Noncontact acoustic optic scanning laser vibrometer for determining the difference between an object and a reference surface

Abstract

A technique and apparatus for non-contact scanning measuring of the dynamic parameters of micro and macro devices using an acousto optic scanning laser vibrometer are disclosed. The system includes an acousto optic deflector to induce scanning in the laser beam. The apparatus also includes either a heterodyne or homodyne system for laser scanning. The heterodyne detection technique involves two acousto optic deflectors driven by a common signal generator. The invention may include an interference technique in which the measuring scanning beam emitted by the acousto optic deflector interferes with the reference-scanning beam. For some applications, this acts as a second measuring beam. With this technique, the frequency shift induced in the laser beam on scanning with the acousto optic deflector is canceled due to fact that the two acousto optic deflector are of same specification and driven by a common driver. The invention may also include an apparatus and technique for homodyne detection. A method adopting single and double acousto optic deflectors in the optical layout is also disclosed for homodyne detection system. The technique also incorporates two axes scanning. The invention may include a computer controller to control the scanning parameters and the data capturing parameters simultaneously. This technique incorporates a parallel scanning beam of small spot size for the purpose of micro device inspection in optical head, hard disk, micro components, etc. The use of the invention for measurement of least fly-height information in hard disk by double axis scanning is also disclosed. The method and apparatus for applying the invention to measuring the dynamic parameters of rotating targets and eliminating the error induced due to pseudo vibration noise is also disclosed.