We present a tunable diode laser spectrometer that can be software-switched between a wavelength modulation spectroscopy (WMS) operation mode and the direct absorption spectroscopy (DAS) technique on a per-scan basis. The new setup allows a direct comparison between the two techniques under equal hardware and system environment conditions. In this work, we compare the impact of analog-to-digital converter resolution for operation in WMS and DAS mode. Example concentration evaluations show that in the DAS approach, relative absorptions smaller than the least significant bit can be measured with scan averaging and FIR-filtering applied. With full resolution, both techniques show similar performance in our setup, with a slight advantage for the WMS implementation.
Fiber Bragg Gratings with a temperature induced chirp are used to control and optimize the power spectral density of a Raman-MOPA by influencing the FWM-based power dependent spectral broadening.
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