Multi-harmonic inline reference cell for optical trace gas sensing
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.
1. A spectroscopic sensor having a sample cell containing a sample, the sensor comprising:
a coherent light source configured to transmit an interrogation light beam along an optical sample path directed towards the sample;
an in-line reference cell located in the optical sample path;
a detector having outputs responsive to absorption signals from the sample and the in-line reference cell;
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; and
circuitry configured to tune the coherent light source across an absorption range of interest and the coherent light source is scanned across the absorption range at a lower frequency and modulated at a higher frequency.
2. The sensor of claim 1 wherein the coherent light source is at least one of a quantum cascade laser, interband cascade laser, vertical cavity laser or semiconductor laser.
3. The sensor of claim 1 wherein the in-line reference cell is 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.
4. The sensor of claim 1 wherein the reference absorption signals are offset spectrally from the sample absorption signals.
5. The sensor of claim 1 wherein the reference cell has a pressure of less than atmospheric pressure.
6. The sensor of claim 5 wherein the reference cell has a pressure of approximately 0.1 atmospheres.
7. The sensor of claim 1 wherein the sample cell further comprises an optical cavity with an optical path length configured to measure the gas of interest at ambient conditions.
8. The sensor of claim 1 wherein the processor is configured to analyze a harmonic of the reference absorption signals.
9. The sensor of claim 8 wherein the processor is configured to analyze a sixth or greater harmonic of reference absorption signals.
10. The sensor of claim 1 wherein the processor is configured to analyze a harmonic of the sample absorption signals.
11. The sensor of claim 10 wherein the processor is configured to analyze a second harmonic of the sample absorption signals.
12. A spectroscopic method of determining a concentration of a sample cell containing a sample, the method comprising:
providing a coherent light source configured to transmit an interrogation light beam along an optical sample path directed towards the sample;
locating an in-line reference cell in the optical sample path; and
detecting absorption signals from the sample and the in-line reference cell;
isolating reference absorption signals from the in-line reference cell and sample absorption signals from the sample cell, generating calibration information based on the reference absorption signals and determining a concentration of the sample based on the sample absorption signals; and
tuning the coherent light source across an absorption range of interest and scanning the coherent light source across the absorption range at a lower frequency and modulating the coherent light source at a higher frequency.
13. The method of claim 12 wherein the coherent light source is at least one of a quantum cascade laser, interband cascade laser, vertical cavity laser or semiconductor laser.
14. The method of claim 12 wherein the in-line reference cell is 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.
15. The method of claim 12 wherein the reference absorption signals are offset spectrally from the sample absorption signals.
16. The method of claim 12 further comprising pressurizing the reference cell at a pressure that is less than atmospheric pressure.
17. The method of claim 12 further comprising pressurizing the reference cell at approximately 0.1 atmospheres.
18. The method of claim 12 wherein the sample cell further comprises an optical cavity with an optical path length configured to measure the gas of interest at ambient conditions.
19. The method of claim 12 further comprising analyzing a harmonic of the reference absorption signals.
20. The method of claim 12 further comprising analyzing a sixth or greater harmonic of reference absorption signals.
21. The method of claim 12 further comprising analyzing a harmonic of the sample absorption signals.
22. The method of claim 21 further comprising analyzing a second harmonic of the sample absorption signals.