Method and apparatus of single-ended polarization mode dispersion measurement
A method and system of measuring the PMD of an optical fiber under test (FUT). A polarization-sensitive optical time domain reflectometer (POTDR) is used to inject into the FUT and from a proximal end thereof, a test signal comprising a series of repeated light pulses and detecting a corresponding polarization-analyzed return light signal coming back from the optical fiber and representing back-reflected light from a light reflector connected to a distal end of the FUT. The plurality of polarization-sensitive acquisitions defines at least one pair of acquisitions performed with mutually different but closely-spaced wavelengths and substantially the same state of polarization (SOP). The process may be repeated for a plurality of pairs of acquisitions performed with at least one of a plurality of mutually-different center wavelengths and a plurality of mutually-different SOPs, and a value of the PMD for the FUT is calculated therefrom.
1 . A method for measuring a polarization mode dispersion (PMD) to characterize an optical fiber under test, the method comprising:
from a proximal end of the optical fiber under test which distal end is connected to a light reflector, performing a plurality of polarization-sensitive acquisitions using a polarization-sensitive Optical Time Domain Reflectometer (POTDR), wherein each acquisition is performed by propagating in the optical fiber under test, a polarized test signal comprising a first series of repeated light pulses and detecting a corresponding polarization-analyzed return light signal coming back from the optical fiber under test and comprising back-reflected light from the light reflector, said return light signal comprising repeated reflected light pulses, wherein said repeated light pulses are together propagated in the optical fiber under test within a fiber round-trip time;
wherein each of said acquisitions is performed with a corresponding wavelength of said test signal,
wherein said plurality of polarization-sensitive acquisitions defines at least one pair of acquisitions performed with mutually different but closely-spaced wavelengths and substantially the same state of polarization (SOP), a center of the wavelengths defining a center wavelength for said at least one pair, and
wherein said plurality of polarization-sensitive acquisitions comprises a plurality of pairs of acquisitions performed with at least one of a plurality of mutually-different center wavelengths and a plurality of mutually-different states of polarization (SOP); and
for each said acquisitions, averaging respective amplitudes of at least part of said repeated reflected light pulses to obtain a value of reflected power;
for each said pairs of said acquisitions, computing a value of a difference, between the two values of reflected powers corresponding to said pair;
computing a mean-square value of the computed values of difference over said at least one of a plurality of mutually-different center wavelengths and a plurality of mutually-different states of polarizations (SOPs); and
from said mean-square value, calculating a value of the polarization mode dispersion of said optical fiber under test.
2 . The method as claimed in claim 1 , wherein said value of reflected power is obtained by averaging respective amplitudes of all of said repeated reflected light pulses.
3 . The method as claimed in claim 1 , wherein each of said acquisitions comprises the propagation of said first series of repeated light pulses and a second series of repeated light pulses, wherein the second series is propagated before detection of the return light signal of the first series.
4 . The method as claimed in claim 1 , wherein each acquisitions comprises the propagation of said first series of repeated light pulses and a second series of repeated light pulses, wherein the second series is propagated after detection of the return light signal of the first series.
5 . The method as claimed in claim 3 , further comprising: normalizing each said value of reflected power before said computing a value of a difference, wherein said normalizing comprises averaging said values of reflected power obtained for said first and said second series of repeated light pulses and dividing each value of reflected power by the average value of reflected power.
6 . The method as claimed in claim 3 , wherein each said first and second series comprises 4 to 100 light pulses.
7 . The method as claimed in claim 1 , wherein said computing a value of a difference comprises computing a second-order joint moment of the difference.
8 . The method as claimed in claim 1 , wherein said averaging respective amplitudes of said repeated reflected light pulses comprises rearranging a timing of each pulse of the series of repeated light pulses in the return light signal and averaging the rearranged return light signal to obtain a value of reflected power.
9 . The method as claimed in claim 1 , wherein said value of the polarization mode dispersion is calculated as a predetermined function of said mean-square value.
10 . The method as claimed in claim 1 , wherein said plurality of polarization-sensitive acquisitions comprises a plurality of pairs of acquisitions performed with a plurality of mutually-different center wavelengths and wherein said plurality of mutually-different center wavelengths is obtained by tuning a laser wavelength of said POTDR in steps between acquisitions.
11 . The method as claimed in claim 1 , wherein said plurality of polarization-sensitive acquisitions comprises a plurality of pairs of acquisitions performed with a plurality of mutually-different center wavelengths and wherein said plurality of mutually-different center wavelengths is obtained by tuning a laser wavelength of said POTDR in continuous.
12 . A system for measuring a polarization mode dispersion (PMD) to characterize an optical fiber under test, the system comprising:
a polarization-sensitive Optical Time Domain Reflectometer (POTDR) acquisition device connectable toward a proximal end of the optical fiber under test for performing a plurality of polarization-sensitive acquisitions toward the optical fiber under test, a distal end of said optical fiber under test being connected to a light reflector,
wherein said POTR acquisition device is configured so that:
each acquisition is performed by propagating in the optical fiber under test, a polarized test signal comprising a first series of repeated light pulses and detecting a corresponding polarization-analyzed return light signal coming back from the optical fiber under test and comprising back-reflected light from the light reflector, said return light signal comprising repeated reflected light pulses, wherein said repeated light pulses are together propagated in the optical fiber under test within a fiber round-trip time,
each of said acquisitions is performed with a corresponding wavelength of said test signal,
said plurality of polarization-sensitive acquisitions defines at least one pair of acquisitions performed with mutually different but closely-spaced wavelengths and substantially the same state of polarization (SOP), a center of the wavelengths defining a center wavelength for said at least one pair, and
said plurality of polarization-sensitive acquisitions comprises a plurality of pairs of acquisitions performed with at least one of a plurality of mutually-different center wavelengths and a plurality of mutually-different states of polarization (SOP); and
a processing unit receiving acquisition data and configured for:
for each said acquisitions, averaging respective amplitudes of at least part of said repeated reflected light pulses to obtain a value of reflected power,
for each said pairs of said acquisitions, computing a value of a difference, between the two values of reflected powers corresponding to said pair,
computing a mean-square value of the computed values of difference over said at least one of a plurality of mutually-different center wavelengths and a plurality of mutually-different states of polarizations (SOPs); and
from said mean-square value, calculating a value of the polarization mode dispersion of said optical fiber under test.
13 . The system as claimed in claim 12 , wherein said POTDR acquisition device comprises:
a light generating assembly comprising a tunable light source and pulse generator and configured to generate said test signal comprising repeated light pulses, at said mutually different wavelengths.
14 . The system as claimed in claim 12 , wherein said POTDR acquisition device comprises:
a light detecting assembly comprising at least one detector configured to detect said return light signal returning from said optical fiber under test from said test signal for each of said acquisitions.
15 . The system as claimed in claim 12 , wherein said POTDR acquisition device comprises a polarization-controller-and-analyzer unit comprising a polarization discriminator, a polarization controller used to control both an input state of polarization and an analyzer state of polarization.
16 . The system as claimed in claim 12 , wherein said POTDR acquisition device is configured such that each of said acquisitions comprises the propagation of said first series of repeated light pulses and a second series of repeated light pulses, wherein the second series is propagated after detection of the return light signal of the first series.
17 . The system as claimed in claim 12 , wherein said POTDR acquisition device is configured such that each acquisitions comprises the propagation of said first series of repeated light pulses and a second series of repeated light pulses, wherein the second series is propagated before detection of the return light signal of the first series.
18 . The system as claimed in claim 16 , wherein said processing unit is further configured for: normalizing each said value of reflected power before said computing a value of a difference, wherein said normalizing comprises averaging said values of reflected power obtained for said first and said second series of repeated light pulses and dividing each value of reflected power by the average value of reflected power.
19 . The system as claimed in claim 12 , wherein said averaging respective amplitudes of said repeated reflected light pulses comprises rearranging a timing of each pulse of the series of repeated light pulses in the return light signal and averaging the rearranged return light signal to obtain a value of reflected power.
20 . The system as claimed in claim 12 , wherein said processing unit is configured for calculating said value of the polarization mode dispersion as a predetermined function of said mean-square value.
21 . The system as claimed in claim 13 , wherein said plurality of polarization-sensitive acquisitions comprises a plurality of pairs of acquisitions performed with a plurality of mutually-different center wavelengths and wherein said tunable light source is configured for tuning a laser wavelength of said POTDR in steps between acquisitions to obtain said plurality of mutually-different center wavelengths.
22 . The system as claimed in claim 13 , wherein said plurality of polarization-sensitive acquisitions comprises a plurality of pairs of acquisitions performed with a plurality of mutually-different center wavelengths and wherein said tunable light source is configured for tuning a laser wavelength of said POTDR in continuous between acquisitions to obtain said plurality of mutually-different center wavelengths.