# Spectral Shaping of a Superluminescent Diode for Terahertz Cross-Correlation Spectroscopy
## 2 Experimental Setup
### Experiment Setup  
For the experiment, 65 different bandpass filters and comb-like structures were created for the POF and amplified afterwards to the constant optical powers of 10, 15 and 20 mW at each antenna (for a detailed list of all filters, see Table 1). The power levels were chosen in a way that all filters can be amplified to the same maximum optical power for all experiments; 100 terahertz time traces were recorded and averaged in the time-domain to increase the SNR. After that, a Blackman window and a fast-Fourier transform was applied to the data, to get the frequency-domain representation. In the following we will give an overview of the treatment of our measured data. Therefore, the optical spectrum, the terahertz time-domain, and the corresponding terahertz spectrum are shown. Important parameters will be highlighted. In Figure 2c, a continuous bandpass filter with the size \(\Delta\)BP = 2 THz was applied to the SLD. The performance for each filter was determined by measuring dynamic range (DR) and peak-to-peak amplitude (\(\Delta\)pp). From this data, a value for the qualitative comparison of detected terahertz power (\(P_{THz}\)) and bandwidth (BW) was derived. In Figure 2a, the recorded terahertz time-domain trace is presented and the parameter \(\Delta\)pp is indicated. The continuous spectrum of the SLD leads to a single terahertz pulse. Figure 2b shows the corresponding terahertz spectrum. Indicated in the picture are the parameters for the peak DR specified at \(\sim\)200 GHz and the range which was used for linearly fitting the slopem of the spectrum. To show the influence on the BW, the slope m was considered for the range between 0.2 THz and 1.2 THz as a parameter for the roll-off. This measurement shows that an increased DR increases the bandwidth of the system when the slope m stays constant. In red, the linear fit for this measurement can be seen in Figure 2b. The BW was determined by this fit as the intersection between the noise floor and the continued linear fit. The noise floor was calculated as the arithmetic average of the frequency-domain signal for frequencies between 2.2-4.0 THz. The result of this method is an approximated bandwidth which makes the bandwidth parameter quantitatively comparable for the following experiments. In Figure 3c, one of the investigated optical comb structures, created with the POF, is depicted. The previously shown 2 THz continuous bandpass filter was shaped into periodic features with a filter period \(\Delta\)f. The filter period describes the distance between the middle of two adjacent features. For the experiment, 20 different filter periods were created with 3 different feature widths, 5, 10 and 20 GHz, and \(\Delta\)f was varied from 30 GHz to 125 GHz in 5 GHz steps (cf. Table 1). Here, the feature width describes the width of the passband of the POF which might deviate from the actual FWHM. Due to the limited resolution of our OSA, we were not able to measure the exact FWHM of the optical features. \begin{table}
\begin{tabular}{c c c c c} \hline
**Optical Power** & **Feature Width** & **Shapes** & \(\Delta\)f** & **Stepsize** \\ \hline
10, 15, 20 mW & 2–3 THz & 5 & - & 200 GHz \\
10, 15, 20 mW & \(\sim\)5 THz & 1 & - & - \\
10, 15, 20 mW & 10 GHz & 20 & 30–125 GHz & 5 GHz \\
10, 15, 20 mW & 20 GHz & 20 & 30–125 GHz & 5 GHz \\ \hline \end{tabular}
\end{table} 
Table 1: Overview of all created filters for the POF and the corresponding antenna power for the measurements. Figure 1: System overview, the light source system part in the red shaded area is the relevant part for the presented experiments. Blue arrows: optical signal; black arrows: electrical signal; red arrows: terahertz signal; dashed black arrow: control signal; diode: superluminescent diode (SLD); optical isolator; programmable optical filter (POF); erbium-doped fiber amplifier (EDFA); optical spectrum analyzer (OSA) and the light source electronics, laser diode controller (LDC) and thermal electric cooler (TEC).