Wavelength modulated diode probe laser for an interferometric cavity-assisted photothermal spectroscopy gas sensor

Abstract

We report on key developments made in advancing Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS) toward a method for rugged and cost-effective gas sensing. In ICAPS, a Fabry Perot Interferometer (FPI) is employed to transduce absorption induced and analyte specific refractive index changes occurring inside the interferometer. Periodic analyte excitation is achieved by modulation of the excitation laser’s wavelength and the photothermal signal is detected by measuring the intensity of the probe laser reflected by the FPI. Maximum sensitivity is achieved when the probe’s wavelength matches the inflection point of the FPI’s transfer function. We describe the development of a locking scheme based on current induced wavelength modulation of a cost-effective diode probe laser to keep its wavelengths at the inflection point of the FPI. Because of the diode lasers characteristics, this locking procedure causes changes in the emitted laser power, which in turn influences the sensor’s sensitivity. Therefore, a normalization procedure had to be developed to correct for this inherent probe power dependency of the ICAPS sensor’s sensitivity. First, a theoretical analysis of the photothermal signal generation is given. Then a strategy is presented for stable operation of the ICAPS sensor with constant sensitivity while actively locking the probe’s wavelength. Experimental results on the example of NO detection in N2 employing a quantum cascade laser (30 mW) emitting at 1900 cm⁻¹ (LOD 2 ppm, NNEA of 3.3⋅10⁻⁶ W cm⁻¹Hz-1⁄2) corroborate the validity of the developed theoretical model and demonstrate its applicability to achieve constant sensor sensitivity at different sensor set-points.