Abstract
A new detection method for Faraday rotation spectra of paramagnetic molecular species is presented. Near shot-noise limited performance in the mid-infrared is demonstrated using a heterodyne enhanced Faraday rotation spectroscopy (H-FRS) system without any cryogenic cooling. Theoretical analysis is performed to estimate the ultimate sensitivity to polarization rotation for both heterodyne and conventional FRS. Sensing of nitric oxide (NO) has been performed with an H-FRS system based on thermoelectrically cooled 5.24 μm quantum cascade laser (QCL) and a mercury-cadmium-telluride photodetector. The QCL relative intensity noise that dominates at low frequencies is largely avoided by performing the heterodyne detection in radio frequency range. H-FRS exhibits a total noise level of only 3.7 times the fundamental shot noise. The achieved sensitivity to polarization rotation of 1.8 × 10(-8) rad/Hz(1/2) is only 5.6 times higher than the ultimate theoretical sensitivity limit estimated for this system. The path- and bandwidth-normalized NO detection limit of 3.1 ppbv-m/Hz(1/2) was achieved using the R(17/2) transition of NO at 1906.73 cm(-1).
Highlights
Since first reported in the 1980s [1], Faraday rotation spectroscopy (FRS) has been used as a sensitive and selective technique for the detection of gas-phase paramagnetic species such as nitric oxide (NO) [1,2,3,4,5,6,7], NO2 [8, 9], O2 [10, 11], and OH radicals [12, 13]
As expected in the low frequency range the laser noise generally follows 1/f trend, and at frequencies >1MHz there are some distinct peaks that can be avoided by selecting an appropriate measurement frequency
We have not studied the origin of those distinct peaks in the noise spectrum between 1 MHz and 10 MHz, but we suspect the noise of the laser current source, power supplies, and thermoelectrically cooled (TEC) controller electromagnetic interference are the most possible causes of this specific spectral noise structure
Summary
Since first reported in the 1980s [1], Faraday rotation spectroscopy (FRS) has been used as a sensitive and selective technique for the detection of gas-phase paramagnetic species such as NO [1,2,3,4,5,6,7], NO2 [8, 9], O2 [10, 11], and OH radicals [12, 13]. In the presence of magnetic field, the transition states of the paramagnetic molecules split due to the Zeeman Effect causing magnetic circular birefringence (MCB, a difference in refractive indices for lefthanded (LHCP), and right-handed (RHCP) circularly polarized components) and magnetic circular dichroism (MCD, a difference in absorption coefficients for LHCP and RHCP). When linearly polarized light, which is a superposition of LHCP and RHCP, propagates in the paramagnetic sample under longitudinal magnetic field, it exhibits rotation of the polarization axis (the Faraday Effect). The polarization rotation angle can be described as Θ = (nR - nL)πL/λ, where L is the effective optical path length within the sample under magnetic field, λ is the wavelength of light and (nR - nL) is a difference between refractive indices for RHCP and LHCP components. Since Faraday rotation Θ is proportional to sample concentration it can be used for quantitative measurements of sample concentration
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