Patent ID: 7920253
Filing Date: 2011-04-05
Classification: G01M

Abstract:
1. A method of measuring cumulative polarization mode dispersion (PMD) along the length of a fiber-under-test (FUT) using a single-end polarization sensitive optical time domain reflectometry means (POTDR), the method comprising the steps of: (i) injecting into an end of the FUT a first group of at least two series of light pulses having at least two different wavelengths, respectively, each of said at least two different wavelengths being closely-spaced from the or each of the other wavelengths by a respective small wavelength difference corresponding to a respective small optical frequency difference, the lowermost and uppermost of said at least two wavelengths defining therebetween a first center wavelength (λ (ii)(a) for each of at least some of the light pulses in each series of light pulses, detecting at least one polarization component of a resulting backreflected signal caused by Rayleigh scattering and/or discrete reflections along the FUT to provide a corresponding impulse-response, said at least one polarization component being the same for each of the said series, and converting each of the impulse-responses into a corresponding electrical impulse-response signal, (ii)(b) for each series of light pulses of said first group, sampling and averaging the electrical impulse-response signals of said each of at least some of the light pulses to provide a corresponding first group of OTDR traces each representing detected backscattered power versus time for the corresponding series of light pulses in said first group; (iii) injecting into said FUT at least a second group of at least two series of light pulses having at least two different wavelengths, respectively, each of said at least two different wavelengths being separated by a small optical frequency difference equal to the corresponding said small optical frequency difference of the said first group, lowermost and uppermost of said at least two wavelengths defining therebetween a second center wavelength (λ (iv)(a) for each of at least some of the light pulses in each of the at least two series of light pulses in said second group, detecting at least one polarization component of a resulting backreflected signal caused by Rayleigh scattering and/or discrete reflections along the FUT to provide a corresponding impulse-response, said at least one polarization component being the same for each of the said series of light pulses of said second group, and converting each of the impulse-responses into a corresponding electrical impulse-response signal, (iv)(b) for each series of light pulses of said second group, sampling and averaging the electrical impulse-response signals of said each of at least some of the light pulses to provide a corresponding second group of OTDR traces each representing detected backscattered power versus time for the corresponding series of light pulses in said second group; and (v) computing: (vi) computing the cumulative PMD value at at least one specific distance z along the FUT corresponding to the said at least one temporal point as a predetermined function of said mean-square difference, said predetermined function being dependent upon the respective difference or differences between the wavelengths corresponding to the normalized OTDR traces or each pair of normalized OTDR traces.