  We present a laser frequency comb based upon a 250 MHz mode-locked
erbium-doped fiber laser that spans more than 300 terahertz of bandwidth, from
660 nm to 2000 nm. The system generates 1.2 nJ, 70 fs pulses at 1050 nm by
amplifying the 1580 nm laser light in Er:fiber, followed by nonlinear
broadening to 1050 nm and amplification in Yb:fiber. Extension of the frequency
comb into the visible is achieved by supercontinuum generation from the 1050 nm
light. Comb coherence is verified with cascaded f-2f interferometry and
comparison to a frequency stabilized laser.
