We report an optical single-frequency synthesizer at the 1.55 μm telecommunications band. Output from a continuous-wave external cavity diode laser is frequency doubled and phase locked to a predetermined component of a Ti:S laser frequency comb. The synthesizer is capable of generating a single user-specified frequency from an atomic time base within the 192-196 THz gain bandwidth of an erbium-doped fiber amplifier. By tuning the repetition rate of the femtosecond laser the synthesized optical frequency can be swept with sub-kilohertz step size. Frequency sweeps of several GHz are realized by automatically re-locking the diode laser to adjacent comb components during frequency sweep. We demonstrate the operation of the device by presenting results of Doppler-free spectroscopy on acetylene using synthesized frequencies.