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.
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.
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.
Dole: 31, 7000 Mons (BELGIUM)Bragg gratings written into polarization maintaining fibres (PM-FBG) are widely used in telecommunication systems and for sensing purposes. They allow simultaneous strain and temperature measurement, which is not possible with a single grating written into conventional single mode fibre. Discrimination between strain and temperature effects implies the measurement of two distinct resonant wavelengths induced by birefringence. Many practical applications such as the cure monitoring of composite materials require miniaturized PM-FBG. Short length PM-FBG can produce the overlap of the resonant peaks so that the measurement is only possible in two steps by means of a polarization controller and a linear polarizer. To avoid this complexity and the measurement errors that it involves, we propose 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. The Stokes parameters represent the state of polarization. They can be deduced from the Jones vector by means of the following equations [I1: S,+, Si=<0_, S2=2Re[] and S3=2Im[]. In these expressions, the notation x(y) represents the slow(fast) axis characterized by an effective refractive index neffx=neff+An/2 (neffy=ne)-An/2) where nL,, is the mean effective index of the fibre and An is the fibre birefringence. If Kqy) and OX(y) are the amplitude and phase angles of the x(y) component of the electric fields, the Jones vectors corresponding to the reflected Er and transmitted E, signals are given by:(E ,)(t M e'4--Px(y) and tX(y) denote respectively the reflection and transmission E= )= p M e and E,= t1= r j1 j where coefficients of the FBG corresponding to the slow(fast) axis and n vE~yt Me . whose expressions are derived from the coupled mode theory [2].Hence, using all the previous equations, it is possible to derive theoretical expressions for the Stokes parameters.The normalized Stokes parameters sl, s, and s3 are then computed from the ratio sj=S,/S0, i=1,2,3. Their expressions will be detailed at the conference. The theoretical evolutions of the Stokes parameters with wavelength will be discussed and compared to experimental evolutions obtained with a PM-FBG by means of a tunable laser source and a polarimeter. Fig. I presents the reflected spectrum of a 2 mm-long uniform PM-FBG written into hydrogen-loaded PANDA fibre through a 1060 nm period phase mask using a frequency-doubled Argon laser. The two resonant peaks overlap and they are represented by dotted curves. The reflected and transmitted Stokes parameters computed using the physical parameters corresponding to this experimental PM-FBG are shown in Fig. 2. ----Fastaxe S 02SI 10 Sowaais -015 o , 09 , o 0.16 A 20 010 A 34 005 014:'2i 0054\(\F\f\')l \INRRR o 0-a80-4 0.16 E~~~~~~~~~~~...
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