Displacement detection device and method

A displacement detection device comprising:

• a light source that generates light;
• an objective lens that condenses the light in such a manner that the light travels toward a medium, the medium being rotatable around an axis of rotation and the axis being orthogonal to an optical axis of the objective lens;
• a beam splitter that separates reflected light into portions, the reflected light being reflected by the medium and then transmitted through the objective lens;
• an aperture limiting element including an aperture limited so as to block a portion of the reflected light;
• and
• a detection device that detects displacement of the medium in a direction of the optical axis based on the reflected light having passed through the aperture limiting element.