FTIR Spectrometer Wavelength Calibration via CO2 Absorption
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Solution Overview
Problem
Existing Fourier transform infrared spectrometers face challenges in accurately calibrating the oscillation wavelength of semiconductor lasers, especially when the accuracy is low, due to the overlap of absorption peaks from water vapor, making it difficult to determine the correct calibration.
Innovation Solution
The Fourier transform infrared spectrometer uses carbon dioxide's distinct absorption peak at 667 cm−1 to calibrate the oscillation wavelength of semiconductor lasers, allowing for easy differentiation from other peaks and improving calibration accuracy, and optionally uses multiple water vapor peaks for further precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If water vapor absorption peaks are used for calibration, then calibration can be performed using ambient air, but the overlapping peaks make it difficult to accurately determine the correct calibration wavelength
Solution Approach 1:
The patent introduces carbon dioxide as an intermediary substance to facilitate wavelength calibration. By adding CO2 to the measurement chamber, a distinct absorption peak at 667 cm-1 is generated, which serves as a clear reference marker for calibration, eliminating the ambiguity caused by overlapping water vapor peaks
Solution Approach 2:
The patent changes the compositional parameter of the calibration environment by introducing carbon dioxide gas. This creates a new, distinct absorption feature at 667 cm-1 that is easily distinguishable from water vapor peaks, enabling accurate wavelength determination through the ratio of absorbance at 667 cm-1 to that at 3700-3750 cm-1
2Volume of moving object
If semiconductor lasers are used to reduce device size, then miniaturization is achieved, but individual differences and aging cause wavelength drift requiring frequent calibration
Solution Approach 1:
The patent implements a feedback mechanism for wavelength calibration by continuously monitoring the absorption peak ratio of carbon dioxide (667 cm-1 to 3700-3750 cm-1). This ratio provides real-time information about wavelength drift, enabling automatic correction and maintaining reliability despite semiconductor laser aging or individual variations
3Reliability
If helium-neon lasers are used for control interferometer, then stable wavelength is achieved, but device size increases
Solution Approach 1:
The patent changes the laser type from helium-neon to semiconductor, accepting the trade-off of reduced inherent stability in exchange for miniaturization. Compensation is achieved through the carbon dioxide absorption ratio method, which corrects for the semiconductor laser's wavelength variations and maintains measurement reliability
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 method enables precise calibration of the oscillation wavelength, enhancing the ability to distinguish absorption peaks and reducing errors, even with low-accuracy semiconductor lasers, and improves the overall accuracy of measurements.
Implementation Method 1
The infrared light generated by the light source is split into two beams by the beam splitter. One beam is reflected by the fixed mirror, while the other beam is reflected by the movable mirror. The reflected beams return to the beam splitter and are merged into one beam
Implementation Method 2
The relationship between the intensity of the infrared light cast onto the sample and the position of the movable mirror can be represented by sinusoidal waves of different wavelengths superposed on each other so that the intensity has the highest value at any wavelength when the infrared beam reflected by the fixed mirror and the infrared beam reflected by the movable mirror has an optical path difference of zero (center burst)
Implementation Method 3
The laser light is introduced into the beam splitter, which splits the light into two beams. One beam is reflected by the fixed mirror, while the other beam is reflected by the movable mirror. The reflected beams return to the beam splitter and are merged together. The merged laser light is detected with a detector which is separately provided from the detector for the main interferometer. The detected laser light becomes interference light whose intensity is maximized when the optical path difference equals an integer multiple of the wavelength of the laser light
Implementation Method 4
an oscillation wavelength calibrator configured to locate an absorption peak of carbon dioxide from peaks in the spectrum created by the spectrum creator
Implementation Method 5
By performing fast Fourier transform (FFT) on this interferogram, a spectrum with the wavenumber (or wavelength) on the horizontal axis and the intensity (e.g. absorbance or transmittance) on the vertical axis is obtained
Data Source
AI summary
In an FTIR 1, a beam splitter 12, fixed mirror 13 and movable mirror 14 are shared by a main interferometer 10 including a multiwavelength infrared light source 11 and a control interferometer 20 including a semiconductor laser 21. A first detector 16 detects infrared interference light generated by the main interferometer 10 and transmitted through or reflected by a sample. A second detector 26 detects monochromatic interference light generated by the control interferometer 20. A spectrum creator 32 determines an optical path difference between an optical path via the fixed mirror 13 and an optical path via the movable mirror 14, based on the intensity and uncalibrated oscillation wavelength of the monochromatic interference light detected by the second detector 26, and creates a spectrum by performing fast Fourier transform on an interferogram which shows a distribution of the intensity of the infrared interference light detected by the first detector 16 with respect to the optical path difference. An oscillation wavelength calibrator 34 locates an absorption peak of carbon dioxide from the peaks in the spectrum created by the spectrum creator 32, and compares a wavenumber or wavelength of the absorption peak with a true absorption wavenumber or wavelength of carbon dioxide to determine a calibrated oscillation wavelength of the semiconductor laser 21.