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.

We propose and experimentally demonstrate a stable homodyne phase demodulation technique in a ϕ-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.

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.

We demonstrate a hybrid distributed acoustic and temperature sensor (DATS) using a commercial off-the-shelf (COTS) distributed feedback (DFB) laser, a single-mode optical fiber, and a common receiver block. We show that the spectral and frequency noise characteristics of the laser, combined with a suitable modulation scheme, ensure the inter-pulse incoherence and intra-pulse coherence conditions required for exploiting the fast denoising benefits of cyclic Simplex pulse coding in the hybrid measurement. The proposed technique enables simultaneous, distributed measurement of vibrations and temperature, with key industrial applications in structural health monitoring and industrial process control systems. The sensor is able to clearly identify a 500 Hz vibration at 5 km distance along a standard single-mode fiber and simultaneously measure the temperature profile along the same fiber with a temperature resolution of less than 0.5°C with 5 m spatial resolution.

We demonstrate a Distributed Acoustic Sensor (DAS) based on Ultra-Weak Fiber Bragg Gratings (UWFBGs) using a scalable homodyne demodulation in direct detection. We show that a distributed interferometric system using delay and mixing of backscattering from consecutive identical gratings can be combined with a Phase-Generated Carrier Differentiate and Cross-Multiply (PGC-DCM) demodulation algorithm to perform dynamic measurements with high SNR, employing a simple narrowband laser and a pin photodiode. The proposed homodyne demodulation technique is suitable for real-time monitoring using distributed measurements, as it does not require computationally costly phase unwrapping common in conventional schemes and is robust against detrimental harmonic distortions, while not requiring additional mechanisms to handle division-by-zero operations. The demodulation scheme is also scalable, as it involves symmetric ordinary differentiation and integration operations suitable for processing with FPGA-based or analogue systems which, thanks to readily realizable schemes for implementing fractional order calculus, are also candidates for small-scale integration. We experimentally demonstrate the effectiveness of the technique by monitoring the dynamic response of a generic 2.5 kHz vibration applied to a PZT placed at the end of a sensing fiber comprised of a 1 km array of 200 UWFBGs each with a reflectivity of ~-43 dB written at a spacing of 5 m, with an SNR of ~34.52 dB.

We propose and experimentally demonstrate a long-range accelerated Brillouin optical time domain analysis (BOTDA) sensor that exploits the complementary noise reduction benefits of adaptive linear prediction and optical pulse coding. The combined technique allows using orders of magnitude less the number of averages of the backscattered BOTDA traces compared to a standard single pulse BOTDA, enabling distributed strain measurement over 10 km of a standard single mode fiber with meter-scale spatial resolution and 1.8 MHz Brillouin frequency shift resolution. By optimizing the system parameters, the measurement is achieved with only 20 averages for each Brillouin gain spectrum scanned frequency, allowing for an eight times faster strain measurement compared to the use of cyclic pulse coding alone.