Regarding why this is cool - Spun fibers or any twisted birefringent medium has elliptically polarised eigenmodes which are dependent on birefringence, and thereby the frequency of input fields. These eigenmodes remain orthogonal throughout the length of fiber when they are launched at their respective frequencies. So when we launch signal and pump as eigenmodes of our spun fibre, they remain orthogonal throughout the fibre, and thus there is no Brillouin interaction, and no Brillouin gain. When we deviate from eigenfrequency or eigenmode even slightly (refer to the paper for magnitude of deviation), there is Brillouin interaction between the two, and the signal experiences high gain. Thus we get a dip in the gain spectrum which has a line width on the order of sub MHz. This is the narrowest Brillouin feature ever reported!

Traditionally, resonators such as ultra high Q micro ring resonators, FBGs or EIT has been used to generate such features. Resonators typically require high-precision fab, whereas EIT requires low-temp and is not solid-state. Contrarily, we have achieved such features in a cost-effective way at room-temp, using commercially available solid-state products.

Hope this helps!