## 4 III-V-on-Silicon Devices for Optical Communication Applications
### III-V-on-Silicon Laser Sources for Optical Communication Applications
#### 4.1.5 1550 nm Mode-Locked Lasers  
Mode-locked laser diodes are gaining interest as compact wavelength division multiplexing (WDM) sources since one single laser can generate a large number of coherent wavelength channels. Also, they are the laser source of choice for optical time division multiplexing applications [39]. A mode-locked laser (MLL) produces a train of short pulses, which corresponds with a wide comb of optical modes in the frequency domain. Low noise, high output power and a broad output spectrum are of great importance for optical communication applications. In order to achieve low-noise performance, it is important to limit the length of the optical amplifier (to reduce spontaneous emission) and realize a low-loss optical cavity. The III-V-on-silicon platform allows using low-loss silicon waveguide structures (both in terms of nonlinear two-photon absorption as well as linear scattering losses) to form the laser cavity. This fact was also recognized by others, demonstrating low phase noise hybrid silicon MLLs. A 15 kHz 3 dB RF line width (at 9.95 GHz) was demonstrated in a linear cavity design with on-chip feedback in [40]. In this section we will discuss three different MLL geometries studied in our work. The first demonstrations were based on classical colliding ring cavity and linear cavity arrangements [41] as shown in Figure 11a,b. Further improvement of the output power and RF spectral purity was achieved with a novel anti-colliding pulse MLL design [42] as shown in Figure 11c. Mode-locking can be realized by placing a saturable absorber (SA) in a standard multimode laser cavity. In our case, the SA is made by electrically isolating a small part of the III-V gain material and reverse biasing that section. In the first two designs, the SA is placed in the center of the laser cavity. In the anti-colliding geometry, the SA is implemented on the low-reflectivity out-coupling mirror of the laser cavity. The optical mode is then coupled to the silicon waveguide using an adiabatic taper structure. The main part of the laser cavity length is realized in the silicon waveguide layer using a spiral waveguide structure. In each case, the length of the cavity is designed to reach a repetition rate of 5 GHz. Details about the device processing can be found in [41,42]. To induce passive mode-locking, the gain section was biased at 160 mA and the SA was reverse biased at \(-1.2\) V for the colliding linear cavity design, 179 mA and \(-1.3\) V for the ring design and 61 mA and \(-0.7\) V for the anti-colliding pulse geometry. The output of the mode-locked laser was coupled to an optical fiber  
Figure 10: (**a**) Cross section of a III-V-on-silicon DFB laser and asymmetric dual-mode DFB laser. (**b**) Optical output spectrum of asymmetric DFB laser at different injection currents. (**c**) Electrical output spectrum of UTC-photodiode illuminated by the dual wavelength laser showing an instantaneous linewidth of 4.2 MHz. (**d**) Integrated trace of electrical signal over thirty seconds. Reproduced from [38]. through a fiber-to-chip grating coupler. Device characterization is carried out on a thermo-electric cooler at 20. Table 1 compares the output characteristics of the three laser geometries. We note that the optical bandwidth of the self-colliding MLL, plotted in Figure 12a, is the widest of the three designs. The cavity of this laser is formed by two broadband DBR gratings etched in the silicon, whereas in the anti-colliding laser design, the bandwidth of the grating underneath the SA limits the bandwidth of the optical comb.