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.