Method and Apparatus for Determining Differential Group Delay and Polarization Mode Dispersion

Measurement instrumentation, for measuring at least one polarization-related characteristic of an optical path (FUT), comprising:

• optical source means for connection to the optical path at or adjacent a proximal end thereof, and
• analyzing-and-detection means for connection to the optical path at or adjacent either the proximal end thereof or a distal end thereof for extracting, analyzing and detecting light that has traveled at least part of the FUT and providing corresponding electrical signals, and
• processing means for processing the electrical signals from the output light means to determine said at least one polarization-related characteristic;
• the optical source means comprising
• light source means for supplying at least partially polarized light at each wavelength in at least two groups of wavelengths, and
• SOP controller means for controlling the state of polarization (I-SOP) of said at least partially polarized light and injecting said light into the FUT, wherein the lowermost (λl) and uppermost (λU) of said wavelengths in each said group of wavelengths are closely-spaced,
• the said group comprises a wavelength pair, said pair in each group corresponding to a small optical-frequency difference and defining a midpoint wavelength therebetween, and
• the SOP of the injected light and A-SOP are substantially constant for each said wavelength in each said group, and wherein at least one of the midpoint wavelength, I-SOP and A-SOP is different between the respective said groups, and
• the analyzing-and-detection means comprising:
• means for extracting corresponding light from the FUT and analyzing the extracted light, and detecting the analyzed light corresponding to at least one transmission axis of the analyzer means (A-SOP) to provide transmitted coherent optical power at each wavelength of the analyzed light in each of said at least two groups of wavelengths, wherein the lowermost (λl) and uppermost (λU) said wavelengths in each said group of wavelengths are closely-spaced;
• the processing means being configured and operable for:
• i. Computing the at least one difference in a measured power parameter corresponding to each wavelength in said wavelength pair for each of the said at least two groups, said measured power parameter being proportional to the power of the said analyzed and subsequently detected light, thereby defining a set of at least two measured power parameter differences;
• and
• ii. Computing the mean-square value of said set of differences;
• and
• iii. Calculating the at least one polarization-related FUT characteristic as at least one predetermined function of said mean-square value, said predetermined function being dependent upon the said small optical frequency difference between the wavelengths corresponding to the said each at least said two pairs of closely-spaced wavelengths;
• and
• iv. outputting the value of said at least one polarization-related FUT characteristic for display, transmission or further processing.