Why does laser interferometer need expansion coefficient?

Laser interferometer needs expansion coefficient to ensure that the distance between two plates is fixed and strictly parallel.

Laser interferometer is a kind of precision measuring instrument, which is used to measure the interference phenomenon of light. In laser interferometer, parallel beams can be formed by using extended light source illumination and lens. In the structure of interferometer, there are usually two parallel glass or metal plates, one of which is fixed and the other can move, thus changing the distance between the two plates. In the process of moving, it is difficult to keep the inner walls of the two plates strictly parallel due to the influence of factors such as thermal expansion and cold contraction of materials. In order to solve this problem, a parallel spacer is usually added to the laser interferometer, which is usually made of indium steel and other materials, and its expansion coefficient is very small. When the temperature changes, the expansion or contraction of the gasket can offset the deformation between the two plates to some extent, thus keeping the distance between the two plates fixed and strictly parallel.