Higher density optical disk system of super parallel optical heads using a two dimensional VCSEL array are described for the higher data transfer rate and technological capability. Optical heads of the VCSEL array and microlens array play a key role to get higher evanescent light from a small aperture for the optical disk system, of which disk surface is coated with a lubricant and protective film on the flat recording medium in order to keep the gap between the super-parallel optical head and the disk surface within 20 nm. Higher throughput efficiency has been obtained in the near-field semiconductor optical probe array head. However, the obtained evanescent light power is about 10μW from the 100nm probe aperture using a VCSEL 1mW laser output power, which is still not enough to write bits on the phase change optical disk medium. One solution for improvement of the writing power is to develop a special nano-fabricated corrugated thin metal film for higher throughput efficiency by surface plasmon polariton enhancement. A metal fine grating fabrication method to get evanescent light wave resonantly enhancement has been studied with a FDTD simulation result.
In order to meet the requirement for an ultrahigh data transfer rate in future optical communication systems, a parallel optical memory system has been developed using a VCSEL array and three-dimensional microoptical adjustment. The concept, theoretical analysis and fabrication process for the array head are discussed with emphasis on the microoptical issues such as the improvement of optical efficiency by surface plasmon enhancement and microlens focusing. The flat-tip microprobe structure was successfully prepared with the small metal aperture of 150 nm and 1.25% optical throughput using metal-aperture Si or GaP semiconductor microprobes. From the finite difference time domain (FDTD) simulation, the optical throughput can be improved using a buried-type microprobe with an asymmetric metal-coated structure since this is a better structure for improving coupling efficiency between the propagating wave and surface plasmon. Since the focused beam promotes better optical throughput in the integrated VCSEL microprobe array, the microlens array is developed on the other side of the microprobe array using the thermal Reflow of photo-resist and a pattern transfer process to the semiconductor substrate. The alignment of optical axis between the optical components is critical to achieving the required optical efficiency in this system. Thus, three-dimensional micro-optical adjustment is being studied to focus a very small spot on the recording media. This three-dimensional approach will provide guidelines for new micro-optical components in future technology in the field of optical memory and other micro-optical systems.
New optical memory system is urgently required to realize high memory capacity and fast data transfer rate in the coming high-speed internet era. To overcome the current capacity barrier of far-field techniques, a novel near-field optical memory using near-field optics has been proposed using vertical cavity surface emitting laser (VCSEL) microprobe array. Arrays of up to 625 microprobes were prepared successfully using newly developed micro-fabrication process, which includes the photolithography and dry etching by ion-milling method. Since etching rate depends on the incident angle of ion beam to etching surface, it is possible to realize higher etching rate for metal layer deposited on top surface compared to side plane by adjusting the angle of ion beam to both surfaces. In case of GaP microprobes, the relative etching rate of top surface to side plane shows maximum value when top surface is exposed to ion beam with 25 degree. After forming apertures on top surface, GaP probe shows triangle surface of around 200 nm in each edge, which strongly depends on its original size of flat-tip surface before the aperture formation. We believe that ion-milling method developed in this research is very effective to prepare all apertures simultaneously in the array system and can be applied to other microprobes prepared in batch process.
ONe of the interesting applications of 2D VCSEL arrays is high density optical data storage. We proposed a micro-metal aperture VCSEL for producing optical near-field. The evanescent wave emitted from a small metal aperture formed on a VCSEL surface is irradiated to an optical disk, such as a phase change optical disk. We carried out a near-field analysis on the radiation from the metal micro aperture loaded on a VCSEL by using 2D finite element method (FEM), showing a possibility of a spot size of below 100 nm. We can recycle the reflected wave from the metal aperture, when we properly design the phase matching between the DBR mirror and the metal. We can expect an improvement in a power conversion efficiency of radiating near field light from the aperture by using a 'photon recycling' effect. An expected efficiency and power density are discussed for 850 nm metal aperture VCSELs. We have fabricated micro-metal aperture VCSELs by using focus ion beam etching. The size of the fabricated apertures ranges from 100 nm and 400 nm. We have realized sub-mA low threshold metal aperture VCSELs. The power density is estimated to be approximately 6 kW/cm2. We will be able to improve the power density by reducing the oxide aperture in the cavity. We also discuss on another way to increase the efficiency and the power density, such as using a surface plasmon effect of a small metal tip formed on the surface. We will discuss a possibility of optical near-field recording by using the proposed metal aperture VCSEL.
A lensless optical floppy disk drive which does’t have any optical lens between the optical head and the phase change (PC) type optical floppy disk medium have been developed. The head is a contact type, which had already been adopted in higher density magnetic floppy disk drives such as the LS120. The main point of this optical head is an antireflection (AR) coating technology of very low reflectivity on an output facet of a low cost laser diode which is as same as the laser diode installed in a commercial CD player. The AR coat of the laser diode and also the optical floppy disk medium are deposited on to both laser diode facet and to thin polyimid floppy disk substrate using an electron cyclotron resonance sputtering (ECR) method.
A very high density optical disk system is newly proposed. The system uses a laser diode array consisting of 100 X 100 (equals 10 thousands) VCSELs (vertical cavity surface emitting lasers) with a very small center hole on each output mirror of each cavity. Each 10 thousand laser diodes can emit evanescent wave on a surface of optical disk medium and can write information bits on it. Each 10 thousand tracks can be written and read at once, by inclining the array about 0.57 degrees toward to a tangential direction of the rotating disk. The Each track that is about 20 nm width doesn't has any borderline between tracks, which means that the total 10 thousand tracks can be traced electrically at once with 199 micron meter width. The new system enable us to write and read information as rapidly as 10 GByte/sec and to store data up to 2 Tera Bytes on one surface of 5.25' disk.
A laser diode chip coated with thin films on both edge- facets is proposed as a main functional device for a lensless magneto-optical (MO) reading head in a new magneto- optical floppy disk system. The head consists of the laser chip with anti-reflection coats, birefringence quarter wave films (QWF), a half mirror film (HM) and a Faraday rotator thin film, a polarizing film (as a combining device for two polarized waves) and a high speed photo diode chip. They are all die-bonded on one pair of heat sink metal which also works as electrodes supplying the injection current to the laser diode chip. An aero-flow suspender keeps the optical head very close toward the floppy disk recording surface. The laser cavity is formed between HM and MO disk. In this optical head the laser can oscillates two orthogonal linearly polarized laser waves in the inside of the chip but they are converted into two circular polarized waves in the outside of the chip through the two QWFs which are coated on the two edge-facets of the laser chip. The MO signal can be got from detecting a difference frequency between the two circular polarized waves which had been biased by the Kerr effect on the surface of the disk.
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