Patent ID: 7045769

Claim:
A measurement device comprising: an acousto-optic modulator adapted to receive a laser beam and modulate the laser beam based upon two or more frequencies of a received input signal to generate two or more modulated laser beams, wherein the input signal comprises at least a first signal having a first frequency and a second signal having a second frequency; a multiple-pitch grating adapted to receive two or more of the modulated laser beams and provide an output laser beam; a photodetector adapted to receive the output laser beam and provide an output signal; at least one filter adapted to filter the output signal at two or more of the frequencies of the input signal and provide a corresponding filtered output signal, wherein the at least one filter comprises at least a first bandpass filter centered at the first frequency to filter the output signal and provide a first filtered output signal and a second bandpass filter centered at the second frequency to filter the output signal and provide a second filtered output signal; and at least one phase detector adapted to determine a phase difference between a phase of the filtered output signal and a phase of a corresponding one of the signals of the input signal, wherein the at least one phase detector comprises at least a first phase detector adapted to provide a first phase difference between the first filtered output signal and the first signal and a second phase detector adapted to provide a second phase difference between the second filtered output signal and the second signal and wherein the phase difference corresponds to a position measurement of the multiple-pitch grating with the first phase difference and the second phase difference providing relative position measurements of the multiple-pitch grating, and a difference between the first and second phase differences provides an absolute position measurement.