Patent ID: 11867809
Assignee: MITUTOYO CORPORATION
Field: Measurement (Instruments)
Classification: CPC G | IPC G

Claim 8:
9. A measurement method for measuring a distance to an object to be measured, the measurement method comprising:
outputting a frequency-modulated laser beam with a plurality of modes from a laser apparatus having an optical cavity;
branching a portion of the frequency-modulated laser beam as a reference light and at least some of the remaining portion as a measurement light;
generating 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;
converting 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;
extracting a signal component of a cavity frequency vc of the optical cavity and being superimposed on the frequency-modulated laser beam;
digitally filtering the signal component at a second sampling rate; and
calculating 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:, d
  =
  
   
    c
    
     2
     ⁢
     
      v
      s
     
     ⁢
     
      v
      c
     
    
   
   ⁢
   
    {
    
     
      
       v
       B
      
      (
      
       m
       ,
       d
      
      )
     
     +
     
      m
      ⁢
      
       v
       c
      
     
    
    }
   
  
 

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.