Plastic Objective Lens Inflection Point Rim Intensity
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Solution Overview
Problem
Existing objective lenses for high-density optical discs, such as Blu-ray discs, face challenges in achieving high rim intensity and transmissivity due to material limitations and manufacturing difficulties with plastic lenses, which are exacerbated by steep surface gradients and inadequate coating uniformity.
Innovation Solution
A single-element plastic objective lens with specific geometric and refractive properties, including an inflection point on the first surface and a two-layer anti-reflection coating on the first surface, and a two to four-layer coating on the second surface, to maintain high numerical aperture and ease manufacturing, measurement, and coating processes without compromising transmissivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of stationary object
If a plastic lens is used instead of glass to reduce weight and cost, then weight and manufacturing cost decrease, but the rim intensity cannot be secured due to small refractive index difference with coating materials
Solution Approach 1:
The patent changes the refractive index parameter of the plastic lens material to optimize the rim intensity. By selecting plastic material with refractive index of 1.58 or more at the minimum wavelength of incident light, and designing the lens with specific curvature radii and thickness, the patent achieves adequate anti-reflection performance without requiring heavy glass material, thus maintaining weight advantage while improving rim intensity.
Solution Approach 2:
The patent employs a composite structure combining plastic lens material with specific coating materials. The lens uses plastic with refractive index of 1.58 or more, and the coating materials are selected to create adequate refractive index contrast, forming a composite system that achieves both weight reduction and adequate rim intensity through the synergistic combination of materials.
2Adaptability or versatility
If the numerical aperture is increased to achieve high density optical disc performance, then recording density improves, but the surface gradient becomes steeper making manufacturing and coating difficult
Solution Approach 1:
The patent optimizes the numerical aperture parameter to be 0.75 or more to ensure compatibility with high-density optical discs. Simultaneously, it adjusts the curvature radii and thickness parameters of the lens surfaces to control the surface gradient, ensuring that even with high numerical aperture, the surface gradient remains manageable for injection molding and coating processes.
Solution Approach 2:
The patent applies different curvature characteristics to different regions of the lens surfaces. By carefully designing the curvature radius of the object-side surface and image-side surface separately, and controlling their respective gradients, the patent achieves high numerical aperture performance in the central region while maintaining gentler gradients at the periphery where coating and manufacturing are most challenging.
3Illumination intensity
If the surface gradient is increased to achieve high numerical aperture, then numerical aperture improves, but coating uniformity deteriorates due to poor vapor deposition adhesion
Solution Approach 1:
The patent controls the surface gradient parameter by optimizing the curvature radii of the lens surfaces. By setting appropriate curvature radii for both the object-side and image-side surfaces, and controlling the thickness distribution, the patent achieves high numerical aperture (0.75 or more) while limiting the maximum surface gradient to values that allow uniform vapor deposition coating with adequate adhesion.
Solution Approach 2:
The patent addresses the coating uniformity issue by considering the three-dimensional shape of the lens surfaces. By designing the sagitta and curvature in multiple dimensions, the patent creates a surface profile that maintains high optical performance while providing adequate surface gradient control in the radial direction, enabling uniform coating application across the entire lens surface.
4Illumination intensity
If the coating thickness is increased to improve rim intensity, then rim intensity improves, but transmissivity decreases due to excessive coating thickness at the perimeter
Solution Approach 1:
The patent applies different coating thicknesses to different regions of the lens surface. By designing a coating structure where the thickness varies radially from the center to the periphery, the patent achieves adequate rim intensity at the perimeter through increased coating thickness where needed, while maintaining high transmissivity in the central region where the coating thickness is reduced, thus optimizing the balance between rim intensity and overall transmissivity.
Solution Approach 2:
The patent introduces radial dimension variation in the coating thickness to solve the trade-off between rim intensity and transmissivity. By controlling the coating thickness as a function of radial position on the lens surface, the patent achieves spatially optimized optical performance with thicker coating at the perimeter for rim intensity and thinner coating in the center for transmissivity.
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 enables the production of a plastic objective lens with improved rim intensity and transmissivity, facilitating easier processing and coating while maintaining optical performance for high-density optical discs.
Implementation Method 1
it is required to increase a rim intensity (i.e., a ratio of an intensity of light passed through a perimeter of the objective lens to an intensity of light passed through a part in the vicinity of an optical axis of the objective lens). Each of Japanese Patent Provisional Publications Nos. 2004-39161A (hereafter, referred to as document #3) and 2005-11494A (hereafter, referred to as document #4) describes that a rim intensity can be increased by forming an anti-reflection coating on a lens surface of the objective lens.
Data Source
AI summary
There is provided an objective lens for an optical pick-up. The objective lens is formed to be a single-element plastic lens having a first surface and a second surface. The first surface is configured to have, within an effective diameter, an inflection point at which a second derivative of a sag of the first surface takes a value of 0. Further, the objective lens having a numerical aperture larger than or equal to 0.75.