Other

Nanyang Technological University

Krishnan Venkatakrishnan

Bryan Kok Ann Ngoi

A system and method for high speed and precision measurement of the distance between at least two near contact surfaces using heterodyne interferometry is disclosed. One of the surfaces is an optically transparent element and the other surface is a substantially non-transparent element. A laser source produces an output having two superimposed orthogonally polarized beams having S and P polarization, with a frequency difference between them. The polarized beams are split into measurement and reference beams without altering the characteristics of the polarized beams. The reference beams are caused to interfere, and a reference photo detector detects the reference beams and provides a reference signal. The measurement beam strikes the object of interest at an oblique angle after passing through a glass plate having a polarization coating on the bottom surface close to the object of interest. The oblique angle is such that the S polarization of the incident beam is reflected from the bottom surface of the polarization coated glass plate and the P polarization refracts through the glass plate. The P polarization reflects from the substantially non-transparent object of interest and refracts to the glass plate. The reflected S and P polarization beams from the bottom surface of the glass plate and the surface of the object are made to interfere. A measurement photo detector detects the measurement beams and provides a measurement signal. The distance between the bottom surface of the glass disk and the object surface based on the phase deference between the measurement and reference signals is determined from the measurement and reference photo detectors.

Heterodyne interferometry for small spacing measurement

A method and apparatus for precision laser scanning suitable for precision machining or cleaning using an ultrashort pulsed laser beam are disclosed. The apparatus employs a laser source that emits a pulsed laser beam, a dispersion compensation scanner that scans the pulsed laser beam, and a focusing unit that focuses the pulsed laser beam from the dispersion compensation scanner on a work piece. The dispersion compensation scanner comprises a first scanning device that scans the pulsed laser beam in a first direction and that causes dispersion of the pulsed laser beam. The dispersion compensation scanner further comprises a first dispersion compensation device that compensates for the dispersion caused by the first scanning device. Effects of polarization of the ultrashort pulsed laser beam on the quality of machining are described. Uses of the invention in direct fabrication of photolithographic masks and in work piece cleaning are also described.

Ultrashort pulsed laser micromachining/submicromachining using an acoustooptic scanning device with dispersion compensation

An apparatus and method for using an acousto optic scanning laser vibrometer for measuring a dynamic parameter of micro and macro components is disclosed. A coherent source of a laser beam of single wavelength and of stabilized frequency is split into two orthogonal polarized beams. One of the beams strikes the surface of investigation and gets reflected back, and the other polarized beam impinges on the reference surface and gets reflected back. The beam reflected from the surface of investigation and the beam from the reference surface are combined, thereby causing them to interfere. At least one photo detector is positioned at the point of interference. The photo detector output signals are input to a signal processor or phase meter to obtain the dynamic parameter information. Information is provided that is based on the phase shift between the beam striking on the object of investigation and the beam striking the reference surface due to the difference in the optical path. The information provided relates to the dynamic parameters of the object under investigation.

Acousto optic scanning laser vibrometer for determining the dynamic properties of an object

A miniaturized stationary optical storage system capable of reading and writing data on an optical storage media is disclosed. In the disclosed invention, the disk is held stationary and the beam is made to scan on both the axes by using an acousto optic technique. The preferred embodiment has a technique of obtaining parallel scanning beam, which is focused on to the optical storage media and the photo detector receives the reflected beam carrying the optical signal. The system has the capability of reading as well as writing on the optical storage media by using the same source or different source. The preferred embodiment also utilizes square as well as circular optical storage media of reduced size, which is achieved, by adopting smaller spot size and higher scanning resolution. The method of magnifying the scan angle of the scanning beam from the acousto optic deflector is also disclosed. The alternative embodiment utilizes phase shifting for reading the signal rather than the intensity of the signal.

Acousto optic data storage system on a stationary and high density data storage media

A technique and apparatus is disclosed for micro machining using an ultra short laser pulse in the range of femto second pulsing. The system is also applicable for smaller or higher pulse rates depending upon the application. The system includes methods for improving the beam quality of the laser beam by filtering. Moreover, it includes the concept of scanning the laser beam using acousto optic deflectors in the X-Y direction rather than conventional mechanical movement of the work piece or deflecting the beam using a mirror. The technique also incorporates means for modulating the ultra short laser pulse in order to control the number of pulses of the ultra short laser pulse which will strike the target surface at each of the target points by a combination of acousto optic modulators. The concept of applying elliptical or circular laser beam spots for machining rather than a circular one is also disclosed.

Three dimensional micro machining with a modulated ultra-short laser pulse

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.

Growing community of inventors

Singapore, Singapore

Krishnan Venkatakrishnan