# Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods
## 3 Optical Methods
Ammonia can be det  
Figure 4: Design and principle of operation of the nondispersive infrared (NDIR) sensor. Figure 5: Diagram of the sensor based on NDIR and fiber-optic evanescent wave spectroscopy (FEWS) principles. absorption spectroscopy (TLAS), multi-pass optical cell (MUPASS), cavity ring down spectroscopy (CRDS), cavity-enhanced absorption spectroscopy (CEAS), and photoacoustic (PAS) approach. TLAS is a very interesting technique for ammonia concentration measuring using tunable diode lasers. The advantage of TLAS over other techniques consists in its ability to achieve low detection limits by using optical path extension techniques and improving the signal-to-noise ratio (SNR). A single-pass TLAS system, operating in direct absorption measurement mode, consists of a tunable laser, transmitting optics, sample cell, receiving optics, photodetector, and a signal detection circuit (Figure 6). The optics matches the laser and photodetector to the gas inside the sample cell. A thermoelectric controller (TEC) is used to set the laser-operating temperature to a value where the desired wavelength can be reached due to injection current tuning. The injection current is usually scanned periodically with a ramp signal, which leads to laser wavelength scanning. The amplitude of the scan should cover the absorption transition of interest. Laser radiation passes through the absorption cell and the transmitted signal is measured using the photodetector. In absence of the absorption, the detector signal represents laser power changes vs. the current. When absorption occurs, a dip in transmission is observed. Ratio of the signals registered at the line center corresponding to the case without absorption (\(I_{0}\)) and with absorption (\(I_{1}\)) can be used to calculate the gas concentration according to the formula (1). Based on TLAS, different components such as NH\({}_{3}\), CO, O\({}_{2}\), CH\({}_{4}\), H\({}_{2}\)O, CO\({}_{2}\), and HCl can be detected with high selectivity and sensitivity. TLAS has been employed for various applications, including industrial process monitoring and its control, environmental monitoring, combustion and flow analyses, trace species measurements, and so on. However, the sensitivity of this technique is usually limited to the absorption coefficient value of \(\sim\)10\({}^{-2}\)-10\({}^{-3}\) cm\({}^{-1}\) (Equation (1)). Wavelength modulation spectroscopy (WMS) is a useful technique providing SNR improvement, which is used in high sensitivity applications [48]. Small modulation (with frequency \(f\approx\) 1-20 kHz) is added to the laser current scan, which is provided in a similar way as in TLAS (Figure 7). The transmitted signal, detected by the photodiode, is fed to a lock-in amplifier. As the laser is scanned across an absorption line profile, the transmitted on-absorption signal changes at the frequency of 2\(f\), while the transmitted off-absorption signal changes at the frequency of \(f\) (Figure 8). Figure 6: Block diagram and principle of operation of the typical tunable laser absorption spectroscopy (TLAS) sensor. Application of high frequency minimizes 1/_f_ noise. Therefore, setting the lock-in band around second harmonic ensures that the sensor becomes more suitable for high sensitivity applications compared to the standard TLAS approach. Typical sensitivity limits the absorption coefficient achievable with WMS and is about \(10^{-4}\) cm\({}^{-1}\). Better limits, about \(10^{-5}\)-\(10^{-7}\) cm\({}^{-1}\), can be obtained for balanced detection-based WMS [49]. This method involves an electrical circuit that subtracts the photocurrents of two detectors: one of them registers the reference laser intensity while the other measures the signal passing through the absorption cell. That provides opportunity to reject common mode laser noise. Small absorptions, which occur due to low densities of molecules in the samples or due to weak line strengths, are usually compensated by extending the optical path with multi-pass cells (White or Herriott) or cavity-enhanced methods.