  A compact and robust coherent laser light source that provides spectral
coverage from the ultraviolet to infrared is desirable for numerous
applications, including heterodyne super resolution imaging[1], broadband
infrared microscopy[2], protein structure determination[3], and standoff
atmospheric trace-gas detection[4]. Addressing these demanding measurement
problems, laser frequency combs[5] combine user-defined spectral resolution
with sub-femtosecond timing and waveform control to enable new modalities of
high-resolution, high-speed, and broadband spectroscopy[6-9]. In this Letter we
introduce a scalable source of near-single-cycle, 0.56 MW pulses generated from
robust and low-noise erbium fiber (Er:fiber) technology, and we use it to
generate a frequency comb that spans six octaves from the ultraviolet (350 nm)
to mid-infrared (22500 nm). The high peak power allows us to exploit the
second-order nonlinearities in infrared-transparent, nonlinear crystals
(LiNbO$_3$, GaSe, and CSP) to provide a robust source of phase-stable infrared
ultra-short pulses with simultaneous spectral brightness exceeding that of an
infrared synchrotron[10]. Additional cascaded second-order nonlinearities in
LiNbO$_3$ lead to comb generation with four octaves of simultaneous coverage
(0.350 to 5.6 $\mu$m). With a comb-tooth linewidth of 10 kHz at 193 THz, we
realize a notable spectral resolving power exceeding 10$^{10}$ across 0.86 PHz
of bandwidth. We anticipate that this compact and accessible technology will
open new opportunities for multi-band precision spectroscopy, coherent
microscopy, ultra-high sensitivity nanoscopy, astronomical spectroscopy, and
precision carrier-envelope phase (CEP) stable strong field phenomena.
