Optical Pickup Magnification Conversion for Multilayer Disc Aberration
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Solution Overview
Problem
Optical pickup devices face challenges in reducing third-order astigmatism when recording or reproducing on multilayer optical discs with three or more information recording surfaces, due to increased spherical and coma aberrations caused by varying light-transmitting layer thicknesses, which affect recording capacity and accuracy.
Innovation Solution
An optical system for optical pickup is designed with a magnification conversion optical element that moves along the optical axis to converge light beams on multiple recording surfaces, using a formula to optimize imaging magnification and numerical aperture, thereby reducing third-order astigmatism and compensating for spherical and coma aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a blue-violet laser beam with high numerical aperture is used to increase recording capacity on multilayer optical discs, then the recording capacity increases, but third-order spherical aberration and third-order coma aberration increase due to varying light-transmitting layer thicknesses
Solution Approach 1:
A collimating lens is introduced as an intermediary element between the laser source and the objective lens. This collimating lens serves as a mediator to adjust the beam parameters (divergence angle, convergence angle, or parallelism) before light enters the objective lens, enabling compensation of third-order spherical aberration caused by variations in light-transmitting layer thickness across multiple recording surfaces
Solution Approach 2:
The patent changes the optical parameters of the laser beam by adjusting the collimating lens position along the optical axis. This modifies the divergence angle, convergence angle, or parallelism of the incident light, thereby compensating for aberrations. The objective lens parameters (focal length, numerical aperture) are also optimized to balance recording capacity with aberration control
2Quantity of substance
If the light-transmitting layer thickness varies across multiple information recording surfaces to increase capacity, then more data can be stored, but third-order spherical aberration increases affecting recording accuracy
Solution Approach 1:
The collimating lens acts as a mediator that compensates for the adverse effects of varying light-transmitting layer thicknesses. By adjusting its position, it modifies the incident beam parameters on the objective lens to counteract the third-order spherical aberration introduced by different layer thicknesses, thereby maintaining recording accuracy across multiple surfaces with different capacities
Solution Approach 2:
The collimating lens is made movable along the optical axis to dynamically adjust beam parameters. This dynamic adjustment allows the system to adapt to different recording surfaces with varying light-transmitting layer thicknesses, optimizing the balance between information storage capacity and recording accuracy for each surface
3Manufacturing precision
If a collimating lens is moved along the optical axis to compensate spherical aberration, then aberration compensation improves, but third-order astigmatism increases
Solution Approach 1:
The patent optimizes the collimating lens parameters (focal length, position range) and objective lens parameters (focal length, numerical aperture) to achieve a balance. By carefully selecting these parameters, the system minimizes the generation of third-order astigmatism while maintaining effective compensation of third-order spherical aberration, preventing the collimating lens movement from introducing excessive harmful aberrations
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 optical system effectively reduces third-order astigmatism, improving recording accuracy and capacity on multilayer optical discs by optimizing imaging magnification and numerical aperture changes with varying light-transmitting layer thicknesses.
Implementation Method 1
an amount of third-order astigmatism which is generated when imaging magnification of the objective lens element is changed
Implementation Method 2
converges an incident light beam through base plates on at least three recording surfaces located parallel to each other, to form spots corresponding to the recording surfaces, respectively
Data Source
AI summary
An optical system for optical pickup includes a magnification conversion optical element moving along an optical axis direction in accordance with each of optical recording medium recording surfaces, and an objective lens element converging a light beam incident through the magnification conversion optical element, to form a spot on a corresponding one of the recording surfaces, and satisfies the following formula: 4.0×10−4<Mn/(tn−tc)×f<6.0×10−4. Here tc is the thickness [μm] of a recording-medium base material at which a third-order spherical aberration occurring when a parallel light beam is incident on the objective lens element is the minimum, f is the focal length [mm] of the objective lens element, and Mn is the imaging magnification of the objective lens element when a spot is formed through a thickness tn [μm]of the base plate.