Patent ID: 7161680

Claim:
A method of interferometrically obtaining measurements for properties associated with a spot on or in an object, said method comprising: receiving a sequence of M optical pulses separated in time, wherein M is an integer; from each pulse in the sequence of M optical pulses, generating an n-tuplet of measurement pulses spaced apart in time, and an n-tuplet of reference pulses that are spaced apart in time, wherein n is an integer, wherein each measurement pulse has a corresponding reference pulse aligned with it in time; from each pulse of each n-tuplet of reference pulses for the sequence of M optical pulses, generating a reference beam; from each pulse of each n-tuplet of measurement pulses for the sequence of M optical pulses, (a) generating a measurement beam; (b) directing the measurement beam onto said spot to thereby produce a return measurement beam from said spot; and (c) combining the return measurement beam with the corresponding reference beam that was derived from the reference pulse corresponding to that measurement pulse to generate an interference beam; wherein the sequence of M n-tuplets of measurement pulses forms n interleaved sequences of M measurement pulses, and wherein said method further comprises, for each of the n interleaved sequences of M measurement pulses, introducing a combination of phase shifts between the measurement beams and corresponding reference beams.