# Spectral Shaping of a Superluminescent Diode for Terahertz Cross-Correlation Spectroscopy
## 4 Conclusions  
A THz-CCS system was characterized with special interest in the parameters dynamic range, peak-to-peak amplitude, THz-power, and bandwidth. The objective was to get a better understanding of the influence of the shape of the optical spectrum on these parameters. We demonstrated that structuring of the optical spectrum increases the dynamic range and bandwidth of the signal. Small feature widths with large filter periods are favorable if an increase in the dynamic range of the system is required. However, the increase comes at the cost of a discrete spectrum which limits the spectral resolution to the filter period. If a high peak-to-peak amplitude is desired, it is favorable to use continuous shapes because they show the highest peak-to-peak amplitude in connection with a high bandwidth and a continuous spectrum. However, the gain in peak-to-peak amplitude is negligible compared to the loss in dynamic range. The structuring of the spectrum comes at a price: due to the periodicity of the spectrum, the frequency resolution is reduced to the filter period. Comparing the results to modern continuous wave terahertz systems, a lack of bandwidth becomes apparent. The emitter and the receiver have a bandwidth up to 2.75 THz for continuous wave excitation [13] and, in a recent publication, [PERSON] et al. (2021) demonstrated a bandwidth of up to 4 THz after averaging the signal 250,000 times, with the same antenna configuration [20]. However, distributed feedback lasers, which have a narrow linewidth, were used as light sources. In this work, a single incoherent broadband light source was used, reducing the bandwidth to about 1.8 THz. The lower bandwidth can be explained by the wider linewidth of the filtered spectrum and lower averaging. It  
Figure 8: (a) Calculated power of the shaped 2 THz bandpass (black line) with 10 GHz (black squares) and 20 GHz (red squares) feature width for different filter periods \(\Delta\)f. (b) App as a function of the filter period for a 10 GHz and 20 GHz feature width. (c) DR for the same feature width; a larger \(\Delta\)f results in a higher peak DR.  
follows that for future laser diode-based terahertz systems (mode-locked or not), special attention must be paid to the optical linewidth. Further, it explains the limited bandwidth of SLD-driven THz-CCS systems.  
In summary, a concept for a THz-CCS system is presented which can be used for prototyping bandpass filters and laser concepts. The results represent a technique to further improve the system performance regarding terahertz power, peak-to-peak amplitude, and peak dynamic range. All measurements were validated with a numerical model. The results give more room for system development which will ultimately lead to compact, mobile, and robust THz-CCS systems which can be optimized for a specific application.  
K.-H.T.: system design, experiment design, data acquisition, preparation, data analysis, editing and writing; K.K.: theoretical modelling, simulation results, data analysis; V.C.: measurement software; B.F.: Third Supervisor, funding acquisition; S.E.: Second Supervisor, review editing; J.C.B.: First Supervisor, experiment design, review editing. All authors have read and agreed to the published version of the manuscript.  
Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) --Project-ID 287022738 TRR 196.  
The data presented in this study are available on request from the corresponding author.  
We acknowledge support by the Open Access Publication Fund of the University of Duisburg-Essen.  
The authors declare no conflict of interest.