92 Final Version of PIERS Proceedings will be available on www.piers.org NO and N 2 O Detection with CEAS Method J. Wojtas 1 , Z. Bielecki 1 , T. Stacewicz 2 , J. Mikolajczyk 1 , R. Medrzycki 1 , and B. Rutecka 1 1 Institute of Optoelectronics, Military University of Technology 2 Kaliskiego Str., Warsaw 00-908, Poland 2 Institute of Experimental Physics, University of Warsaw 69 Hoza Str., Warsaw 00-068, Poland Abstract— The article describes an application of cavity enhanced absorption spectroscopy in nitric oxide and nitrous oxide sensors. For detection of both gases the vibronic molecular transitions were used. The wavelength ranges of these transitions are situated in mid-infrared spectra spectrum of radiation. To achieve optimal sensitivity and selectivity interferences by absorption lines from other gases commonly present in atmosphere (like H 2 O or CO 2 ) should be minimised. The best results were obtained in the spectral regions of 5.23 μm–5.29 μm for NO and of 4.46 μm–4.54 μm for N 2 O. A setup of the sensors consists of pulsed laser sources, optical cavities and photodetectors. As a radiation sources single mode quantum cascade lasers (QCL) were applied. Their narrow emission lines were precisely tuned to the wavelengths of interest. The optical cavities were built with spherical mirrors of high reflectance. The optical signal from the cavities outputs were registered with specially developed low noise detection modules. Thanks to this, the detection limits of single ppb were obtained. 1. INTRODUCTION Nitric oxide (NO) and nitrous oxide (N 2 O) are important compounds of the air. According to HI- TRAN database, in the standard atmosphere their concentrations are as follows: NO — 0.3 ppbv, N 2 O — 320ppbv [1]. However, in real ambient air the concentrations differ strongly due to influ- ence of various emission sources (anthropogenic and natural). Both NO and N 2 O are important greenhouse gases that have a large influence on environment, living organisms and human health. Acid rains occur due to reactions of these compounds with H 2 O contained in the air. Moreover, NO and N 2 O are produced during a decomposition process of specific explosive materials so these gases can be used as markers [2, 3]. Therefore these gases monitoring is of great importance for various applications: from routine air monitoring in industrial area and regions of intensive traffic, detection of explosives in airports, finally in medicine investigation, for health care, etc. There are many techniques for NO and N 2 O detection. For example, in the case of gas chromatography (GC) and mass spectrometry (MS), a detection limit of a few dozen ppb is re- ported [4, 5]. Detection methods using the photoacoustic phenomenon provide a sensitivity of about 20 ppb [6]. In gas detection applications, a special role is played by optoelectronic methods. They are characterized by both high selectivity and high sensitivity (low limit detection). In practice, cavity enhanced absorption spectroscopy (CEAS) belongs to the most sensitive sensors. In this technique, off-axis direction of laser beam to the optical cavity is applied (Figure 1). Inside the cavity the light is repeatedly reflected by the mirrors. In comparison to cavity ring-down spec- troscopy (CRDS), the light spots on the mirrors surfaces are spatially separated and weak mode structure is occurred. It causes that the entire system is much less sensitive to instability in the cavity and to instability in laser frequencies. Additionally, due to off-axis illumination of the front mirror, the source interference by the optical feedback from the cavity is eliminated. The CEAS sensors can obtain a detection limit of about 10 -9 cm -1 [7]. Therefore, this method makes the best opportunity to develop a portable optoelectronic sensor of nitrogen oxides. In this technique determination of the concentration of the examined gas can be carried out during two-step process. In the frame of the first step, measurement of the signal decay time Figure 1: CEAS idea.