Method for measurement of first-and second-order polarization mode dispersion vectors in optical fibers

Abstract

The present invention consists of a method and apparatus for measuring first and higher order PMD vectors in optical fibers. For each first-order PMD vector determination, two distinct polarization states are sequentially injected into an optical device under test for each of a pair of frequencies &ohgr; and &ohgr;+&Dgr;&ohgr; . A Stokes vector s representing the first polarization state must not be parallel or anti-parallel to a Stokes vector s representing the second polarization state, but the relative angle between s and s need not be known. The frequency interval &Dgr;&ohgr; is large to obtain a high signal-to-noise ratio. Thus, four light beams are injected, the first at frequency &ohgr; and polarization s , the second at frequency &ohgr; and polarization s , the third at frequency &ohgr;+&Dgr;&ohgr; and polarization s , and the fourth light at frequency &ohgr;+&Dgr;&ohgr; and polarization s . The output polarizations of these beams are measured and four corresponding output Stokes vectors are determined. A first-order PMD vector is then calculated from the four input Stokes vectors and four output Stokes vectors. Each subsequent PMD vector determination is then made using a pair of frequencies which differs from the previous pair of frequencies by a frequency interval &Dgr;&ohgr; which is small compared to &Dgr;&ohgr; . Since &Dgr;&ohgr; can be small, high spectral resolution of the PMD vector can be obtained, while still maintaining good signal-to-noise ratio provided by using a large &Dgr;&ohgr; . The good signal-to-noise ratio and high spectral resolution of the PMD vector allow the accurate determination of second- and higher-order PMD vectors.