Patent ID: 8868379

Claim:
An apparatus which obtains a digital reference signal from reference light modulated by a modulation unit at a first frequency, obtains a digital measurement signal from measurement light reflected by a measurement target object irradiated with light modulated at the first frequency, and processes the digital reference signal and the digital measurement signal to measure a position of the measurement target object, the measurement light being modulated at a second frequency along with movement of the measurement target object in addition to the modulation at the first frequency, comprising: a first synchronized detection unit which multiplies the digital measurement signal by a sin signal synchronized with the digital reference signal, and outputs a signal having the second frequency and harmonic components; a second synchronized detection unit which multiplies the digital measurement signal by a cos signal synchronized with the digital reference signal, and outputs a signal having the second frequency and harmonic components; a first decimation filter which filters, at a decimation frequency, the signal output from said first synchronized detection unit to attenuate the harmonic components; a second decimation filter which filters, at the decimation frequency, the signal output from said second synchronized detection unit to attenuate the harmonic components; and a calculation unit which calculates the position of the measurement target object based on a signal output from said first decimation filter and a signal output from said second decimation filter, wherein letting fr be the first frequency, fd be the second frequency, and fm be the decimation frequency, frequencies of the harmonic components are given by (2fr±fd), the first frequency and the decimation frequency satisfy a relation of fr=n×fm (n=one of 1/4, 2/4, 3/4, 4/4, 5/4, . . . ), and said first decimation filter and said second decimation filter attenuate the harmonic components having frequencies represented by (2fr±fd) by a notch filter characteristic in which a gain attenuates at a frequency of k×fm (k=one of 1/2, 2/2, 3/2, 4/2, 5/2, . . . ).