Sub-100 fs all-fiber broadband electro-optic optical frequency comb at 1.5 µm

A sub-100 fs all-fiber broadband optical frequency comb seeded from a 12.5 GHz electro-optic modulated pulse is presented. Combining pulse reshaping, nonlinear mixing with dispersion compensation processes, a frequency comb with the main pulse width of 86 fs was achieved. The frequency comb has a 6 dB spectral bandwidth spanning over 150 nm which corresponding to more than 1500 comb tones. The measured average power of the broadband comb is over 550 mW, and the calculated average power of each comb line is roughly -4 dBm. To illustrate the whole spectral broadening process, a numerical investigation was also brought out, showing a very good match with the experiments. With a delayed self-heterodyne interferometer, the evolutions of the seed comb linewidths and the broadened comb linewidths were measured revealing the same parabolic trend. Specifically, the linewidths of the 20 seed comb lines are less than 10 kHz, while the linewidths of the 400 broadened comb lines are less than 1 MHz. The results also indicate that the nonlinear mixing led to an accumulation of the phase noise with respect to the comb line number, indicating that a low phase noise RF source or phase locking technique is essential to produce ultra-low phase noise broadband electro-optic combs.