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 μm, resulting in a spectrometer that is inexpensive, simple, and robust. It provides a very large spectral bandwidth (1.45-1.65 μm) for investigation of a wide variety of molecular absorptions. Strong molecular absorptions at 1.5 μm allow for detection at sensitivities approaching the 1 part in 109 volume level. We provide a detailed description of our spectrometer and present measurements of the rovibrational spectra for CO, NH3, and C2H2 with an absorption sensitivity of 2 × 10-8 cm-1 Hz -1/2 per detection channel.