TaeSun Song

Aspherical Solid Immersion Lens of Integrated Optical Head for Near-Field Recording.

New solid immersion lenses (SILs) have been studied for a high-density optical storage system by the near-field process that can overcome the far-field diffraction limit. We have proposed the aspherical SILs, named elliptic SIL (ESIL) and Cartesian SIL (CSIL) according to geometrical optics. The SILs have a high numerical aperture (NA), for instance, the NAs of the ESIL and the CSIL are over 1, with a refractive index of 1.56 of the disc cover layer. The SILs that include the function of the objective lens are able to read/write the signals inside the disc substrate. The optical heads employing an internal recording method are expected to be utilized in an unsealed environment. Experimental results of the application of an ESIL are presented. The replicated ESIL (RESIL) has been proposed to solve critical issues such as the problems of the thickness error of the SILs or the disc substrate. These problems need to be solved for the commercialization of the near field recording (NFR) technology.

Elliptic solid immersion lens for NFR; compensation for disk thickness variation and disk tilt

In the last decade of optical data storage development, near-field recording (NFR) technology has been suggested for achieving higher areal density. However, there remain several serious and practical issues. The major defects in NFR due to the use of a solid immersion lens (SIL) are dust and thermal problems compared to orthodox optical storage technologies. The existing SIL is delicate in an unsealed environment due to heat, dust, contamination etc. CISD released a new SIL concept called elliptic solid immersion lens (ESIL) at the last ISOM (T.S. Song et al, ISOM01 p. 130, 2001). The new ESIL is capable of resolving some critical problems that are possibly caused by the existing SIL which records on the disk surface; in addition, the ESIL does not require an objective lens and with its simple structure, it has achieved improvements in mechanical tolerances of the alignment process and in pickup actuator dynamics. Although the ESIL possesses the potential to reduce present restrictions from the conventional SIL, it is necessary to establish a focusing servo system against variation in thickness of the cover layer. Also, a gap maintenance and tilt compensation system using an optical sensor and PZT was proposed in this paper.

Gap maintenance system using near-field optics and piezoelectric materials for near-field recording

In this paper, our aim is to develop a system that can not only maintain stable gap distance under 100nm but also compensate the tilting between a pick-up head and disk surface for near-field recording (NFR), which is known as a key technology for a next generation optical storage. Applying total internal reflection (TIR) to the air gap measurement, we design an optical sensor to measure the gap distances at three points. Stack and bimorph piezoelectric actuators are utilized for high precision control with nanometer resolution. To understand dynamic characteristics of the system, an analytical method for boundary coupled beam model is performed and verified by comparison with the results of the finite element method (FEM).

Design of Cover-Layer-Incident Dual Layer Near-Field Recording Optics Using Elliptical Solid Immersion Lens

We propose the design of novel optics for media within a dual-layer near-field recording (NFR) device using an elliptical solid immersion lens (E-SIL). E-SIL-based near-field optics is designed for cover-layer-incident dual focus positions with a high numerical aperture of 1.4 in the region around λ= 405 nm. In this design, to improve the small optical tolerance for each imaging position and to insure the ability to fabricate the device, an additional aspheric lens surface is added on top of the hemispheric lens, and it is combined with zoom optics which are composed of two single lenses. To compensate for serious chromatic aberrations which happen seriously in optics using blue laser diodes, a diffractive optical element (DOE) is used. Using zoom optics, an additional aspheric lens surface, and DOEs together, wavefront aberration is effectively reduced over a broad range of variation in cover layer thickness and wavelength. In addition, in this study, an air-gap-induced aberration in the designed NFR optics is briefly observed.

Novel Cover-Layer-Incident Dual-Layer Near-Field Recording Optics Using Hemispherical Solid-Immersion-Lens

A novel for the design optics for using cover layer-incident dual-layer near-field recording (NFR) is proposed. Using two aplanatic positions of a hemispherical solid immersion lens (HMS), dual-layer NFR optics is designed and optimized. To achieve a high system numerical aperture (NA) of 1.45 for the dual recording layers, a prefocusing lens having NA of 0.7 and a solid immersion lens (SIL) with a refractive index of 2.086 are used. Critical spherical aberrations caused by variations in thickness and chromatic aberrations are effectively suppressed by zoom optics, an additional aspheric lens surface, and a diffractive optical element (DOE). The gap-induced aberration of the designed NFR optics is also investigated.

Effect of Elliptical Gaussian Beam in Solid-Immersion-Lens-Based Near-Field Recording Optics

In this study, we summarized the characteristics of beam spot profiles, exit pupil images, the gap error signal and the push–pull signal obtained when using a Gaussian beam in solid immersion lens (SIL) based near-field recording (NFR) optics. As the rim intensity of the Gaussian beam decreases and the tracking direction shift of the SIL assembly increases, the intensity of the beam spot is decreases. The exit pupil image is distorted asymmetrically by the tracking direction shift of the SIL assembly. We confirmed that the tracking direction shift of the SIL assembly affects the gap error signal and push–pull signal. When using linearly polarized incident light, it is observed that a tracking direction shift of 500 µm induces a gap error signal reduction of 3.2%. The push–pull signal is offset by 18.7% in the case of a tracking direction shift of when the tracking direction is shifted by 20 µm.

Optical performance evaluation of hybrid micro lens with NA 0.85 for small form factor optical pickup

As a popularity of a portable digital device such as a cellular phone, a digital camera and a MP3 player is spreading, the demand of the mobile storage device increases rapidly. A Blu-ray technology using 406nm laser diode and 0.85 NA(Numerical Aperture) satisfies a miniaturization and a high capacity which are the requirements of the portable device. Because a Blu-ray optical storage device can store 1.5Gbyte on a 3cm disk, it is possible to distribute the media in great quantities and with cheaper price. In this paper, we analyze the optical performance of the hybrid micro lens which is manufactured using a micro fabrication technology. The measurements of the optical power, the spot size and the wavefront error are performed to evaluate the hybrid micro lens with 0.85 NA and the designed wavelength of 406nm. Using the measured data, we estimate if the performance of hybrid micro lens corresponds to the designed performance. Also, the processes of measuring an optical performance of hybrid micro lens, which is composed of the refractive lens and the diffractive lens, are proposed using laser diode of 406nm and finite optics.

Compensation of Aberrations Due to Shift of Solid Immersion Lens for Media Inside Recording

In ultra high density optical recording such as near field recording, large amount of SIL shift from optical axis due to tracking direction eccentricity of disk degrade spot quality on recording layer as well as reduction of overall optical efficiency. In this paper, we suggest aberration compensation method for SIL shift using galvano actuator to improve read and write characteristics.

Design of Elliptic Solid Immersion Lens for Dual Layer Near Field Recording

As well as advantage of data protection, inside near-field recording has prominent technology for multi-layer recording for ultra high data capacity. We suggest dual layer near-field recording concept using elliptic solid immersion lens.