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We present a nondestructive method that allows the spectral birefringence characterization of single-mode erbium-doped fibers within the amplification band. Using the same optical set up and the predicted polarization eigenmodes, the quality of the results has been verified. The results obtained for three commercial erbium-doped fibers with different erbium concentrations are shown. For two of them, the values measured for the elliptical birefringence shift, for different signal wavelengths, from having a dominant circular component to exhibit a dominant linear component.

Using the Poincare sphere and wavelength scanning it is possible to determine if the fiber birefringence corresponds to that of a linear, circular or elliptical retarder, as well as to obtain an approximate measurement of the polarization beatlength. This method is useful for low birefringence single-mode fibers. It is applied to erbium-doped fibers.

In this work we present the theoretical basis of two methods applied to the birefringence characterization of single--mode optical fibers and devices. Both methods are based on classical polarization optics and make use of the geometric properties of the Poincare sphere. One of them, used to identify the dominant polarization characteristics, is based on the wavelength scanning technique; i.e. assumes that wavelength dependence can be neglected. The second one, with a higher accuracy, avoids the uncertainty introduced by birefringence dispersion using a monochromatic linear signal to determine the polarization eigenmodes for that specific wavelength. The results obtained for some common cases are presented and discussed.

Erbium-doped monomode optical fibers are widely used to build amplifiers, lasers, incoherent light sources and sensors. Since the performance of these devices can be modified by its spectral birefringence, it is important to characterize it. In the case of an elliptical retarder the determination of the sign of the linear birefringence and the relative value of the linear and circular distributed birefringence is not obvious. In this work we analyze the anisotropic response of erbium fibers assuming that they are uniform and behave as a distributed retarder with linear and circular birefringence. Taking advantage of the geometric properties of the Poincare sphere it is possible to separate the linear birefringence contribution from that of the circular birefringence and to determine their signs and relative values.

In this work we present a polarimetric method that uses a polarization description based on the Poincare sphere. The erbium fiber is modeled assuming that it is uniform and behaves as a distributed retarder with linear and circular birefringences. We show that taking advantage of the geometric properties of the Poincare sphere it is possible to separate the linear birefringence contribution from that of the circular birefringence while the measurement precision is optimized. The experimental procedure here presented provides reproducible results for the measurement of the linear and circular distributed birefringences in erbium-doped fibers.

The relative insensitivity of erbium doped fiber amplifiers to the state of polarization of the input signal is considered as an advantage when these devices are used within communication networks. On the other hand, in order to use them in combination with interferometric optical fiber sensor, they should be able to maintain the input polarization state. In this work we presented the results obtained for the performance characterization of an EDFA built with a helically wound monomode erbium fiber. The use of a regular winding allows the control of the evolution of the state of polarization of the optical signal to be amplified. We include results of the geometrical and birefringence characterization of two helices built with tow different commercial erbium fibers. The amplification of polarized signals has been evaluated for the C band. It presents a high gain for small as well as for saturated signals. In regard with the noise due to the amplified spontaneous emission, it was evaluated also for small as well as for saturated signals. The values we measured are below the theoretical predictions based on typical EDFAs built with an irregularly wound erbium fiber.

The volution of the state of polarization along an out-of- plane trajectory has been widely studied for monomode optical fibers. To demonstrate the validity of our proposal we compare the results predicted by our model with experimental result obtained for helically wound optical fibers and the conoscopic patterns obtained for GRIN lenses when oblique illumination is used.

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 Poincare's 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.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In a helically wound single-mode fiber the mode degeneracy is lifted by the geometric form and the photo-elastic effect. Fiber bending induces linear birefringence' and the presence of torsion is responsible for circular birefringence.2 Considering the geometrical properties of a helix and using Jones calculus, it has been shown3 that a helically wound fiber can be described as the combination of two distributed homogeneous retarders: a linear retarder and a circular retarder. Since linear and circular retardation can be easily followed on the Poincare sphere, we use Mueller calculus to describe the polarization optics of helical fiber structures.4 Because of the strong absorption and the subsequent emission that active fibers present within the amplification band, the signal becomes depolarized and it is not possible to characterize the birefringence properties of active fibers at these wavelengths. In this work, the birefringence performance of active fibers is evaluated in the neighborhood of the amplification band, using signals with a high signal to noise ratio at the active fiber output. We present the birefringence characterization of a helically wound erbium fiber. This helical fiber structure was built with a commercial fiber. Since the birefringence parameters we measured agree with the values predicted by the theoretical model, we propose that we can make use of this model to design the helix structure and, to select the input polarization states of the pump and the signal that can be used to control the polarization evolution of the amplified signal as it propagates along the fiber.

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).