VCSEL based Faraday rotation spectroscopy with a modulated and static magnetic field for trace molecular oxygen detection |
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Authors: | S G So E Jeng G Wysocki |
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Institution: | 1.Electrical Engineering Department,Princeton University,Princeton,USA |
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Abstract: | Faraday Rotation Spectroscopy (FRS) is a useful technique for quantification of paramagnetic trace gases with significantly
higher sensitivity when compared to direct absorption techniques. Our prototype system based on the openPHOTONS sensor core
measures the concentration of molecular oxygen (O2) in the A band using a 763-nm vertical cavity surface emitting laser. We provide detailed analysis of two measurement methods
based on FRS using the same sensor configuration: one with a modulated magnetic field, and one with a static magnetic field
in combination with wavelength modulation. Our spectra signal-to-noise ratios agree well with our simulations via modeling
of the FRS signal. For alternating magnetic field, we achieve an equivalent minimum detectable absorption (MDA) of
8.86×10-7/Hz\frac128.86\times 10^{-7}/\mathrm{Hz}^{\frac{1}{2}} resulting in a minimum detection limit of 30 ppmv⋅m/
Hz\frac12\mathrm{Hz}^{\frac{1}{2}} of O2, limited by detector noise and laser noise. For the same system configuration in the static field case, parasitic etalon
fringes limited the MDA to
4.8×10-6/Hz\frac124.8\times 10^{-6}/\mathrm{Hz}^{\frac{1}{2}}. In both cases, we describe methods to improve signal-to-noise ratio based on our data and models. |
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