  Laser frequency combs based on erbium-doped fiber mode-locked lasers have
shown great potential for compact, robust and efficient optical clock
comparisons. However, to simultaneously compare multiple optical clock species,
fiber laser frequency combs typically require multiple amplifiers and fiber
optic paths that reduce the achievable frequency stability near 1 part in
$10^{16}$ at 1s. In this paper we describe an erbium-fiber laser frequency comb
that overcomes these conventional challenges and supports optical frequency
synthesis at the millihertz level, or fractionally $3 x 10^{-18}$ $tau^{-1/2}$
by ensuring that all critical fiber paths are within the servo-controlled
feedback loop. We demonstrate the application of this frequency comb as a
synthesizer for optical clocks operating across a wavelength range from 650 nm
to 2100 nm.
