## 3 System Characterization
Figure 4a compares TD-SNR values of the ECOPS system at 1600 Hz and 200 Hz to values obtained with the TeraFlash pro. As before, we present values for single-shot measurements and 10, 100 and 1000 averages. Since a single measurement does not have any standard deviation, we calculate the TD-SNR from 10 consecutive runs, and apply the same method to the averaged data (for instance, the TD-SNR value for 100 averages is calculated from 10 runs, each run comprising 100 averaged time traces). We find that TD-SNR values from 10 single-shot runs amount to 50 dB and 55 dB for ECOPS at 1600 Hz and 200 Hz, respectively, and 74 dB for the TeraFlash pro. Akin to the TD-DR, we find that a factor of 10 in the number of averages improves the TD-SNR by ~10 dB, in agreement with refs [30; 31; 32]. Whilst the higher TD-SNR values of the TeraFlash pro are expected, the difference between the two datasets obtained with ECOPS is still subject to further investigation. Tentatively, the higher measurement speed (1600 Hz as compared to 200 Hz) results in an increased timing jitter, which has no effect on the background noise (cf. Figure 3) but on the reproducibility of the signal amplitude and consequently, the TD-SNR. Figure 4b depicts the same TD-SNR data as a function of the measurement time. Viewed that way, the performance of the different systems is comparable: they all achieve similar TD-SNR values in the same measurement time. Yet another relevant parameter for terahertz TDS systems is the spectral bandwidth. Figure 5 shows terahertz power spectra, calculated by a fast Fourier transform of time traces obtained with our ECOPS setup at 1600 Hz. In a single-shot measurement, the system achieves a peak dynamic range (PDR) of 38 dB and the signal reaches the noise floor at 3.4 THz. All of the dips observed in the spectrum correspond to literature values of water vapor absorption lines, as documented in the HITRAN database [33]. With 1000 averages, the PDR increases to 68 dB and the detectable signal extends to ~4.8 THz, still within a measurement time of only 0.6 s. Please note that the number of averages affects the spectral bandwidth but not the resolution, which only depends on the scan range. Figure 6 illustrates the interplay between the scan range and the measurement speed of our ECOPS instrument. Depending on the experimental requirements, the measurement speed can be flexibly selected between 200 and 1600 terahertz pulse traces per second. The increase in measurement  
Figure 4: (a) TD-SNR as a function of the number of pulse-trace averages. Blue triangles: ECOPS at 1600 traces per second; red circles: ECOPS at 200 traces per second, green squares: TeraFlash pro. (b) Same data versus measurement time. speed is achieved by reducing the travel of the piezo in the slave-laser cavity, corresponding to a reduction of the scan range. Set to the lowest speed, the ECOPS system achieves a scan range of about 700 ps. This large range enables the assessment of thick structures (~7 cm in reflection, assuming a double pass through a sample with refractive index of 1.5), or spectroscopic measurements with high resolution (~1.4 GHz). Even at this speed, the system still exceeds the limits of our conventional TDS system by a factor of ~5. Vice versa, at maximum speed, the scan range of the ECOPS system amounts to 200 ps. The ECOPS technology is then ~40 times faster than our state-of-the-art TDS unit with mechanical delay. Figure 5: Dynamic-range spectrum of the terahertz power for a single-shot measurement (black trace), and for 10 (red), 100 (green), and 1000 (blue) averages of terahertz pulse traces. Spectra are obtained with the ECOPS system at 1600 traces per second. Vertical grey lines indicate water vapor absorption frequencies according to the HITRAN database [33]. Figure 6: Scan range of the ECOPS system vs. measurement speed.