Optical Recording Medium Layer Thickness Design
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Solution Overview
Problem
Optical recording media with multiple information surfaces face issues with back focus problems and coherence between reflected lights, leading to deteriorated servo and reproduction signals due to equal thicknesses between surfaces and lack of consideration for refractive indices, resulting in mixed light signals and reduced reflectance.
Innovation Solution
An optical recording medium with a cover layer and intermediate layers of varying refractive indices, where the thicknesses are adjusted to ensure a minimum difference of 1 μm between any two values, preventing back focus issues and coherence between reflected lights, thereby improving servo and reproduction signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the thicknesses between information recording surfaces are made equal, then the manufacturing process is simplified, but back focus problems occur and coherence between reflected lights increases, deteriorating servo and reproduction signals
Solution Approach 1:
The patent applies local quality by making the intermediate layer thicknesses different from each other (t2 ≠ t3 ≠ t4) while maintaining a specific relationship with the cover layer thickness (t1). This local differentiation in thickness prevents back focus problems and reduces coherence between reflected lights from different information recording surfaces, thereby improving signal quality without requiring complete thickness uniformity throughout the entire structure.
Solution Approach 2:
The patent introduces asymmetry by setting the intermediate layer thicknesses to be unequal (t2 ≠ t3 ≠ t4) and specifically requiring that the sum of the first and second intermediate layer thicknesses differs from the third intermediate layer thickness by at least 1 μm. This asymmetric thickness configuration effectively prevents the formation of coherent reflected light paths that cause back focus problems, while still allowing for manufacturable variations in thickness.
2Device complexity
If the refractive indices of intermediate layers are not considered in thickness design, then the design process is simplified, but coherence between reflected lights increases, reducing reflectance and degrading signal quality
Solution Approach 1:
The patent applies parameter changes by establishing specific mathematical relationships between the thicknesses of the cover layer and intermediate layers, and by considering the refractive indices of these layers in the design. The thicknesses are designed to satisfy conditions such as t1 + t2 ≠ t3 + t4 and specific differences between adjacent layer thicknesses, while the refractive indices are selected to optimize optical performance and prevent coherent reflection, thereby improving signal quality through coordinated parameter optimization.
3Quantity of substance
If the distance between the surface and the closest information recording surface is small, then the recording capacity per unit volume is increased, but damage or smear on the surface directly affects the recording quality
Solution Approach 1:
The patent applies preliminary action by designing the intermediate layer thicknesses to be sufficiently large (t2, t3, t4 ≥ 10 μm) to provide optical isolation and physical separation between the surface and the first information recording surface. This preliminary structural design prevents surface damage or smear from directly affecting the recording process, while still maintaining high recording capacity through efficient use of the remaining vertical space in the multi-layer configuration.
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 effectively prevents light from forming images on the backside of the optical recording medium, reducing coherence between reflected lights and enhancing the quality of servo and reproduction signals, while allowing for a larger distance between the surface and the closest information recording surface to mitigate damage or smear effects.
Implementation Method 1
a cover layer having a refractive index nr1 and formed between the medium surface and the first information recording surface; a first intermediate layer having a refractive index nr2 and formed between the first information recording surface and the second information recording surface
Implementation Method 2
The beam 70 reflected on the fourth information recording surface 401d is transmitted through the objective lens 56 and the quarter wavelength plate 54, converted into linearly polarized light along an optical path displaced by 90 degrees with respect to the outward path
Data Source
AI summary
An object of the invention is to provide an optical recording medium and an optical information device that enable to improve the quality of a servo signal and a reproduction signal. In the case where shape-wise thicknesses tr1, tr2, tr3, and tr4 of a cover layer, a first intermediate layer, a second intermediate layer, and a third intermediate layer are respectively converted into thicknesses t1, t2, t3, and t4 of the respective corresponding layers each having a predetermined refractive index “no”, a defocus amount with respect to a layer having a refractive index nrα and a thickness trα (satisfying: 1≦α≦n (where α is a positive integer and n is an integer of 4 or more)), and a defocus amount with respect to a layer having the refractive index “no” and a thickness tα (satisfying: 1≦α≦n (where α is a positive integer and n is an integer of 4 or more)) are equal to each other; and the thicknesses t1, t2, t3, and t4 satisfy |t1−(t2+t3+t4)|≧1 μm, a difference between any two values of the thicknesses t1, t2, t3, and t4 is set to 1 μm or more in any case, and |(t1+t2)−(t3+t4)|≧1 μm.