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Adaptive Spectral Phase Optimization of High Repetition Rate Electro-Optic Frequency Combs for Enhanced Nonlinear Spectral Broadening

Frequency combs are used in optical communications, arbitrary RF waveform generation and astronomical spectrograph calibration [1]. Combs with tunable repetition rate can be generated by electro-optic modulation of lasers but are limited in bandwidth (BW) [2]. Wide band combs can be generated by cascaded four-wave mixing with two lasers in highly nonlinear fibre (HNLF) [3], but the spectral spacing of the two pumps determines both bandwidth and repetition rate limiting the independent control of the two. Spectral broadening of electro-optic combs can achieve tunable repetition rate and wide bandwidth. However, the temporal and/or spectral profile is not suitable for spectral broadening. Different methods have been used such as pulse compression in SMF [4] and pulse shaping prior to nonlinear broadening. However, such open loop methods resulted in poor flatness in the central region of comb and are not robust to varying operating and drive conditions of the modulators, initial spectral phase profile, modulator bias, length, dispersion profile of HNLF and power. In this work, we utilise an adaptive spectral phase optimization based on adaptive step size random search algorithm in a closed loop to optimize the spectral phases to obtain the broadest comb. We demonstrate bandwidth scaling by over 13 times of an initial 25GHz comb with 9lines (0.2THz) to 121 lines (3THz).