| 摘要 |
PURPOSE:To eliminate a variation in an optical distance between mirrors by providing two types of member having different linear expansion coefficients for determining the distance to cancel the variation in the optical distance generated upon temperature change of the refractive index of a medium between the mirrors. CONSTITUTION:Two parallel mirrors 3, 4 and members 1, 2 for determining the distance between the two mirrors are provided, and the two members are so composed as to combine at least two types of materials having different linear expansion coefficients in such a manner that the optical distance represented by the product of the refractive index of the medium between two mirrors and the distance is substantially constant irrespective of the temperature change of the environment. That is, the distance l1 from the connecting surface 5 of mirror holding members 1, 2 to the mirror 3 is not equal to that l2 from the surface 5 to the mirror 4, but the distance l1-l2 between the mirrors can be set to constant by equalizing the temperature changes of the both by utilizing the fact that the linear expansion coefficients are different with each other.
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