DCM/DiffD is proposed here as a new transmission and detection technique which provides longer transmission distance than the conventional technique. This technique uses amplitude subtraction between original data bits and their inversion to provide a simple dynamic decision making scheme. The inverted data basically serve as the threshold in place of the conventional externally injected voltage. DCM/DiffD technique is shown here to double the eye opening and improves the performance significantly. BER of as good as 10⁻¹⁴ at STM-16 rate over 150 km transmission distance with 0 dBm transmit power is demonstrated in this report. The new technique also shows a much slower degradation with distance than that of the conventional IM/DD systems.