We numerically study the synchronization of two chaotic semiconductor lasers in a master-slave con guration. To synchronize the lasers a small amount of output power from the master laser is injected, after propagating through an optical ber, into the slave laser. We show that the output of the master laser can be used as a chaotic carrier to encode a digital message which can be recovered after propagating long distances. We also check the robustness of this scheme when the two lasers are slightly di erent.