Acousto-optic modulator, position detector using it, and projection exposure apparatus

摘要

In a position detecting apparatus, a light beam from a white light source is guided through a collimator lens etc. to be converted into a parallel beam, and this parallel beam is incident to a first acousto-optic modulator (AOM) and a second acousto-optic modulator positioned with a spacing by which a phase difference between diffracted light is approximately m pi (where m is an integer). Traveling waves of slightly different frequencies opposite to each other are supplied in opposite directions to the two acousto-optic modulators, and the position detecting apparatus executes position detection by the heterodyne interference method as guiding a beam comprised of +first-order diffracted light from the acousto-optic modulators and a beam comprised of -first-order diffracted light from the acousto-optic modulators through an objective lens to project them with a predetermined intersecting angle onto a diffraction grating mark.