Point Diffraction Interferometer with Null Seeking Servo
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Solution Overview
Problem
Conventional interferometers face challenges in producing a 'perfect' reference wave due to beam splitters and other optics, leading to artifacts in the interferogram, and struggle with precise phase shifting, especially in low light level conditions.
Innovation Solution
A point diffraction interferometer system with Fourier transform lenses, a bead that radiates a spherical wave, and a null seeking servomechanism using controllable light transmissive elements and a processor to compensate for aberrations and phase errors, supported by a multimode fiber preamplifier for enhanced sensitivity and precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If beam splitters and other optics are used to create a reference wave, then the reference wave can be generated, but artifacts are introduced into the interferogram
Solution Approach 1:
The patent extracts the reference wave generation process from the conventional beam splitter-based optical system and relocates it to a computational domain. By using a digital micromirror device (DMD) to spatially modulate the test wave and subsequently extracting the reference wave information through Fourier transform and inverse Fourier transform operations, the system eliminates physical beam splitters and associated optical artifacts while maintaining reference wave functionality.
2Measurement precision
If conventional phase shifting methods are used, then phase shifting can be achieved, but the system becomes sensitive to minute mirror motions and requires complex optical arrangements
Solution Approach 1:
The patent replaces mechanical phase shifting methods (which require precise mirror translations and are sensitive to vibrations) with a computational approach. Phase shifting is achieved through digital processing of the spatially modulated interferogram using Fourier transform techniques, eliminating the need for mechanical mirror movements and associated complexity while improving stability and precision.
3Measurement precision
If a point diffraction interferometer is used to generate a perfect reference wave, then measurement accuracy improves, but phase shifting between reference and test waves becomes difficult
Solution Approach 1:
The patent introduces dynamic control through a digital micromirror device (DMD) that can be electronically programmed to create various spatial modulation patterns. This dynamic element enables flexible phase shifting and reference wave generation without requiring physical movement of optical components, making the system both precise and easy to operate.
4Measurement precision
If Fourier transform techniques are used to process the interferogram, then measurement precision improves, but the system requires additional optical components and processing complexity
Solution Approach 1:
The patent makes the digital micromirror device (DMD) a multi-functional element that performs both spatial modulation of the test wave and enables Fourier transform processing. By programming the DMD with appropriate patterns, the system achieves both reference wave generation and phase measurement capabilities through a single programmable component, reducing overall system complexity while maintaining high precision.
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 setup enables high-contrast interferograms and accurate phase shifting, effectively removing aberrations and achieving precise measurements even at low light levels by using a self-referencing, thermally and vibrationally insensitive system with electronic fringe tracking.
Implementation Method 1
The light diffracting around the occluding disk or through the pinhole, unencumbered by beam splitters, mirrors, lens, or other optical elements, closely approximates the ideal spherical wave desired for a reference wave
Implementation Method 2
The test and reference waves are recombined to generate an interference pattern or interferogram
Data Source
AI summary
An apparatus in one example has: first and second Fourier transform lenses, the first Fourier transform lens receiving an incident plane wavefront and focusing the plane wavefront down to a focal point in a focal plane, and the second Fourier transform lens reimaging the focused down plane wavefront to an output plane wavefront; a bead located substantially at the focal point and that is illuminated by radiation that comes in from the first lens, the bead reradiating a spherical wave, which interferes with light that passes around the bead to produce a diffraction pattern; an array of controllable light transmissive elements that support the bead in the focal plane; and a null seeking servomechanism for assigning an electrical value of phase departure of the incident plane wavefront from a reference thereof, the null seeking servomechanism controlling the light transmissive elements to produce phase shifts in light that passes around the bead to thereby remove aberrations in the incident plane wavefront.