The impact of varying the phase relationship between adjacent OTDM channels is investigated in 80 Gbit/s transmission experimentally and numerically. A fiber-based coherent multiplexer is proposed for OTDM experiments - a phase shifter in the multiplexer and an external phase control circuit are used to set and maintain the phase difference. It is demonstrated that the optimum modulation format for maximum transmission distance strongly depends on pulse width, e.g. 120 degrees-RZ provides the best performance for pulse width of 8 ps; however, 90 degrees-RZ is advantageous when pulse width is reduced to 2 ps. Power in 'zero' bit slots and amplitude jitter are calculated to demonstrate that the performance variation is due to intra-channel four-wave mixing (IFWM) and different receiver sensitivity at back-to-back. We also show that phase modulation formats are sensitive to optical filtering.