  Microcombs are revolutionizing optoelectronics by providing parallelized,
mutually coherent wavelength channels for time-frequency metrology and
information processing. To implement this essential function in integrated
photonic systems, it is desirable to drive microcombs directly with an on-chip
laser in a simple and flexible way. However, two major difficulties are
preventing this goal: 1) generating mode-locked comb states usually requires a
significant amount of pump power and 2) the requirement to align laser and
resonator frequency significantly complicates operation and limits the
tunability of the comb lines. Here, we address these problems by using
microresonators on an AlGaAs on-insulator platform to generate dark-pulse
microcombs. This highly nonlinear platform dramatically relaxes fabrication
requirements and leads to a record-low pump power of less than 1 mW for
coherent comb generation. Dark-pulse microcombs facilitated by
thermally-controlled avoided mode-crossings are accessed by direct DFB laser
pumping. Without any feedback or control circuitries, the comb shows good
coherence and stability. This approach also leads to an unprecedented wide
chirping range of all the comb lines. Our work provides a route to realize
power-efficient, simple and reconfigurable microcombs that can be seamlessly
integrated with a wide range of photonic systems.
