Low-Threshold and Sub-kHz-Linewidth Brillouin Laser Based on Chalcogenide Fiber at 2 $\mu$m

We experimentally demonstrate a Brillouin fiber laser (BFL) with a low threshold and narrow linewidth at 2
$\mu$
m employing a chalcogenide As
$_{2}$
S
$_{3}$
fiber as the Brillouin gain fiber. The As
$_{2}$
S
$_{3}$
fiber exhibits a Brillouin gain coefficient of 1.76
$\times 10^{-9}$
m/W at 2
$\mu$
m, which is two orders of magnitude higher than that of the silica fiber, together with a Brillouin frequency shift of 6.308 GHz and a gain linewidth of 10.2 MHz. By theoretically optimizing the fiber length adopted in the Brillouin ring cavity, a 3-m-long segment of the As
$_{2}$
S
$_{3}$
fiber is used to realize a BFL with excellent performance in terms of laser power threshold and linewidth. The BFL shows a total cavity loss of 6.18 dB, corresponding to a cavity feedback parameter of 24.1
$\%$
. The lowest laser power threshold of 39 mW and narrowest laser linewidth of
$\sim$
619 Hz so far, to the best of our knowledge, are simultaneously achieved for the BFL at 2
$\mu$
m. These results benefit from not only the extremely high Brillouin gain coefficient and small effective mode field area of the As
$_{2}$
S
$_{3}$
fiber but also a suitable Brillouin gain fiber length and low cavity loss of the BFL. The relative intensity noise and temporal stability of the BFL are also investigated. Such a low-threshold and sub-kHz-linewidth laser will facilitate the development of practical applications such as gas sensing and coherent LIDAR in the 2
$\mu$
m spectral range.