US 11,867,809 B2
Measurement apparatus and measurement method
Yoshimasa Suzuki, Tsukuba (JP); Shinichi Hara, Tsukuba (JP); Shinji Komatsuzaki, Mito (JP); Ryusuke Kato, Tsukuba (JP); Hiroki Ujihara, Tsukuba (JP); Masayuki Nara, Tsukuba (JP); and Tomotaka Takahashi, Tsukuba (JP)
Assigned to Mitutoyo Corporation, Kawasaki (JP)
Filed by Mitutoyo Corporation, Kawasaki (JP)
Filed on Jul. 6, 2020, as Appl. No. 16/921,839.
Claims priority of application No. 2019-125541 (JP), filed on Jul. 4, 2019.
Prior Publication US 2021/0011155 A1, Jan. 14, 2021
Int. Cl. G01S 17/08 (2006.01); G01S 7/481 (2006.01); G01S 7/48 (2006.01)
CPC G01S 17/08 (2013.01) [G01S 7/4808 (2013.01); G01S 7/4814 (2013.01)] 10 Claims
OG exemplary drawing
 
1. A measurement apparatus for measuring a distance to an object to be measured, the measurement apparatus comprising:
a laser apparatus that has an optical cavity and outputs a frequency-modulated laser beam with a plurality of modes;
a branch that branches a portion of the frequency-modulated laser beam output by the laser apparatus as a reference light and at least some of the remaining portion of the frequency-modulated laser beam as a measurement light;
a beat signal generator that generates a beat signal by mixing the reference light and a reflected light that is reflected by radiating the measurement light onto the object to be measured;
a converter that converts the beat signal into a digital signal at a first sampling rate and then outputs a frequency vB(m, d) of the beat signal by a frequency analysis on the digital signal;
an extractor that extracts a signal component of a cavity frequency vc of the optical cavity and being superimposed on the frequency-modulated laser beam output from the laser apparatus;
a digital filter that digitally filters the extracted signal component at a second sampling rate; and
calculation circuitry configured to calculate a difference in a propagation distance between the reference light and the measurement light by substituting the frequency vB(m, d) of the beat signal and the digitally filtered extracted signal,
the cavity frequency vc of the optical cavity into the following equation:

OG Complex Work Unit Math
where m is an interval between longitudinal mode numbers of the reflected and reference beams of the frequency-modulated laser light, c is optical velocity, and vs is frequency shift amount of the frequency-modulated laser beam.