Focusing error detection by the astigmatic method, an important measurement technique, is used widely due to its simple operation and low cost. However, the focus error signal (FES) is prone to noise with the control circuit and the signal conditioning method. To address this, an astigmatic detection system was constructed with a modified digital video disc (DVD) optical head. By designing a stable laser drive control circuit using a low-noise signal conditioning method, the developed system can reduce the FES of background noise. Additionally, by analyzing the data interpolation and approximation model, the corresponding defocus error signal is extracted. The exact optical design of the method has been verified through simulations. Moreover, an experiment is set up to verify the feasibility of the developed method. It is concluded that the method is effective to reduce the background noise 2 nm and the standard deviation 2.15 mV, with PZT working frequencies ranging from 300 Hz to 2 kHz.

Reducing the residual focus error signal in optical pickup head astigmatism displacement systems using the signal conditioning method.

In optical disk drives, interactions between the electromechanical actuators and the servo error generation systems can cause instabilities in the operation of individual servo loops. We examine the effects of these interactions on the servo loops of an optical disk drive in which a galvano fine-tracking actuator, a push-pull tracking error detection scheme, and an astigmatic focus error detection method are used. Also, we describe a digital servo controller that effectively cancels the effects of the deleterious interactions, resulting in stable operation.

Interactions between Servo Loops of an Optical Disk Drive

Publisher Summary This chapter discusses the role of modeling in engineering and applied science, both in a general sense and as specifically applied to MO recording. The chapter catalogs the type of physical models needed to treat the MO recording process, considering inputs and outputs for each function to cover the process—the recording stylus, the head optics, light interaction with domains, multilayers, and topographical features. The chapter summarizes multilayer optical design for optimization of media signal-to-noise ratio performance; the optical considerations were concluded with a discussion of the degradation in MO recording performance due to substrate birefringence. The chapter also presents a detailed treatment of thermal modeling. Finally, the chapter provides a brief overview of noise models that have been developed for MO recording media. The models have considered media noise spectral content and response as perceived by a scanning optical head. Other simulations have dealt with representations of reflectivity and Kerr effect variations. A micromagnetic model of thermomagnetic writing with overlap readout assessed several contributions to signal jitter arising from the drive and media.

Modeling the Magneto-Optical Recording Processes

To remove sensitivity difference in focusing error signal between land and groove in order to optimize focusing servo gains simultaneously for land and groove. The laser beam reflected by an optical disk is diffracted by a holographic element, and the ±1st order diffracted beams are detected by a photo-detector which comprises a detecting portion for detecting the +1st order diffracted beam and another detecting portion for detecting the −1st order diffracted beam. Focusing error is generated on the basis of the difference between a focusing error signal obtained from the +1st order diffracted beam spot and another focusing error signal obtained from the −1st order diffracted beam spot.

Optical head for land and groove recording

A refractive astigmatic lens is employed to generate a focus error signal (FES) for servo control in general optical pickup. In
this paper, a diffractive optical element (DOE) is designed to implement optical functions of astigmatic focusing for a
MEMS-based miniature optical pickup at 650nm wavelength. We present a diffractive astigmatic lens with its internal zones
and external zones quantized into four, two phase levels, respectively based on fabrication consideration. The optical
simulation based on scalar diffraction theory shows the efficiency of this quantized lens for astigmatic focusing and a
comparison of the FES in the optical pickup system has been plotted.

Design and analysis of a diffractive astigmatic lens for DVD pickup

The astigmatic-focusing/push-pull tracking-error detection method is an elegant and sensitive optical servo technique. Unfortunately the formation of error signals far from either line focus of the astigmat (for relaxing alignment tolerances and broadening the servo’s acquisition range) gives rise to undesired diffraction effects in the focus servo channel owing to track crossings of the pregrooved disk by the optical stylus, especially if certain aberrations are present. These undesired effects are given several names: pattern noise, optical servo cross talk, and feedthrough. By combining two astigmatic lenses and their associated detectors, one can configure a differential variant of the astigmatic technique. This double-astigmatic method greatly reduces pattern noise caused by the presence of spurious astigmatism oriented with its line foci at ±45° to the disk tracks. In this paper we present numerical modeling and experimental data that demonstrate the effectiveness of this focusing/tracking technique in feedthrough suppression.

Characterization of a novel focusing/tracking technique with increased feedthrough immunity for optical-disk applications: the double-astigmatic method

Characterization of a novel focusing/tracking technique with increased feedthrough immunity for optical-disk applications: the double-astigmatic method.

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Characterization of a novel focusing/tracking technique with increased feedthrough immunity for optical-disk applications: the double-astigmatic method

Characterization of a novel focusing/tracking technique with increased feedthrough immunity for optical-disk applications: the double-astigmatic method.