Optical frequency comb-based cavity-ringdown spectroscopy has recently enabled high-sensitivity absorption detection of molecules over a broad spectral range. We demonstrate an improved system based on a mode-locked erbium-doped fiber laser source centered at $1.5\mu \mathrm{m}$ , resulting in a spectrometer that is inexpensive, simple, and robust. It provides a very large spectral bandwidth $(1.45–1.65\mu \mathrm{m})$ for investigation of a wide variety of molecular absorptions. Strong molecular absorptions at $1.5\mu \mathrm{m}$ allow for detection at sensitivities approaching the 1 part in ${10}^9$ volume level. We provide a detailed description of our spectrometer and present measurements of the rovibrational spectra for CO, $\mathrm{N}{\mathrm{H}}_3$ , and ${\mathrm{C}}_2{\mathrm{H}}_2$ with an absorption sensitivity of $2\times {10}^{-8}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1∕2}$ per detection channel.