## 3 Experimental Results
Similar to the tunable laser, we observe a differential resistance of 6 \(\Omega\)  
Figure 3: (**a**) Propagation losses of the strip (red trace) and rib (blue trace) Si waveguides as a function of the optical wavelength. Colored zones correspond to the experimental deviation from the average value. Insets show the fundamental TE-like mode simulation for the strip (left) and the rib (right) waveguides. (**b**,**c**) present the V-I characteristic of the thermal heaters of the Vernier filter and the laser’s gain section. (**d**) P-I curve of a test widely tunable laser device containing a vertical grating coupler output. Insets in (**b**–**d**) show 3D schematic views of each device and an SEM cross-section view of the laser’s gain section. and a similar voltage range. Figure 4b presents the amplified spontaneous emission (ASE) as a function of the injected current in the SOA booster. The curve peaks at I = 110 mA. It is worth remarking that no lasing emission was observed up to a current of I = 120 mA, which denotes the absence of significant back-reflections and, hence, validates the design of adiabatic tapers and the efficacy of the anti-reflection coating at the output facet. Figure 4c displays the fiber-coupled output power of the co-integrated system (\(y\)-axis) as a function of the injected current in the laser's gain section (\(x\)-axis) and for different injected current values in the SOA booster. Each curve corresponds to a different driving current in the SOA, in steps of 20 mA up to 110 mA. Similar to the test widely tunable laser device, several kinks in the P-I curve, due to the passive-active phase mismatch, are seen. A consistent increase in the P-I curve is observed for consecutive values of injected current in the SOA booster, with a maximum of 10 mW (10 dBm) for an injected current of I = 110 mA in both the SOA and the laser's gain section (see the peak value of the pale red curve). Bearing in mind that the butt-coupling configuration induces 3 dB of fiber coupling losses, this value corresponds to a waveguide-coupled optical power of 20 mW (13 dBm). From this value on, the fiber-coupled optical power quickly saturates and decreases steadily for higher SOA current values, denoting the kick-off of nonlinear and/or thermal effects. This observation is in agreement with the peak ASE value in Figure 4b. From this curve, we estimate an on-chip optical gain for the SOA of 10 dB, i.e., 18 dB/mm. This value is comparable with similar III-V/Si SOAs recently reported [40]. Figure 4d shows a superposition of the measured lasing spectra for different driving currents in the Vernier filter. A wavelength tuning range of 50 nm is obtained over a span from 1524 nm to 1574 nm, with a side-mode suppression ratio (SMSR) over 50 dB for the entire wavelength range. To evaluate the coherence of our system, we have also measured its intrinsic linewidth. Figure 4e shows the measured RF spectrum of the co-integrated widely tunable laser-SOA booster device in a delayed self-heterodyne measurement using an acousto-optic modulation frequency of 40 MHz [41]. The measurement was performed with the SOA booster driven under optimum operation conditions, i.e., at \(\text{I}_{\text{SOA}}\) = 110 mA, while increasing the laser current gradually from \(\text{I}_{\text{Laser}}\) = 60 mA up to 110 mA (driving current for the  
Figure 4: **(a)** V-I characteristic of the SOA booster. **(b)** Fiber-coupled ASE optical power in function of the SOA injected current. **(c)** P-I curves of the widely tunable laser for different driving current values in the SOA booster, in steps of 20 mA. **(d)** Overlaid spectra of the co-integrated device. Each spectrum corresponds to a different driving operation point at the Vernier filter. **(e)** Measured RF delayed self-heterodyne spectrum for different driving current values in the widely tunable laser (for a single \(\text{I}_{\text{SOA}}\) = 110 mA). Blue traces correspond to the Voigt fitting used to extract the laser linewidth. The curves have been up-shifted for convenience.