Absolute Mode Number Determination of Frequency Combs

We demonstrate a method to determine the absolute mode number of a frequency comb when it is used for high precision laser frequency measurement, without the help of wavemeters. Our technique involves changing the repetition rate of the frequency comb in a two-steps process. Guidelines for choosing the correct repetition rates for different laser linewidths are given. As a demonstration, the absolute frequencies of two lasers with different linewidths are measured with our method.