Tiziana Tambosso

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.

Measurement of birefringence correlation length in long, single-mode fibers

Measurements of birefringence correlation length performed on long single-mode telecommunication fibers are reported. The proposed technique relies on the statistical properties of the backscattered-field polarization, which was measured by means of a polarization-sensitive optical time-domain reflectometer. Experimental results are reported that show good agreement with the underlying theory based on stochastic differential equations.

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.

Single-end polarization mode dispersion measurement using backreflected spectra through a linear polarizer

Routine characterization of polarization mode dispersion of single-mode fibers in installed cables requires simple and fast techniques. All standardized techniques use both fiber ends, one connected to the optical source and the other one for signal detection. Clearly, this causes several drawbacks in field tests because the two fiber ends are usually far from each other. Among standardized techniques, the fixed-polarizer method (also called wavelength scanning method) is one of the simplest to implement. In this work we present a new single-end measurement scheme based on the fixed polarizer method applied to the signal backreflected by the fiber far-end. We report analytical equations and numerical solutions that permit to calculate the mean value of the differential group delay measuring the crossings and/or extreme densities of the spectrum transmitted through a linear polarizer. We also show that the mean value of the differential group delay can be calculated using the Fourier transform of the detected signal. Finally, experimental results on cascades of single-mode step-index fibers confirm the robustness and easiness of our proposal for polarization mode dispersion measurements.

Speckle Pattern Errors in Self-Mixing Interferometry

We analyze the random errors occurring in interferometric measurements because of the speckle pattern regime, when the remote target is a diffusing surface. First, we review the statistical properties of speckle and discuss amplitude fading that is affecting the self-mixing interferometer (SMI) signal and methods to alleviate it. Second, we derive intra-speckle phase errors using the bivariate conditional probability, and find that the noise-equivalent-displacement for small displacement Δ is proportional to the ratio of Δ to speckle longitudinal size sl. Last, we extend the analysis to inter-speckle displacements (Δ > s1) and, after deriving speckle systematic and random errors, show that operation up to meters on a diffusing surface target is possible with a small (≈ λ) error. Results are mainly focussed on SMI, yet they have general validity for any configuration of interferometry.

Spatially resolved chromatic dispersion measurement by a bidirectional OTDR technique

A technique for spatially resolved chromatic dispersion measurement of installed optical fiber links is presented. The method is based on the analysis of bidirectional optical time-division reflectometer (OTDR) traces at different wavelengths obtained by a specifically designed tunable external source. Measurement accuracy is improved through a particular algorithm that corrects the OTDR nonlinearity error. Experimental data on spatially resolved chromatic dispersion of installed dispersion-shifted and nonzero-dispersion fibers are reported.

Continuous-wave backreflection measurement of polarization mode dispersion

Experimental results of continuous-wave polarization mode dispersion (PMD) measurements are presented. The proposed technique is based on the backscattered signal given by the Fresnel reflection at the fiber far-end. Experimental statistical distribution of the differential group delay of the round-trip is reported. It presents a Rayleigh distribution whose mean value is simply related to the fiber mean differential group delay. Finally, the statistical distribution of the round-trip dispersion vector is found experimentally and numerically. The proposed method can be easily used for one-end measurement of PMD in installed optical cables.

Measurement of beat length and perturbation length in long single-mode fibers by backscattered signal analysis

We measured beat length and birefringence correlation length of several types of long single-mode fibers, using a new backscattering technique. Results represent the first experimental characterization of the random birefringence affecting long single-mode fibers.

Improving the accuracy of the wavelength-scanning technique for PMD measurements

An improvement of the wavelength-scanning polarization mode dispersion (PMD) measurement technique is presented. It consists in the evaluation of the level crossing density for three or more levels, instead of only one. Experimental results show that measurement uncertainty can be reduced more than 40%.