  Tunable lasers emitting at a 2-3 $\mu$m wavelength range and compatible with
photonic integration platforms are of great interest for sensing applications.
To this end, combining GaSb-based semiconductor gain chips with Si$_3$N$_4$
photonic integrated circuits offers an attractive platform. Herein, we exploit
the low-loss features of Si$_3$N$_4$ waveguides and demonstrate a hybrid laser
comprising a GaSb gain chip with an integrated tunable Si$_3$N$_4$ Vernier
mirror. At room temperature, the laser exhibited a maximum output power of 15
mW and a tuning range of 80 nm (1937-2017 nm). The low-loss performance of
several fundamental Si$_3$N$_4$ building blocks for photonic integrated
circuits is also validated. More specifically, the single-mode waveguide
exhibit transmission loss as low as 0.15 dB/cm, the 90$^\circ$ bend has 0.008
dB loss, and the 50/50 Y-branch has an insertion loss of 0.075 dB.
