We have analyzed numerical code modulation of semiconductor microcavity lasers by using rate equations. It is found that microcavity lasers have high speed pulse code response and minimum error probability. An excellent eye diagram can be obtained with the microcavity lasers modulated by a 50 Gbit/s current if their spontaneous emission factors equal 0.1. The lasers will open vast vistas in future optical connection. Furthermore, we have simulated fiber communication systems with microcavity lasers as the light sources 10 Gbit/s numerical codes modulated and have given the received eye diagrams sfter transmission of different distances. It is proved that the system can stand for 60 kilometers'transmission.