# Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods
## 3 Optical Methods
They enable the optical path to be extended. If instead of a single-pass chamber we apply the multi-pass cell (Figure 9), the sensitivity of the sensor would be improved by the factor of the optical path lengthening. A key parameter of multi-pass cells is the ratio of path length to volume. Moreover, this system provides an opportunity for the simultaneous application of the WMS approach and _2f_ detection. Figure 8: Idea of first and second harmonic detection with tunable diode lasers. Figure 7: The principle of operation of the sensor using a combination of TLAS and wavelength modulation spectroscopy (WMS) techniques. Unlike conventional configurations, which involve at least a pair of mirrors separated by exactly defined distances, circular multi-pass cells have been developed (Figure 10) [50]. Thanks to the single piece, the cell is especially robust against thermal expansion. Minimizing the cell size is desired for the development of fast and portable gas sensors. Cavity ring-down spectroscopy (CRDS) exploits the sample cells in the form of optical cavities (resonators) built with mirrors of very high reflectivity. A simplified scheme of such experiment is presented in Figure 11. Laser pulse injected into the cavity through one of the mirrors is then reflected many times among them. Its wavelength must be tuned to the NH\({}_{3}\) spectral line in order to measure the absorption coefficient of ammonia contained inside. The light transmitted through the exit mirror is monitored by a detector. Analysis of the output signal by the acquisition system provides opportunity to determine the Q-factor of the cavity which is limited due to diffraction and mirror losses as well as due to the light absorption or scattering inside the cell. As far as the Q-factor is inversely proportional to the signal decay time, its value is found due to analysis of the photoreceiver signal by the acquisition system. The majority of the approaches exploit a two-step procedure consisting of the decay time measurement: first, when the cavity is empty (\(\tau_{0}\)), and then, when the cavity is filled with the tested gas (\(\tau_{A}\)). These decay times depend on the mirror reflectivity, resonator length, and extinction factor (absorption and scattering of light in the cavity) [51]. Comparing both decay times, the absorber concentration can be found. \[N=\frac{1}{c\sigma(\lambda)}\left(\frac{1}{\tau_{A}}-\frac{1}{\tau_{0}}\right) \tag{2}\]  
where \(c\) is the light speed and \(\sigma(\lambda)\) is the absorption cross-section. Figure 3:0 The circular multi-pass—small volume cell. Figure 3:1 Scheme of a multi-pass experimental system for multi-pass optical cell (MUPASS)–WMS spectroscopy. There are various approaches to CRDS with pulsed lasers or with continuously operating AM modulated ones. Using these techniques, the absorption coefficients \(\alpha=\sigma N<10^{-9}\) cm\({}^{-1}\) can be observed. The detection limit is mainly related to the resonator quality (determined by \(\tau_{0}\)), but also by the precision of decay time measurement. The advantage of CRDS over other absorption spectroscopy approaches consists not only in dominant sensitivity. Their superiority also results from minimized impacts of light source intensity fluctuations or detector sensitivity changes on the measurement results. This method is extremely sensitive but requires that the laser frequency is precisely matched to the cavity mode. In this way, it is possible to obtain a high Q-factor of the resonator and efficient storage of optical radiation. On the other hand, small mechanical instabilities cause changes in the cavity mode frequency and significant output signal fluctuations, which is the main disadvantage of this method [52]. It can be minimalized by use of cavity length stabilization [53,54], or by application of the cavity with dense mode structure, called cavity-enhanced absorption spectroscopy (CEAS) [55]. In CEAS, the off-axis arrangement of the laser and cavity is applied.