# Remote Spectral Identification in the THz Band with Reflection Spectroscopy in an Open Atmosphere
## 2 Materials and Methods  
### Technical Analysis  
In this study, selection of the key components of the experimental setup was of the greatest importance. To ensure the best experimental results, the criteria and requirements for the functionality were defined. As the main components of the setup, the authors selected the source of the radiation and the detector. In Table 1, a comparison of the THz radiation sources is presented, and in Table 2, a comparison of the THz detectors is presented. All the considered devices were selected based on their commercial availability and practical aspects for exploitation. For the sources, two main properties were selected--the power of the generated radiation and the spectral range. The spectral range contained cases of both broadband sources or sources with tuning capabilities. The first position in Table 1--semiconductor diodes--refers to the many types of semiconductor sources, including Gunn diodes, IMPATT diodes, TUNNETT diodes, or resonant tunnel diodes. Although these sources produce radiation with different properties, none of them outperforms the others in combined terms of optical power and bandwidth. There is another class of sources that gives high-power broadband THz radiation with high repeatability. The underlying principle for electromagnetic radiation generation is \begin{table}
\begin{tabular}{l l c} \hline \hline
**Source Type** & **Spectral Range** & **Output Power** \\ \hline Semiconductor Diodes [12; 13; 14; 15; 16] & Limited & Average \\ Difference Freq. Generation [17; 18] & Limited & High \\ Optical Rectification [19; 20] & Good & Good \\ Optical Parametric Oscillators [21; 22] & Good & High \\ Frequency Multilyergers [23] & Limited & Average \\ Photoconductive Antennas [24; 25] & Good & Low \\ Photomixing [26; 27] & Good & Low \\ Optically Pumped FIR Lasers [28] & Limited & Good \\ Quantum Cascade Lasers [29; 30] & Limited & Average \\ Backward Wave Oscillators [31; 32] & Limited & Good \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the currently available THz sources [11]. manipulating charged particle beams. These sources--gyrotrons, synchrotrons, and free electron lasers [32; 33]--produce radioactive radiation as a side effect, so they would not meet the health requirements for practical application. These were also omitted in the analysis because of the size and cost. They are typically institution-level devices often built as nationally funded projects. Another class of THz generators are sources based on THz laser-induced generation devices producing broadband, pulsed signals [34]. Broadband sources, especially pulsed, are an interesting solution but they produce significant technical difficulty in terms of detection. To detect broadband pulse in most cases, a delay line is incorporated. The length of the delay line provides some margin for geometrical configuration of the setup, but unfortunately, providing capabilities to change according to the length of the optical path varying from 5 to 15 m in an elastic way is a significant technical complication. This applies to both broadband sources and systems operating in a TDS configuration. As for the selection of the detectors, three main properties were selected: sensitivity, speed, and spectral range. Photoconductive antennas, photomixers, and electro-optic detection work best in a TDS configuration. Although they have good parameters, their practical applications are limited. The evaluation was conducted as objectively as possible. Even though some of the assessments may seem disputable, one should keep in mind that the evaluation was conducted from the angle of possible applications in reflection spectroscopy. For the purpose of identification, at least one of the devices must provide information about the frequency. From the point of view of the whole setup, the dynamic range is a crucial parameter. A more sensitive detector and stronger source were preferred, since the dynamic range can be expanded by externally attenuating radiation from the source.