This paper analyze the capacity performance in term of bit rate for free space optical (FSO) communication between the conventional intensity modulation-direct detection on off keying (IM/DD OOK) and new dual diffuser modulation (DDM) technique. This technique employs two transmitters and two receivers with differential mode detection at the receiver. This technique can improve the threshold decision process and expand the diffuser effect. As a result, the bit rate of FSO can be increased with 6 magnitude of BER or equal to 100% improvement percentage with considering at 2.5Gbps bit rate. Meanwhile at the effective power transmit for the comparison between 0dBm and 5dBm, the result of BER is at 3.45x10^-9 and 2.26x10^-12 respectively. This indicate that the DDM technique can performance better than conventional modulation technique with able to operate at low power transmit under the acceptable error floor BER 10^-9.