## 4 III-V-on-Silicon Devices for Optical Communication Applications
### III-V-on-Silicon Laser Sources for Optical Communication Applications
#### 4.1.3 1550 nm Tunable and Multi-Wavelength Lasers Integrated on Silicon PICs  
Once having a high performance III-V silicon hybrid optical amplifier available, it is possible to combine it with the rich variety of building blocks offered by the silicon photonics platform to construct more complex laser devices as illustrated in Figure 8. Figure 8a shows some of the typical building blocks being used: the hybrid amplifier itself, DBR-mirrors used as on-chip reflectors and tunable ring resonators and wavelength multiplexers for providing wavelength selectivity. The DBR-mirrors used as on-chip reflectors define the laser cavity and replace traditional cleaved facets. It is important that they are well designed to have controllable reflection and low transmission loss, especially at the output side of the device. They could e.g., be implemented using a shallow etch step in a locally widened 220 nm high waveguide as described in [27]. Figure 8: Schematic layout of complex hybrid III-V silicon tunable and multi-frequency lasers. (**a**) Some basic building blocks in our tool box, including a III-V on silicon hybrid amplifier, a standard and high reflection DBR-mirror, a tunable ring resonator and an integrated wavelength multiplexer. (**b**) Different implementations for tunable lasers. (**c**) A multi-frequency laser. Figure 7: (**a**) Small signal response at different bias currents and large signal eye diagram at 28 Gbps with 2.1 dB extinction ratio using 2\({}^{11}\)-1 pattern length (inset). (**b**) bit error ratio (BER) measurements for back-to-back and 2 km DSSMF configuration. Reproduced from [26]. Integrating the optical amplifier with a tunable ring resonator allows building a tunable laser as shown in Figure 8. To ensure wide tunability, narrow linewidth and a relatively straightforward implementation thermal tuning is generally preferred. Using a single ring resonator, tuning of around 10 nm typically can be reached. E.g., in [8] we demonstrated up to 8 nm tuning by applying 25 mW of electrical power to the ring resonator (Figure 9a). This device had a threshold current of 45 mA and output power of up to 10 mW at room temperature. The linewidth was measured to be around 1.7 MHz (at 80 mA). In addition, more complex ring resonator based reflectors have been demonstrated [28]. To increase the tuning range, multiple ring resonators with slightly different free spectral range (FSR) could be integrated, exploiting the so-called Vernier effect. Several different configurations have been proposed in literature (e.g., a 35 nm tuning range was demonstrated in [29]). The exact placement of the ring resonators can influence the maximal attainable output power and care has to be taken in optimizing the overall design. The ring resonator based tunable lasers described above typically require a rather complex tuning scheme and are designed to generate only a single wavelength channel at a time. To alleviate this problem, so-called digitally tunable lasers or multi-frequency lasers were demonstrated [27]. They consist of an amplifier array, in this case a hybrid III-V on silicon amplifier array, integrated with a wavelength multiplexing device such as an arrayed waveguide grating (AWG), a series of ring resonators or a planar concave grating (PCG). Turning on a single amplifier generates a single well-defined wavelength channel without further control. Turning on multiple amplifiers allows generating a comb of wavelengths with well-defined channel spacing in a single output waveguide. Figure 9b shows the output spectrum for a 4-channel device realized by integrating 4 amplifiers with a silicon AWG. Very good suppression of the side modes is obtained. A similar device was realized integrating the amplifier array with a demultiplexer consisting of four ring resonators with slightly increasing radii coupled to a common bus waveguide [27]. Similar threshold currents, output powers and linewidths were obtained as for the single ring tunable laser described above.