Optical frequency measurement comparison using fiber laser combs between CMS and NMIJ

An Er-fiber laser comb with repetition frequency of 250 MHz made by the Center for Measurement Standards (CMS), Taiwan was shipped to the National Metrology Institute of Japan (NMIJ) to compare with the NMIJ fiber laser comb by simultaneously measuring the frequency of an optical cavity-stabilized Nd:YAG laser at 1064 nm. The measured average frequency difference is 32 mHz and corresponds to a relative difference of 1.1×10
^-16
, which is limited by the measurement time. Furthermore, method for determining the comb mode number using two combs with large difference of repetition rate is proposed.