Patent ID: 7738112

Claim:
A displacement detection apparatus configured to detect a displacement on the basis of received-light intensity of an interference beam of two polarized beams obtained by splitting a laser beam, the displacement detection apparatus comprising: a light source configured to emit the laser beam to be split into the two polarized beams; a reflective diffraction grating configured to receive the two polarized beams, said reflective diffraction grating including: (a) a first grating disposed on a first line; and (b) a second grating disposed on a second line orthogonally intersecting with the first line on a plane on which the first grating is disposed, the first grating and the second grating being disposed two-dimensionally symmetrical on the plane; two polarization altering elements configured to alter the polarization states of two diffracted beams of the two polarized beams obtained at the reflective diffraction grating; two mirrors configured to reflect the beams whose polarization states have been altered at the two polarization altering elements and to guide the reflected beams to the two polarization altering elements, each of the mirrors corresponding to the polarization altering elements; and a polarization beam splitter provided as a unit with at least the two polarization altering elements, the polarization beam splitter including: (a) a polarizing and splitting surface configured to split the laser beam to generate the two polarized beams, said polarizing and splitting surface of the polarization beam splitter including: (i) a first polarizing and splitting surface configured to generate two first polarized beams; and (ii) a second polarizing and splitting surface configured to generate a second polarized beam, the second polarizing and splitting surface being disposed on a plane different from a plane on which the first polarizing and splitting surface is disposed; and (b) a transmissive area configured to guide: (i) the two polarized beams generated at the polarizing and splitting surface to the reflective diffraction grating; (ii) two return beams from the two polarization altering elements to the polarizing and splitting surface via the reflective diffraction grating so as to obtain the interference beam; (iii) the first polarized beams generated at the first polarizing and splitting surface to the first grating; and (iv) the second polarized beams generated at the second polarizing and splitting surface to the second grating.