| 摘要 |
PROBLEM TO BE SOLVED: To measure variation of a reflective index of a gas in an optical passage with high accuracy by accurately securing coaxial property of third and fourth lights without influence of variation of an emission direction of a first light. SOLUTION: When an emission direction of a light having a frequencyω2 from a light source 31 is varied with respect to a designed optical axis, lights having frequenciesω2 ',ω3 ' emitted from a frequency modulation section 101 are similarly affected by variation of the emission direction of the light having the frequencyω2 . As a result, since a coaxial property of the lights having the frequenciesω2 ',ω3 ' is attained with high accuracy, a real displacement quantity D of a moving mirror 5 is obtained with high accuracy insusceptibly of variation of the emission direction of the light having the frequencyω2 from the light source 31. Thus, since the coaxial property of the lights having the frequenciesω2 ',ω3 ' is attained with high accuracy insusceptible of variation of the emission direction of the light from the light source 31, it is possible to measure variation of a reflective index of a gas in an optical passage with high accuracy by using a heterodyne interference method.
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