  We consider optical fibers as detectors for scalar ultralight dark matter
(UDM) and propose using a fiber-based interferometer to search for scalar UDM
with particle mass in the range $10^{-17} - 10^{-13}$ eV/$c^2$ $\left(10^{-3}-
10 \text{ Hz}\right)$. 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 well-suited to probe for a Solar halo of dark matter with a
sensitivity exceeding that of previous DM searches over the particle mass range
$7\times 10^{-17} - 2\times 10^{-14}$ eV/$c^2$.
