Faraday rotation spectrometer with sub-second response time for detection of nitric oxide using a cw DFB quantum cascade laser at 5.33 μm

Abstract

Lewicki et al. [4]. However, since that system is based upon an external cavity laser it requires stable running conditions to obtain the optimum detectability, which restricts its use in industrial applications. In addition, it utilizes a laser source with a moderate output power (3 mW) that is modulated through a mechanical actuator.Here, we present a Faraday modulation spectrometer for sensitive detection of NO that is based upon a distributed feedback (DFB) QCL with an output power of 60 mW and a Peltier cooled mercury-cadmium-telluride (MCT) detector. A DFB laser provides more output power than an external cavity laser, which increases the sensitivity of the spectrometer. It is also more 3 compact and it is solely scanned through the injection current of the laser, which makes it less susceptible to drifts (as no mechanical parts are involved in scanning the laser wavelength).Furthermore, application of a rapid magnetic field modulation (in the kHz regime) and a fast wavelength scan allow for further enhancement of the signal-to-noise ratio and at the same time yields a sub-second response time of the spectrometer. In addition, the concentration of NO is extracted via curve fitting, which is an advantage when high accuracy assessments are to be made since it reduces the risk for contributions from structured or drifting background signals.All these features make the presented spectrometer suitable for industrial and field applications.An Allan variance analysis demonstrates that the system has an excellent stability, i.e. a white noise behavior, for up to several hours of operation, with a detection limit of 4.5 ppb for a response time of 1 s. Theory ExperimentThe experimental set-up is shown in Figure 1. A room temperature cw QCL DFB laser emitting /W. The photocurrent from the reference detector was amplified by a DC-coupled transimpedance amplifier and sent to a DAQ board (NI-USB 6251). The direct absorption signal from the reference cell was used for accurate determination of the laser frequency in the signal processing. 6The main part of the beam was transmitted through a CaF 2 wedge and passed two wire grid polarizers stacked in series to clean up and define the plane of polarization from the laser.After the polarizers, the laser beam passed through a 25 cm long absorption cell with an outer diameter of 12 mm. The cell tube was made of borofloat glass whereas the optical windows were made of 2 mm thick CaF 2 plane parallel discs (with a diameter of 12 mm). The cell was connected via a pressure controller to a gas mixing device (HOVAGAS). The cell was inserted in a 15 cm long solenoid with an inner diameter of 12.5 mm. The solenoid, which consisted of 765 turns of 1 mm thick copper wire with 0.3 mm thick isolation, coupled in series with a capacitor stack, was driven by a power audio amplifier. The resonant circuit was tuned to obtain a maximum magnetic field at a frequency of 7.4 kHz. We found this choice of modulation frequency to be a good compromise between the performance of the instrument ...

Spectroscopic monitoring of NO traces in plants and human breath: applications and perspectives

“…Recently, the development of high performance FRS spectrometers has been reported [15,23]. The ultimate sensitivity levels achieved in these experiments was 4.7 and 4.5 ppbv for 1 s averaging time, respectively.…”

“…First, the NO absorption strength at the wavelength of the available QCL is twice as low as the most sensitive transition for FRS (1875.81 cm −1 ) [15,21,22]. The power of the QCL is only 1 mW compared to those reported in the literature: 3 mW and 60 mW, respectively [15,23]. In addition, the detectivity (D * ) of the detectors used in these studies is 2 orders of magnitude higher than in the present experiment.…”

A Review of Signal Enhancement and Noise Reduction Techniques for Tunable Diode Laser Absorption Spectroscopy

“…14 NO has been achieved. The isotope ratio (δ 15 N) has been determined with a precision (1σ ) of 0.52 at 800-s averaging time for 100 ppm NO gas with a time resolution of 2 s. In 2011, a Faraday modulation spectrometer for sensitive and fast detection of nitric oxide at 5.33 μm utilizing a room temperature CW distributed feedback QCL was presented by Kluczynski et al (114). The spectrometer provides a detection limit of 4.5 ppb for a response time of 1 s. Most related work on the use of diode laser (VCSEL and QCL)-based Faraday rotation spectroscopy for trace gases sensing (O 2 , NO, and OH radicals) was carried out by Wysocki's group at Princeton University and their collaborators (115)(116)(117).…”

Analytical expression for the nth Fourier coefficient of a modulated Lorentzian dispersion lineshape function

“…Examples of such techniques are Faraday modulation/rotation spectroscopy (FAMOS/FRS), which is used for detection of paramagnetic gaseous species, in particular NO [1][2][3][4][5][6][7][8][9], and wavelengthmodulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, commonly referred to as wm-NICE-OHMS [10]. It has been shown that the signals from these techniques can be written in terms of such first Fourier coefficients [1,2,7,[11][12][13].…”