Fringe Count Error Correction in Interferometer Spectra
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Solution Overview
Problem
Interferometers face accuracy issues due to uncertainty in fringe waveform reversals, leading to variations in optical path difference, which affect the quality of interferograms and transformed spectra, especially in systems like the Cross-Track Infrared Sounder (CrIS) that require high spectral resolution.
Innovation Solution
A method using recursive least squares (RLS) to extract phase errors by forming a phase extraction function from earth scene and background spectra, determining the sign of a square root term, and extrapolating phases to correct fringe count errors, ensuring accurate optical path difference measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the position of the optical path difference varies between each scan, then the absolute optical path difference of points read by the ADC becomes uncertain, but this variation significantly affects the accuracy of added interferograms and the quality of transformed spectra
Solution Approach 1:
The patent uses the reference fringe signal as feedback to continuously track and determine the exact times at which the ADC in the interferogram channel is read. This feedback mechanism allows the system to adjust for variations in optical path difference between scans, ensuring that the absolute optical path difference remains accurately known despite mechanical variations in the movable mirror position.
Solution Approach 2:
The patent replaces reliance on purely mechanical positioning accuracy with an optical-based reference measurement system. Instead of depending on the mechanical stability of the movable mirror, the system uses optical interference fringes from a reference laser to precisely determine the optical path difference, substituting mechanical measurement with optical measurement.
2Measurement precision
If changes in scan direction occur at slightly different optical path difference values in different sweeps, then the timing of ADC readings becomes uncertain, but this leads to degraded quality of transformed spectra
Solution Approach 1:
The reference fringe signal provides continuous feedback about the actual optical path difference during each scan. By monitoring the phase and timing of these reference fringes, the system can precisely determine when ADC readings occur relative to the optical path difference, compensating for variations in scan direction timing between different sweeps.
Solution Approach 2:
The patent dynamically adjusts the timing parameters for ADC readings based on the measured reference fringe signal. Instead of using fixed timing intervals, the system modifies the sampling timing to align with the actual optical path difference progression in each scan, accounting for variations in scan speed and direction changes.
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 enhances the accuracy of interferometric measurements by correcting phase errors, improving the quality of spectra and reducing uncertainties in optical path differences, thereby enhancing the overall performance of interferometric systems.
Implementation Method 1
An interferometer of the Michelson type splits an input light beam into a reflected beam and a transmitted beam using a beam splitter
Implementation Method 2
At the beam splitter, the return split beams recombine along a common output path leading to a photodetector
Data Source
AI summary
A method includes extracting phase due to fringe count error (FCE) in spectra formed by an interferometer. The exemplary method includes the steps of: (a) forming an earth scene spectrum; (b) forming a background reference spectrum; and (c) forming a phase extraction function, Rk, where k is a kth channel of the interferometer. The phase extraction function is formed from the earth scene spectrum and background reference spectrum. The method may also include the steps of (d) using recursive least squares (RLS) to extract phase from the Rk function; and (e) providing the extracted phase to a user to correct the FCE. Step (c) may include forming a term in the Rk function that includes a positive or a negative sign of a square root. Step (d) may include determining whether the sign is negative or positive using the RLS.