# Widely-Tunable Ring-Resonator Semiconductor Lasers
## 1 Introduction  
Chip-scale widely-tunable lasers have been of interest for some time [1]. Applications range from communications, especially wavelength-division multiplexing systems (WDM), to a number of sensing applications, such as wavelength-steered light detection and ranging (LIDAR)for autonomous driving [2]. There are also numerous other sensor applications that rely upon the ability to sweep the laser frequency over a wide wavelength range, such as spectroscopy or backscatter reflectometers. A fully-integrated widely-tunable semiconductor laser has a number of advantages compared to the mechanically tuned counterparts--size, weight and cost are most obvious ones, but equally important, is the ability to integrate them with other components to provide added functionality on the same chip. Integration using monolithic or heterogeneous processes assembles many devices or optical functionalities on a single chip, so that all optical connections are on-chip and require noexternal alignment. The need for external alignment can become a bottleneck at large enough volumes, and result in increased cost. Photonic integrated circuits (PIC) can be realized in a number of platforms, but for efficient on-chip light generation, a direct bandgap semiconductor is needed. Historically, the first integrated photonic circuits with lasers were realized on gallium arsenide (GaAs) or indium phosphide (InP) substrates for the same reason [3]. The performance of lasers on native III-V substrate is considered state-of-the-art, but the cost due to the lower size of wafers, and higher cost of substrates, still presents a barrier to some markets. More recently, optical components are being integrated on silicon (Si) substrates for fabricating large-scale photonic integrated circuits that co-exist with micro-electronic chips. Additionally, the economy of scale in the microelectronics industry offers much a larger size (300 mm) and lower cost wafers, which consequently offers cheaper cost per unit die area, provided the volumes are high enough to offset higher initial investment [4]. Furthermore, the maturity of Si processing is vastly superior as shown with the latest generation of microprocessors having more than a few billion transistors densely packed in an area of \(\sim\)450 mm\({}^{2}\). This is many orders of magnitude more complex than the most advanced InP PICs. As the complexity of PICs increase, yield can be a limiting factor, and that is one more reason why the Si platform is actively being pursued for complex PICs. In the past 15 years, a whole range of photonic components, including filters, (de) multiplexers, splitters, modulators, and photodetectors, have been demonstrated, mostly in the silicon-on-insulator (SOI) platform [5]. The SOI platform is especially suited for standard communication bands at 1300 nm and 1550 nm, as silicon (\(n=3.48\)) and its oxide SiO\({}_{2}\) (\(n=1.44\)) are both transparent, and form high-index contrast, high-confinement waveguides ideally suited for medium to high-integration PICs. In the shorter term, the rationale of silicon photonics is a reduction of the cost of photonic systems, most probably the ones targeted to the large volume data center market, but in the longer term, photonics should help solve the communications bottleneck, with the introduction of optics inside a high-performance electronic chip [6]. However, electrically pumped efficient sources on silicon remain a challenge due to the indirect bandgap of silicon. There have been some demonstrations with, e.g., strained Ge [7], but the performance of such sources is still very far from practical products. A large number of current Si photonic PICs use hybrid approaches in which Si PIC is coupled to III/V PIC that provides gain and lasing. Such an approach requires precise alignment, and arguably has scaling limitations. An alternative approach, most commonly called heterogeneous integration, bonds pieces of III/V materials on patterned Si wafer and then processes them using the same lithography tools.