  We present a compact, reliable, and robust free-running
all-polarization-maintaining erbium (Er) single-cavity dual-comb laser
generated via polarization multiplexing with gain sharing. Polarization
multiplexing exploits the fast and slow axes of the fiber, while modelocking is
achieved through a nonlinear amplifying loop mirror scheme using readily
available components. The laser operates at a repetition rate of around 74.74
MHz with a tuning capability in the difference in repetition rates from 500 Hz
to 200 kHz. This tunability makes the system more flexible for dual-comb
spectroscopy experiments. Consequently, using this laser, we demonstrated a
proof-of-principle dual-comb spectroscopy of carbon monoxide (CO), operating
without any active stabilization.
