Patent ID: 8529460

Claim:
A measuring apparatus comprising: a driving unit that outputs a first driving signal and a second driving signal having a phase opposite to that of the first driving signal; a first semiconductor laser device, driven by the first driving signal, that emits a first laser light beam to an object to be measured; a second semiconductor laser device, disposed in proximity to the first semiconductor laser device and driven by the second driving signal, that emits a second laser light beam to the object to be measured; a first detection unit that detects a first electrical signal, the first electrical signal corresponding to the intensity of the first laser light beam modulated due to the self-coupling effect; a second detection unit that detects a second electrical signal, the second electrical signal corresponding to the intensity of the second laser light beam modulated due to the self-coupling effect; a calculation unit that calculates a difference between the first electrical signal and the second electrical signal; and a measuring unit that measures a change in the state of the object to be measured based on the calculated difference; wherein first and second detection units are configured to calculate the intensities of the first and second laser light beams modulated due to the self-coupling effect as expressed by Equations (1) and (2) respectively I 1 ( t )= I 0 +ΔI ( t )+ rαI 0 cos [(Δω+Ω) t−kΔd] (1) I 2 ( t )= I 0 −ΔI ( t )+ rαI 0 cos [(−Δω+Ω) t−kΔd] (2) where, if the intensity of the laser light emitted to the object to be measured is I(=I 0 +ΔI), then I 0 is the reference oscillation intensity of the laser light beam, ΔI is the amount of modulation from the reference oscillation intensity I 0 , r is the reflectivity of the object to be measured, α is the self-coupling efficiency of the laser light, Δω is the difference between the oscillation frequency of the laser light beam emitted from the semiconductor laser device and the frequency of the reflected light, Ω is the amount of phase-shift due to the Doppler effect upon the reflection from the object, Δd is the amount of displacement of the object to be measured, and k is the wave number vector of the reflected light; and the calculation unit is configured to calculate the difference between the first electrical signal and the second electrical signal as expressed by Equation (3); I 1 −I 2 =2Δ I +2α I 0 sin [(Ω t−kΔd ) t ] sin(Δ wt ) (3) where the reference oscillation intensity I 0 >> the amount of modulation ΔI.