Optical system for measuring linear or angular displacements

TL;DR: An optical system for measuring linear or angular displacements uses a monochromatic radiation source and an acousto-optic modulator, which acts as a Bragg modulator and is driven at a frequency which is caused periodically to vary.

Abstract: An optical system for measuring linear or angular displacements uses a monochromatic radiation source and an acousto-optic modulator, which acts as a Bragg modulator and is driven at a frequency which is caused periodically to vary. The modulator emits a first beam having the same wavelength as the source and a second beam, whose wavelength and emission direction vary with the modulating frequency. The second beam is collimated. For measuring angular displacements, the collimated beam is recombined with the first beam after traversing a plate with a plane parallel faces, to which plate the rotation of a moving detent is transmitted, and a photodetector generates a beat signal whose frequency depends on the angular position of the plate. For measuring linear displacements, the two beams are directly sent to a detector moving with the moving element.