Optical Information Storage In Glass By Using Femtosecond Laser

An intense femtosecond laser pulse can provide a powerful electric field that exceeds the electric field force of valence band that holds the electrons to their ionic cores and thus can results tremendous changes in the electronic structures of an atom or a molecule. In the processes of light-matter interaction, a femtosecond laser pulse exhibits different feature compared with a picosecond and a nanosecond laser pulse, for instance, it has little or no thermal effect, the breakthrough of optical diffraction limit and well space-selectivity. These features can have many important applications in various fields, such as ultra-precision machining, fabrication of micro-photonic devices, nano-operation in bioengineering, three-dimension optical storage and etc.An intense femtosecond pulse tightly focused inside the transparent material will induce a multi-photon absorption process, leaves permanent structure changes with micrometer precision in the material. These changes can be various with the increasing power of the laser pulse. From the refractive index changes caused by the formation of color centers to the micro-voids caused by micro-explosions. The dense optical storage can therefore be achieved by these array of micro-voids localized inside the bulk of the material.Optical hologram is the interference pattern formed by the object wave and the reference wave superimposition and recorded in the mediums. An object image can be sampled first and then Fourier transformed by a computer, so that he complex amplitude and phase distribution can be encoded to form a computer-generated hologram (CGH). In this thesis, the CGH was recorded directly inside silica glass step by step controlled by PC using a tightly focused femtosecond laser pulses with the mechanism of micro-explosion. The fabricated CGH was permanent because it was written inside the bulk of silica glass directly. This process needs no annealing treatment of the material and therefore without surface damage of the material. The CGH cells formed by micro-voids carried the information of the object wave amplitude and the phase. The whole image could be reconstructed with a collimated He-Ne laser beam. By changing the...