Metrology

We report on the investigation of distributed chromatic dispersion (CD) and distributed nonlinear coefficient (NLC) measurements based on phase mismatched four-wave mixing in dispersion-shifted fibers (DSFs). Experimental results of the distributed CD maps for low Polarization-mode dispersion (PMD) DSF fibers are discussed. We also report how nonnegligible values of PMD can adversely affect the distributed CD measurements. A new method to measure the distributed NLC map in low-PMD DSF fibers is also proposed and demonstrated experimentally.

Using a method based on the detection of the Kerr phase shift by a self-aligned interferometer, we present measurements of the nonlinear coefficient n(2)/A(eff) for standard single-mode fiber (SMF), dispersion-shifted fibers, and dispersion compensating fibers. The presence of a Faraday, mirror in the interferometer makes the setup very robust, and different test fibers can be measured without any further readjustments. Interlaboratory comparisons show that the values found with our method are in good agreement with the other ones. Further, analysis of a SMF fiber with large chromatic dispersion shows a good reproducibility of the n(2)/A(eff) measurements as a function of fiber length.

A method for measuring the nonlinear coefficient n(2)/A(eff) in telecom fibres at 1550nm is presented. The method is based on determining the Kerr phase shift detected by a self-aligned interferometer incorporating a Faraday mirror. This makes the setup very robust, and different test fibres can be measured without the need for any further readjustments.

We present both a theoretical and experimental analysis of nonlinear polarization rotation in an optical fibre. Starting from the coupled nonlinear Schrodinger equations an analytical solution for the evolution of the state of polarization, valid for fibres with large linear birefringence and quasi cw input light with arbitrary polarization, is given. It allows us to model straightforwardly go-and-return paths as in interferometers with standard or Faraday mirrors. In the experiment all the fluctuations in the linear birefringence, including temperature- and pressure-induced ones, are successfully removed in a passive way by using a double pass of the fibre under test with a Faraday mirror at the end of the fibre. This allows us to use long fibres and relatively low input powers. The match between the experimental data and our model is excellent, except at higher intensities where deviations due to modulation instability start to appear.

For critical Erbium-doped fiber amplifier (EDFA) design, e.g., gain tilt optimization in WDM booster amplifiers, knowledge of the gain distribution within the active fiber can present a valuable information. Among the different techniques to evaluate the distributed gain in active fibers, the technique of optical frequency domain reflectometry seems most promising as it is a nondestructive measurement method well matched to the task due to its dynamic range, resolution, and range, Moreover, background light from ASE or residual pump light is strongly rejected due to the coherent detection scheme employed. Using different Erbium-doped fibers with strongly varying doping levels and confinements, we demonstrate the excellent accuracy and reproducibility of the technique.