Two advanced approaches for a semi-automatic, octave-spanning optical frequency counter

This paper demonstrates two approaches to determine the absolute mode number of the comb line for a semi-automatic optical frequency counter based on two mode-locked fiber laser combs. In the first approach, two laser combs were operated at three different repetition rates with two of them are so close to each other that their beating mode numbers are the same. In the second approach, the laser combs were operated at two different repetition rate and monochromators were used to offer an approximate mode number to determine the exact mode number from the two measured beat frequencies.