## 3 Experimental Results  
The dependence of the optical power and the voltage versus the currents \(I_{\text{G1}}\) and \(I_{\text{G2}}\) injected into the two DFB lasers are shown in Figure 4a for \(T=25\)\({}^{\circ}\)C and \(35\)\({}^{\circ}\)C. These temperatures enable the selection of two adequate pairs of online and offline wavelengths. The current to the front section is set to \(I_{\text{F}}=10\) mA, whereas the currents to the Y-coupler are set to \(I_{\text{Y1}}=I_{\text{Y1}}=0\) mA to avoid crosstalk between both branches. Thus, the Y-coupler is optically pumped to near transparency, which is possible because of low non-saturable optical losses, due to a free carrier absorption if the order \(1\) cm\({}^{-1}\) and the absence of intervalence-band absorption as well as high differential gain and low transparency carrier density of the compressively strained InGaAs QWs. At currents of \(250\) mA and at \(T=25\)\({}^{\circ}\)C, output powers of \(P=96\) mW and \(93\) mW are reached for each branch. At \(T=35\)\({}^{\circ}\)C, the optical power decreases to \(90.0\) mW and \(86.6\) mW. The slope efficiency is \(0.60\) W/A at \(25\)\({}^{\circ}\)C and \(0.57\) W/A at \(35\)\({}^{\circ}\)C for the first branch and \(0.54\) W/A at \(25\)\({}^{\circ}\)C and \(0.50\) W/A at \(35\)\({}^{\circ}\)C for the second branch. Optical spectra are measured with an optical spectrum analyzer (OSA) having a resolution of \(10\) pm and a dynamic range of \(60\) dB (Advantest Q8384). Figure 4b shows the optical spectra having peak wavelengths of \(940.02\) nm and \(967.50\) nm at a current of \(I_{\text{G1}}=I_{\text{G2}}=130\) mA corresponding to a power of about \(30\) mW, later used as input power for the MOPA system. A side-mode suppression ratio (SMSR) of about \(55\) dB and a spectral full \(3\) dB width of \(10\) pm (limited by the OSA) are obtained. The dependence of the optical spectra on the injection current to the two branches with fixed \(I_{\text{F}}=10\) mA and \(I_{\text{Y1}}=I_{\text{Y2}}=0\) mA is shown in Figure 4c as a pseudo-color mapping at \(T=25\)\({}^{\circ}\)C and \(T=35\)\({}^{\circ}\)C. Each DFB laser branch operates in a single longitudinal mode over the whole current range from \(80\) mA to \(250\) mA with a constant distance between the two peak wavelengths of \(3.48\) nm. It is smaller than the designed value (\(4\) nm) probably since the dispersion of the effective index is not considered in the design of the grating periods. The increase of the peak wavelength with rising current is caused by self-heating, which leads to an increase in the modal refractive index and thus, in the Bragg wavelength. At \(T=25\)\({}^{\circ}\)C, the tuning range, due to a change in the current from \(80\) mA to \(250\) mA, is \(0.13\) nm, corresponding to \(0.52\) pm/mA for both branches, i.e., from \(964.02\) nm to \(964.15\) nm for the first branch and from \(967.42\) nm to \(967.56\) nm for the second one. At \(T=35\)\({}^{\circ}\)C, the corresponding tuning ranges are \(964.29\)-\(964.42\) nm and \(967.75\)-\(967.88\) nm, respectively. As shown in [15], the output power of the PA remains almost constant when the input power varies in the range between \(15\) mW and \(50\) mW. In this work, an input power of about \(20\) mW corresponding to currents \(I_{\text{G1}}=I_{\text{G2}}=130\) mA is chosen. The PA is operated at \(\tau_{\text{OG}}=8\) ns, \(I_{\text{OG}}=100\) mA, \(\tau_{\text{Delay}}=3.5\) ns, \(\tau_{\text{TS}}=15\) ns, \(I_{\text{TS}}=18.4\) A,\(f=25\) kHz, \(T_{\text{PA}}=25\)\({}^{\circ}\)C.  
The results for a seed wavelength of 964.02 nm, corresponding to an operation of the first branch of the Y-DFB laser (\(I_{\rm G1}=130\) mA, \(I_{\rm G2}=0\) mA), are shown in Figure 5. An optical pulse width of 8 ns and a plateau optical peak power of the MOPA system of 16.3 W is obtained. At the leading and training edges of the optical pulse, small amounts of ASE are visible in Figure 5a.