Francisco J. Mendieta-Jimenez

Multiplexing of grating-based fiber sensors using broadband spectral coding

The development of a new fiber Bragg grating multiplexing technique is reported. The technique is based on a twin Bragg grating sensor configuration and exploits a combination of wavelength- and frequency-domain multiplexing. We demonstrate that very weak Bragg gratings with reflexivity of approximately 0.5 percent or less can be efficiently used by this technique. Experimental results presented in the paper and theoretical estimates indicate that the interrogation of more than 100-element serial array sensor system is feasible.

Spectral birefringence characterization of helically wound monomode erbium-doped fibers

In this work we apply a simple, non-destructive method we developed for the birefringence characterization of helically wound passive fibers to monomode erbium-doped optical fibers. This method is based on the Jones matrix model developed by JN Ross for helically wound optical fibers. In the case of passive fibers Ross model is correct if the polarization evolution of light is measured with respect o an input local reference frame defined by the helix geometry; but in order to use a fixed reference frame it is necessary to consider the rotation of the plane of polarization introduced by parallel transport along the fiber. The use of Poincares method and Mueller calculus simplifies the physical interpretation of the results. The birefringence properties of two helically wound erbium fibers are tested in the neighborhood of the amplification band showing that in this case the spectral response has a much stronger variation than in the case of passive step- index monomode fibers. Despite the topological contribution due to the parallel transport of the reference frame, with method where presented provides an easy way to measure the total linear retardation induced by the fiber curvature and the total circular retardation indued by the fiber torsion. Experimental results obtained for two commercial EDF are presented.

Birefringence evaluation of helically wound optical fibres

Abstract A simple, non-destructive method is presented for the characterization of the birefringence properties of a helically wound mono-mode optical fibre. It is shown that the Jones matrix model developed by J.N. Ross can be applied to describe them, if the polarization evolution of light is measured with respect to a local reference frame (Serret-Frenet frame). In this case the Ross model is equal to the matrix model developed by R.C. Jones for a twisted crystal. In order to use a fixed reference frame (laboratory reference frame) it is necessary to consider the rotation of the reference frame at the fibre input face produced by the fibre topology. The orientation of the helical fibre axes at the input is defined following the geometrical criteria proposed by Ross. The use of Poincarés method and Mueller calculus to rewrite the Ross model simplifies the physical interpretation of the results. The method presented provides an easy way to measure the total linear retardation induced by the fibre curvature, and to separate the circular retardation associated with the photo-elastic response of the fibre, from the topological rotation of the reference frame defined only by the geometry of the helically wound fibre.

Transmission/reflection analysis for localization of loss-inducing perturbations in distributed fiber optic sensor based on Rayleigh backscattering

We present a simple method for the localization of a loss region for a fiber-optic alarm-condition sensor based on the measurement of transmitted and Rayleigh backscattered power. Bending the sensing fiber affects both the transmitted and backscattered power of unmodulated continuous-wave light that is launched into the fiber. The position of the loss region is determined from unique relationships between normalized transmitted and backscattered powers for different locations of the disturbance along the test fiber. The localization of a strong disturbance with an estimated accuracy of few meters along a few km-length single-mode test fiber was demonstrated.

Simple method for the birefringence evaluation of helicoidal optical fiber structures

The polarization optical properties of single mode fibers are important in those applications based on the use of coherent polarized light, such as fiber optic interferometric sensors and coherent communication systems. Various birefringence mechanisms and combinations of them have been developed to control the polarization evolution of light along the fiber. Since 1977 when A.Papp and H.Harms1 suggested the application of helical core fibers for this purpose, their polarization properties have been studied by several authors.24 In I 984 J.N. Ross2 showed that helically wound monomode fibers behave as the combination of a distributed linear retarder and a distributed circular retarder. Since linear and circular retardation can be easily followed on the Poincare sphere, this representation of polarized light results adequate for helical fibers. In this work the non-destructive evaluation of the equivalent optical activity and the equivalent linear retardation of a fiber helix are performed using the trajectory described on the Poincare sphere when the orientation of the linearly polarized light at the input rotates 360°. The results we obtain are compared with the values determined for the linear and circular retardation using Ross model and an input circular or linear polarization. We present preliminary results obtained for the birefringence characterization of two helically wound fibers built with .-l4m and —27m of a telecommunications fiber ( I 550nm).

Polarization-dependent performance of helically wound EDFAs

The polarization dependent performance of EDFA has been explained in terms of polarization hole burning, considering that the erbium sites in the silica matrix preserve its random nature. Although typical EDFAs are built with an erbium fiber longer than 10 m (hence the active fiber is necessarily wound) the influence of the bending has not been included in the investigation of this effect. To verify the relevance of bending induced birefringence, a helically- wound active fiber has been used to build an EDFA. The characterization of this helically-wound optical amplifier has shown that the polarization dependent performance was enhanced. The stronger link between the states of polarization of the signal wave, the pump wave and the active fiber birefringence axes was used to show that a selective excitation of the erbium ions by a polarized pump and a selective de-excitation of those ions by a polarized signal are the origin of the polarization dependent performance of EDFAs.

Adaptive interferometer measurement of the piezoelectric and electro-optic coefficients of PZT films

In this paper we present simple interferometric technique for thin film testing with GaAs:Cr adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force. The technique needs no special vibroinsulation and automatically adjusts and keeps the operation point of the interferometer. Two different interferometric schemes with GaAs:Cr adaptive photodetectors are reported. A modified Mach-Zehnder interferometer with adaptive photodetector is used for the measurement of piezoelectric coefficient of the thin film. A two-beam polarization interferometer with adaptive photodetector is used for the measurement of effective differential Pockels coefficient re = r33-(n0/ne)3r13. It is shown that two-beam polarization technique allows measurement of the Pockels coefficient of thin film with a strong Fabry-Perot effect that usually presents in ferroelectric thin film. Strong hysteresis effect with a slightly asymmetric form of the hysteresis loop was observed at the dependence of d33 and re coefficients of the PZT thin film versus DC electric field. The values of d33 and re are in agreement with known data.