Optical Head Objective Lens Focus Servo Signal Integrity
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Solution Overview
Problem
Existing optical heads with diffraction structures fail to effectively implement the focus servo function due to the generation of a third focus error signal between normal and unwanted diffracted order lights, causing the waveform of the focus error signal to lose its S-shape necessary for accurate servo function operation.
Innovation Solution
An optical head with an objective lens that converges laser light of different wavelengths, featuring a diffraction structure that generates normal diffracted order light on the information recording surface and collects unwanted diffracted order light at a position away from the surface, with a virtual collecting point distance from the normal diffracted order light convergent point being twice or more of the focus error signal pull-in range, thereby avoiding interference from virtual diffracted order light.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If an objective lens with a diffraction structure is used to converge laser light on optical discs with different protective substrate thicknesses, then compatibility with multiple disc types is improved, but unwanted diffracted order light is generated that interferes with the focus error signal
Solution Approach 1:
The patent extracts and removes the harmful unwanted diffracted order light from the optical system by positioning it at a different focal plane than the information recording surface, separating the useful normal diffracted order light from the harmful unwanted diffracted order light
Solution Approach 2:
The patent introduces a beam splitter as an intermediary element that separates the optical paths of normal and unwanted diffracted order light, directing them to different destinations to eliminate interference with the focus error signal
2Reliability
If the collecting position of unwanted diffracted order light is placed far from the information recording surface, then the focus error signal waveform is maintained, but the distance requirement increases system complexity
Solution Approach 1:
The patent resolves the distance constraint by transitioning to a different dimensional approach - using a beam splitter to separate light paths in the optical path dimension rather than relying solely on axial distance separation, thereby maintaining focus servo reliability without excessive distance requirements
3Manufacturing precision
If a diffraction structure is added to the objective lens, then convergence on information recording surfaces is improved, but a third focus error signal is generated that distorts the waveform
Solution Approach 1:
The patent extracts the harmful third focus error signal from the detection system by using a beam splitter to separate the optical paths, removing the distorted signal component while preserving the useful focus error signal for servo control
Solution Approach 2:
The beam splitter acts as an intermediary that filters out the third focus error signal generated by the diffraction structure, allowing the system to maintain convergence precision without waveform distortion affecting the focus servo function
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 ensures effective implementation of the focus servo function by maintaining the S-shape waveform of the focus error signal, reducing interference between detection light spots, and achieving high-quality information recording and reproduction on optical discs with different protective substrate thicknesses.
Implementation Method 1
an objective lens provided with a diffraction structure (a step structure having a step for giving an optical path length difference with respect to an incident light flux) generates diffracted light (normal diffracted order light) of normal order which is converged on an information recording surface of an optical disc, and generates diffracted light (unwanted diffracted order light) of unwanted order which is not converged on the information recording surface of the optical disc
Implementation Method 2
an objective lens configured to converge the laser light of the first wavelength λ1, the laser light of the second wavelength λ2, and the laser light of the third wavelength λ3 on the information recording surface of the first information recording medium, the information recording surface of the second information recording medium, and the information recording surface of the third information recording medium
Data Source
AI summary
An objective lens (101) is configured to converge laser light of a first wavelength λ1 (390 [nm]≦λ1≦430 [nm]), laser light of a second wavelength λ2 (630 [nm]≦λ2≦680 [nm]), and laser light of a third wavelength λ3 (750 [nm]≦λ3≦810 [nm]) on an information recording surface of a first information recording medium, an information recording surface of a second information recording medium, and an information recording surface of a third information recording medium, and is configured such that the distance from a convergent point of normal diffracted order light, to a virtual collecting point formed by virtual diffracted order light between the convergent point of normal diffracted order light and a collecting point of unwanted diffracted order light is twice or more of the pull-in range of a focus error signal to be obtained in recording or reproducing information with respect to the third information recording medium.