Optical Pickup Shading Unit for Multilayer Disc Crosstalk
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Solution Overview
Problem
Multilayer optical discs face challenges in tracking stability due to crosstalk noise from adjacent layers, particularly in Blu-ray Discs, where the larger numerical aperture and groove depth lead to false tracking error signals, affecting the accuracy of tracking error detection.
Innovation Solution
An optical pickup and optical disc device are designed with a photodetector having a light receiving face divided into four regions, and a shading unit that blocks diffracted rays from adjacent layers, improving tracking stability by reducing crosstalk noise and false tracking error signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single beam push pull method is used for tracking control, then device complexity is reduced, but measurement precision deteriorates due to crosstalk noise from other layers
Solution Approach 1:
The photodetector light receiving face is divided into four regions (first, second, third, and fourth regions) to enable separate detection of signal light and crosstalk light. This segmentation allows the system to distinguish between useful signal and harmful noise, improving measurement precision without adding complex external components.
Solution Approach 2:
The patent introduces an intermediary signal processing approach by calculating a corrected tracking error signal that combines detection signals from multiple photodetector regions with weighted coefficients. This intermediary calculation method filters out crosstalk noise while preserving tracking information.
2Manufacturing precision
If the numerical aperture of the objective lens is increased to improve focus precision, then manufacturing precision of the optical disc increases, but crosstalk noise from other layers increases
Solution Approach 1:
The photodetector is segmented into four distinct regions that correspond to different spatial locations on the optical disc surface. This segmentation enables selective detection of light from the target layer versus other layers, allowing the system to maintain high NA focus precision while filtering out crosstalk noise through regional signal differentiation.
Solution Approach 2:
Different regions of the photodetector are assigned different detection functions - the first and second regions detect signal light from the target layer, while the third and fourth regions detect crosstalk light from other layers. This local quality differentiation allows the system to optimize for both high NA performance and noise rejection.
3Measurement precision
If the groove depth is increased to improve tracking error signal amplitude, then measurement precision improves, but false tracking error signals increase around boundary areas
Solution Approach 1:
The photodetector light receiving face is divided into four regions, allowing separate measurement of tracking error signals from different spatial locations. This segmentation enables the system to identify and exclude false signals generated by deep grooves in boundary areas while maintaining accurate tracking error detection in stable regions.
Solution Approach 2:
The patent employs feedback mechanisms through the calculation unit that processes detection signals from multiple photodetector regions with weighted coefficients to generate a corrected tracking error signal. This feedback processing filters out false signals and maintains signal accuracy even when groove depth variations cause spurious signals.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enhances tracking stability and accuracy in multilayer optical discs, enabling higher data density and reducing false tracking errors, thus improving the performance of optical disc recording and playback systems.
Implementation Method 1
an objective lens configured to focus light from the light source onto one signal face of an optical disc
Implementation Method 2
a shading unit disposed at an incident side of the photodetector and configured to block, of returning rays from a signal face other than the one signal face of the optical disc, at least a diffracted ray that is diffracted by the groove
Implementation Method 3
a photodetector configured to receive returning rays from the optical disc
Data Source
AI summary
There is provided an optical pickup including a light source, an objective lens configured to focus light from the light source onto one signal face of an optical disc, the optical disc including two or more signal faces, each of the signal faces having a groove formed therein, a photodetector configured to receive returning rays from the optical disc, the photodetector having a light receiving face divided into four light receiving regions by a dividing line that is substantially parallel to a track direction of the optical disc and by a dividing line that is substantially perpendicular to the track direction, and a shading unit disposed at an incident side of the photodetector and configured to block, of returning rays from a signal face other than the one signal face of the optical disc, at least a diffracted ray that is diffracted by the groove.