Position detection method and optical module

A position detection method for detecting a position of a movable portion which moves along a predetermined direction, comprising:

• splitting detection light into first light and second light in an interference optical system including a movable mirror coupled to the movable portion and a beam splitter having a fixed position, wherein the first light is incident on a circulating optical path formed in the interference optical system by the movable mirror and the beam splitter, and a portion of the first light is transmitted through the beam splitter and a remaining portion of the first light is reflected by the beam splitter to reach the beam splitter through the movable mirror every time the first light reaches the beam splitter through the movable mirror in the circulating optical path;
• combining the first light transmitted through the beam splitter from the circulating optical path and the second light to generate multiple interference light in the interference optical system;
• extracting a second interference light signal having a wavelength of 1/p (p is a natural number) of a wavelength of the detection light from a first interference light signal of the multiple interference light;
• and
• calculating the position of the movable portion in the predetermined direction based on the second interference light signal.