High dimensional, broadband chaotic optical signal can be generated by a semiconductor laser with external disturbance, which is an ideal carrier for high-speed secure communications. The performances of chaotic optical secure communication systems are numerically studied by encrypting the message by chaos modulation (CMO) or chaos masking (CMS). The variation of the signal-to-noise ratio (SNR) and Q-factor are demonstrated when sinusoidal and digital messages are transmitted separately by CMO or CMS. The correlation coefficient as a function of modulation depth (MD) is discussed. The results show that the system performance varies for different MD or modulation frequency/rate and the performance of CMO system is better than that of CMS.