Investigation Of The Three-Dimensional Optical Storage In Transparent Material Using Femtosecond Laser Pulses

An intense femtosecond laser pulse with nJ energy, 800nm, 150-fs at repetition rate of 1 -kHz is tightly focused inside the transparent material, inducing the energy of pulse absorbed by material, leaving a permanent structure changes with sub-micrometer precision in the material. The size of the structure increases with the energy of laser pulse-from small density variations, to voids. The array of these voids in different layers is localized inside the bulk of the material, three-dimensional optical storage is recorded inside the sample. The refractive index changes can be used for micromachining and fabrications of optical devices.In this dissertation, the main aim is to study experimentally some difficult problem in three-dimensional high-density optical storage in transparent materials using femtosecond laser pulse. The context and conclude is summarized as follow:(1) A creative work that a train of laser pulses with 1kHz frequency is modulated by digital signal is achieved for the first time.(2) It is set up a system to write and read, including a software system and a hardware system. This system can be used for micromachining and fabrications of optical devices.(3) It is investigated for pulse with different energy, the bulk structural change morphology inside PMMA (polymethyl methacrylate) and fused silica induced by tightly focused fs laser pulses, and the scale change of the voxels with different depth beneath the surface. It is presented thatdepth range of the storage bits that can be used for 3-D storage in PMMA and fused silica using 800nm, 150fs laser pulses with different energy. The theory explanation for some experiment results is given.(4) It was achieved the practical demonstrate of "3-D optical storage in transparent materials using fs laser pulses" for the first time. The storage format was designed for readout in parallel using transmitted light CCD (Charge Coupled Device) image. After computer image processing, distinguish the array of binary digit from the location of voids, encoding and coding, it was achieved that recording and readout of 5 layers information recorded in PMMA with 2.5jnm separation between two bits and 15|j.m separation between two layers. The storage volumetric density is 1010bits/cm3. On the assumption that recording depth is lmm, the capability of this kind storage device with the same area as CD is 1600 larger than that of a CD.(5) It was investigated experimentally writing of internal diffraction grating, and numerical fitting computing. It is presented that a way to increase the diffracive efficiency.