Study of Optical Destruction Techniques for Optical Discs

Abstract

The topic of this dissertation is on the investigation of optical techniques for completely destroying data stored in optical discs. Complete and secure destruction of data is important when disposal of an optical disc containing sensitive and possibly classified information is concerned, since some information can be retrieved even from a fragment of a disc. After several candidate techniques and related systems are examined and fundamental system components are identified, an optical data destruction (ODD) system using a focused high power laser beam is devised, and a prototype system is designed and constructed. The ODD system uses a high power laser diode (HPLD) to expose data marks on optical discs and maintains the focused laser beam on a data layer by a focus servo using a diffractive optical element (DOE).The optical characteristics of the beam emitted from an HPLD are thoroughly investigated, and a few methods of modeling an HPLD beam in an optical system are studied. With the understanding of the HPLD beam properties, a limited-divergence raytracing (LDRT) model is developed to predict the propagation behavior of the HPLD beam in an optical system and shows good agreement with the real HPLD beam. This LDRT method is used to model the HPLD beam in the ODD system and simulate the resultant focus error signal with and without fabrication errors.The DOE focus sensor overcomes the problems in conventional focus sensors associated with the use of an intense line beam. The DOE comprising two angled gratings is designed to use only two weak 3rd order beams for focus sensing and fabricated on a chrome-coated glass substrate using a maskless lithography tool.The constructed ODD system is then used to perform destruction tests on various optical discs, which are examined using static and dynamic methods of data observation and retrieval. The observations show that data marks are optical invisible or completely covered with numerous micro-bubbles. These test results demonstrate that secure and complete destruction of data on optical discs is achieved using an ODD system. Successful destruction, however, depends greatly on exposure conditions and the type of optical media.