Relative position detection means and displacement detection device

A displacement detection device for optically detecting a displacement of an object to be measured along a measuring direction, comprising:

• a light source for irradiating light;
• a light source side beam splitter for splitting the light from the light source into two;
• a relative position detection means for detecting relative position of the displacement of the object to be measured along the measuring direction based on a change of polarization state of reflected light with respect to one light split by the light source side beam splitter;
• an absolute position detection means for detecting absolute position of the displacement of the object to be measured along the measuring direction based on a change of light quantity of reflected light with respect to another light split by the light source side beam splitter,
• wherein the absolute position detection means and the relative position detection means are arranged on in-line with respect to the measuring direction of the object to be measured,
• wherein the relative position detection means comprising:
• an optical member mounted on the object to be measured and irradiated with light from the light source;
• a light receiver for detection of relative position for receiving light by changing polarization state of reflected light at the optical member with respect to the light;
• and
• a relative position information output unit for outputting relative position information based on the displacement of the optical member along the measuring direction based on a change of polarization state of the reflected light received at the light receiver for detection of relative position,
• wherein the optical member comprises:
• a reflector mounted on the object to be measured;
• and
• a plurality of birefringent members provided on the reflector, the plurality of birefringent members including a first birefringent member having a thickness changing from a tip to a base end along the measuring direction, and a second birefringent member having a thickness changing from a tip to a base end along a direction different from the measuring direction.