# Spectral Shaping of a Superluminescent Diode for Terahertz Cross-Correlation Spectroscopy
## 3 Results and Discussion
### Analysis and Comparison of Structured Spectra to Continuous Spectra  
In this section, the results from the comb-like shapes as shown in Figure 3 are presented and analyzed. The shapes were created out of the 2 THz continuous bandpass  
Figure 5: (**a**) The peak DR is slowly saturating for increased optical power. (**b**) The slope m is slightly reduced for powers larger than 15 mW. Powers equal to 15 mW or lower lead to a smaller bandwidth and a higher slope. (**c**) The fitted BW parameter is saturating with increased optical power at the antennas. The improvement in BW is decreased for optical powers \(>\) 25 mW. [(cf. Figure 2c)] by periodically shaping 5 GHz, 10 GHz, and 20 GHz broad features in the continuous bandpass. This was done by blocking the output for the considered frequency span of 30 GHz to 125 GHz between the features, producing a periodic comb-like spectrum [(cf. Figure 3c)]. Figure 6a-c depicts the results for \(P_{\text{THz}}\), \(\Delta\)pp and DR, respectively, for different filter periods \(\Delta\)f for 10 GHz and 20 GHz broad features at 20 mW incident optical power per antenna. For comparison with the continuous spectrum, the respective result for the 2 THz bandpass is indicated as a black line. The best value for each feature width and filter period is indicated by an arrow and the value is written in the figure. Figure 6a shows a sharp increase in the calculated power for the structured spectrum, which correlates with the filter period \(\Delta\)f. Regardless of feature size, the calculated power of the structured spectrum is always greater than the power from the continuous bandpass filtered spectrum. In addition, a smaller feature width results in higher power. For a feature width of 10 GHz and a filter period of 0.095 THz, a power increase by a factor of 4 is observed. From the point of view of a mode-locked laser, this behavior is counterintuitive. A dense mode spectrum, i.e., a small free spectral range or filter period, results in pulses with a higher peak power. Since a high peak power is considered beneficial for THz-TDS systems, a different explanation is required for a THz-CCS system. If you think of the continuous spectrum as a spectrum of discrete lines with an infinitesimally small spacing, you get many mixing products with a very small difference frequency. These low-frequency components can neither be radiated nor detected by the antenna structure. Consequently,  
Figure 6: (**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**) \(\Delta\)pp 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.  
part of the optical power does not contribute to the terahertz generation. From this point of view, it becomes obvious why the structuring of the optical spectrum increases the radiated power: less power is used for non-radiable frequency components. The same is true for the filter period and feature width. Within a feature, there are also non-radiable difference frequency components. This effect is more pronounced for 20 GHz than for 10 GHz and explains the reduced calculated power. If the number of features is also reduced, which corresponds to an increase in the filter period, the low-frequency components become correspondingly fewer and the radiated or detected power is increased. In Figure 6b, it can be seen that a comb structure can only outperform the continuous bandpass in terms of \(\Delta\)pp, if the number of peaks is high, respectively, the filter period is small within the 2 THz span. Further, measurements with 20 GHz feature width show a broader range for the filter period where \(\Delta\)pp is larger than the continuous BP compared to the measurements with 10 GHz feature width. The largest increase in \(\Delta\)pp was 2 nA for 10 GHz and 4 nA for 20 GHz.