## 2 Devices and Experimental Setup  
A scheme of a Y-branch DFB laser is shown in Figure 1. The vertical layer structure grown on an n-GaAs substrate in one epitaxial step is given in Figure 1a. It includes an n- GaAs substrate, n-Al\({}_{0.35}\)Ga\({}_{0.65}\)As cladding, 1.6 \(\upmu\)m n-Al\({}_{0.25}\)Ga\({}_{0.75}\)As waveguide, double quantum well (QW) active region consisting of InGaAs QWs and GaAsP barriers, 0.8 \(\upmu\)m p-Al\({}_{0.25}\)Ga\({}_{0.75}\)As waveguide, p-Al\({}_{0.85}\)Ga\({}_{0.15}\)As cladding and finally, p-GaAs contact layer. The lateral-longitudinal structuring starts with the definition of 80 th order Bragg gratings in the DFB lasers by i-line wafer stepper lithography. Subsequently, the gratings are transferred by reactive ion etching through the p-cladding into the p-waveguide layer. The use of surface gratings avoids the need for sophisticated grating overgrowth. The large periods of the Bragg grating of about 11.5 \(\upmu\)m are used to enable the realization of the current injection between the etched grating grooves. The effective duty cycle of the grating (ratio between unetched region to period) is extremely large (\(\sim\)0.99). In order to achieve good laser performance, a coupling-coefficient length product \(\kappa L=0.3\dots\ 0.5\) is aimed at, where \(\kappa\) is the coupling coefficient of the grating and \(L\) is its length, which is achieved by a proper adjustment of the etch depth. The exact grating periods are 11,520.0 nm and 11,567.6 nm to ensure emission wavelengths of around 964 nm and 968 nm, respectively, with an intended spacing of 4 nm. A top view of the Y-branch DFB laser is shown in Figure 1b. The 4 mm long device consists of two branches and three sections, all of them electrically separated, enabling currents \(I_{\rm G1}\), \(I_{\rm G2}\) to each DFB laser, \(I_{\rm Y1}\), \(I_{\rm Y2}\) to each S-bend of the Y-coupler and a current \(I_{\rm F}\) to the section at the front facet. The section lengths are 2.0, 1.6 and 0.4 mm for the DFB lasers, Y-coupler and the front section, respectively. Lateral waveguiding is provided by a ridge waveguide with a width of the ridge of 5 \(\upmu\)m. The spatial distance between the DFB lasers is 80 \(\upmu\)m. Details of the devices manufactured are described in Refs. [18; 22; 23]. The Y-branch-DFB laser is mounted p-side up on a standard C-mount. The power amplifier (PA) schematically shown in Figure 2 consists of n-GaAs substrate, n-Al\({}_{0.45}\)Ga\({}_{0.55}\)As cladding, 4 \(\upmu\)m n-Al\({}_{0.35}\)Ga\({}_{0.65}\)As waveguide, InGaAs/GaAsP double quantum well as the active region, 0.8 \(\upmu\)m p-Al\({}_{0.35}\)Ga\({}_{0.65}\)As waveguide, p-Al\({}_{0.85}\)Ga\({}_{0.15}\)As cladding and p-GaAs contact layer (Figure 2a). The asymmetrical super large optical cavity  
Figure 1: Epitaxial layer structure (**a**) and top view (**b**) of the Y-branch DFB laser. results in a small vertical far-field divergence of only 14\({}^{\circ}\) full-width at half maximum (FWHM) [24; 25]. The PA has a total length of 6 mm and consists of two electrically separated sections, namely, a 2 mm long index-guided RW section having a 4 \(\upmu\)m wide ridge and a 4 mm long gain-guided tapered section (TS) having a full flare angle of 6\({}^{\circ}\) (Figure 2b). The front and the rear facets are passivated and anti-reflection coated to achieve a residual reflectivity of \(R\leq 5\times 10^{-4}\) in the spectral range from 950 nm to 980 nm. The device is mounted p-side up on a C-mount, allowing to drive the two sections electrically independent from each other. The electro-optical characteristics were investigated in detail in [17].