Optical Head Device Offset-Free Tracking Multilayer Discs
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Solution Overview
Problem
Conventional optical head devices face challenges in generating a stable tracking signal for multilayer optical discs due to stray lights from other layers, leading to offset issues and unstable tracking control.
Innovation Solution
The optical head device incorporates a diffraction optical system with regionally divided holographic elements and strategically arranged light receiving portions on the photodetector to effectively shield stray lights, ensuring that the light receiving portions for diffracted signals are positioned to avoid stray light interference, thereby generating an offset-free tracking signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If light receiving portions are used to detect diffracted light for tracking signals in a conventional optical head device, then tracking control can be performed, but stray lights from other layers cause offset errors and unstable tracking control in multilayer optical discs
Solution Approach 1:
The holographic element is divided into multiple regions (first main region, second main region, first subregion, second subregion) by dividing lines. Each region directs diffracted light to specific light receiving portions, enabling selective detection while blocking stray lights from other layers through strategic positioning of detection regions.
Solution Approach 2:
Different regions of the holographic element have different optical functions: main regions detect tracking errors from the focused layer while subregions detect stray lights from adjacent layers. The light receiving portions are positioned to receive light from specific regions only, creating local detection zones that eliminate cross-layer interference.
2Measurement precision
If a holographic element is used to diffract light for generating tracking signals, then tracking information can be obtained, but stray lights from adjacent layers are also diffracted and incident on light receiving portions causing offset errors
Solution Approach 1:
The holographic element is segmented into main regions and subregions with different diffraction characteristics. Main regions are optimized for tracking signal detection while subregions handle stray light detection, allowing the system to distinguish between useful diffracted light and harmful stray lights through spatial separation.
Solution Approach 2:
The divided holographic element acts as an intermediary that selectively diffracts light from different layers to different destinations. By positioning light receiving portions to receive light only from main regions, the system mediates between the multilayer disc and detector to eliminate stray light interference while preserving tracking signal integrity.
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
This configuration allows for stable tracking control even on multilayer optical discs by preventing stray light interference, ensuring accurate signal detection and minimizing offset errors.
Implementation Method 1
The light beam having passed through the beam splitter 103 is incident on a holographic element 108 to be partly diffracted, thereby becoming a 0th-order light 110 that is not diffracted and a 1st-order light 111 that is diffracted
Implementation Method 2
The light beam having passed through the holographic element 108 passes through a detection lens 109 to be incident on a photodetector 120
Implementation Method 3
a focusing optical system for focusing the light beam emitted from the light source into a convergent light to be incident on an information recording medium
Data Source
AI summary
An offset-free tracking signal enables stable tracking control even if an optical disc is a multi-layer disc having three or more layers. Light receiving portions of a main region light receiving portion group are arranged between a projection line of a third dividing line on a photodetector and a projection line of a fourth dividing line on the photodetector by stray lights from information layers adjacent to the one, on which a light beam is focused, out of a plurality of information layers. Further, light receiving portions of a subregion light receiving portion group is arranged between a projection line of a first dividing line on a photodetector and a projection line of a second dividing line on the photodetector by the stray lights from the information layers adjacent to the one, on which a light beam is focused, out of the plurality of information layers.