High-rate femtosecond pulse generation via line-by-line processing of phase-modulated CW laser frequency comb

Introduction: High repetition rate optical pulses in a few picosecond range and below are playing an increasingly important role in high speed optical fibre communication systems. Such high repetition rate short pulses are routinely generated by harmonically modelocked lasers [1], but with the following limitations: (i) complicated feedback control is required; (ii) the modelocked frequency comb has limited tunability; (iii) at high repetition rates, these frequency combs often suffer from frequency instability. Alternatively, applying a strong periodic temporal phase modulation to a CW laser can generate a well-defined, broad frequency comb [2], which can support short pulse generation after appropriate control. For example, a modulated CW laser followed by singlemode fibre or other dispersion control components has been demonstrated for pulse generation [3–5]. Recently we demonstrated spectral line-by-line pulse shaping on frequency comb derived from a modulated CW laser to generate 12 ps pulses at 9 GHz [6]. Line-by-line pulse shaping significantly extends the capability of optical processing with a modulated CW laser since the intensity and phase of all individual spectral lines can be independently controlled [7]. Very recently an arrayed waveguide grating device was used to manipulate 20 spectral lines also generated from a modulated CW laser, resulting in pulses as short as 4.7 ps [8]. The number of spectral lines to be controlled is one of the most important parameters for all applications: (i) for short pulse generation, more spectral lines correspond to shorter pulses; (ii) generally for optical arbitrary waveform generation [9], the number of spectral lines determines the degree of complexity of the generated optical waveforms. In this Letter we report line-by-line pulse shaping on a phase-modulated CW laser to generate and control over 40 lines, resulting in 2.76 ps pulse generation at 9 GHz repetition rate. These pulses are further compressed down to 324 fs using a commercially available dispersion decreasing fibre. Compared with harmonically modelocked lasers, short pulses generated in this way have significant advantages including simple control, low cost, and a tunable and a stable optical frequency comb.