Patent ID: 8636668

Claim:
A measuring apparatus comprising: a semiconductor laser device that emits a laser light beam to an object to be measured; a driving unit that provides a driving signal for modulation drive of the semiconductor laser device; a detection unit that detects a first electrical signal that corresponds to the intensity of the laser light beam modulated due to the self-coupling effect, in a first half-cycle of the driving signal, and that detects a second electrical signal that corresponds to the intensity of a second laser light beam emitted by the semiconductor laser device and modulated due to the self-coupling effect, in a second half-cycle of the driving signal being in a phase opposite to that of the first half-cycle; the first electrical signal corresponding to a first amount of variation between the intensities of the laser light beam at a first time and at a second time, and the second electrical signal corresponding to a second amount of variation between the intensities of the laser light beam at a third time and at a fourth time; 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 the first amount of variation and the second amount of variation is expressed in Equations (1) and (2) respectively; I 1 ( Q 2 −Q 1)= I 0 +ΔI ( Q 2 −Q 1)+ rαI 0 cos [(Δ w +Ω) t−kΔd] (1) I 2 ( Q 4 −Q 3)= I 0 −ΔI ( Q 4 −Q 3)+ rαI 0 cos [(−Δ w +Ω) t−kΔd] (2) where Q 1 , Q 2 , Q 3 , and Q 4 are the first, second, third, and fourth time, respectively, and if the intensity of the laser light beam 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 beam, 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 to be measured, 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 =2ΔI+2αI 0 sin [(Ωt−kΔd) t )sin(Δ wt ) (3) where the reference oscillation intensity I 0 >> the amount of modulation ΔI, and Q 2 −Q 1 ≈Q 4 −Q 3 << 1 .