The effects of placing a shading band in the collection path of a magneto-optical data storage device are presented. The shading band consists of an opaque region in the center of the collection aperture, which reduces the low-frequency response of the system without affecting the high-frequency response. This partially equalizes the system transfer function. Dynamic measurements indicate that the noise is also reduced in the low-frequency region, thus not significantly affecting the narrow-band carrier-to-noise ratio (CNR). The integrated signal-to-noise ratio (SNR) shows slight improvement in the high-frequency region.
We describe a differential wax-wane focus servo technique for use in optical data storage. A combination of scalar diffraction modeling and experiment is used to quantify performance. Our results indicate that the differential technique is superior to the single-detector wax-wane technique with respect to gain, linearity, and lock-on range. We present modeling results that show the effects of aberrations and detector misalignment. The differential system was found to be robust. It can also reject many common pattern noise effects such as tracking cross talk, which was reduced from 0.7 µm in a single channel to < 0.1 µm in the optimized differential channel.
We present studies of crosstalk cancellation using the differential wax-wane focus servo technique. A scalar diffraction model is employed to accurately predict its performance. The measured wavefront is implemented in the diffraction model, and excellent agreement between experimental and modeling results is achieved. With measured system aberration, the tracking crosstalk is reduced from 0.7 µm in a single channel to less than 0.1 µm in the optimized differential channel. Comparisons to the astigmatic and pupil obscuration focus error detection techniques are given. Without any aberration, the differential wax-wane technique has a tracking crosstalk amplitude of 0.04 µm compared to about 0.25 µm in other techniques.
There are several techniques that can be used for focus-error detection in an optical data storage device. Astigmatic, knife-edge, critical-angle prism, pupil obscuration, and spot-size detection are common techniques (1)(2)(3). These methods sense the focus error by manipulating reflected light from the disk and creating an electrical focus-error signal (FES) with sectioned detectors. If a continuously pregrooved disk is used, the reflected light also contains diffracted orders that are used to provide a tracking-error signal (TES). It is difficult to completely separate the focuserror information from the tracking-error information, regardless of the focus-error detection method. The residual amount of TES observed in the FES is called cross talk. Prikryl (4) has modeled the sensitivity of several focus-error detection methods to sources of cross talk. Stahl (5) has modeled the sensitivity of astigmatic focus-error detection. The works of Prikryl and Stahl indicate that spot-size focus-error detection is probably a good method for eliminating cross talk. In this paper, we discuss the characteristics of a differential spot-size measurement technique, which has better cross-talk rejection than the single-detector spot-size measurement technique. Similar differential techniques have been presented in the literature (6), but they have not been analyzed with respect to cross talk.
An autocollimatoris used to align and detect collimation of a laser diode in an optical data storage head. High wavefront quality is achieved without interferometric equipment.
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