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.
We report on the broadening of the optical bandwidth of a distributed feedback quantum cascade laser (QCL) caused by the application of radio frequency (RF) noise to the injection current. The broadening is quantified both via Lamb-dip spectroscopy and the frequency noise power spectral density (PSD). The linewidth of the unperturbed QCL (emitting at ∼5.3 μm) determined by Lamb-dip spectroscopy is 680±170 kHz, and is in reasonable agreement with the linewidth of 460±40 kHz estimated by integrating the PSD measured under the same laser operating conditions. Measurements with both techniques reveal that by mixing the driving current with broadband RF noise the laser lineshape was reproducibly broadened up to ca 6 MHz with an increasing Gaussian contribution. The effects of linewidth broadening are then demonstrated in the two-color coherent transient spectra of nitric oxide.
We report on the observation of saturation effects in Intracavity Faraday Modulation Spectroscopy (INFAMOS). A quantum cascade laser operating at ∼5.3 µm is used to probe the 2 Π 3/2 and 2 Π 1/2 R(3.5) transitions in the fundamental band of nitric oxide. With average intracavity intensities up to 450 W cm −2 , the saturation of these molecular transitions is observed up to a total pressure of ∼240 Torr. The experimental data are interpreted by incorporating saturation into a model for the INFAMOS line shape in the homogeneously broadened limit.
cascade lasers, QCLs. The high resolution available using single-mode devices provides high sensitivity with minimum detection limits in the range of ppbv in some applications. The limited range of mode-hop-free, MHF, tuning of most single-mode lasers, however, often restricts TDLAS methods to detection of a single species unless, fortuitously, two species have transitions lying within the MHF range. Detection of more than one species using TDLAS usually involves multiplexing several lasers and detector systems [4]. Widely tunable external cavity diode lasers, emitting narrow line or single-mode outputs, have recently been demonstrated in the 4-10 μm mid-infrared range based on quantum cascade devices but the relatively slow tuning rates limit application in rapidly changing environments [5]. A variety of broadband spectroscopy methods have been developed that allow detection of multiple species over a wide spectral range but not all are available in the mid-IR. These include use of multi-section diodes, broadband or wavelength agile light sources and correlation spectroscopic methods [6][7][8][9][10][11][12]. The wide bandwidth "frequency combs" available from mode-locked lasers has been exploited for broadband spectroscopy but usually involve complex laser systems and high resolution dispersion optics [13,14]. Extension of comb-based spectroscopy to the mid-infrared beyond 4 μm is becoming possible with the development of frequency combs based on QCL devices. A comb of width 100 cm −1 has recently been demonstrated at 7 μm [15]. Heterodyne methods, with potential for multi-species sensing, have also been demonstrated to allow detection in the RF region at the cost of some additional complexity and restriction in width of the spectrum covered [16].The technique of multi-mode absorption spectroscopy, MUMAS, addresses some of the limitations of TDLAS, specifically by providing wide spectral coverage whilst Abstract Detection of multiple transitions in NO and H 2 O using multi-mode absorption spectroscopy, MUMAS, with a quantum cascade laser, QCL, operating at 5.3 μm at scan rates up to 10 kHz is reported. The linewidth of longitudinal modes of the QCL is derived from pressure-dependent fits to experimental MUMAS data. Variations in the spectral structure of the broadband, multi-mode, output of the commercially available QCL employed are analysed to provide accurate fits of modelled MUMAS signatures to the experimental data.
Background Sepsis and infection are the leading cause of morbidity and mortality after surgery, but the inflammatory response to the trauma of surgery can make diagnosis challenging. Carbon-13 breath delta value (13C BDV) has been shown to be discriminant as a novel diagnostic marker for sepsis and infection. We aim to assess biomarkers including BDV following major hepatopancreatobiliary (HPB) surgery to diagnose postoperative infection/sepsis. Methods 20 participants undergoing HPB surgery were recruited prospectively. Breath samples were collected from baseline preoperatively, and on postoperative days (POD) 1–9, with plasma and peripheral blood mononuclear cell (PBMC) samples preoperatively, POD1, 4 and 8. Breath samples were analysed using infrared laser spectroscopy to generate BDV (per mil). Plasma was analysed for cytokines using MesoScale Discovery immunoassay, Procalcitonin (PCT) using ELISA, and Cell surface marker expression on monocytes was phenotyped using flow cytometry. Differences between groups who did and did not develop infective complications was analysed using two-way ANOVA and Mann Whitney U test. Results 5/20 participants developed infective complications, with a mean day of diagnosis POD5. There was no difference between BDV in patients with or without infection. Monocyte count was increased in infected participants at all timepoints, and monocyte expression of PD-L1 on POD1, and CD155 and HLA-DR on POD4 was upregulated, +13.09% (95%CI 1.59 to 24.61, p<0.05), +422.5 MFI (95% CI -770.9 to -73.68, p<0.05), and +19.02% (95% CI 37.58 to 0.45, p<0.05) respectively. Conclusions Monocyte expression of PD-L1, HLA-DR and CD155 is associated with infection, +13.09% (p<0.05), +422.5 MFI (p<0.05), and +19.02% (p<0.05) respectively. The innate immune system is dysregulated in infection following major HPB surgery. In this cohort BDV did not predict infection. Further investigation of novel biomarkers including BDV and soluble monocyte markers is required.
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