In recent years, the hybrid silicon platform (HSP) has emerged as an important platform for the realization of large-scale photonic integrated circuits (PICs), which is attributed to the transparency of silicon at the telecom wavelength and the mature complementary metal oxide semiconductor (CMOS) technology that can be used for fabricating photonic devices with sub-micron features. Currently, state-of-the-art passive waveguide circuits, low power consumption resonant modulators, and high-speed photodetectors are available on this platform. However, the integration of a coherent light source on the silicon platform remains an issue, due to the indirect band gap of silicon.
For an on-chip light source in silicon, many approaches to light emission have been demonstrated, including Raman lasers, the nano-patterning laser, and Ge-on-Si laser. Currently, heterogeneous integration of III-V materials and SOI waveguides (including molecular bonding and adhesive bonding) is a principal approach to obtain an on-chip light source in silicon, owing to the high-density integration and avoiding costly active alignment required in the case of packaged lasers. With the use of bonding technology, a number of hybrid lasers have been successfully demonstrated, like the Fabry-Perot (FP) laser[7, 8], the racetrack laser[9, 10], the distributed feedback (DFB) laser, the distributed Bragg reflector (DBR) laser and the micro-cavity laser[13, 14, 15, 16]. However, these lasers require defining a resonant cavity by cleaving the Si wafer, except for the racetrack laser, DBR laser and micro-disk laser, which makes it a difficulty for an on-chip light source in silicon. Although the hybrid laser by etching DBR on Si can obtain a single-mode light source in silicon, its large size and mode instability block its further development. Besides, an on-chip racetrack resonator laser without facet polishing and dicing for defining the laser cavity has been demonstrated. However, it was a pity that only multimode lasing was obtained. In order to overcome this problem and obtain single longitudinal mode operation, our team has designed and demonstrated the single mode laser (SMSR of larger than 20~dB in the CW regime) through integrating a racetrack ring with slots to build a mode-selection mechanism in III-V/SOI hybrid architectures. Despite the realization of single-mode lasing, the footprint of this laser was relatively large, resulting in the high threshold current and high power consumption. Therefore, how to reduce the footprint of a racetrack ring has been an attractive subject for improving the characteristics of the laser.
Diode lasers with ring or disk resonator geometries are one of the most attractive on-chip light sources for PICs, because of their low threshold current and low power consumption[13, 14]. In the past few decades, the development of hybrid III-V/silicon micro-cavity lasers (ring or disk) have acquired remarkable achievements[13, 14, 15, 16, 17]. Electrically pumped hybrid micro-disk and micro-ring lasers were realized separately, which all demonstrated low threshold current and power consumption. However, the single mode operation and directional emission in micro-disk lasers were greatly limited by the symmetry of the micro-disk, and much work was focused to get directional emission microlasers[15, 16]. Currently, one of the most popular approaches is to add an integrated optical reflector, such as a teardrop reflector }$or DBR, at one end of the bus waveguide. These approaches have been shown to obtain directional output[15, 16]. However, stable directional operation requires strongly high requirements for photolithography and etching, in order to reduce the sidewall roughness and minimize back-reflection. In addition, the SMSR of these structures was less than 20 dB, which showed a bad single mode output. Another approach is to break the rotational symmetry by using deformed microcavities or connecting an output waveguide to increase the directionality of emission and power collection efficiency. In fact, a deformed micro-disk laser with an output waveguide is an excellent choice to realize the single mode lasing and directional emission. An AlGaInAs/InP octagonal resonator microlaser connected to an output waveguide has been designed and demonstrated, which obtained the SMSR of 24 dB.
In this letter, an electrically-pumped hybrid III-V/silicon deformed micro-disk laser connected to a Si output waveguide has been designed and demonstrated. In order to ensure the light lasing from the III-V active area efficiently couples to the silicon waveguide, a taper mode converter structure is introduced. Furthermore, the deformed design of the micro-disk laser has realized unidirectional emission and single-longitudinal-mode operation from a Si waveguide, which can be utilized as an effective light source for silicon-based optical interconnects. In the experiments, an output power of 0.31 mW and an SMSR of 27 dB in the CW regime is obtained.