Nakyeong Kim

Increasing the storage density of a page-based holographic data storage system by image upscaling using the PSF of the Nyquist aperture.

We introduce an image upscaling method that reduces bit errors caused by Nyquist apertures. Nyquist apertures used for higher storage densities generate optical aberrations and degrade the quality of the image that is recorded on the medium. Here, to correct the bit errors caused by the Nyquist aperture, an image upscaling method is used to restore the degraded image in the enhanced spatial frequency domain using its point spread function (PSF) as a restoration filter. The proposed method reduces the bit error rate (BER) significantly and hence allows higher storage densities.

Line Tracking Applied Data Acquisition Method for Holographic Data Storage System

In a holographic data storage device, data are stored and retrieved as two-dimensional images. These images can be distorted in various ways. Generally, sync marks are used to acquire exact pixel values in a distorted image. However, this inevitably results in some loss of data density. In this research, a virtual detector that follows the rows of an image like a compact disk (CD) track is used to acquire the data pixels in a distorted image without sync marks. The proposed method can compensate for image distortion without any loss of data density. The feasibility of the new technique is demonstrated by simulation. The procedure is also evaluated on the basis of signal-to-noise ratio and bit error ratio.

Design of 2-axis compensation servo system for angle multiplexing Holographic Data Storage

Holographic Data Storage System, one of the next generation data storage devices, is a 2-dimensional page oriented memory system using volume holograms in writing and retrieving process. Recently photopolymer with disc type substrate was selected as a media for the Holographic Data Storage System. The disc tilt occurs when the media rotates and the external disturbance applies. The disc tilt causes the change of the angle between the reference beam and the media, the data cannot be retrieved with the right angle or other data page is retrieved. The tilt is generated in a 2-axis direction (tangential, radial). The tangential tilt direction is the same with the multiplexing plane, while the radial tilt direction is a perpendicular to the multiplexing plane. In this research we propose 2-axis tilt angle servo system. The tilt errors are measured by using external photo detector and the additional red laser. Then the tangential direction tilt is compensated by using the galvano mirror. Also the radial direction tilt is compensated by the rotating prism between the relay lens in the reference field. Finally we confirm the compensation results through the Signal to Noise Ratio(SNR) and Bit Error Rate(BER).

Retrieving Angle Searching Method using Pre-scan for Holographic Data Storage

In retrieving process in angle multiplexing Holographic Data Storage(HDS), It is important to search the exact recording angles. In this paper, we propose the retrieving angle searching method using pre-scan. Pre-scan method is means that rotating the galvano mirror before the retrieving process. Therefore, retrieving signals are maximized when between the galvano mirror and recoded angle are exactly matched. Through the measured signals, we will find the exact angle and compensate tangential tilt angle. The experimental results shows propose method is enough to apply the HDS.

Inter Pixel Interference Reduction using Interference Ratio Mask for Holographic Data Storage

Holographic Data Storage System (HDSS), one of the next generation data storage devices, is a 2-dimensional page oriented memory system using volume hologram. HDSS has many noise sources such as crosstalk, scattering and inter pixel interference, etc. The noise source is changing intensity of the light used for carrying the data signal in HDSS. The inter pixel interference results in decrease of Signal to Noise Ratio and increase of Bit Error Rate. In order to improve these problems, this paper proposes to compensate the inter pixel interference with simple interference mask.

Angle Servo Method for Holographic Data Storage System using Angle Multiplexing

A control method for the angle servo of reference beam during reading recoded data images of a holographic data storage system is presented. The recording scheme with angle offset is adopted in order to verify the proposed angle servo. Using this recording scheme, the angle error signal is generated and we can implement the angle servo. Experiments have been performed on recording and reading data images to compensate Bragg angle mismatch.

Focusing Error Detection Method in Microholographic Data Storage System Using Polarization Characteristics

We present a disk-focusing error detection method for microholographic data storage systems using polarization characteristics. In high-speed optical storage systems, disturbances exist along the vertical direction of a disk. The diffraction efficiency of a readout signal depends on the distance between the focusing point of the reference beam and the vertical center of the microholographic grating. The proposed disk focusing error detection method uses only one laser source, unlike most existing systems, which use two laser sources. By using polarization characteristics, disk-focusing and reading beams are inserted in the media without interference, and the proposed system achieves the simultaneous reading of microholograms and disk-focusing control. By evaluating the focus error signal, we verified the reliability of the optical path and tolerable S-curve (FES) balance (1.8%) for the proposed system. It is possible to achieve a diffraction efficiency of over 90% at the best focus position using the proposed disk-focusing method.