Patent ID: 7265837

Claim:
A method, comprising: using a first partial polarization beam splitter to split by reflection a fraction of light in one of first and second mutually orthogonal polarization directions from an input beam to produce a first monitor beam; using a second partial polarization beam splitter to split by reflection a fraction of said light in said one of said first and second mutually orthogonal polarization directions from said input beam to produce a second monitor beam, wherein said first and second partial polarization beam splitters are oriented to have their polarization axes to be 90 degrees with each other; converting said first and said second monitor beams into first, and second detector signals, respectively; using a difference between said first and said second detector signals to indicate an amount and a direction of a deviation in a polarization of said light from a known direction; and using a third partial polarization beam splitter to split by reflection a fraction of said light in said one of said first and second mutually orthogonal polarization directions from said input beam to produce a third monitor beam, wherein said third partial polarization beam splitter is oriented between polarization axed of said first and second partial polarization beam splitters at 45 degrees.