Multi-harmonic Inline Reference Cell for Trace Gas Sensing
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Solution Overview
Problem
Conventional laser-based trace gas sensors require separate reference cells or interrupt the measurement for calibration, leading to complications due to differing optical pathlengths and environmental conditions, and provide only intermittent calibration, which is inadequate for rapidly changing conditions.
Innovation Solution
A spectroscopic sensor and method using an in-line reference cell with multiharmonic wavelength modulation spectroscopy to calibrate and determine the concentration of a sample gas in real-time, without the need for separate reference cells, by isolating and analyzing reference absorption signals to generate calibration information and separate target gas signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If separate reference cells are used for calibration, then calibration can be performed, but the optical pathlengths and environmental conditions differ between sample and reference cells complicating the calibration relevance
Solution Approach 1:
The patent merges the reference cell and sample cell into a single integrated optical path, eliminating separate reference cells. The reference gas and sample gas coexist in the same optical path, ensuring identical optical pathlengths and environmental conditions for both reference and sample measurements, thereby resolving the calibration relevance issue while reducing system complexity
Solution Approach 2:
The patent introduces a wavelength modulation technique as an intermediary method to separate reference absorption signals from sample absorption signals in the frequency domain. By modulating the laser wavelength and analyzing harmonics, the system can distinguish between reference gas absorption and sample gas absorption even when they occupy the same optical path, enabling accurate calibration without physical separation
2Measurement precision
If measurement is interrupted for calibration, then calibration can be performed, but the calibration is only intermittent and inadequate for rapidly changing conditions
Solution Approach 1:
The patent enables continuous calibration by performing reference measurements simultaneously with sample measurements in the same optical path. The wavelength modulation technique allows the system to continuously extract reference absorption signals at specific harmonic frequencies while simultaneously measuring sample absorption, eliminating the need to interrupt measurements for calibration and providing continuous calibration under rapidly changing conditions
Solution Approach 2:
The patent employs periodic wavelength modulation of the laser source as a carrier signal to encode reference absorption information. By modulating the wavelength periodically and detecting specific harmonic components, the system can continuously separate reference signals from sample signals in real-time, enabling uninterrupted simultaneous calibration and measurement
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
Enables accurate, continuous, and unattended measurements of trace gases under ambient conditions, addressing the limitations of conventional calibration methods by providing online and in-situ calibration, accounting for system drift and noise, and improving precision and accuracy in trace gas monitoring.
Implementation Method 1
a detector having outputs responsive to absorption signals from the sample and the in-line reference cell
Implementation Method 2
The coherent light source may be scanned across the absorption range at a lower frequency and modulated at a higher frequency
Implementation Method 3
The in-line reference cell may be a closed path, in-line reference cell configured with angled windows or windows with an anti-reflective coating to minimize back reflections from the windows
Implementation Method 4
The processor may be configured to analyze a harmonic of the reference absorption signals. The processor may be configured to analyze a sixth or greater harmonic of reference absorption signals
Data Source
AI summary
A spectroscopic sensor and a spectroscopic method of determining a concentration of a sample are disclosed. The sensor is used in connection with a sample cell containing a sample. The sensor includes a coherent light source configured to transmit an interrogation light beam along an optical sample path directed towards the sample. The sensor also includes an in-line reference cell located in the sample path. The sensor also includes a detector having outputs responsive to absorption signals from the sample and the in-line reference cell. The sensor also includes a processor configured to isolate the reference absorption signals from the in-line reference cell and sample absorption signals from the sample cell and generate calibration information based on the reference absorption signals and determine a concentration of the sample based on the sample absorption signals.