Two laser diodes (slave lasers) have been frequency-locked at the comb resonance frequencies of a Fabry-Perot interferometer. The measured frequency spacing stability is in the order of 10-11(τ=10s). Moreover the Fabry-Perot interferometer used as the reference in the experiment has been frequency-locked at a master laser beam. This technique can be applied to frequency division multiplexing coherent optical fiber communication.