Coded optical probe pulses in return-to-zero/non-return-to-zero (RZ/NRZ) formats are used for long-range distance sensing based on a differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) in order to enhance the spatial resolution and measurement accuracy. It is found that using the RZ format maintains the Brillouin spectral shape, enhances the sensing range and leads to a higher signal-to-noise ratio compared to a single-pulse Brillouin optical time-domain analysis. With 512 bit RZ-coded pulse pairs of 60/55 ns for the DPP-BOTDA, a spatial resolution of approximately 0.5 m and a strain resolution of 12 microepsilon (which is equivalent to a 0.7 MHz Brillouin frequency shift) have been achieved over a 50 km large effective area fiber.

In this paper, we demonstrate the possibility of using a polarization optical time domain reflectometry (POTDR) system as an intrusion sensor. This system measures the evolution of the state of polarization (SOP) along the fiber length. In the typical application of fence monitoring, an intrusion leads to a modification of the SOP, which appears, after a temporal averaging, as a detectable flattening of the POTDR trace starting from the intrusion position. Experiments show that an intrusion leading to a 1.5 cm fiber displacement can be detected, using a 5 s averaging and 50 ns pulses with an error on the intrusion localization of the order of 5 m.

In this paper we propose a novel kind of multi-point vibration sensor based on the polarization properties of light. Its principle relies on the combination of mechanical transducers with fiber Bragg gratings. When subject to vibrations, the mechanical transducers induce birefringence variations within the fiber and in turn modify the state of polarization, which appears as a power variation after going through a polarizer. The FBGs reflect light from different positions of the sensing fiber and provide wavelength multiplexing. We show that this sensor can provide the vibration frequencies in a quasi-distributed manner.

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In this paper, the characteristics of a polarization-based vibration sensor are theoretically and experimentally analyzed with a focus on its sensitivity and linearity. It is shown that this sensor can correctly recover the vibration frequency spectrum (i.e., with limited distortions) up to an acceleration of 140 m/s(2), with a sensitivity equal to 9.98 mV/(m/s(2)).

This paper presents the design of a mechanical transducer for an optical-fiber accelerometer based on polarization variation. Several transducers can be imagined using either bending, twist, stretching or crushing of the fiber. The transducers are modelled analytically and are compared through the curve representing the sensitivity in function of the sensor resonant frequency. It turns out that the use of crushing shows a sensitivity several orders of magnitude higher than the other deformations. In this latter case, experimental results confirm the analytical computation of the sensitivity.

A new type of highly birefringent microstructured optical fiber has been tested for vibration measurements using a polarimetric technique. This technique takes advantage of the stress-induced phase shift between the two orthogonally polarized fiber eigenmodes. Comparison of three different fiber types shows that standard single-mode fibers do not provide stable measurements and that conventional polarization-maintaining fibers lead to a significant cross-sensitivity to temperature. However, for highly birefringent microstructured fibers specifically designed to provide a temperature-independent birefringence, our experiments show repeatable vibration measurements over a frequency range extending from 50 Hz to 1 kHz that are unaffected by temperature variations (up to 120 °C).

The limitation of long distance sensing (>20km) based on BOTDA (Brillouin optical time doman analysis) with centimetre spatial resolution, and high strain or temperature resolution include 1) gain saturation of the Stokes signal; 2) pump depletion induced the Brillouin spectrum distortion. The coded pulse offers the best solution to reduce above limiting factors and to improve the signal to noise ratio (SNR). In this paper, two most commonly used pulse formats: non-return-to-zero (NRZ) and return-to-zero (RZ) are used for BOTDA, and it is found that RZ coded pulses offer minimum distortion in the time domain waveform and the Brillouin spectra while NRZ coded pulses introduce spatial broadening which has reduced spatial resolution. For SMF gain saturation occurs at much shorter length (<20km with 20ns coded pulses) due to one peak Brillouin spectrum, while for 50km LEAF fibre with 20ns coded pulse, no gain saturation is observed due to three Brillouin peaks occurring and we have seen the lower and more uniform Brillouin gain across the fibre length. Using RZ coded pulses of differential Brillouin gain to realize DPP-BOTDA, we achieved 50cm spatial resolution with the strain resolution of 12 .