Hugo F. Martins

Coherent Noise Reduction in High Visibility Phase-Sensitive Optical Time Domain Reflectometer for Distributed Sensing of Ultrasonic Waves

Phase-sensitive optical time domain reflectometry (φOTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. Up to now, φOTDRs have been used mostly for the measurement of sub-kHz vibrations, normally in the context of intrusion sensing. In this paper, the authors present an experimental and theoretical description of a high-visibility φOTDR and its performance when used for ultrasonic vibration measurements. The use of a semiconductor optical amplifier in the setup allows to suppress coherent noise and also to improve the spectral response of the pump pulses. These two advantages greatly decrease the detected intra-band noise thus allowing frequency measurements in the limits set by the time of flight of the light pulses while maintaining the simplicity of the scheme, as no post-processing, extremely high coherence lasers or coherent detection methods are required. The sensor was able to measure vibrations of up to 39.5 kHz with a resolution of 5 m over a range which could go up to 1.25 km. This is the first time to our knowledge that a fully distributed measurement of ultrasonic waves was achieved. The statistical behavior of the system was also described theoretically and characterized experimentally.

Modulation instability-induced fading in phase-sensitive optical time-domain reflectometry

Phase-sensitive optical time-domain reflectometry (φOTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. We show in this Letter that modulation instability (MI) can induce a position-dependent signal fading in long-range φOTDR over conventional optical fibers. This fading leads to a complete masking of the interference signal recorded at certain positions and therefore to a sensitivity loss at these positions. We illustrate this effect both theoretically and experimentally. While this effect is detrimental in the context of distributed vibration analysis using φOTDR, we also believe that the technique provides a clear and insightful way to evidence the Fermi-Pasta-Ulam recurrence associated with the MI process.

Phase-sensitive Optical Time Domain Reflectometer Assisted by First-order Raman Amplification for Distributed Vibration Sensing Over >100 km

In this study, the authors present an experimental and theoretical description of the use of first order Raman amplification to improve the performance of a Phase-sensitive optical time domain reflectometer (φOTDR) when used for vibration measurements over very long distances. A special emphasis is given to the noise which is carefully characterized and minimized along the setup. A semiconductor optical amplifier and an optical switch are used to greatly decrease the intra-band coherent noise of the setup and balanced detection is used to minimize the effects of RIN transferred from the Raman pumps. The sensor was able to detect vibrations of up to 250 Hz (close to the limits set by the time of flight of light pulses) with a resolution of 10 m in a range of 125 km. To achieve the above performance, no post-processing was required in the φOTDR signal. The evolution of the φOTDR signal along the fiber is also shown to have a good agreement with the theoretical model.

Single-shot distributed temperature and strain tracking using direct detection phase-sensitive OTDR with chirped pulses

So far, the optical pulses used in phase-sensitive OTDR (ΦOTDR) were typically engineered so as to have a constant phase along the pulse. In this work, it is demonstrated that by acting on the phase profile of the optical pulses, it is possible to introduce important conceptual and practical changes to the traditional ΦOTDR operation, thus opening a door for new possibilities which are yet to be explored. Using a ΦOTDR with linearly chirped pulses and direct detection, the distributed measurement of temperature/strain changes from trace to trace, with 1mK/4nε resolution, is theoreticaly and experimentaly demonstrated. The measurand resolution and sensitivity can be tuned by acting on the pulse chirp profile. The technique does not require a frequency sweep, thus greatly decreasing the measurement time and complexity of the system, while maintaining the potential for metric spatial resolutions over tens of kilometers as in conventional ΦOTDR. The technique allows for measurements at kHz rates, while maintaining reliability over several hours.

Distributed Vibration Sensing Over 125 km With Enhanced SNR Using Phi-OTDR Over a URFL Cavity

We describe the use of a phase-sensitive optical time domain reflectometer (ΦOTDR) over an ultra-long Raman fiber laser cavity allowing fully distributed detection of vibrations over 125 km. Compared to a first-order Raman-assisted ΦOTDR, this scheme shows an enhanced signal-to-noise ratio (SNR). This is due to the fact that the relative intensity noise introduced by the Raman amplification is mostly transferred to a lower frequency range, where the balanced detection implemented in the setup provides better suppression of the common-mode noise. The sensor was able to measure vibrations of up to 380 Hz (limit set by the time of flight of light pulses) in a distance of 125 km with a resolution of 10 m and an average SNR of 8 dB with no postprocessing. This implies a >3 dB improvement in SNR over a first-order Raman-assisted setup with similar characteristics.

Distributed phase birefringence measurements based on polarization correlation in phase-sensitive optical time-domain reflectometers

In this paper a technique to measure the distributed birefringence profile along optical fibers is proposed and experimentally validated. The method is based on the spectral correlation between two sets of orthogonally-polarized measurements acquired using a phase-sensitive optical time-domain reflectometer (ϕOTDR). The correlation between the two measured spectra gives a resonance (correlation) peak at a frequency detuning that is proportional to the local refractive index difference between the two orthogonal polarization axes of the fiber. In this way the method enables local phase birefringence measurements at any position along optical fibers, so that any longitudinal fluctuation can be precisely evaluated with metric spatial resolution. The method has been experimentally validated by measuring fibers with low and high birefringence, such as standard single-mode fibers as well as conventional polarization-maintaining fibers. The technique has potential applications in the characterization of optical fibers for telecommunications as well as in distributed optical fiber sensing.

300 km-ultralong Raman fiber lasers using a distributed mirror for sensing applications

Several configurations of ultralong Raman fiber lasers (URFL) based on a distributed mirror combined with Bragg gratings or fiber loop mirrors are studied. Two continuous-wave URFL configurations, with single and cascaded cavities using fiber Bragg gratings as mirrors are explored for a 300 km long fiber. For optical sensing, the cavity length was optimized for 250 km using one of the gratings an intensity sensor. Another URFL configuration based in a fiber loop mirror is also reported. For optical sensing using a 300 km long fiber it is shown that the best choice is a hybrid configuration. The sensitivity of the FBG laser sensor range was from (76 ± 2) × 10⁻⁶ με⁻¹ (for lower strain) to (9.0 ± 0.4) × 10⁻⁶ με⁻¹ (for higher strain).

Real time dynamic strain monitoring of optical links using the backreflection of live PSK data

A major cause of faults in optical communication links is related to unintentional third party intrusions (normally related to civil/agricultural works) causing fiber breaks or cable damage. These intrusions could be anticipated and avoided by monitoring the dynamic strain recorded along the cable. In this work, a novel technique is proposed to implement real-time distributed strain sensing in parallel with an operating optical communication channel. The technique relies on monitoring the Rayleigh backscattered light from optical communication data transmitted using standard modulation formats. The system is treated as a phase-sensitive OTDR (ΦOTDR) using random and non-periodical non-return-to-zero (NRZ) phase-shift keying (PSK) pulse coding. An I/Q detection unit allows for a full (amplitude, phase and polarization) characterization of the backscattered optical signal, thus achieving a fully linear system in terms of ΦOTDR trace coding/decoding. The technique can be used with different modulation formats, and operation using 4 Gbaud single-polarization dual PSK and 4 Gbaud dual-polarization quadrature PSK is demonstrated. As a proof of concept, distributed sensing of dynamic strain with a sampling of 125 kHz and a spatial resolution of 2.5 cm (set by the bit size) over 500 m is demonstrated for applied sinusoidal strain signals of 500 Hz. The limitations and possibilities for improvement of the technique are also discussed.

Toward Prevention of Pipeline Integrity Threats Using a Smart Fiber-Optic Surveillance System

This paper presents the first available report in the literature of a system aimed at the detection and classification of threats in the vicinity of a long gas pipeline. The system is based on phase-sensitive optical time-domain reflectometry technology for signal acquisition and pattern recognition strategies for threat identification. The system operates in two different modes: 1) machine+activity identification, which outputs the activity being carried out by a certain machine; and 2) threat detection, aimed at detecting threats no matter what the real activity being conducted is. Different strategies dealing with position selection and normalization methods are presented and evaluated using a rigorous experimental procedure on realistic field data. Experiments are conducted with eight machine+activity pairs, which are further labeled as threat or nonthreat for the second mode of the system. The results obtained are promising given the complexity of the task and open the path to future improvements toward fully functional pipeline threat detection systems operating in real conditions.

A Novel Fiber Optic Based Surveillance System for Prevention of Pipeline Integrity Threats

This paper presents a novel surveillance system aimed at the detection and classification of threats in the vicinity of a long gas pipeline. The sensing system is based on phase-sensitive optical time domain reflectometry (ϕ-OTDR) technology for signal acquisition and pattern recognition strategies for threat identification. The proposal incorporates contextual information at the feature level and applies a system combination strategy for pattern classification. The contextual information at the feature level is based on the tandem approach (using feature representations produced by discriminatively-trained multi-layer perceptrons) by employing feature vectors that spread different temporal contexts. The system combination strategy is based on a posterior combination of likelihoods computed from different pattern classification processes. The system operates in two different modes: (1) machine + activity identification, which recognizes the activity being carried out by a certain machine, and (2) threat detection, aimed at detecting threats no matter what the real activity being conducted is. In comparison with a previous system based on the same rigorous experimental setup, the results show that the system combination from the contextual feature information improves the results for each individual class in both operational modes, as well as the overall classification accuracy, with statistically-significant improvements.