## 1 Introduction  
The development of ultrafast physics has made tremendous progress in the past decade. This includes the development of broadband coherent optical sources which are desirable in ultrafast source development as well as in a wide range of research areas including laser spectroscopy and quantum optics [1]. While it is possible to produce ultrashort pulses via Fourier waveform synthesizer at optical frequencies that leads to a periodic train of single-cycle attosecond pulses [2; 3; 4], more attention has been given to isolated pulse generations in the femtosecond and attosecond time domain. This is because an isolated pulse can be employed readily for experiments in probing the electronic dynamic in atoms and molecules [5]. The common approaches that have been utilized for generating broadband multi-octave coherent optical sources include self-phase-modulation/cross phase-modulation of femtosecond laser pulses in gases and solids [6; 7], driving Raman resonances to produce sidebands by four-wave mixing with nanosecond lasers or femtosecond lasers [8; 9; 10], synchronously pumped femtosecond optical parametric oscillators [11] or supercontinuum seeded optical parametric amplifiers [12]. A common feature of these approaches is that they use a high power pulsed laser as the pump source. Although there are impressive demonstrations of using these sources as femtosecond waveform synthesizers [13; 14], stringent requirement on phase stability dictates that the synthesized waveform easily varies with time unless elaborate feedback control is installed in the system [13].  
The opposite extreme to isolated pulses is a continuous train of sub-cycle optical pulses. Since cw lasers are intrinsicaly more stable and are better managed than pulsed lasers, a train of sub-cycle pulses at \(\sim\)100 THz can offer unprecedented precision and accuracy to as much as one part in \(10^{14}\) in ultrafast event timing, ranging, quantum control, and for metrologic applications. A Fourier waveform synthesizer using a phase-locked cw laser has a main attribute that pulsed lasers do not have. It can maintain highly precise phase and amplitude stability for each comb component to yield waveforms that are nearly free from waveform variation [3]. A cw periodic waveform can therefore be used to produce ultra-stable novel shaped field potentials for trapping neutrals and charged particles and study their short-range dynamical behavior.  
There are several new developments toward a broadband coherent optical source for the purpose of making a cw Fourier waveform synthesizer. A cavity-enhanced molecular modulation scheme has been demonstrated [15]. The spectral components of the molecular modulator were extended to span from 0.8 \(\upmu\)m to 3.2 \(\upmu\)m via four-wave mixing in a gas medium. A 4-THz cw frequency comb at 1.56 \(\upmu\)m based on cascading quadratic nonlinearities has also been realized [16]. Yet the power of the frequency bands produced in these developments are in the mW level so that their potential applications are limited. Here we describe an approach that utilizes reliable high power cw fiber lasers. The approach employs a master-oscillator fiber-laser power amplifier pumped frequency division-by-three parametric oscillator [17; 18] to provide the first three comb components and then adopt quasi-phase-matched (QPM) nonlinear mixing to generate the next three higher harmonics to give a highly-stable commensurate sixcomponent cw comb at the watt level. The approach we describe here encompasses advantages of QPM based parametric oscillators and frequency conversions which are compact and have inherently high conversion efficiency.