Frédéric Lhommé

Autocorrelation demodulation technique for fiber Bragg grating sensor

In this letter, we describe a very accurate and simple demodulation technique for fiber Bragg grating sensors. The technique is suitable for both single and twin Bragg gratings. A twin grating is composed of two identical gratings located at different positions in the same single-mode fiber. Our demodulation technique evaluates the wavelength position of the reflection spectrum with respect to the spectrum of an undisturbed sensor. To calculate the spectrum shift, it computes the autocorrelation product between the two reflection spectra. The demodulation method, which is very fast, has been tested experimentally with temperature sensors. It gives absolute measurements and provides high accuracy compared to a conventional temperature probe.

Synthesis of fiber Bragg grating parameters from experimental reflectivity: a simplex approach and its application to the determination of temperature-dependent properties

A simple, accurate, and fast method to synthesize the physical parameters of a fiber Bragg grating numerically from its reflectivity is proposed and demonstrated. Our program uses the transfer matrix method and is based on a Nelder-Mead simplex optimization algorithm. It can be applied to both uniform and nonuniform (apodized and chirped) fiber Bragg gratings. The method is then used to synthesize a uniform Bragg grating from its reflectivity taken at different temperatures. It gives a good estimate of the thermal expansion coefficient and the thermo-optic coefficient of the fiber.

Simultaneous bend and temperature sensor using tilted FBG

We propose a simple, very reliable and fast optical sensor based on a tilted fiber Bragg grating for the simultaneous measurement of temperature and macro bending. The transmitted spectrum of a tilted Bragg grating is composed of numerous discrete dips which have two distinct origins: the dip at the longest wavelength comes from the self coupling of the core mode while the others are due to the backward coupling with the cladding modes. We apply a different demodulation technique to each of these two contributions in order to realize a dual sensor. This sensor allows the detection of small curvatures and provides a good accuracy.

Characterization of twin Bragg gratings for sensor application

Twin gratings are important devices for sensor application. They are formed by two gratings inscribed in cascade in a single fiber. We present here a detailed study of twin Bragg gratings. We first show the evolution of twin Bragg gratings spectra versus the spacing between the gratings and their length. Owing to the coupled mode theory, we explain the particular shape of the various spectra. Then we demonstrate the influence of the reflectivity of each Fiber Bragg Grating on the global spectral response of the two cascaded FBGs. All the gratings we have inscribed are simulated using the Transfer Matrix Method in order to retrieve their parameters (grating length, period, index modulation and distance between gratings)from experimental reflection spectra. Finally we characterize these FBGs for temperature sensing. We discuss the design suitable to sensor applications and describe the interrogation technique used in this case. We use a digital algorithm based on Fast Fourier Transform. It offers precise, fast and absolute measurements.

Simultaneous strain and temperature fibre Bragg grating sensor based on the measurement of one Stokes parameter

In this paper, we proposed a sensor based on the measurement of the first Stokes parameter of the transmitted signal by a short length PM-FBG. Since it is important to characterize the polarization properties of PM-FBG for a lot of applications, we also present theoretical and experimental studies of the reflected and transmitted Stokes parameters. Experimental evolutions are obtained with a PM-FBG by means of a tunable laser source and a polarimeter. It is concluded that the technique is compatible with frequency multiplexing and can be used in the case of quasi-distributed sensors.

Numerical reconstruction of induced birefringence in uniform fiber Bragg gratings through polarization properties measurement

We present an efficient numerical method to evaluate the induced birefringence generated during the inscription process of an FBG. We use the experimental transmitted spectrum and the wavelength evolution of the corresponding first normalized Stokes parameter to obtain the value of the birefringence. Our reconstruction technique is based on the Nelder-Mead simplex algorithm.

Use of tilted Bragg gratings to simultaneously measure sugar concentration and temperature during the production process of sugar

We will demonstrate the potential use of a 2 cm long 6° tilted Bragg grating for simultaneous refractive index and temperature sensing during the production process of sugar from sugarbeets. This sensor is easy to use and allows realtime measurements. Its accuracy falls within ± 0.1 % and ± 1°C for sugar concentration and temperature, respectively.

Novel technique for the simultaneous measurement of strain and temperature using polarization maintaining fiber Bragg gratings

When a short Bragg grating is written into a polarization maintaining fiber, the resonant peaks produced by birefringence can overlap. In this case, the simultaneous measurement of strain and temperature is only possible by means of polarization control devices. To avoid this complexity, we propose a demodulation technique allowing to accurately estimate the resonant wavelengths from the reflected spectrum measured using an unpolarized light. We use this technique to discriminate between strain and temperature effects with a 2 mm long grating written into PANDA fiber. Our method allows real-time functioning. Its accuracy falls within ±2°C and ±12 µε in the ranges 25-90°C and 0-1800 µε, respectively.

Influence of etching on the refractive index sensitivity of tilted fiber Bragg gratings and long period gratings

Tilted fiber Bragg gratings and long period gratings enhance the coupling to the cladding modes and constitute excellent refractometers. In this paper, we study how the cladding diameter influences the coupling to the cladding modes for these two types of gratings. Our experiments are made on different tilted Bragg gratings, characterized by different tilt angles. For every grating, we study the evolution of the sensitivity to the surrounding refractive index as function of the cladding radius. We also investigate two ways to correlate the spectral evolution of a tilted fiber Bragg grating with respect to the external refractive index. The first method is based on the local monitoring of the transmitted spectrum while the second one makes use of the correlation between a reference spectrum and a perturbed spectrum. These methods offer both very good accuracy and repeatability.

FBG temperature sensor based on the measurement of the reflected signal polarization properties

We analyze the polarization properties of uniform fiber Bragg gratings written into polarization maintaining fibers by studying the evolution with wavelength of the normalized Stokes parameters and the degree of polarization for both the reflected and transmitted signals. Theoretical expressions are derived for the Stokes parameters. Numerical simulations and experimental results are also reported. We demonstrate the possible realization of a sensor using the information contained in the evolution of teh degree of polarization in reflection. This new demodulation technique has been experimentally tested with temperature sensors.