  The optical properties of the superconducting single crystal
Rb$_{0.8}$Fe$_{1.68}$Se$_2$ with $T_{c}$ $\simeq$ 31 K have been measured over
a wide frequency range in the $ab$ plane. We found that the optical
conductivity is dominated by a series of infrared-active phonon modes at
low-frequency region as well as several other high-frequency bound excitations.
The low-frequency optical conductivity has rather low value and shows quite
small Drude-like response, indicating low carriers density in this material.
Furthermore, the phonon modes increase continuously in frequency with
decreasing temperature; specifically, the phonon mode around 200 cm$^{-1}$
shows an enhanced asymmetry effect at low temperatures, suggesting an
increasing electron-phonon coupling in this system.
