Multi-order Diffractive Fresnel Lens Achromatizing Compensation
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Solution Overview
Problem
Diffractive Fresnel lenses suffer from large chromatic dispersion and focal length variations with wavelength, leading to increased focal range and background levels, which are detrimental to imaging systems.
Innovation Solution
A multiple-order diffractive Fresnel lens (MOD-DFL) with an achromatizing compensation mechanism, where the surface profile is designed with transitions based on integer multiples of optical path difference, combined with a single-order diffractive Fresnel lens to reduce refractive dispersion and focal range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of stationary object
If a diffractive Fresnel lens is used to achieve size and weight reduction, then the optical element becomes very thin and lightweight, but chromatic dispersion increases and focal length varies dramatically with wavelength
Solution Approach 1:
The patent combines a diffractive Fresnel lens (DFL) with a Fresnel zone plate (FZP) into a single hybrid optical element. The DFL provides the primary focusing function with high diffraction efficiency, while the FZP component suppresses background levels and reduces chromatic dispersion effects. This merging allows the optical element to maintain its thin, lightweight form factor while mitigating the harmful chromatic aberrations inherent in pure diffractive lenses.
Solution Approach 2:
The hybrid lens employs a composite structure where regions of the optical element have different phase modulation characteristics - some regions function as DFL zones with continuous phase profiles while others function as FZP zones with binary transmission. This composite approach enables simultaneous achievement of high diffraction efficiency and reduced chromatic dispersion, as the two components have complementary optical properties that compensate for each other's weaknesses.
2Use of energy by moving object
If a diffractive Fresnel lens is used to focus light, then diffraction efficiency into the primary focal order increases, but background levels at the primary focus increase due to other focal positions
Solution Approach 1:
The patent extracts and eliminates the harmful background levels generated by the diffractive Fresnel lens by incorporating a Fresnel zone plate component. The FZP is designed to block or redirect light from secondary focal positions while allowing light from the primary focus to pass through. This extraction of the harmful background component preserves the high diffraction efficiency of the DFL while removing the detrimental background illumination that degrades image quality.
3Length of stationary object
If a multi-order diffractive Fresnel lens is used to reduce focal range, then focal range is reduced by a factor of three, but device complexity increases due to multiple surface profiles
Solution Approach 1:
The patent segments the optical element into distinct functional zones - inner zones with one surface profile and outer zones with another surface profile. Each segment is optimized for a specific function: the inner zones provide primary focusing with high efficiency while the outer zones contribute to chromatic dispersion compensation. This segmentation allows the complex multi-order diffractive functionality to be implemented in a modular manner, reducing the overall focal range while keeping the manufacturing process more manageable through clear zone demarcations.
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 MOD-DFL with achromatizing compensation significantly reduces focal range by almost a factor of three, improving image quality and reducing post-processing overhead required for image reconstruction.
Implementation Method 1
Diffractive optical elements perform lens-like functions by utilizing principles of wave combination. Constructive wave combination, where two waves combine with crests and troughs arriving at the same time, leads to increased amplitude.
Implementation Method 2
Light transmitted through an aperture that is illuminated by a distant star can be divided into equal-area Fresnel zones that identify which parts of the transmitted light interfere constructively at an on-axis observation point
Implementation Method 3
since the constructive or destructive nature of the wave combination depends on wavelength, the focal point changes dramatically with different wavelengths, with a focal length proportional to 1/λ, where λ is the wavelength
Data Source
AI summary
An optical device is provided that comprises a multi-order diffractive Fresnel lens (MOD-DFL) and an achromatizing compensation mechanism that reduces refractive dispersion created by the MOD-DFL, thereby reducing the focal range of the MOD-DFL. A method is also provided of using the optical device in an image processing system to obtain images of an object and processing the images to perform image enhancement.