Frequency comb from a microresonator with engineered spectrum

We demonstrate that by varying the ratio between the linewidth and dispersion of a whispering gallery mode resonator we are able to control the number N of free spectral ranges separating the first generated comb sidebands from the pump. We observed combs with N = 19 and N = 1. For the comb with N = 1 we have achieved a span of over 200 nm using a 0.4 mm MgF₂ resonator pumped with 50 mW at 1560 nm. This pump power is a factor of 10 lower than previously reported for combs with comparable bandwidth.