# Photonic Technology for Precision Metrology
## 2 From the Beginning of Spectroscopy to High-Spectral-Resolution Measurements  
Atomic and molecular spectroscopy appeared earlier than quantum mechanics, after [PERSON]'s fundamental study of the spectra of sunlight in 1672, and the demonstration of a narrow slit, instead of a circular hole, for generating spectral lines by [PERSON] in 1802 and [PERSON] in 1814, who also constructed transmission diffraction grating (an array of slits) and made the first observations of star emission spectra and measurements of dark line wavelengths in the solar spectrum. In 1859, [PERSON] (physicist) and [PERSON] (chemist) explained that the structure of these spectra is the result of absorption by the components of the Sun's cooler atmosphere of the continuous spectrum emitted by the hotter interior of the Sun. They recognized that each atom and molecule has its own characteristic spectrum (a so-called fingerprint) and established spectroscopy as a scientific tool for analyzing the composition of materials. Another eminent scientist, [PERSON]. [PERSON], proposed the use of continuous curves to illustrate absorption spectra in various media in 1827. Thanks to these and many other achievements, optical spectrometers have found wide application in many fields as a very common analytical tool for the analysis of both terrestrial and stellar objects. Fraunhofer described 574 lines of solar spectrum [2], but, to date, there are over 25,000 lines that have been identified [3]. From the first spectrometer, constructed by [PERSON] and [PERSON], to the present day, spectroscopy has become one of the most important tools in the study of matter. Thanks to the development of optomechanical technologies, optical sources and detectors, as well as analog and digital electronics, today's spectrophotometers achieve resolutions that, until recently, were only offered by simulation programs. Figure 1 shows an example of absorption spectra simulated and measured in a mid-infrared range of wavelength, which is very common for gas detection research. There is a comparison of the nitrous oxide (N\({}_{2}\)O) absorption spectra obtained in the commonly used HITRAN database and measured with the use of the Bruker IFS 125 HR spectrophotometer, which currently offers one of the highest spectral resolutions of absorption spectrum measurements, amounting to 16 \(\times\) 10\({}^{-4}\) cm\({}^{-1}\). On the basis of the obtained spectra, it is possible to determine the maximum values of the absorption coefficient for a given wavelength, temperature and pressure. Additionally, it should be noted that, in the oscillatory spectrum, one can distinguish the narrow component lines lying very close to each other, which form the so-called rotational spectrum related to the rotations of molecules. Both types of vibrations, together, constitute the so-called ro-vibronic spectrum. The separation of the rotational spectrum lines was possible, only thanks to the high-resolution measurements. Therefore, the spectra obtained in this way are important from the point of view of the development of optical systems for detecting trace amounts of substances based on laser absorption spectroscopy. Resolution, apart from accuracy and precision, is one of the most important and most frequently reported metrological factor-characterizing instruments. It is defined as the smallest change that can be measured. As can be observed in Figure 1b, the higher spectral resolution allows the identification of N\({}_{2}\)O absorption lines that are invisible at the smaller ones, which is of great importance in the construction of laser gas sensors. This issue will be explained later in the article. Accuracy, in turn, can be described as a certain amount of measurement uncertainty, with respect to an absolute standard. In other words, this means that an accurate instrument will provide measurements closest to the true value or standard (low uncertainty). Precision describes the reproducibility of the measurement.