Diffraction Grating Point Symmetry Stabilizes Optical Pickup Servo
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Solution Overview
Problem
Existing optical pickup devices face instability in tracking control due to large variations in tracking error signal amplitudes, leading to unreliable data recording and reproduction, especially when the objective lens is displaced, causing DC-offset issues and reduced tracking control gain.
Innovation Solution
An optical pickup device with a diffraction grating having six areas arranged in point symmetry and shifted centers, which stabilizes servo signals by minimizing amplitude variations of tracking error signals across the disc's radial positions, even when the disc's rotating center is not aligned with the optical pickup's displacement direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional diffraction grating is used for tracking error signal detection, then the optical pickup device can detect tracking error signals, but large amplitude variations occur in the tracking error signal when the objective lens is displaced, causing unstable tracking control
Solution Approach 1:
The diffraction grating is divided into six distinct detection areas (first through sixth areas) with specific functional assignments. The first and second areas detect light beams containing tracking components, while the third and fourth areas detect light beams primarily containing DC-offset components. This segmentation allows separate processing of tracking signals and DC-offset signals, enabling stable tracking control despite lens displacement.
Solution Approach 2:
The diffraction grating employs an asymmetric configuration where the center parts of the first and second areas are spaced by a distance d in the radial direction, while the third and fourth areas are positioned to capture DC-offset components. This asymmetric arrangement ensures that tracking components are detected in areas (first and second) that are optimized for tracking sensitivity, while DC-offset components are detected in separate areas (third and fourth), preventing amplitude variations from degrading tracking control stability.
2Ease of operation
If the objective lens is displaced to adjust tracking position, then tracking alignment can be achieved, but DC-offset variations occur in the tracking error signal, reducing tracking control gain
Solution Approach 1:
The diffraction grating separates DC-offset detection (third and fourth areas) from tracking component detection (first and second areas). This allows the system to measure and compensate for DC-offset variations caused by lens displacement without affecting the tracking control gain, maintaining reliable tracking control even when the lens is displaced for tracking adjustment.
3Adaptability or versatility
If the disc rotating center is not aligned with the optical pickup displacement direction, then the optical pickup can still read the disc, but amplitude variations in tracking error signals increase, causing unstable tracking control
Solution Approach 1:
The six-area diffraction grating configuration with point-symmetric arrangement allows the optical pickup to maintain stable tracking control even when the disc rotating center is not aligned with the pickup displacement direction. The segmentation into tracking-specific areas (first and second) and DC-offset-specific areas (third and fourth) ensures that misalignment-induced amplitude variations do not degrade tracking control stability.
Solution Approach 2:
Different areas of the diffraction grating are optimized for different detection functions: the first and second areas are optimized for tracking component detection with high sensitivity to radial position changes, while the third and fourth areas are optimized for DC-offset detection. This local quality differentiation ensures that amplitude variations from misalignment affect only the DC-offset measurement, not the tracking control stability.
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 provides stable servo signals and robust tracking control by reducing amplitude variations and DC-offsets, ensuring reliable data recording and reproduction across different radial positions on the optical disc.
Implementation Method 1
a grating for branching a light beam reflected from the second optical disc
Data Source
AI summary
A pickup device splits and detects a light beam reflected from a disc with the use of a grating. The grating has first to six areas, wherein the first area and the second area, the third area and the fifth area, and the fourth area and the sixth area are arranged in point symmetry with respect to the center of the grating, respectively. The first area is interposed between the fourth area and the fifth area and the second area is interposed between the third area and the sixth area. Further, the centers of the first area and the second area are arranged to be spaced by a distance d in a direction perpendicular to the displacement direction of the optical pickup device. Even though the center of an optical disc is not located on a straight line in the displacement direction of the optical pickup device, objective lens can obtain a stable servo signal.