  Portable mid-infrared (mid-IR) spectroscopy and sensing applications require
widely tunable, narrow linewidth, chip-scale, single-mode sources without
sacrificing significant output power. However, no such lasers have been
demonstrated beyond 3 $\mu$m due to the challenge of building tunable, high
quality-factor (Q) on-chip cavities. We demonstrate a tunable, single-mode
mid-IR laser at 3.4 $\mu$m using a high-Q silicon microring cavity with
integrated heaters and a multi-mode Interband Cascade Laser (ICL). We show that
the multiple longitudinal modes of an ICL collapse into a single frequency via
self-injection locking with an output power of 0.4 mW and achieve an oxide-clad
high confinement waveguide microresonator with a loaded Q of $2.8\times 10^5$.
Using integrated microheaters, our laser exhibits a wide tuning range of 54 nm
at 3.4 $\mu$m with 3 dB output power variation. We further measure an
upper-bound effective linewidth of 9.1 MHz from the locked laser using a
scanning Fabry-Perot interferometer. Our design of a single-mode laser based on
a tunable high-Q microresonator can be expanded to quantum-cascade lasers at
higher wavelengths and lead to the development of compact, portable,
high-performance mid-IR sensors for spectroscopic and sensing applications.
