  Dual-comb interferometry is based on self-heterodyning two optical frequency
combs, with corresponding mapping of the optical spectrum into the
radio-frequency domain. The dual-comb enables diverse applications, including
metrology, fast high-precision spectroscopy with high signal-to-noise ratio,
distance ranging, and coherent optical communications. However, current
dual-frequency-comb systems are designed for research applications and
typically rely on scientific equipment and bulky mode-locked lasers. Here we
demonstrate for the first time a fully integrated power-efficient
dual-microcomb source that is electrically driven and allows turnkey operation.
Our implementation uses commercially available components, including
distributed-feedback and Fabry--Perot laser diodes, and silicon nitride
photonic circuits with microresonators fabricated in commercial multi-project
wafer runs. Our devices are therefore unique in terms of size, weight, power
consumption, and cost. Laser-diode self-injection locking relaxes the
requirements on microresonator spectral purity and Q-factor, so that we can
generate soliton microcombs resilient to thermal frequency drift and with
pump-to-comb sideband efficiency of up to 40\% at mW power levels. We
demonstrate down-conversion of the optical spectrum from 1400 nm to 1700 nm
into the radio-frequency domain, which is valuable for fast wide-band Fourier
spectroscopy, which was previously not available with chip-scale devices. Our
findings pave the way for further integration of miniature microcomb-based
sensors and devices for high-volume applications, thus opening up the prospect
of innovative products that redefine the market of industrial and consumer
mobile and wearable devices and sensors.
