We propose a flexible computational EDFA dynamic model for amplified modulated probes used in long-distance distributed sensors based on pulse coding, employing Rayleigh, Raman and Brillouin Scattering phenomena. The EDFA dynamics is studied numerically solving the differential equations of the reservoir model for Erbium ion concentrations. The model allows one to identify the optimal choice of gain parameters as well as the gain control mechanism needed for gain stabilization, minimizing both the code-words output power fluctuations and avoiding nonlinear effects which can degrade the distributed sensor performance when using pulse coding techniques such as Simplex, Golay and Cyclic- Simplex. The technique allows distributed sensors to be robust against fluctuations induced by long amplified code sequences, ensuring at the same time enough optical gain to enhance their performance, especially in terms of sensing distance, without overcoming the threshold of nonlinear propagation effects.
We propose a method to address the issue of handling the large amount of data involved in Distributed Acoustic Sensing (DAS) by designing and implementing a data storage system for a benchmark DAS scheme for performing continuous monitoring over a 100 km range at meter-scale spatial resolution. We employ the DynamoDB functionality of Amazon Web Services (AWS) which allows highly expandable storage capacity with latency of access of a few tens of milliseconds. In addition, the scalability of the DynamoDB-based data storage scheme is evaluated for linear and nonlinear variations of number of batches of access and a wide range of data sample sizes corresponding to sensing ranges of 1km-110 km. The results show latencies of 40 msec per batch of access with low standard deviations of a few milliseconds, and latency per sample decreases for increasing sample size paving the way toward the development of scalable, cloud-based data storage services integrating additional post-processing for more precise feature extraction. The technique greatly simplifies DAS data handling in key application areas requiring continuous, large-scale measurement schemes such as remote environmental & railways infrastructure monitoring and precision agriculture.
Phase-Sensitive Optical Time Domain Reflectometry (Φ-OTDR) is the most common implementation of a Distributed Acoustic Sensing (DAS) system and employs the observation of speckles resulting from Rayleigh Backscattering from coherent pulses in an optical fiber. Since they are sensitive to local disturbances altering the intensity and phase of light, perturbations induced by events cause changes in the speckle pattern whose precise measurement gives information on the amplitude and frequency of vibrations distributed along the fiber. Demodulation of the local phase change is key to the precise measurement of events since it is more linearly related to the strain applied to the fiber. One of the key issues in distributed sensing is that phase demodulation schemes usually require additional post-processing algorithm runs for each spatial location, which introduces delays, and hence reductions in dynamic sensing capability when scaled along the whole sensing distance. In this contribution, we analyze the impact of the post-processing in different phase demodulation techniques employing Phase-Generated Carrier (PGC) on the bandwidth of distributed feature extraction in a typical DAS system by quantifying the total computation time needed for a benchmark, 10-km sensing range at meter-scale spatial resolutions. We then design, implement, and analyze a signal processing scheme for phase extraction in Φ-OTDR enabling real-time dynamic measurements based on a Fast Hilbert Transform (FHT). Particular focus is given to the choice of this demodulation scheme for optimized bandwidth of distributed feature extraction using parallel processing of adjacent blocks in such a way that the overall throughput of spatially resolved concurrent demodulation allows dynamic vibration sensing at speeds relevant to most structural health monitoring applications.
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.
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.
In this paper, we experimentally demonstrate a hybrid distributed acoustic and temperature sensor (DATS) based on
Raman and coherent Rayleigh scattering processes in a standard singlemode fiber. A single commercial off-the-shelf
DFB laser and a common receiver block are used to implement a highly integrated hybrid sensor system with key
industrial applications. Distributed acoustic sensing and Raman temperature measurement are simultaneously performed
by exploiting cyclic Simplex pulse coding in a phase-sensitive OTDR and in Raman DTS using direct detection. Suitable
control and modulation of the DFB laser ensures inter-pulse incoherence and intra-pulse coherence, enabling accurate
long-distance measurement of vibrations and temperature with minimal post-processing.
We propose and experimentally demonstrate a Distributed Acoustic Sensor exploiting cyclic Simplex coding in a phase-sensitive OTDR on standard single mode fibers based on direct detection. Suitable design of the source and use of cyclic coding is shown to improve the SNR of the coherent back-scattered signal by up to 9 dB, reducing fading due to modulation instability and enabling accurate long-distance measurement of vibrations with minimal post-processing.
We propose and experimentally demonstrate a fully hybrid distributed sensing scheme that uses a single narrow-band laser to perform fast measurement of the BFS using BOTDA and simultaneous temperature measurement based on spontaneous Raman scattering over 10 km of single mode fiber. The use of cyclic pulse coding effectively reduces the pump peak power levels required for accurate Raman-based distributed temperature measurement, enhancing at the same time the speed of the BFS measurement in the BOTDA technique.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.