Measuring polarization mode dispersion
A device for measuring polarization mode dispersion (PMD) in a device under test (DUT) includes a polarized light source for launching a test beam through the DUT, and a PMD analyzer, which uses one of a plurality of known techniques, e.g. fixed analyzer-Fourier transform (FA-FT) or interferometric, to calculate the PMD from the DUT. A passive depolarizer, made up of a plurality of birefringent elements is disposed between the light source and the PMD analyzer to generate a plurality of carrier frequencies that correspond to the delays imposed by the birefringent elements. The PMD content of the DUT is present around each of the carriers, and the plurality of PMD measurements can be averaged to obtain a more accurate and repeatable measure of the PMD.
1. A device for measuring polarization mode dispersion (PMD) of an optical device under test (DUT) comprising:
a polarized light source for launching an optical test signal, having a polarization state, through the DUT;
light depolarizing means between the polarized light source and the optical device, comprising:
a first birefringent element having an eigenaxis oriented at an acute angle to the polarization state of the optical test signal; and
a second birefringent element having an eigenaxis oriented at an acute angle to the eigenaxis of the first birefringent element and the polarization state of the optical test signal;
whereby the optical test signal includes a plurality of carrier components associated with time delays from the first and second birefringent elements; and
analyzing means for calculating the average PMD of the DUT by taking an average of the PMD from the plurality of carrier components.
2. The device according to claim 1 , wherein the average is a weighted average based on the power of each carrier.
3. The device according to claim 1 , wherein the average is obtained by merging the plurality of carriers into a resultant peak, and calculating the RMS width of the resultant peak.
4. The device according to claim 1 , wherein the analyzing means comprises a fixed axis-Fourier transform set-up including:
a polarizer for analyzing the optical test signal exiting the DUT;
an optical spectrum analyzer for generating an optical spectrum of a polarization transmission versus optical wavelength from the optical test signal exiting the polarizer; and
a computer for Fourier transforming the optical spectrum and calculating the average PMD from the plurality of carrier components.
5. The device according to claim 1 , wherein the analyzing means comprises an interferometric set-up including:
an interferometer for interfering portions of the optical test signal forming a transmitted signal;
a detector for measuring intensity of the transmitted signal; and
a computer for calculating the average PMD from the plurality of carrier components.
6. The device according to claim 1 , wherein the plurality of carrier components comprise:
a first carrier component based on a difference between ordinary and extraordinary optical path lengths through the first birefringent element;
a second carrier component based on a difference between ordinary and extraordinary path lengths through the second birefringent element;
a third carrier component based on a difference between ordinary and extraordinary optical path lengths through the combination of the first and second birefringent elements; and
a fourth carrier component based on a difference between ordinary and extraordinary optical path lengths through a difference between the first and second birefringent elements.
7. The device according to claim 6 , wherein the first, second, third and fourth carrier components are within 5 ps to 30 ps of adjacent carrier components.
8. The device according to claim 6 , wherein at least one of carrier components is 50 ps or more from adjacent carrier components for measuring PMD up to 10 ps, and at least one of the carrier components is within 5 to 30 ps of each other for measuring PMD up to 5 ps.
9. The device according to claim 6 , wherein the eigenaxis of the first birefringent element is oriented at ±π/6 with respect to the polarization state of the optical test signal; and wherein the eigenaxis of the second birefringent element is oriented at ±π/4 to the eigenaxis of the first birefringent element.
10. The device according to claim 6 , further comprising a third birefringent element having an eigenaxis oriented at an acute angle to the eigenaxis of the first and second birefringent elements and the polarization state of the optical test signal providing additional carrier components; wherein a plurality of the carrier components are from 5 ps to 30 ps from each other for measuring low range measurements; and wherein a plurality of the carrier components are from 30 ps to 100 ps from each other for measuring higher range measurements.
11. The device according to claim 1 , wherein the eigenaxis of the first birefringent element is oriented at ±π/6 with respect to the polarization state of the optical test signal; and wherein the eigenaxis of the second birefringent element is oriented at ±π/4 to the eigenaxis of the first birefringent element.
12. The device according to claim 1 , further comprising a third birefringent element having an eigenaxis oriented at an acute angle to the eigenaxis of the first and second birefringent elements and the polarization state of the optical test signal providing additional carrier components; wherein a plurality of the carrier components are from 5 ps to 30 ps from each other for measuring low range measurements; and wherein a plurality of the carrier components are from 30 ps to 100 ps from each other for measuring higher range measurements.