Phased Holographic Array for Large Optical Surface Testing
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Solution Overview
Problem
Existing interferometry techniques for testing optical surfaces, such as the Hindle test, face challenges with large and costly auxiliary optics, and are susceptible to environmental errors, particularly when measuring large convex aspheric surfaces.
Innovation Solution
An array of computer-generated holograms (CGHs) is used, arranged in a matrix format, to form a phased holographic array testplate (PHAT) that covers the optical surface, allowing for the testing of large optics using smaller individual holograms, which are calibrated to act as a single large hologram, combined with mechanical motions for complete surface mapping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a single large hologram is used to cover the entire optical surface, then measurement coverage is improved, but manufacturing cost and difficulty increase significantly
Solution Approach 1:
The patent divides a large hologram into multiple smaller holograms arranged in an array. Each individual hologram in the array can be manufactured separately with standard fabrication techniques, avoiding the need to create one extremely large hologram. The array of smaller holograms collectively covers the entire optical surface area needed for measurement.
2Area of stationary object
If auxiliary optics are made very large to test large optical surfaces, then measurement capability is improved, but system complexity and environmental susceptibility worsen
Solution Approach 1:
The patent replaces complex mechanical auxiliary optics with a computer-generated hologram array. Instead of using large physical optical components that require precise mechanical alignment and are susceptible to environmental errors, the system uses computationally generated holographic patterns that can be programmed and adjusted without physical realignment.
3Measurement precision
If a single large hologram is used, then measurement precision is improved, but alignment accuracy becomes more difficult to maintain
Solution Approach 1:
The patent segments the large hologram into multiple smaller holograms in an array. Each small hologram can be manufactured and positioned with standard precision, and any alignment errors are corrected through computational phase adjustment. The relative positions and phases of individual holograms are calibrated to achieve the precision equivalent of a single large hologram.
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 approach reduces the cost and size limitations of holograms, provides accurate surface measurements by forming a null wavefront, and minimizes alignment and environmental errors through the use of a PHAT, enabling efficient testing of large optical surfaces.
Implementation Method 1
The CGHs usually include patterns of lines which act as diffraction gratings. These patterns are usually written onto, or etched into glass substrates.
Implementation Method 2
Interferometry techniques are used for testing aspheric surfaces. In one test configuration, known as a null configuration, a reference wavefront and a test wavefront are formed to produce an interference pattern.
Data Source
AI summary
An apparatus for testing an optical surface comprising an array of holograms. The array includes a plurality of individual holograms arranged in an M×N format, in which M is the number of rows and N is the number of columns in the array. The array of holograms is positioned between the optical surface and a wavefront sensor. The array of holograms reflects a reference beam back to the wavefront sensor, and transmits a test beam to the optical surface. The array of holograms also receives the test beam reflected from the optical surface and transfers the test beam back to the wavefront sensor.