  Recently developed techniques for generating precisely equidistant optical
frequencies over broad wavelength ranges are revolutionizing precision physical
measurement [1-3]. These frequency "combs" are produced primarily using
relatively large, ultrafast laser systems. However, recent research has shown
that broad-bandwidth combs can be produced using highly-nonlinear interactions
in microresonator optical parametric oscillators [4-11]. Such devices not only
offer the potential for developing extremely compact optical atomic clocks but
are also promising for astronomical spectroscopy [12-14], ultrashort pulse
shaping [15], and ultrahigh-speed communications systems. Here we demonstrate
the generation of broad-bandwidth optical frequency combs from a
CMOS-compatible integrated microresonator [16,17], which is a fully-monolithic
and sealed chip-scale device making it insensitive to the surrounding
environment. We characterize the comb quality using a novel self-referencing
method and verify that the comb line frequencies are equidistant over a
bandwidth that is nearly an order of magnitude larger than previous
measurements. In addition, we investigate the ultrafast temporal properties of
the comb and demonstrate its potential to serve as a chip-scale source of
ultrafast (sub-ps) pulses.
