Contents 1. Introduction
Silicon photonics has become a mainstay of photonic integration for a wide range of applications [1]. One of the remaining challenges is the integration of light sources on the silicon platform. Several approaches to overcome this obstacle have been explored in the past. Electrically pumped Ge-on-Si lasers show an interesting path towards large-scale monolithic integration [2], but the performance of these devices still cannot compete with that of conventional InP-based light sources. As an alternative, epitaxial growth and wafer bonding of III-V semiconductors on silicon substrates have shown promising results [3], [4], but elaborate front-end processing outside established CMOS fabrication workflows is required. A simpler approach relies on mounting of pre-processed laser dies on silicon photonic chips [5]. This concept allows integration of known-good InP devices on the silicon platform, but requires high-precision alignment and leads to a large on-chip footprint. These disadvantages can be overcome by the concept of photonic wire bonding [6], which enables low-loss single-mode connections across chip boundaries. Photonic wire bonding does not require precise mechanical alignment of chips, and is therefore well suited for automated production. The viability of the technique has been demonstrated by coupling silicon photonic chips to each other [6] or to multi-core fibers [7]. The concept is also applicable to multi-chip integration of InP-based light sources, where photonic wire bonding of horizontal-cavity surface-emitting lasers (HCSEL) [8] to silicon photonic circuitry has previously been demonstrated [9]. However, coupling to HCSEL still requires dedicated surface-emitting coupling structures based on proprietary fabrication processes, whereas commercially available InP photonic circuitry usually relies on butt-coupling interfaces. Moreover, only single interfaces have been realized up to now, whereas photonic wire bonding lends itself to a co-integration of device arrays. Further, considering the fact that the laser is coupled to the silicon photonic chip without an intermediate optical isolator, it was still unclear if back-reflections into the laser cavity impact the laser linewidth.
