Optical Pickup Offset Compensation via Phase-Shifting Shield
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Solution Overview
Problem
The optical pickup device faces issues with lowering the modulation degree of the tracking error signal and unstable tracking control when the width of regions on the hologram surface is increased to improve light intensity sensitivity, leading to leakage of first-order diffracted beams into regions intended for zeroth-order diffracted beams only.
Innovation Solution
Incorporating a partial light shielding element between the objective lens and the polarization hologram element, which partially transmits the second laser beam and shifts its phase, allowing the first laser beam to pass completely while preventing first-order diffracted beams from leaking into regions intended for zeroth-order diffracted beams, thus maintaining the modulation degree of the tracking error signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the width of regions on the hologram surface is increased to improve light intensity sensitivity, then the sensitivity to light intensity improves, but first-order diffracted beams leak into regions intended for zeroth-order diffracted beams only
Solution Approach 1:
The patent divides the optical path into distinct regions by introducing a partial light shielding element that segments the hologram surface into regions for zeroth-order diffracted beams and regions for first-order diffracted beams. This segmentation prevents beam leakage while maintaining the enlarged region widths needed for improved light intensity sensitivity.
Solution Approach 2:
The partial light shielding element acts as an intermediary component between the hologram surface and the light detection system. It selectively transmits zeroth-order diffracted beams while blocking first-order diffracted beams, thereby preventing harmful beam leakage without compromising the sensitivity improvements gained from enlarged region widths.
2Measurement precision
If regions are enlarged to receive both zeroth-order and first-order diffracted beams, then light intensity sensitivity improves, but tracking control stability deteriorates
Solution Approach 1:
The patent extracts the first-order diffracted beams from the regions intended for zeroth-order diffracted beams by using a partial light shielding element. This extraction allows the regions to be enlarged for improved sensitivity while preventing the harmful effect of first-order beam contamination that would degrade tracking control stability.
Solution Approach 2:
The partial light shielding element serves as an intermediary that enables the regions to be enlarged for improved sensitivity while simultaneously protecting the tracking control system from beam leakage that would cause instability. It mediates between the conflicting requirements of sensitivity and stability.
3Measurement precision
If the width of regions is increased, then more light is captured improving sensitivity, but positional displacement between objective lens and polarization hologram element causes offset in tracking error signal
Solution Approach 1:
The patent applies preliminary action by pre-compensating for the offset in the tracking error signal that arises from positional displacement. The offset compensation signal is generated in advance based on the position detection signal, allowing the system to maintain accurate tracking control despite manufacturing tolerances in the alignment between the objective lens and polarization hologram element.
Solution Approach 2:
The patent implements a feedback mechanism where the position detection signal is used to generate an offset compensation signal that is fed back to correct the tracking error signal. This feedback loop compensates for positional alignment errors and maintains tracking accuracy despite the enlarged region widths that increase sensitivity.
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 stabilizes tracking control and prevents the reduction in modulation degree of the tracking error signal, even with positional displacement between the objective lens and the polarization hologram element, by compensating the offset component in the tracking error signal.
Implementation Method 1
The optical element causes the light beam having the first wavelength to be transmitted therethrough, and causes the light beam having the second wavelength to be partially transmitted therethrough
Implementation Method 2
The optical element causes the light beam having the second wavelength to be partially transmitted therethrough
Implementation Method 3
for receiving a zeroth-order diffracted beam from the optical recording medium in the first region, and for receiving the zeroth-order diffracted beam and plus and minus first-order diffracted beams from the optical recording medium in the second region
Data Source
AI summary
When first-order diffracted beams leak into a region, which is for receiving only a zeroth-order diffracted beam from an optical disc, due to positional displacement between an objective lens and a hologram element, an offset compensation signal includes an AC component, the offset compensation signal preferably including a DC component only. Accordingly, there may be caused deterioration in a modulation degree of the tracking error (TE) signal. A partial light shielding element 110 is formed on a hologram surface 112a along boundaries between a light receiving region (121a), which receives a zeroth-order diffracted beam, and light receiving regions (121b, 121c), which receive the zeroth-order diffracted beam and first-order diffracted beams, so as to cover the light receiving region (121a). Further, the partial light shielding element 110 shifts phases of transmitted light beams by π, whereby the TE signal is offset-compensated, and the modulation degree can be improved.