High repetition rate sub-picosecond pulse generation through compression of adaptively optimised frequency combs based on phase-modulated continuous wave lasers

High repetition rate pulsed lasers are used for applications such as highspeed optical communications, nonlinear optics and optical sampling. Conventionally, mode locked lasers are used as pulsed sources. However, they suffer from low repetition rate that is not tunable. Electro-optic modulation allows generation of frequency combs with tunable high repetition rate. These combs can be compressed to generate pulses, but the pulse widths are large owing to the limited bandwidth of electro-optic frequency combs. Though, modulators can be cascaded to scale the bandwidth, it is only a linear enhancement and requires additional RF components. Spectral broadening of these combs in nonlinear fibers results in marginally improved bandwidths. The broadening achieved at a given optical power can be enhanced several times by suitably modifying the temporal profile of the comb before spectral broadening with a pulse-shaper. In this work, electro-optic intensity and phase modulators are driven at 25GHz to generate an initial comb. A pulse-shaper adaptively optimises the temporal profile of the electro-optic frequency comb to enhance the spectral broadening in highly nonlinear fiber (HNLF). The optimised comb is power scaled in an Er-Yb co-doped fiber amplifier before HNLF. The adaptively optimised comb is compressed with single mode fiber and is characterized with zero delay implementation of spectral shearing interferometry to obtain the spectral phase and temporal profile of the pulses. The output pulses have 1 dB bandwidth of ~0.6 ps and 3 dB bandwidth of ~ 1 ps with high repetition rate of 25GHz. This adaptive technique is shown to be immune to drifts and changes in modulator drive conditions.