibre is proposed. Its operation builds on the use of Raman optical time-domain reflectometry and wavelength-tunable quasi-monochromatic Rayleigh reflectometry implemented using a microelectromechanical filter (MEMS). The sensor configuration includes independent Raman and Rayleigh scattering channels. Our experiments have demonstrated that, at a sensing fibre length near 8 km, spatial resolution of 1 – 2 m, and measurement time of 10 min, the noise level (standard deviation) is 1.1 μϵ (μm m−1) for the measured tension change (at small temperature deviations) and 0.04 °C for the measured temperature change, which allows for effective sensing of mechanical and temperature influences with improved accuracy.

Ultrahigh Temperature Raman-Based Distributed Optical Fiber Sensor With Gold-Coated Fiber

An ultrahigh temperature distributed sensor based on a Raman optical-time-domain-reflectometry (ROTDR) and two types of fibers: a standard multimode fiber and multimode gold-coated fiber are experimentally validated in this paper. A calibration technique has been implemented to correct the dynamic variation of the optical loss in the gold-coated fiber. Distributed temperature measurements up to 600 $^{\circ }$ C have been carried out.

Integrated distributed fiber optic sensing technology-based structural monitoring of the pound lock

Summary In this paper, an integrated distributed fiber optic sensing technology, which includes Raman optical time-domain reflectometry (ROTDR), Brillouin optical time-domain analysis (BOTDA), and fiber Bragg grating (FBG) sensing technologies, is adopted to monitor the temperature and the stress/strain variations of a reinforced concrete pound lock structure during the construction process. The Raman optical time-domain reflectometry was used to monitor the internal temperature variation throughout the concrete curing process when concrete hydration heat was released in the base plate of the lock head. The FBG temperature sensors were adopted to measure the surface temperature of the concrete, and the temperature data were used to compensate the results that are measured by BOTDA sensing technology to get the real concrete strain of the base plate. To better understand the stress/strain state of the base plate before and after filling water in the lock, the BOTDA sensing cable and FBG temperature sensors monitoring continued throughout the whole construction process. The observation provided a positive indication that the proposed integrated distributed fiber optic sensing technology may have great potential in performance monitoring of hydraulic structures.

High-performance hybrid Raman/fiber Bragg grating fiber-optic sensor based on simplex cyclic pulse coding

We propose and experimentally demonstrate the use of cyclic pulse coding to improve the performance of hybrid Raman/fiber Bragg grating (FBG) fiber-optic sensors, for simultaneous measurement of distributed static temperature and discrete dynamic strain over the same sensing fiber. Effective noise reduction is achieved in both Raman optical time-domain reflectometry and dynamic interrogation of time-division-multiplexed fiber FBG sensors, enhancing the sensing range resolution and providing real-time point dynamic strain measurement capabilities. The highly integrated sensor scheme employs broadband apodized low-reflectivity FBGs, a single narrowband optical source, and a shared receiver block.

Hybrid Raman/fiber Bragg grating sensor for distributed temperature and discrete dynamic strain measurements

We propose and experimentally demonstrate a hybrid fiber optic sensing technique that effectively combines Raman optical time domain reflectometry and in-line time-division-multiplexing for fiber Bragg grating (FBG) dynamic interrogation. The highly integrated proposed scheme employs broadband apodized low reflectivity FBGs with a single narrowband optical source and a shared receiver block, allowing for simultaneous measurements of distributed static temperature and discrete dynamic strain, over the same sensing fiber.

Input Power Conditions for Distributed Fiber Optic Temperature Sensors in Raman Optical Time-Domain Reflectometry

The critical power of the input pulse in which spontaneous Raman scattering is not seriously disturbed by stimulated Raman scattering in a fiber optic temperature sensor has been investigated experimentally and theoretically. The critical power determines the critical distance, which is nearly equal to the distance where optical time-domain reflectometry (OTDR) signal of the stimulated Raman scattering becomes maximum. From this fact, a new method to determine the critical input power with the OTDR system has been proposed, which may be applicable to most distributed fiber optic temperature sensors. It has been determined that the critical power of the input pulse is 4 watts for an infinitely long silica fiber.

Ultra-long graded-index multimode optical-fiber fault location using Raman optical time-domain reflectometry

Ultra-long graded-index (GI) multimode fiber fault location has been carried out using Raman optical time-domain reflectometry (ROTDR), where the Stokes' light pulses generated by the stimulated Raman scattering effect in a measured fiber are used as probe pulses. For basic investigation, spectral power distributions of the Stokes' pulses in a multimode fiber pumped by a 1.06-μm <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switched Nd:YAG laser have been measured. Maximum locatable distance for a fiber break using the ROTDR technique has been estimated by the measurement results, and it has been shown that a more-than-100-km-long multimode fiber break is locatable. On the basis of these results, a 86-km-long multimode fiber fault-location experiment has been carried out, and the break without Fresnel reflection has been located.