Multiple gas detection systems based on tunable diode laser absorption spectroscopy
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Simultaneous detection of multiple gases is useful in many applications. In this thesis, the simultaneous detection of CO and O2 based on tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation spectroscopy (WMS) has been studied. Interference-free, fiber based schematics for both high and low temperature applications have been suggested and prototype instruments have been built and tested. The Laser Gas II electronics and software based on second harmonic detection from Norsk Elektro Optikk (NEO) were used for laser modulation, digital signal processing and calculations of the desired gas parameters. The chosen schematic was based on wavelength division multiplexing (WDM) as the spectral range between the absorption lines to be detected were large. Since fiber coupled lasers have high noise levels which can not be reduced by external components, fiber couplers were introduced to the schematics. The fiber coupler divided the original laser beams into nearly identical beams with a predetermined power ratio. One of the beams, the signal beam, was guided through and modulated by the gas before it was detected. The other beam, the reference beam, was detected directly with an identical photodetector. When these two beams were subtracted, the laser noise should be removed and only the alteration due to the gas modulation should be left for calculations. However, as seen in this thesis, additional noise from the different photodetectors, the electronics and the components used to guide the laser beams limited the sensitivity gain seen by this method. Also the timing of when the signal and reference beam was measured was found to be important for the signal subtraction. This was because the noise from the lasers and their laser couplings fluctuated fast, especially for the O2 laser. The signal subtraction for the CO detection was found to increase the sensitivity by a factor 3 for both the set-ups for high and low temperature applications. The achieved detection limit for the CO detection was 200ppm per meter when signal subtraction was used. For the O2 detection, the increase in the sensitivity from the signal subtraction was not visible due to the electronic noise from the Laser Gas II equipment. The higher electronic noise level for this detection was due to low output power from the laser. For the schematic for the low temperature applications, a signal subtraction with an averaged reference beam was however found to increase the sensitivity by a factor 1.5. This gave a detection limit equal to 0.11% per meter for the O2 detection for the low temperature applications. This was almost 8 times worse than for the CO detection. The sensitivity for both systems was however limited by slowly fluctuating etalon noise in the signal beam.