Patent ID: 8922785

Claim:
A method for measuring a displacement, the method comprising: generating a first reflected beam and a transmitted beam from an incidence of a parallel beam upon a first diffraction grating and a semi-reflective mirror in sequence, the first diffraction grating and the semi-reflective mirror being disposed along an optical axis, wherein the first reflected beam is generated by a reflection of the parallel beam by the semi-reflective mirror and the transmitted beam is generated by a transmission of the parallel beam by the semi-reflective mirror; generating a second reflected beam by a reflection of the transmitted beam by a total reflection mirror movable along the optical axis; dividing the first reflected beam into a first 0 th order beam propagating in a direction same as that of the first reflected beam and a first ±n th order beam having a diffraction angle with respect to the first 0 th order beam by an incidence of the first reflected beam upon the first diffraction grating; dividing the second reflected beam into a second 0 th order beam and a second ±n th order beam having a diffraction angle with respect to the second 0 th order beam by an incidence of the second reflected beam upon the first diffraction grating; and measuring a first displacement in a direction of the optical axis from a first light intensity obtained by receiving an interference beam of the first ±n th order beam and the second ±n th order beam.