Intracavity Faraday modulation spectroscopy (INFAMOS): A tool for radical detection

Abstract

We present the intra-cavity Faraday modulation spectroscopy technique, whereby optical feedback cavity-enhanced spectroscopy is coupled with Faraday modulation spectroscopy to greatly enhance the interaction path length of a laser beam with a paramagnetic sample in a magnetic field. We describe a first prototype based upon a cw quantum cascade laser targeting a selection of fundamental rovibrational R-branch transitions of nitric oxide (1890 cm), consisting of a linear cavity (finesse F=6300) and a water-cooled solenoid. We demonstrate a minimum detectable Verdet constant of V=4.7×10 rad cm G Hz (at SNR = 1), corresponding to a single-pass rotation angle of 1.6×10 rad Hz and a limit of detection of 0.21 ppbv Hz NO.

Sensitive detection of HO2 radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy

“…4 in the work of Westberg et al 49 or Fig. 3 in the work of Gianella et al 43 ). Figure 4 shows that the maximum is not reached at the field strength of 694 G, meaning that even larger fields could further increase the sensitivity of the spectrometer.…”

“…Faraday rotation cavity ring-down spectroscopy (FR-CRDS) or polarization-dependent cavity ring-down (PDCRD) was first demonstrated for molecular oxygen detection using a pulsed laser system at 628 nm and magnetic field of 200 G. 35 Numerous examples of coupling high finesse optical cavities to measure small amounts of Faraday rotation or, more generally, optical rotation exist in the literature. [37][38][39][40][41][42][43] Recently, Westberg and Wysocki 34 refined the technique to measure the ring-down times of two orthogonally polarized beams simultaneously to detect molecular oxygen at 762 nm, achieving a polarization rotation sensitivity of 1.3 nrad Hz −1/2 (per cavity round-trip). One major advantage of this latter implementation is that it puts the Faraday rotation angle on an absolute scale if the ring-down times of two orthogonal, linearly polarized beams are measured.…”

Optical frequency comb Faraday rotation spectroscopy

“…A theoretical model of the normalized FRS spectrum is presented, which shows good agreement with the measurements. A concentration detection limit for NO of 2.5 ppm•m is achieved at 1000 s, and the system remains stable for more than 1000 s. Further improvements in sensitivity can be obtained by improving the extinction ratio of the polarizers or increasing the interaction length through multiple passes [30] or by cavity enhancement [31,32]. While in this first demonstration we used an FTS to acquire broadband OFC-FRS spectra, the technique is compatible with other detection schemes of comb spectroscopy, for example, with continuous Vernier filtering [33,34], which should allow faster acquisition and thus faster modulation of the magnetic field.…”

Spectral Methods of Control of Impurities, Their Flows and Localization in an Equilibrium Low-Temperature Plasma of Low Pressure

“…The technique of phase measurements at deep modulation without loss of resolution was developed in [109,110,114]. The studies are underway to measure the concentrations of paramagnetic particles (О 2 , ОН, NO, HO 2 …) from the difference between the rotation angles of the s-and p-polarizations of radiation in the resonant cells under the application of the magnetic field [115][116][117]. According to the estimates made in the review [33], in the future such a method can provide the sensitivity of polarization measurements in the magnetic field of ~10-1000 G, e.g., [НО 2 ] radicals ~10 7 cm -3 , while the measurements carried out by traditional CRDS methods and its "off-axis ICOS" version showed the sensitivity of ~[НО 2 ] ~ 10 10 cm -3 [117].…”