## 2 Devices and Experimental Setup
An average power of 1.4 mW or a pulse power of 3.8 W of amplified spontaneous emission (ASE) was reached if a current pulse with a length of \(\tau_{\text{TS}}=15\) ns and an amplitude of \(I_{\text{TS}}=18.4\) A was injected into the TS at a repetition rate of \(f=25\) kHz and a temperature of \(T=30\)\({}^{\circ}\)C. The peak wavelength and 3 dB full width of the ASE spectrum were about \(\lambda=969\) nm and 18 nm, respectively. No lasing modes were observed. The experimental setup of the MOPA configuration under study is shown in Figure 3. The emission from the Y-branch DFB laser, used as MO, is collimated with an anti-reflection coated aspheric lens having a focal length of 3.1 mm and a numerical aperture (NA) = 0.68 (Thorlabs; 352330-B). The light passes a two-stage optical isolator (60 dB isolation; Qioptiq FI-1060-TI) and is focused, using another aspheric lens (Thorlabs; 352330-B) into the OG section of the power amplifier. To obtain stable online/offline wavelength pairs in the 964 nm range with a small spectral linewidth, the MO is operated in CW mode. The RW section and the TS of the PA are operated in pulsed mode for generating optical pulses with high peak power and low ASE. The RW section of the PA shown in Figure 3 serves as an optical gate (OG), where the coupled CW beam of the MO is absorbed if no current is injected. On the other hand, the OG becomes transparent, and the optical beam can pass the OG if a current pulse of defined width \(\tau_{\text{OG}}\) and amplitude \(I_{\text{OG}}\) is injected. In this work, the amplitude of the current pulse is \(I_{\text{OG}}=100\) mA. The generated optical pulse is then subsequently amplified in the tapered section, where a current with  
Figure 3: Scheme of experimental MOPA configuration. Figure 2: Epitaxial layer structure (**a**) and top view (**b**) of the power amplifier with integrated optical gate (OG) and tapered section (TS). an amplitude of \(I_{\text{TS}}\leq 18.4\) A is injected. Due to the targeted optical pulse width of \(8\) ns a pulse width of \(\tau_{\text{OG}}=8\) ns for the gate current is chosen. To obtain a top-hat profile of the optical pulse, the pulse width \(\tau_{\text{TS}}\) has to be longer than \(\tau_{\text{OG}}\), but not too long in order to prevent the generation of ASE after the emission of the optical pulse. The time needed to accumulate enough excess carriers in the active region amounts to several times the carrier lifetime, which is of the order of \(1\) ns. Hence, the electrical pulse injected into the TS is turned on \(3.5\) ns before the pulse through the OG is injected as discussed in Refs. [16; 17]. Thus, the pulse injected into the OG is delayed by a time \(\tau_{\text{Delay}}=3.5\) ns with respect to the TS pulse. Similarly, the TS pulse is kept on for a further \(3.5\) ns after turning off the OG pulse. Therefore, a pulse width of \(\tau_{\text{TS}}=15\) ns is selected. The repetition rate of the current pulses injected into both sections (OG and TS) is \(f=25\) kHz. The current pulses injected into the OG and the TS are generated by driver circuits based on GaN transistors as described in Ref. [26]. In the following, the electro-optical characteristics, including the spectral properties of the MO as well as the MOPA configuration are given.