Method and apparatus for compensating for polarization mode dispersion (PMD) using a Mach-Zender interferometer

A polarization mode dispersion (PMD) compensator for compensating for PMD occurring in an optical input transmission line comprising:

• optical circuitry for receiving from the optical input transmission line an input signal comprising first and second PMD generated optical pulses having arbitrarily oriented first and second principal states of polarization, respectively, and for selectively adjusting the aligning of the received arbitrarily oriented first and second principal states of polarization to predetermined fixed first and second principal states of polarization required for directing the first and second PMD optical pulses onto respective first and second output paths;
• an interferometer arrangement comprising first and second paths that are coupled to the first and second output paths, respectively, of the optical circuitry for propagating the respective first and second PMD optical pulses, the interferometer arrangement measuring any PMD induced differential delay between the first and second principal states of polarization of the respective first and second PMD optical pulses, and selectively compensating for the measured differential delay;
• an optical combiner for optically combining the signals from the first and second paths of the interferometer arrangement to generate a combined output signal;
• and
• a control arrangement for selectively controlling both the compensation for the measured differential delay between the first and second polarization states in the interferometer arrangement in response to at least a portion of the combined output signal from the optical combiner, and for selectively adjusting the aligning of the received arbitrarily oriented first and second polarizations to the required predetermined fixed first and second polarizations in the optical circuitry.

A method of compensating for polarization mode dispersion (PMD) produced in a transmission line comprising the steps of:

• (a) receiving an input optical signal comprising first and second PMD associated optical pulses having arbitrary orientations of their first and second principal states of polarization, respectively;
• (b) adjusting the arbitrarily oriented first and second principal states of polarization in a polarization controller to match predetermined fixed first and second principal states of polarization of a polarization beam splitter;
• (c) directing the first and second PMD optical pulses with their adjusted first and second principal states of polarization, respectively, from step (b) onto respective first and second output paths of the beam splitter;
• (d) rotating one of the first and second principal state of polarization of the first and second PMD associated optical pulses with a waveplate coupled in one of-the first and second paths so that the first and second principal states of polarization are aligned;
• (e) selectively compensating for a measured differential delay in an interferometer arrangement comprising first and second paths that are coupled to the first and second output paths, respectively, of the polarization beam splitter the selective compensation including delaying one of the first and second PMD associated optical pulses with an optical delay element coupled in one of the respective first and second paths in order to synchronize the first and second PMD associated pulses propagating in the first and second paths;
• and
• (f) optically combining the signals from the first and second paths of the interferometer arrangement with an optical combiner to generate a combined optical output signal.