Apparatus and method for improving the accuracy of polarization mode

Abstract

An apparatus and method for simultaneously taking measurements that are used for determining PMD vectors. This reduces the time interval over which all measurements are taken and reduces inaccuracy caused by PMD variation during the time measurement interval. The apparatus and method may be used in conjunction with any technique for calculating PMD, such as the Poincare Sphere Technique or Jones Matrix Eigenanalysis. The apparatus simultaneously produces multiple light beams. To distinguish each light beam from the others, each beam is given a distinct modulation. All the beams are then combined and passed through the optical device under test. A polarization measuring device then measures the output polarization of the combined beam and outputs one or more composite electrical signals that describe the Stokes components of the output polarization of the combined beam and that have the same modulations present in the original combined beam. Using modulation filters having the same modulations as given to each original light beam, electrical signals may be extracted from the composite electrical signals that describe the Stokes components of the output polarization corresponding to each original light beam. Also, measurement and control of the frequency difference between light beams used for determining PMD vectors is improved by receiving light beams of two distinct frequencies using a detector that then outputs a signal having a frequency equal to the difference in frequencies of the light beams. In addition, a feedback loop is used to better control the frequency difference between two light sources. Also, a frequency shifter is used to generate two light beams separated by a specific frequency interval.