Optical Head Diffractive Element Offset Cancellation
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Solution Overview
Problem
Conventional optical head devices face challenges in canceling offset components due to objective lens shift and mitigating stray light in multilayer optical discs, particularly with complex configurations and weak tracking error signals.
Innovation Solution
An optical head device with a photodetector having a simple light-receiving surface pattern and a diffractive optical element that diffracts returning light beams, separating zero-order and ±1-order components to generate an offset-free tracking error signal without losing optical intensity, using a primary and secondary diffraction region configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional single-beam push-pull method is used for tracking error detection, then the system is simple to implement, but an offset component is added to the push-pull signal when the objective lens shifts radially
Solution Approach 1:
The light-receiving surface is divided into multiple regions: a first light-receiving region for receiving the main beam, and second and third light-receiving regions for receiving sub-beams. This segmentation allows separate detection of main beam and sub-beam positions, enabling offset cancellation while maintaining system simplicity.
Solution Approach 2:
A diffractive optical element is introduced as an intermediary component to split the returning light beam into main beam and sub-beams. This element enables the generation of tracking error signals by diffracting light without requiring complex mechanical adjustments or additional moving parts.
2Illumination intensity
If the light intensity of sub-beams is reduced by diffraction grating separation, then the main beam intensity is preserved for high-speed recording/reproduction, but the tracking error signal quality deteriorates due to stray light from other information recording layers
Solution Approach 1:
Different regions of the photodetector are assigned different functions: the first light-receiving region detects the main beam for high-speed recording/reproduction, while the second and third light-receiving regions detect sub-beams for tracking error detection. This local differentiation allows optimization of each region for its specific purpose.
Solution Approach 2:
The stray light from other information recording layers, which was previously harmful, is now detected by the second and third light-receiving regions. By measuring the stray light intensity in these regions and using it to correct the tracking error signal, the harmful effect is converted into a useful correction mechanism.
3Measurement precision
If a polarization hologram is used to separate zero-order and ±1-order light beams for offset cancellation, then the push-pull signal offset is eliminated, but the device configuration becomes complex and manufacturing cost increases
Solution Approach 1:
The patent replaces the expensive and complex polarization hologram with a simpler diffractive optical element that can be manufactured using standard semiconductor fabrication techniques. This substitution reduces manufacturing cost and device complexity while achieving the same offset cancellation function.
Solution Approach 2:
The invention changes the operational parameters of the diffractive optical element to optimize performance. By adjusting the diffraction grating pitch and depth, the system achieves high diffraction efficiency for sub-beams while maintaining sufficient main beam intensity, eliminating the need for polarization-sensitive components.
4Quantity of substance
If the spacing between adjacent information recording layers is reduced to increase recording capacity, then more layers can be fitted on the disc, but the light intensity of stray light from other layers increases
Solution Approach 1:
The system uses feedback from the second and third light-receiving regions to correct the tracking error signal. By continuously monitoring the stray light intensity in these regions and using it to adjust the tracking control, the system compensates for the increased stray light effect caused by reduced layer spacing.
Solution Approach 2:
The diffractive optical element is designed in advance to separate the main beam and sub-beams before they reach the photodetector. This preliminary separation allows the system to prepare for stray light interference by directing sub-beams (including stray light components) to dedicated detection regions, enabling proactive correction rather than reactive compensation.
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 cancels offset components and reduces stray light interference, improving tracking error signal quality and maintaining optical intensity, thus enhancing the recording and reproduction capabilities of multilayer optical discs.
Implementation Method 1
a diffractive optical element for diffracting a returning light beam that has been reflected by the optical disc and has passed through the objective lens and outputting a transmissively diffracted light beam
Implementation Method 2
a photodetector for receiving the transmissively diffracted light beam
Data Source
AI summary
An optical head device mounted in an optical disc device. The optical head device is provided with a diffractive optical element and a photodetector. The diffractive optical element has: a primary diffraction region at a location on which the positive and negative first-order components and some of the zero-order component of a reflectively diffracted light beam are incident; and secondary diffraction regions at locations on which the rest of the zero-order component but none of the positive or negative first-order components of the reflectively diffracted light beam are incident. A main light-receiving section of the photodetector receives the zero-order component of a transmissively diffracted light beam that has passed through the primary diffraction region and the secondary diffraction regions. Secondary light-receiving sections receive the positive first-order component and/or the negative first-order component of the transmissively diffracted light beam that has passed through the secondary diffraction regions.