Three Mirror Anastigmat Spectrograph with Field Lens
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Solution Overview
Problem
Echelle spectrographs face challenges in achieving high spectral resolution and broad wavelength coverage due to chromatic aberration and high f/number, which limits their application in Raman spectroscopy and other fields requiring low f/number optics, and they struggle with ambiguity in wavelength determination due to overlapping diffraction orders.
Innovation Solution
The design incorporates a three-mirror anastigmat (TMA) configuration with a concave primary mirror, convex secondary mirror, and spheroidal tertiary mirror, along with a field correcting lens and a rotatable diffraction grating, which improves angular field correction, reduces optical aberrations, and allows for higher throughput and broader wavelength coverage by optimizing the mirror positions and grating orientations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a lens-based spectrograph is used to achieve good resolution and high throughput, then spectral resolution and throughput are improved, but chromatic aberration limits spectral resolution when operating over broad wavelength ranges
Solution Approach 1:
The patent replaces lens-based optical systems with an all-reflective three-mirror anastigmat configuration. By using mirrors instead of lenses, the system eliminates chromatic aberration entirely since mirrors reflect all wavelengths equally, while maintaining high spectral resolution and throughput across broad wavelength ranges through precise mirror positioning and configuration.
Solution Approach 2:
The patent changes the optical parameters by using an all-reflective design with specific mirror configurations (concave primary, convex secondary, concave tertiary mirrors) to achieve both high resolution and broad wavelength coverage without the chromatic aberration that plagues lens-based systems.
2Measurement precision
If a high f/number camera focusing optics is used in echelle spectrographs to achieve good image quality, then spectral resolution is improved, but total light reaching the image plane decreases
Solution Approach 1:
The patent changes the f/number parameter by optimizing the three-mirror anastigmat configuration to achieve low f/number operation (f/2 or lower) while maintaining high spectral resolution. This is accomplished through precise control of mirror curvatures, spacing, and orientations, enabling high throughput without sacrificing image quality.
Solution Approach 2:
The patent employs adjustable and optimizable mirror positions and orientations in the three-mirror anastigmat system, allowing dynamic optimization of the f/number to balance light throughput and spectral resolution based on specific application requirements.
3Measurement precision
If a standard ruled grating in 1st order is used to achieve good resolution, then spectral resolution is improved, but wavelength coverage is limited
Solution Approach 1:
The patent employs an echelle grating that operates in multiple high diffraction orders (not just 1st order), enabling the single spectrograph to cover a broad wavelength range across many orders. The three-mirror anastigmat optics are designed to handle and focus light from multiple orders simultaneously, providing universal wavelength coverage while maintaining high resolution in each order.
Solution Approach 2:
The patent uses a second cross-dispersing element to spatially separate the overlapping diffraction orders in the perpendicular dimension. This allows multiple high-order spectra to be simultaneously captured and analyzed without ambiguity, extending wavelength coverage while preserving spectral resolution through the use of higher diffraction orders.
4Adaptability or versatility
If overlapping diffraction orders are used to enhance performance with echelle gratings, then wavelength coverage is improved, but ambiguity in wavelength determination increases
Solution Approach 1:
The patent introduces a second cross-dispersing element as an intermediary that spatially separates the overlapping diffraction orders. This element disperses light in the perpendicular direction, creating distinct spatial positions for each order and wavelength combination, thereby eliminating ambiguity in wavelength determination while preserving the benefits of broad wavelength coverage from multiple orders.
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 enhances spectral resolution, increases wavelength coverage, and reduces optical aberrations, enabling the spectrograph to achieve higher throughput and better image quality, making it suitable for applications requiring low f/number optics and improved wavelength determination.
Implementation Method 1
Light incident upon any ruled grating is split into several different diffraction orders
Implementation Method 2
An echelle spectrograph is an optical instrument that uses an echelle grating to diffract light with high dispersion and utilizes higher diffraction orders
Implementation Method 3
a second cross-dispersing element is used to spatially separate the orders
Implementation Method 4
a field correcting lens comprising a first lens surface in combination with a second lens surface (positive meniscus lens)
Implementation Method 5
a primary mirror having a concave-shaped reflective mirror surface, a secondary mirror having a convex-shaped reflective mirror surface and positioned to receive light reflected by the primary mirror, a tertiary mirror having a spheroidal (spherical concave) reflective mirror surface
Data Source
AI summary
A spectrograph that includes camera focusing optics with a primary mirror having a concave-shaped reflective mirror surface, a secondary mirror having a convex-shaped reflective mirror surface and positioned to receive light reflected by the primary mirror, a tertiary mirror having a concave reflective mirror surface and positioned to receive light reflected by the secondary mirror, and a field correcting lens comprising a convex lens surface in combination with a concave lens surface, wherein light received by said field correcting lens from said tertiary mirror enters said convex lens surface, traverses said field correcting lens, and exits from said concave lens surface. The optional field correcting lens is positioned such that the primary mirror, secondary mirror, tertiary mirror, and the field correcting lens share the common parent vertex axis.