Searching for scalar ultralight dark matter via refractive index changes in fibers

We propose using an optical fiber-based interferometer to search for scalar ultralight dark matter (UDM) with particle mass in the range 10-17 - 10-13 eV/c2 o10-3 - 10 Hz thorn . Composed of a solid core and a hollow core fiber, the proposed detector would be sensitive to relative oscillations in the fibers' refractive indices due to scalar UDM-induced modulations in the fine-structure constant alpha. We predict that, implementing detector arrays or cryogenic cooling, the proposed optical fiber-based scalar UDM search has the potential to reach new regions of the parameter space. Such a search would be particularly wellsuited to probe for a solar halo of dark matter with a sensitivity exceeding that of previous dark matter searches over the particle mass range 7 x 10-17 - 2 x 10-14 eV/c2.