# Charge-Line Dual-FET High-Repetition-Rate Pulsed Laser Driver
## 4 Test Results
### Pulsed Laser Driver Integrated with Infra-Red Laser  
The final tests of the presented solution were performed with the Osram SPL PL90_3 NIR laser [15]. Parameters of this high-power optical source are listed in Table 2. The pulse characterization was performed using a high-speed (f\({}_{\text{BW}}\) = 5 GHz) THORLABS DET08 CFC photodetector [16]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline Threshold current & I\({}_{\text{th}}\) & 0.75 A \\ Operating forward voltage & U\({}_{\text{F}}\) & 9 V \\ Wavelength & \(\lambda_{\text{peak}}\) & 905 nm \\ Maximum output power & P\({}_{\text{o max}}\) & 75 W \\ Rise/fall time & t\({}_{\text{r}}\)/t\({}_{\text{f}}\) & 1 ns/1 ns \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Near Infra-Red (NIR) laser catalogue typical parameters (room temperature). Figure 8: The pulse amplitude compression as a function of repetition rate. Figure 7: Preliminary tests of the pulsed laser driver: (**a**) Pulse width regulation; (**b**) pulse peak current regulation. The effect of the pulse width regulation obtained with the designed driver is shown in Figure 9a. The shortest measured pulse has a symmetrical Gaussian shape and duration of 322 ps, which confirms the high-speed and high-current switching performance of the solution. The maximum pulse width is limited to 940 ps, because of the particular laser's turn-on delay time. However, all measured optical pulses are characterized with rise time <220 ps and fall time <680 ps, which is much shorter than the laser manufacturer's specification (see Table 2). The optical pulse power regulation is shown in Figure 9b. The maximum pulsing current for U\({}_{+\text{HV}}\) = 80 V was measured to be approximately 1.3 A and is sufficient do drive most of the market-available 1 W NIR lasers. The measurements shown in Figure 9 prove that the developed laser driver is capable of independent regulation of the pulse width and amplitude, although one of the key parameters of the developed laser driver is a very low timing noise. In Figure 10a, family of time domain waveforms corresponding to multiple optical pulses generated by the laser diode are shown. The \"blur\" observed at the falling edges represents the phase fluctuations of the signal (noise in the time domain)--jitter. The maximum measured jitter is \(\sigma_{\text{t}}\) = 28 ps for \(\text{t}_{\text{FWHM}}\) = 500 ps. This ensures stable and reproducible optical pulse generation, regardless of the width of the optical pulse. Figure 10: Jitter measurements of three different optical pulse widths: (**a**) \(\text{t}_{\text{FWHM}}\) = 500 ps, \(\sigma_{\text{t}}\) = 28 ps; (**b**) \(\text{t}_{\text{FWHM}}\) = 750 ps, \(\sigma_{\text{t}}\) = 24 ps; (**c**) \(\text{t}_{\text{FWHM}}\) = 1 ns, \(\sigma_{\text{t}}\) = 22 ps. Figure 9: Tests of the developed pulsed driver coupled with the NIR laser diode: (**a**) Pulse width regulation; (**b**) pulse peak current regulation. The pulse-to-pulse optical signal amplitude stability measurements are shown in Figure 11. The amplitude fluctuations-over-time parameter \(\sigma_{\rm A}\) (std. dev.) is not affected by \(\rm{t_{FWHM}}\) or \(\rm{f_{rep}}\) adjustments. However, as listed in Table 3, the optical pulse amplitude instability increases with higher modulation currents. To present amplitude instability relative to peak pulse amplitude, the \(\rm{S_{A}}\) and \(\rm{S_{C}}\) parameters were included in Table 3. The \(\rm{S_{A}}\) ratio is not constant and, compared to the \(\rm{S_{C}}\), measured with resistive load \(\rm{R_{L}}\) = 33 \(\Omega\); we suspect that both the NIR laser and detection system exhibit an additional 1/f or excess noise. The long term stability was measured with an optical peak power of 750 mW and \(\rm{t_{FWHM}}\) = 750 ps, while the repetition rate was set to 1 MHz. Figure 12 presents the data set after 12 h of continuous optical signal sampling. The laser driver showed a high and reproducible performance with negligible pulse width and amplitude drift.