Optical Recording Medium Interlayer Thickness Design for Signal Coherence
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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 in existing disc structures.
Innovation Solution
An optical recording medium with a specific interlayer structure, including a cover layer and intermediate layers with varying refractive indices, where the thickness differences between layers are set to at least 1 μm to prevent back focus issues and coherence, improving servo and reproduction signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If equal thicknesses are used between multiple information surfaces, then manufacturing is simplified, but back focus problems and coherence between reflected lights occur leading to deteriorated servo and reproduction signals
Solution Approach 1:
The patent applies local quality by assigning different thickness values to different intermediate layers (first, second, and third intermediate layers) while maintaining uniform thickness for the cover layer. This localized differentiation in layer thicknesses prevents back focus problems and coherence issues between reflected lights from multiple information surfaces, thereby improving servo and reproduction signal quality without significantly complicating the overall manufacturing process.
Solution Approach 2:
The patent changes the thickness parameter of intermediate layers to resolve the technical contradiction. Specifically, it sets the first intermediate layer thickness to 10-20 μm, the second intermediate layer thickness to 20-30 μm, and the third intermediate layer thickness to 30-40 μm, while keeping the cover layer thickness at 50-100 μm. This parameter variation eliminates coherence between reflected lights and prevents back focus problems, thereby improving signal quality while maintaining manufacturing feasibility.
2Reliability
If larger distances are set between information surfaces, then coherence between reflected lights is reduced, but the overall disc thickness increases
Solution Approach 1:
The patent optimizes the thickness parameters of intermediate layers to achieve the right balance. By setting the first intermediate layer to 10-20 μm, the second to 20-30 μm, and the third to 30-40 μm, it creates sufficient distance between information surfaces to reduce coherence between reflected lights while controlling the overall disc thickness to remain practical for manufacturing and usage.
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 proposed structure 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 larger distances between surfaces to mitigate damage or smear effects.
Implementation Method 1
the beam 70 is converted into a convergent beam through an objective lens 56, transmitted through a transparent substrate of the optical recording medium 401, and collected on one of the first information recording surface 401a, the second information recording surface 401b, the third information recording surface 401c, and the fourth information recording surface 401d
Implementation Method 2
The beam 70 incident into the polarized beam splitter 52 is transmitted through the polarized beam splitter 52, and converted into circularly polarized light while being transmitted through a quarter wavelength plate 54
Implementation Method 3
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, and then reflected on the polarized beam splitter 52
Implementation Method 4
Astigmatism is imparted to the beam 70 while the beam 70 is transmitted through the cylindrical lens 57
Implementation Method 5
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, and then reflected on the polarized beam splitter 52. The beam 70 reflected on the polarized beam splitter 52 is converted into convergent light while being transmitted through a light collecting lens 59, and incident into a photodetector 320
Data Source
AI summary
An optical recording medium and an optical information device that improve the quality of a servo signal and a reproduction signal. In the optical recording medium, when shape-wise thicknesses tr1, tr2, tr3, and tr4 of a cover layer (42), a first intermediate layer (43), a second intermediate layer (44), and a third intermediate layer (45) 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. Further, 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, and |(t1+t2)−(t3+t4)|≧1 μm.