Luca Palmieri

Statistical characterization of fiber random birefringence

The statistical properties of the random birefringence that affects long single-mode fibers are experimentally evaluated by means of a polarization-sensitive optical time-domain reflectometry. The measurements are in good agreement with theoretical predictions and show, for what we believe is the first time, that the components of the local birefringence vector are Gaussian random variables.

Measurements of beat length and perturbation length in long single-mode fibers

Experimental results of measurement of the beat length and the differential group delay of several types of long single-mode fiber are presented. The proposed measurement technique is based on a polarization-sensitive analysis of the backscattered signal and allows one to calculate the correlation length of the random birefringence affecting the fiber.

Polarization mode dispersion characterization of single-mode optical fiber using backscattering technique

This paper presents a completely new method able to characterize polarization mode dispersion (PMD) properties of randomly birefringent single-mode fibers, using polarization sensitive backscattering technique. We show analytical relationships between evolution of polarization state of backscattered signal with respect to state of polarization of forward one. Our technique allows one to measure differential group delay, beat length, and correlation length at the same time over long single-mode fibers using only one fiber end. Experimental data fit very well with numerical results, confirming the capability of our technique for fast routine characterization of PMD during cabling, before and after installation.

Optimized spinning design for low PMD fibers: an analytical approach

It is known that the differential group delay (DGD) due to polarization mode dispersion (PMD) can be effectively reduced by spinning the fiber during drawing. In this paper, we propose an analytical approach that allows optimization of the spinning design. The fundamental idea is that, in the absence of polarization coupling, an optimized spinning profile can balance the effects of the intrinsic linear birefringence so that the differential group delay can be forced to be periodic and, consequently, have a limited amplitude as a function of distance. Our approach Is independent of the spin profile. In other words, with a fixed set of parameters that characterize a particular spin function, we are able to find analytically the values corresponding to a periodic DGD in a deterministic regime. Numerical results based on waveplate model confirm the analytical prediction and show that PMD can be reduced by about two orders of magnitude with respect to the same fiber without spinning, even after the introduction of random polarization coupling.

Spatially resolved PMD measurements

Spatially resolved measurements of polarization properties of fiber optic link-such as birefringence, polarization-mode dispersion (PMD) and polarization dependent loss (PDL)-may be effectively performed using polarization sensitive reflectometric techniques. In particular, this paper focuses on polarization-OTDR and reviews its theory and applications. Special emphasis is given to the use of optical reflectometry as a tool to characterize fiber birefringence. This allows to inspect the fiber while cabled and, consequently, to test and improve the cabling process. In addition, it also allows to define reliable mathematical models, which are essential for the design of low polarization mode dispersion fibers. The application of polarization-OTDR to polarization mode dispersion and polarization dependent loss measurements is also discussed.

Beat length characterization based on backscattering analysis in randomly perturbed single-mode fibers

This paper presents analytical and numerical results on the statistical properties of the backscattering signal of a randomly perturbed, linearly birefringent, single-mode fiber. Our theory is based on the study of amplitude evolution of the backscattered field passed through a linear polarizer, aligned with the state of polarization of the input signal. In particular, we show that the mean value of the beat length can be calculated using two methods based, respectively, on a level crossing rate analysis of the backscattered signal and on the evaluation of the standard deviation of its spectral density.

Measure of twist-induced circular birefringence in long single-mode fibers: theory and experiments

Production defects and external perturbations cause standard telecommunication fibers to be randomly birefringent. Fiber birefringence is the origin of the well-known polarization mode dispersion (PMD), which degrades system performances. The knowledge of birefringence properties may be crucial, especially when problems like development of low-PMD fibers or PMD interaction with optical nonlinearities in very high-capacity systems are faced. Some techniques are known to measure birefringence, and useful results have been obtained for both installed and wound-on-drum fibers. However, measurement of the circular component of birefringence still presents difficulties. In this paper, a new method for circular birefringence measurement is proposed that applies to long single-mode twisted fibers. The technique is based on polarization-sensitive optical time-domain reflectometry. Experimental results are in good agreement with the theoretical analysis.

Distributed Optical Fiber Sensing Based on Rayleigh Scattering

Optical fiber sensors offer unprecedented features, the most unique of which is the ability of monitoring varia- tions of the observed physical field with spatial continuity along the fiber. These distributed optical fiber sensors are based on the scattering processes that originate from the interaction between light and matter. Among the three different scatter- ing processes that may take place in a fiber—namely Rayleigh, Raman and Brillouin scattering, this paper focuses on Rayleigh-based distributed optical fiber sensors. For a given optical frequency, Rayleigh-based sensors exploit the three main properties of light: intensity, phase and polarization. All these sensing mechanisms are reviewed, along with basic principles, main acquisition techniques and fields of application. Emphasis, however, will be put on polarization-based distributed optical fiber sensors. While they currently represent a niche, they offer promising unique features worth being considered in greater detail.

Measurement of local beat length and differential group delay in installed single-mode fibers

We present beat length and polarization mode dispersion (PMD) measurements performed on installed fibers. Results regard three different kinds of fibers: standard step index, dispersion shifted and nonzero dispersion (NZD). After a historical comparison with standard differential group delay measurement collected four years ago on the same fibers, we perform a spatial-resolved measurement of the beat length by analyzing the state of polarization of the backscattered field. We compare PMD properties of different fibers and calculate the statistical distribution of the beat length. The differential group delay (DGD) and the beat length statistics depend strongly on fiber type and on fiber position along the link. The influence of the beat length on the DGD is also discussed.

Analytical treatment of polarization-mode dispersion in single-mode fibers by means of the backscattered signal

We present analytical results on the polarization-mode dispersion characteristics of single-mode birefringent fibers based on statistical properties of the backscattered signal. In particular, we calculate exactly the relationship between polarization-mode dispersion characteristics in forward propagation with respect to round-trip propagation in terms of dynamical equations, differential group delays, correlation lengths, and second-order effects. The theory applies to fibers affected by superposition of linear and circular birefringence in both the short-length and the long-length regimes.