In high bit-rate retum-to-zero (RZ) on-off keying (OOK) systems operating at 40Gb/s and beyond ghost pulses induced by intrachannel four-wave mixing (IFWM), and intrachannel cross-phase modulation (IXPM) are dominant factors affecting the performance of a high-speed optical transmission system. As the creation of ghost pulses is a phase sensitive effect, the common approach to deal with IFWM is to remove short-term phase coherence by employing a proper modulation format. Another approach, known as constrained or line coding, is to identify the most troublesome sequences and to forbid them during encoding process. This paper presents an efficient method for testing the efficiency of a modulation techniques for countering the effects of IFWM by calculating the sum of all the contributors at a given resonance position. For a modulation technique to counter these effects the initial pulses phases must be chosen properly so that different contributors to the ghost pulse creation cancel each other in bit being in "resonance". In addition to this method, we are proposing a modulation technique that shows a significant Q-factor improvement of up to 7.5dB (depending on the number of spans and the phase sequence implemented) over the uncoded RZ-OOK as will be demonstrated. Finally, implementation of this phase modulation technique is very efficient in extending the non-linearity tolerance and enhancing the performance of a high-speed optical transmission system.