System for detecting rotation angles of a diffracting portion using a absorption cell sealed with a plurality kinds of gases

An apparatus for detecting rotation angles of a diffracting portion comprising:

• a diffracting portion which is composed of optical elements including a predetermined optical element and diffracts light;
• a drive unit which rotates the predetermined optical element of the diffracting portion;
• an angle detector which detects the rotation angle of the predetermined optical element;
• a reference wavelength light source which includes a light source and an absorption cell and emits the reference light of a specific wavelength determined depending on said absorption cell toward said diffracting portion;
• a reference photodetector which receives a diffracted light emitted in a predetermined direction from the diffracting portion receiving the reference light from the reference wavelength light source, and converts the diffracted light into an electric signal;
• and
• a signal processor which receives a rotation angle to be issued by said angle detector when detecting a predetermined value of the electric signal from said reference photodetector by rotating the predetermined optical element by said drive unit, and determines the predetermined rotation angle corresponding to the specific wavelength,
• wherein said absorption cell is sealed with a plurality of gases of different kinds having mutually different in the wavelength region in which absorption lines exist, and
• said signal processor is designed to determine plural specific rotation angles corresponding to the specific wavelength determined depending on the plurality of gases of different kinds sealed in the absorption cell, and also determine the rotation angle between plural specific rotation angles by predetermined complementary calculation including linear complement.