Holographic Optical Element for Wavefront Error Compensation
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Solution Overview
Problem
Optical tape systems face challenges with limited field of view and higher wavefront error in single aspheric lenses, leading to degraded performance in direct read after write (DRAW) functions and increased costs with multi-element lens assemblies.
Innovation Solution
Incorporating a holographic optical element, specifically a volume phase holographic grating, to split laser beams into +1 and -1 order beams and introduce wavefront error that offsets the error introduced by the aspheric objective lens, resulting in reduced wavefront error and improved optical performance for satellite spots.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single aspheric objective lens is used, then device complexity is reduced, but wavefront error increases leading to degraded optical performance
Solution Approach 1:
A holographic optical element is introduced as an intermediary component between the laser source and the aspheric objective lens. This HOE pre-corrects the wavefront error that will be introduced by the lens, allowing the use of a simpler single-element aspheric lens while maintaining high optical performance. The holographic element acts as a compensatory mediator that offsets the aberrations of the simple lens.
Solution Approach 2:
The holographic optical element performs preliminary correction of wavefront error before the light reaches the aspheric objective lens. By introducing the opposite phase error through the HOE, the system pre-compensates for the aberrations that the simple lens will introduce, resulting in net low wavefront error at the tape surface.
2Manufacturing precision
If multi-element lens assemblies are used, then wavefront error is reduced, but device complexity and cost increase
Solution Approach 1:
The holographic optical element serves as a cost-effective intermediary that replaces the need for complex multi-element lens assemblies. Instead of using multiple expensive precision lenses to correct wavefront error, the system uses a single aspheric lens combined with a holographic element that achieves the same correction at lower cost and complexity.
Solution Approach 2:
The invention changes the approach to wavefront correction by using a holographic optical element with specific phase modulation parameters. The HOE is designed with grating structures that introduce controlled phase shifts to compensate for lens aberrations, representing a parameter-based solution rather than a structural multi-element approach.
3Productivity
If satellite spots are used for DRAW functions, then productivity is improved, but optical performance degrades due to wavefront error
Solution Approach 1:
The holographic optical element performs preliminary correction of wavefront error for satellite spots before they are formed. By pre-compensating the phase errors in the diffracted beams that create satellite spots, the system enables these spots to be used for DRAW functions with sufficient optical performance for reliable direct read after write verification.
Solution Approach 2:
The optical system is designed to provide acceptable performance across multiple functions using a single configuration. The holographic element corrects wavefront error for both main spots and satellite spots, enabling the satellite spots to serve dual purposes: as optical references for positioning and as functional spots for DRAW verification, enhancing system versatility.
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 solution enables optical tape systems to maintain acceptable optical performance at lower costs by compensating wavefront error in satellite spots, enhancing the ability to perform DRAW functions and reducing the need for more complex and costly multi-element lens assemblies.
Implementation Method 1
The holographic optical element splits a laser beam into +1 and -1 order beams
Implementation Method 2
The holographic optical element introduces wavefront error of specified polarity and magnitude into each of the +1 and -1 order beams
Implementation Method 3
The aspheric objective lens focuses the +1 and -1 order beams onto optical tape
Implementation Method 4
The aspheric objective lens introduces wavefront error into each of the +1 and -1 order beams
Data Source
Figure 1~2
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Figure 6~9
AI summary
An optical tape pick up unit includes a holographic optical element and an aspheric objective lens. The holographic optical element splits a laser beam into first order beams and introduces pre-compensating wavefront error into the first order beams. The aspheric objective lens focuses the first order beams onto optical tape and introduces wavefront error into the first order beams having a magnitude similar to and polarity opposite that of the pre-compensating wavefront error.