Nicolas Linze

High-resolution DPP-BOTDA over 50 km LEAF using return-to-zero coded pulses.

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.

Development of an Intrusion Sensor Based on a Polarization-OTDR System

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.

Development of a multi-point polarization-based vibration sensor

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.

Temperature-insensitive polarimetric vibration sensor based on HiBi microstructured optical fiber

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).

Linearity considerations in polarization-based vibration sensors.

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)).

Quasi-distributed vibration sensor based on polarization-sensitive measurement

In this paper we propose a novel kind of quasi-distributed vibration sensor based on the measurement of the polarization state of light. Its principle is based on the combination of mechanical transducers -which transform the mechanical perturbation into a birefringence variation- with fiber Bragg gratings. We show that several vibrations can be detected and localized at each transducer position with a frequency resolution of 1.25 Hz.

Signal-to-noise ratio improvement in Brillouin sensing

The signal-to-noise ratio (SNR) enhancement using a coded pulse in Brillouin Optical Time Domain Analysis (BOTDA) is studied theoretically and experimentally and compared with the one pulse case. This new technique is based on simplex coding and provides a theoretical SNR enhancement of 3.6 dB. We find that the experimental SNR improvement is around 3.8 dB, which is in good accordance with the theory. We also observe that the Differential Pulsewidth Pair - Brillouin Optical Time Domain Analysis (DPP-BOTDA) method can be applied. This allows to combine high spatial and frequency resolutions1 and higher SNR.

Design of an Optical-Fiber Accelerometer Based on Polarization Variation Due to Crushing of the Fiber

This paper presents the design of a novel kind of accelerometer. The sensor is based on the modification of the polarization state of the light inside the fiber by the means of a mechanical transducer. The transducer presented uses crushing to deform the fiber. Measurements were done with an acceleration varying from 1 to 40 m/s² and a frequency range varying from 100 to 1,000 Hz. The measured output is the ratio between the variable part and the constant part of the optical power at the end of the fiber. A sensitivity of the order of 6E-4/ms² was found. Numerical simulations were also performed with the Abaqus software. The obtained sensitivity is quite greater than the measured one which can be explained by the hypothesizes we made for the simulations.

Design of a mechanical transducer for an optical fiber accelerometer based on polarization variation

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.

Recent Advances in Polarization Sensitive OTDR for Sensing Applications

Firstly used for the characterization of telecommunication fibers, POTDR systems are now proposed for distributed sensing. Intrusion, vibrations, magnetic field and high current sensing can indeed be performed from the analysis of POTDR traces.