# Photonic Technology for Precision Metrology
## 3 Some Practical Aspects of High-Resolution Measurements of Absorption Spectra  
Nitrous oxide is primarily known as a laughing gas and a gas used to improve the power of car engines (during racing). However, the most important motivation is that N\({}_{2}\)O is an explosives marker. It is used to assess their condition (aging process) as well as for detection [5, 6]. From a chemical point of view, most of the modern explosives are organic nitro compounds of aromatic, heterocyclic or aliphatic series, having one or more nitro groups linked to the molecular skeleton through carbon (C-NO\({}_{2}\)), nitrogen (N-NO\({}_{2}\)) or oxygen (O-NO\({}_{2}\)). It is well known that, as a result of the thermal decomposition of nitrates and nitro compounds, nitrogen oxides are formed (including N\({}_{2}\)O), and therefore an indirect method of explosives detection may be the analysis of gases released during their heating. The composition of gaseous products of thermolysis, including the amount and type of nitrogen oxides, depends primarily on the conditions of its operation (e.g., temperature and state of aggregation), but also on the structure of the explosive compound undergoing decomposition, i.e., on the number of nitro groups and the method of their connection with the skeleton carbon molecule. For this reason, the results of the analysis can also be used to identify the compound to be degraded (or at least a group of compounds) [7, 8]. Extensive research on the decomposition products of various explosives, where N\({}_{2}\)O has been identified as a common marker, is described in refs. [6, 9]. Figure 3 shows the examples of the research results conducted in this area at the Institute of Optoelectronics MUT. The graph shows the N\({}_{2}\)O spectra recorded during the decomposition of propylnitroguanidine (PrNQ), which is a future TNT substitute. Figure 2: Plot of differences between the centers of the 53 highest N\({}_{2}\)O absorption lines in the range of 4.4–4.7 \(\upmu\)m, determined on the basis of calculations from the HITRAN database and measured with IFS 125 HR. Taking this into account, it makes sense to develop sensors capable of detecting N\({}_{2}\)O. It should also be taken into consideration that these are trace concentrations. and therefore the sensor must have the highest possible sensitivity and excellent parameters, which were previously described. Therefore, the sensors that use laser absorption spectroscopy (LAS) are ideal for this purpose [8,9]. The fast response, non-invasive measurement and high reliability of these sensors can be highlighted alongside their other advantages. However, in order to achieve such features, one of the fundamental conditions that must be met is to match the wavelength of the radiation source to the absorption spectrum of the test substance. Therefore, lasers that have narrow spectral lines (e.g., FWHM < 0.001 cm\({}^{-1}\)) are used. As a result, a precise adjustment of the laser radiation wavelength to the selected absorption bands (lines), the half-width of which is also often below 0.001 cm\({}^{-1}\), can be achieved. Moreover, in order to ensure the possibility of detecting the lowest possible concentration of the molecules of a given substance, the bands with the largest absorption coefficient are selected. However, to ensure the high accuracy of the detection system, the interference of the absorption bands of other substances in the vicinity of the analyzed sample should also be taken into account. As can be observed in Figure 1b, in the case of low-resolution absorption spectra, it may occur that the laser is tuned between the absorption lines due to the apparent shape of the absorption lines formed as envelopes from the points included in different and independent absorption lines. The accurate and precise measurements of the absorption spectra have another very important meaning. If the sensor is not perfectly tuned at the maximum of the absorption line, not only will it decrease its sensitivity, but it will also be exposed to a highly non-linear operation.