Low-Threshold, High-Power On-Chip Tunable III-V/Si Lasers with Integrated Semiconductor Optical Amplifiers  
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Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/](https://www.mdpi.com/journals/)]([https://www.mdpi.com/journals/](https://www.mdpi.com/journals/))  
still a long way to go before it can be considered for industrial purposes. Alternatively, microtransfer printing has recently been subject of intense research due to the foreseeable advantages over other technologies [15, 16]. Microtransfer printing enables parallel device processing of both III-V and silicon photonics devices/circuits on their native substrates, minimizes the material waste, and permits substrate reuse. Moreover, microtransfer printing can be performed over virtually any kind of substrate, which provides a wealth of opportunities that will certainly promote exploratory research in the years to come. For now, however, this technology is still in maturation; therefore, this III-V/Si microtransfer printing activity is mostly localized in academia, although a drift towards industry could be expected in the long-term. Thus, in the short-term, heterogeneous integration remains the most suitable option to implement III-V-on-Si lasers and amplifiers with high volume production and a high technology readiness level [17]. Apart from the CMOS compatibility and the dense device integration, this technology offers an easy alignment between III-V active waveguides with the passive Si photonic circuits using only standard photolithography tools, provided that robust broadband optical transitions are employed. At the device level, research efforts have been mostly driven to improve the static and dynamic performance of individual III-V-on-Si lasers/amplifiers in terms of laser threshold, output saturation power, spectral and electro-optical bandwidth, chirp or laser linewidth. Tremendous progress has been made since the first demonstration of evanescently coupled III-V/Si lasers and SOAs in 2006 and 2007, respectively [18, 19]. A non-exhaustive list of remarkable breakthroughs include the demonstration of a distributed feedback hybrid silicon laser [20, 21]; a widely tunable and narrow linewidth laser with a tuning range of 120 nm and an emission linewidth of 140 Hz [22]; a high-speed membrane laser with 108 GHz bandwidth [23]; the demonstration of high-speed evanescent quantum-dot lasers on Si [24]; or a temperature-robust coarse wavelength division multiplexing transceiver composed by four single-wavelength lasers capable of delivering 20 mW of output power at 80 \({}^{\circ}\)C [25]. Further details on the recent progress in III-V/Si lasers and amplifiers are available elsewhere [26, 27, 28, 29]. In the last few years, increased research activity on the integration of multiple III-V/Si light sources and amplifiers on a single chip is being carried out. Several demonstrations of complex III-V/Si photonic circuits have been reported, such as a microwave generator, an interferometric optical gyroscope driver, a two-dimensional beam scanner or a network-on-chip [30]. In all these cases, the design of compact devices with small footprints and the sequential placement of optical amplifiers along the photonic circuit to compensate for the insertion loss and relax the on-chip routing constraints are key points to continue growing in that direction [31]. In this work, we report on the co-integration of low-threshold, widely tunable C-band lasers with compact semiconductor optical amplifiers (SOAs) on a heterogeneous III-V-on-SOI platform. Optimum performance of each individual component has been pursued through a thoughtful optical mode engineering of the heterogeneous waveguide modes.