Yoshinori Honguh

Readout Signal Analysis of Optical Disk Based on Approximated Vector Diffraction Theory

A simple approximation method is proposed for readout signal simulation of an optical disk based on a vector diffraction model. Tensor reflectivity is introduced to deal with the boundary condition at the recording surface. Preliminary calculations were carried out for stadium-shaped recording pits periodically arranged on a recording surface. Calculations of the push-pull signal were also carried out for guide grooves with a trapezoidal intersection. The results of the frequency response, readout signal waveform, and push-pull signal showed reasonable polarization characteristics. The calculation time was only several tens of times (typically 20–80 times) that required for the scalar model calculation. This method is expected to be useful for extensive studies of readout characteristics necessary for the development of high-density optical storage systems.

Diffraction analysis of optical-disk readout signal deterioration caused by mark-profile fluctuation

A noise power-spectrum estimation method for mark-profile fluctuation is presented for optical-disk readout signals. The diffraction by marks is modeled by use of the Fraunhofer diffraction theory, and mark-profile fluctuations are taken into account through statistical averaging. The change in the readout signal is assumed to be linear to the change in the mark profile. The calculated results for a high-definition video disk agree well with the experimental results. Fluctuations in the mark-edge position is the major source of noise in the present high-definition video-disk system.

Fast-laser-power control for high-density optical disk systems

An optical head and a fast laser power control electro-optics using short wavelength laser diode was developed. This laser power control method reduces much of the laser noise, and, at the same time, rapidly regulate the laser power in order to compensate for mark shape. The relative intensity noise of the laser, lasing at 670 nm, was reduced -127 dB/Hz. This noise level is enough low to use the short wavelength laser diode for high density recording. This method is able to set various writing power waveforms to avoid mark distortion. The recorded mark shapes were got in case of (1,7) mark edge modulation, the minimum mark length is 0.8 micrometers . This high recording density is equivalent to 1.7 bit/micrometers .

Equalizer Design Based on Diffraction Analysis

A design procedure is described that determines the optimum frequency characteristics of an equalizer, considering the intersymbol interference (ISI) and crosstalk, under the influence of the tilt of the disk substrate. A scalar-wave diffraction model was employed to calculate the readout signals. A finite impulse response (FIR) filter was assumed for the equalizer. The jitter was calculated as a function of the equalizer parameters, and the equalizer parameters were determined so that the calculated jitter took its minimum value. The entire process of the equalizer design was performed on an computer, and the resultant characteristics were similar to those determined through experiments. Therefore, the method provides a useful tool for the basic design of optical disk systems.

Focusing-Error Detection Using a Mixed-Aberration-Generating Holographic Optical Element

A new focus error detecting method for an optical disk drive has been developed using a holographic optical element (HOE). The groove pattern on the HOE was designed using the ray-tracing and least-squares-fitting methods to attain a symmetric beam profile. It has been proven successfully that this method of focus error detection is insensitive to wavelength fluctuations, objective lens shifts, and tracking grooves.

Diffraction analysis of groove noise in optical disk readout signal

The noise power spectrum was calculated for groove wall undulation based on the Fraunhofer diffraction theory. Groove profile fluctuations were modeled as fluctuations in the groove profile parameters. Changes in the diffracted light field were assumed to vary linearly with the changes in the groove profile. The expression for the noise power spectrum was obtained by statistical averaging, and the contribution of the undulation to the noise power spectrum was found to depend on the depth of the undulating area.

Development of a captured image simulator for 199 nm mask inspection tools

Recently, technologies of ArF laser exposure tools and alternating phase shifting masks (Alt-PSM) are expected to be used in actual production. To utilize such newly developed technologies, it is inevitable to develop a mask inspection technology to check them properly. But it is currently difficult to check them precisely because sufficient image contrast is hard to obtain with any conventional mask inspection tools. It is not well understood whether we can get sufficient sensitivity with conventional optical setups and wavelength with the assistance of some kind of resolution enhancement techniques (RET), or we should move toward inspection using revolutionary new optics or shorter inspection wavelength. To study precisely the sensitivity of inspection optics for common types of defects, we have made a captured image simulator based on the RCWA (Rigorous Coupled Wave Analysis) method with which we can take into account the effect of the three-dimensional structure of a mask. We tried to calculate captured images for some mask structures at two different wavelengths, namely 199 nm and 257 nm. We made certain that no significant differences were observed for larger scale defects, but that a considerable difference of image contrast was observed for small scale defects around 50 nm in size. Thus we confirmed that this simulator is effective for evaluating and designing optical systems of mask inspectors, in order to achieve a high sensitivity for the detection of small defects in Alt-PSMs.

Development of a captured image simulator for the differential interference contrast microscopes aiming to design 199 nm mask inspection tools

Recently, technologies of ArF laser exposure tools and alternating phase shifting masks (Alt-PSM) are expected to be used in actual production. To utilize such newly developed technologies, it is inevitable to develop a mask inspection technology to check them properly. But it is currently difficult to check them precisely because sufficient image contrast is hard to obtain with any conventional mask inspection tools. Among many observation methods, the differential interference contrast (DIC) is one of a few methods that can be used to observe a differentiated phase shift of transmitted light of an object with high resolution. To study precisely the performance of this optical configuration, we built a new captured image simulator in which Wollaston prisms were modeled as a kind of phase modulation plates. We built this simulator as an extension of the captured image simulator we reported formerly), which is based on Rigorous Coupled- Wave Analysis (RCWA) to calculate diffractions; this enables us to properly treat effects of polarization, high NA, and 3-dimensional mask structures. We applied this simulator to see sensitivities of DIC against bumps and divots with various sizes. We found that the image contrast for small phase defects 20 to 50 nm in sizes is much higher in DIC microscopes than in conventional optical setup with coherence factor less than 1. We also found the dependence of captured images on polarizations and optical axis directions. We expect our simulator to be a useful tool for studying, designing, and developing mask inspection tools.

Verification of Validity of Approximated Vector Diffraction Model

The validity of an approximated vector diffraction model was studied for an optical-disk readout system by comparing the results with those obtained by a rigorous calculation model. It was confirmed that the approximation model provides useful information for sidewall angles up to about 40°.

Characteristics of an autofocus system on a grating with period smaller than the focus-beam wavelength

In this paper, characteristics of an autofocus system on a grating with period smaller than the focus beam wavelength are investigated. The through-the-lens autofocus system, which has a visible light source of 670nm wavelength and light radiation mechanism for causing light to be obliquely incident on a sample surface, has been prepared for experiments. It is shown in experiments that the focusing error for a grating with 0.6-micrometer period is larger than 0.2 micrometer, and polarization of reflected light is changed from circular to elliptic. By performing RCWA simulations, the qualitative correspondence of theoretical expectations with experimental results is obtained. Based on the result of experiments and simulations, methods of reducing the focusing error are proposed. One of the methods is to use the polarization information to correct the focusing error. The method is evaluated experimentally and is shown to achieve autofocus accuracy of ± 0.1 micrometer.