Recent Advances in Distributed Acoustic Sensing Based on Phase-Sensitive Optical Time Domain Reflectometry

Distributed acoustic sensing (DAS) using coherent Rayleigh backscattering in an optical fiber has become a ubiquitous technique for monitoring multiple dynamic events in real time. It has continued to constitute a steadily increasing share of the fiber-optic sensor market, thanks to its interesting applications in many safety, security, and integrity monitoring systems. In this contribution, an overview of the recent advances of research in DAS based on phase-sensitive optical time domain reflectometry (ϕ-OTDR) is provided. Some advanced techniques used to enhance the performance of ϕ-OTDR sensors for measuring backscattering intensity changes through reduction of measurement noise are presented, in addition to methods used to increase the dynamic measurement capacity of ϕ-OTDR schemes beyond conventional limits set by the sensing distance. Recent ϕ-OTDR configurations which significantly enhance the measurement spatial resolution, including those which decouple it from the probing pulse width, are also discussed. Finally, a review of recent advances in more precise quantitative measurement of an external impact based on frequency shift and phase demodulation methods using simple direct detection ϕ-OTDR schemes is given

application of raman and brillouin scattering phenomena in distributed optical fiber sensing

We present a review of the basic operating principles and measurement schemes of standalone and hybrid distributed optical fiber sensors based on Raman and Brillouin scattering phenomena. Such sensors have been attracting a great deal of attention due to the wide industrial applications they offer, ranging from energy to oil and gas, transportation and structural health monitoring. In distributed sensors, the optical fiber itself acts as a sensing element providing unique measurement capabilities in terms of sensing distance, spatial resolution and number of sensing points. The most common configuration exploits optical time domain reflectometry, in which optical pulses are sent along the sensing fiber and the backscattered light is detected and processed to extract physical parameters affecting its intensity, frequency, phase, polarization or spectral content. Raman and Brillouin scattering effects allow the distributed measurement of temperature and strain over tens of kilometers with meter-scale spatial resolution. The measurement is immune to electromagnetic interference, suitable for harsh environments and highly attractive whenever large industrial plants and infrastructures have to be continuously monitored to prevent critical events such as leakages in pipelines, fire in tunnels as well as structural problems in large infrastructures like bridges and rail tracks. We discuss the basic sensing mechanisms based on Raman and Brillouin scattering effects used in distributed measurements, followed by configurations commonly used in optical fiber sensors. Hybrid configurations which combine Raman and Brillouin-based sensing for simultaneous strain and temperature measurements over the same fiber using shared resources will also be addressed. We will also discuss advanced techniques based on pulse coding used to overcome the tradeoff between sensing distance and spatial resolution affecting both types of sensors, thereby allowing measurements over tens of kilometers with meter-scale spatial resolution, and address recent advances in measurement schemes employing the two scattering phenomena

A cost-effective distributed acoustic sensor using a commercial off-the-shelf DFB laser and direct detection phase-OTDR

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Development of a Pulsed Laser for Hybrid Optical Fiber Sensors for Simultaneous Distributed Temperature and Dynamic Point Measurements

In Raman based distributed or hybrid Raman/FBG optical fiber sensors, the pulsed laser source plays a key role and must be optimized in terms of output power, spectral width in order to provide high back-scattered light without coherent Rayleigh noise for suitable measurement of physical parameters. In this thesis work, a pulsed laser system for use as a source in such a hybrid fiber-optic sensing scheme has been designed, using narrowband semiconductor laser and amplifier, also considering various optimizations of the used components. The pulsed optical source has been designed specifically for hybrid Raman/FBG optical fiber sensor systems, allowing for the use of optical pulse coding to improve sensing range and static/dynamic resolution of distributed temperature & dynamic point measurements. The effectiveness of the source has been experimentally verified in this hybrid Raman/FBG sensors; the pulsed laser characteristics are also useful for other sensor schemes

A compact source for a distributed acoustic sensor using a miniaturized EYDFA and a direct digital synthesis module

Distributed Acoustic Sensing (DAS) is a ubiquitous technique which enables concurrent, real-time measurement of fault or event-induced vibrations over long distances. Although there has been focused research in increasing the performance of DAS based on Phase-Sensitive Optical Time Domain Reflectometry (Φ-OTDR), the cost of conventional schemes remains high due to the complexity of the opto-electronic components in the sources used in the interrogator for high coherent Rayleigh scattering visibility, which rely on optical amplifiers designed for wideband telecom networks and multi-purpose waveform generators. However, probes in DAS use narrow linewidth lasers, whose fluctuations are well below the bandwidth of a single ITU grid and the driving waveforms can be generated by compact RF sources. In this contribution, we propose and experimentally demonstrate the design of a compact DAS interrogator using a miniaturized Erbium-Ytterbium-Doped Fiber Amplifier (EYDFA) commonly used in CATV networks together with an integrated Direct Digital Synthesis (DDS) module which can generate readily programmable waveform probes with a bandwidth of up to 1.4 GHz. The DDS module is suitable for use with any digital acquisition system for real-time acquisition of traces. Optical pulse probes generated with the DDS an a miniaturized EYDFA were used to obtain coherent Rayleigh backscattering traces with high SNR and interference visibility, allowing the measurement of a generic vibration at the end of a 10-km fiber. The proposed technique enables the simplification of DAS systems and paves the way toward their scalable development for wider use in among others environmental, seismic and structural health monitoring systems

Stable dynamic phase demodulation in a das based on double-pulse Φ-OTDR using homodyne demodulation and direct detection

Distributed Acoustic Sensing (DAS) is a technology with interesting features for real-time safety and security monitoring applications, and constitutes a steadily growing share of the optical fiber sensing market. Recently, the quantitative measurement of disturbances using DAS schemes based on Phase-Sensitive Time Domain Reflectometry (Φ-OTDR) has become a focus of investigation. In this contribution, we propose and experimentally demonstrate a stable homodyne phase demodulation scheme in a fiber optic Φ-OTDR sensor using a double pulse probe and a direct detection receiver. We show that a carrier for the distributed dynamic phase change induced by an external perturbation can be generated by selective phase modulation of one of the probing pulses. The local phase is then retrieved from the backscattering signal using a demodulation technique robust against light intensity disturbances, which have been limiting factors in existing phase demodulation schemes. In addition, the method is independent of the phase modulation depth and does not require computationally costly multi-dimensional phase unwrapping algorithms necessary when using I-Q demodulation in DAS, and is a suitable candidate for analogue signal processing. We demonstrate the capacity of the sensor to measure the distributed dynamic phase change induced by a nonlinear actuator generating a 2 kHz perturbation at a distance of 1.5 km with an SNR of ∼24 dB. The demodulated multi-frequency response is also shown to be consistent with one obtained using a point senor based on an FBG and a commercial reading unit

Dynamic phase extraction in a modulated double-pulse Ï\u86-OTDR sensor using a stable homodyne demodulation in direct detection

We propose and experimentally demonstrate a stable homodyne phase demodulation technique in a Ï\u86-OTDR using a double-pulse probe and a simple direct detection receiver. The technique uses selective phase modulation of one of a pair of pulses to generate a carrier for dynamic phase changes and involves an enhanced phase demodulation scheme suitable for distributed sensing by being robust against light intensity fluctuations, independent of the modulation depth, and convenient for analogue signal processing. The capability of the technique to quantify distributed dynamic phase change due to a generic external impact is experimentally demonstrated by measuring the phase change induced by a nonlinear actuator generating a 2 kHz perturbation at a distance of 1.5 km on a standard singlemode fiber with an SNR of ~24 dB. The demodulated nonlinear response is shown to have a spectrum consistent with one obtained using an FBG sensor and a commercial reading unit