Chemical detection and laser wavelength stabilization employing spectroscopic absorption via laser compliance voltage sensing
Systems and methods are disclosed that provide a direct indication of the presence and concentration of an analyte within the external cavity of a laser device that employ the compliance voltage across the laser device. The systems can provide stabilization of the laser wavelength. The systems and methods can obviate the need for an external optical detector, an external gas cell, or other sensing region and reduce the complexity and size of the sensing configuration.
1. A method for identifying at least one analyte present within an external cavity of an external cavity quantum cascade laser (ECQCL), comprising the steps of:
supplying a drive current to a laser device disposed within the ECQCL;
setting an operating wavelength of the ECQCL to a value corresponding to an absorption feature of the at least one analyte; and
measuring a compliance voltage across the laser device to identify the at least one analyte.
2. The method of claim 1 , further including comparing the compliance voltage to a known value thereof to determine the at least one analyte.
3. The method of claim 2 , wherein the comparing includes comparing the compliance voltage to a known value thereof measured in the absence of an analyte.
4. The method of claim 2 , wherein the comparing includes comparing the compliance voltage to a known value thereof measured with a selected concentration of at least one analyte in the ECQCL.
5. The method of claim 1 , wherein the measuring includes sweeping the operating wavelength across a selected wavelength range or a selected set of wavelength values to obtain a compliance voltage spectrum.
6. The method of claim 5 , wherein supplying the drive current includes modulating the current at a selected frequency, waveform, and amplitude.
7. The method of claim 5 , wherein the sweeping includes simultaneously modulating the operating wavelength at a selected frequency, waveform, and amplitude.
8. The method of claim 5 , wherein the measuring includes demodulating the compliance voltage at a selected demodulation frequency, waveform, and amplitude to obtain a demodulated compliance voltage spectrum.
9. The method of claim 5 , wherein identifying the at least one analyte includes identifying one or more features of the compliance voltage spectrum corresponding to absorption features of the at least one analyte.
10. The method of claim 9 , wherein identifying the at least one analyte includes obtaining a corrected compliance voltage spectrum by applying a mathematical function to the compliance voltage spectrum.
11. The method of claim 10 , wherein identifying the at least one analyte includes determining a concentration of the at least one analyte.
12. The method of claim 11 , wherein identifying the at least one analyte includes a least squares fit analysis or a weighted least squares fit analysis.
13. The method of claim 10 , wherein identifying the at least one analyte includes comparing the one or more features of the corrected compliance voltage spectrum with absorption features appearing in at least one known spectrum from a spectral database of known analytes.
14. The method of claim 5 , wherein the sweeping includes supplying a sweep signal to a wavelength-selective element disposed within the external cavity of the ECQCL.
15. The method of claim 14 , wherein the wavelength-selective element is a diffraction grating.
16. The method of claim 14 , wherein the wavelength-selective element is a mirror acting in concert with a diffraction grating.
17. The method of claim 14 , wherein the wavelength-selective element is a piezo-electric element.
18. The method of claim 1 , wherein supplying the drive current to the laser device includes modulating the current at a selected modulation frequency, waveform, and amplitude.
19. The method of claim 1 , wherein setting the operating wavelength includes modulating the operating wavelength at a preselected frequency, waveform, and amplitude.
20. The method of claim 1 , wherein measuring the compliance voltage includes demodulating the compliance voltage at a selected demodulation frequency, waveform, and amplitude to obtain a demodulated compliance voltage.
21. The method of claim 20 , wherein measuring the compliance voltage includes displaying and/or processing the compliance voltage as a function of the operating wavelength.
22. The method of claim 1 , wherein the at least one analyte includes a gas, a liquid, a solid, a plasma, or combinations thereof.