Arbitrary and endless polarization controller and polarization-mode dispersion compensator using the same and arbitrary and endless polarization controlling method

A polarization-mode dispersion compensator, comprising:

• a variable polarization light element having an optical axis and a phase shift, the variable polarization light element changing a polarization state of light input to the variable polarization light element by changing an azimuth of the optical axis or the phase shift;
• a polarization controller, the polarization controller compensating for polarization-mode dispersion of the input light by changing the azimuth of the optical axis or the phase shift of the variable polarization light element;
• a polarization state monitor, the polarization state monitor monitoring the polarization state of the input light and a polarization state of the output light of said variable polarization light element;
• a differential group delay generator for giving a differential group delay to the output light of said variable polarization light element;
• a degree of polarization measuring instrument for measuring a degree of polarization of the output light of said differential group delay generator;
• and
• a controller, the controller controlling said azimuth and said phase shift so that the degree of polarization measured at said degree of polarization measuring instrument becomes a maximum value, the controller changing the azimuth of said optical axis or inverting a changing direction of said phase shift when absolute values of Stokes parameters of the polarization state of the input light are substantially equal to absolute values of Stokes parameters of the polarization state of the output light.

A polarization-mode dispersion compensator comprising:

• a variable polarization light element having an optical axis and a phase shift, the variable polarization light element changing a polarization state of light input to the variable polarization light element by changing an azimuth of the optical axis or the phase shift;
• a polarization controller, the polarization controller compensating for polarization-mode dispersion of the input light by changing the azimuth of the optical axis or the phase shift of the variable polarization light element;
• a differential group delay generator for giving a differential group delay to output light of said variable polarization light element;
• a degree of polarization measuring instrument for measuring the degree of polarization of the output light of said differential group delay generator;
• and
• a controller, the controller when said degree of polarization measured by said degree of polarization measuring instrument is kept even if a minute amount change of said phase shift and a minute amount rotation of said azimuth of said variable polarization light element are repeated, the controller judging that absolute values of Stokes parameters of the polarization state of the input light are substantially equal to absolute values of Stokes parameters of the polarization state of the output light, and rotating said azimuth of said variable polarization light element or inverting a changing direction of said phase shift.

An polarization controlling method for controlling a polarization state of light, comprising the steps of:

• providing a variable polarization light element having an optical axis, the variable polarization light element changing a polarization state of input light by changing an azimuth of the optical axis and a phase shift of the variable polarization light element;
• monitoring a polarization state of the input light and a polarization state of output light of said variable polarization light element;
• and
• rotating the azimuth of said variable polarization light element or inverting the changing direction of said phase shift, when absolute values of Stokes parameters of the polarization state of the input light are substantially equal to absolute values of Stokes parameters of the polarization state of the output light.

An polarization controlling method for controlling a polarization state of light comprising the steps of:

• providing a variable polarization light element having an optical axis, the variable polarization light element changing a polarization state of input light by changing an azimuth of the optical axis and a phase shift of the variable polarization light element;
• measuring a degree of polarization of output light of said variable polarization light element by a degree of polarization measuring instrument;
• and
• controlling a differential group delay generator for giving a differential group delay to the output light of said variable polarization light element so that the degree of polarization measured at said degree of polarization measuring instrument becomes a maximum value,
• rotating the azimuth of said variable polarization light element or inverting a changing direction of said phase shift when absolute values of Stokes parameters of the polarization state of the input light are substantially equal to the absolute values of Stokes parameters of the polarization state of the output light.

An polarization controlling method for compensating polarization-mode dispersion, comprising the steps of:

• providing a variable polarization light element having an optical axis, the variable polarization light element changing a polarization state of input light by changing an azimuth of the optical axis and a phase shift of the variable polarization light element;
• measuring the degree of polarization of output light of said variable polarization light element by a degree of polarization measuring instrument;
• and
• judging that the absolute values of Stokes parameters of the polarization state of the input light are substantially equal to the absolute values of Stokes parameters of the polarization state of the output light,
• keeping said degree of polarization measured by said degree of polarization measuring instrument even if a minute amount change of said phase shift and a minute amount rotation of said azimuth of said variable polarization light element are repeated, and rotating said azimuth of said variable polarization light element, or inverting a changing direction of said phase shift.