## 4 III-V-on-Silicon Devices for Optical Communication Applications
### 1550 nm Microdisk Lasers  
Heterogeneously integrated microdisk lasers have been studied for about a decade. A schematic of such a microdisk laser is shown in Figure 13, together with a SEM picture of a typical device cross section. Single mode, CW operating lasers with a typical diameter of 7 \(\upmu\)m can be realized with low threshold currents of 0.5 mA and output powers of a few tens of \(\upmu\)W coupled to a silicon waveguide circuit. The low threshold currents are possible thanks to the small volume of the InP membrane and the low losses of the whispering gallery mode. These losses can be minimized by using a tunnel junction and limiting the overlap of the mode with the top contact using careful alignment of the top contact, limiting the scattering loss due to sidewall surface roughness by optimizing the etching process and by designing a small evanescent coupling of the whispering gallery mode to the underlying silicon waveguide [44; 45; 46]. Figure 13: (**a**) Schematic view of an InP microdisk laser heterogeneously integrated onto SOI. (**b**) SEM picture of the device cross section. Reproduced from [44]. A special feature of microdisk lasers and ring lasers in general is the possibility of unidirectional lasing. This feature has potential applications in all-optical flip-flops and in low-power optical signal regeneration. In a symmetrical microdisk laser, without coupling between clockwise and counter clockwise modes, a bistable unidirectional behaviour is generally found just above the threshold current, with the lasing direction being determined by initial conditions or by externally injected optical pulses. This bistable unidirectional behaviour is possible because the cross gain suppression is twice as large as the self-gain suppression [47]. Due to sidewall surface roughness and reflections inside the silicon bus waveguide, there is usually coupling between clockwise and counter clockwise modes. In this case, bistable unidirectional operation is still observed in a symmetrical microdisk configuration, but only for sufficiently high bias currents (determined by the amount of coupling). Asymmetric microdisk configurations (with different coupling from clockwise to counter clockwise than from counter clockwise to clockwise) behave unidirectional in a direction determined by the strongest of the two coupling coefficients. In particular, if coupling from the clockwise to the counter clockwise mode is the largest, then lasing will be predominantly in the counterclockwise mode. The unidirectionality becomes progressively better with increasing bias current, again owing to the larger cross gain saturation [48]. The bistable unidirectional behavior found in symmetric microring or microdisk device configurations can be exploited to implement all-optical flip-flops. The lasing direction can be changed by injecting short optical pulses (with a wavelength sufficiently close to the lasing wavelength in order to obtain injection locking). Switching has been demonstrated with 100 ps pulses with energy as low as 1.8 fJ and switching times are less than 60 ps [49]. The unidirectional asymmetric microring lasers have been demonstrated to be suitable for low power all-optical 2R regeneration [50]. The regeneration relies on the fact that the injection of light with a wavelength within the injection locking bandwidth is physically equivalent to an extra reflection of the laser light. Hence, by externally injecting light into the direction opposite to the lasing direction, one can modify the equivalent coupling coefficients such that the lasing direction switches to the direction of the injected light signal. Figure 14 shows measured bit error rate curves at 10 Gbps without and with regeneration in a microdisk laser as a function of the received power, demonstrating this regenerative capacity. Figure 14: Measured BER curves at 10 Gbps, before and after regeneration with a microdisk laser. Reproduced from [50].