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In this paper, we examine the performance of two different phase modulation methods in coherent optical code-division multiple-access (OCDMA) systems. As optical spreading sequences, a recently introduced prime code family hereby referred to as double-padded modified prime code has been employed and analyzed. The OCDMA system operates under synchronous homodyne dual-balanced detection scheme. In this analysis, binary phase-shift keying format is deployed where the phase is modulated by either a Mach-Zehnder interferometer external phase-modulator or an injection-locking of distributed feedback laser diode's driving current. The performances of the two phase-modulation techniques in the presence of multiple-access interferences in a shot-noise limited regime are studied. Phase limitations and reasonable consequence associated with the limited phase excursion are also investigated.